Volume 07 - Powder Metal Technologies and Applications Part 17 docx

Powder Metallurgy Electrical Contact Materials Composite Manufacturing Methods The methods used to manufacture composite contact materials can be classified into three major categories:

Circuit breakers, arcing tips

Complex composite contacts

(a) PSR, press-sinter-re-press; INF, press-sinter-infiltrate; PS, press-sinter; PSE, press-sinter-extrude; IO, internal oxidation; PPSE,

preoxidize-press-sinter-extrude; SF, oxidized from one direction

(b) A: Advance Metallurgy, Inc., McKeesport, PA C: Contacts, Materials, Welds, Inc., Indianapolis, IN E: Englehard Industries, Plainville, MA G:

Gibson Electric Inc., Delmont, PA S: Stackpole Carbon Co., St Marys, PA T: Texas Instruments Inc., Attleboro, MA M: Metz Degussa, South

Plainville, NJ W: Art Wire-Duduco, Cedar Knolls, NJ

(c) Annealed

(d) Cold worked

Table 2 presents the compositions and properties of various composite contact materials Because manufacturing methods affect the properties of materials with the same composition, the manufacturing methods are also given in Table 2 The most common methods of producing composite electrical contact materials are described in the section "Composite Manufacturing Methods" in this article

Data published by contact manufacturers usually include density, hardness, and electrical conductivity (Table 2) These data provide designers of electrical devices with the basic properties of a composite contact Other properties, such as contact resistance, may depend on operational parameters such as force (Fig 3)

Fig 3 Contact resistance versus force for fine silver and Ag-CdO contacts Unarced contacts were 12.7 mm (

in.) in diameter with a 38 mm (1 in.) spherical radius Resistance measurements were made with ac current

at 50 A and 60 Hz

Characteristics that relate directly to failure modes such as arc erosion or material transfer are usually described in a qualitative manner Very few quantitative data pertaining to these characteristics have been published because these properties depend on several test parameters For instance, the arc erosion rate is affected by various mechanical factors:

• Opening force and opening speed

• Closing force and closing speed

• Bouncing of the movable contact

• Power factor (inductive/capacitive)

Because each variable can greatly affect the arc erosion rate of a composite, it is virtually impossible to define a universal test to evaluate erosion rate

Published data on erosion rate and welding frequency usually are collected under very specific conditions They are valid only for qualitative description in a specific set of circumstances and cannot be extrapolated to suit other applications The only means of learning how a composite will perform in a specific application is to test it extensively in the device in which it will be used Examples of test data are given in Fig 3, 4, 5, and 6

Fig 4 Contact erosion characteristics of silver and silver-tungsten contacts Test conditions were 115 V, 60 Hz,

and 1.0 power factor for 100,000 operations at 60 operations/min Closing and opening speeds were 38 mm/s (1 in./s) Closing force was 980 mN (0.22 lbf) and opening force, 735 mN (0.165 lbf)

Fig 5 Contact welding characteristics of silver and silver-tungsten contacts Operation characteristics are the

same as for Fig 4

Fig 6 Results of short-circuit tests on silver-tungsten, silver-molybdenum, and silver tungsten carbide

Refractory Metal and Carbide-Base Composites

Refractory metals and their carbides are distinguished by high melting and boiling points and high hardness, but poor electrical and thermal conductivities and poor oxidation resistance In pure elemental form, refractory metals perform well only under low-current conditions

Forming a composite can compensate for these drawbacks For example, the development of composite contact materials involving silver or copper with tungsten or molybdenum or their carbides has resulted in materials that can withstand higher currents and more arcing than other contact materials, without experiencing sticking or rapid erosion The refractory metal content can vary from 10 to 90%, although 40 to 80% usually is used in air- and oil-immersed circuit breaker devices Refractory metals offer good mechanical wear resistance and resistance to arcing The silver and copper provide the good electrical and thermal conductivities

Because silver and copper do not alloy with tungsten, molybdenum, or their carbides, P/M processes are required in fabrication Depending on the composition, refractory metals containing silver or copper contact materials are made either

by pressing and sintering or by the press-sinter-infiltrate method When infiltration is used, either all refractory metal powder is compacted to shape, or a small amount of silver or copper powder is blended with the refractory metal, compacted, and sintered in a reducing atmosphere The sintered compact is then returned to the furnace; silver or copper

is added to act as the infiltrant

Most infiltrated composite contacts use silver as the infiltrant because of its excellent thermal and electrical conductivities, as well as its superb oxidation resistance Copper infiltrant, which costs less but has very poor corrosion resistance, is used for composites that operate in noncorrosive environments such as oil, vacuum, or inert atmospheres At temperatures above the melting point of the infiltrant, the liquid metal penetrates and fills the interconnecting voids of the pressed-and-sintered compact Densities of 96 to 99% of theoretical can be achieved by this process Infiltrated contact materials find use as current-carrying contacts in air- and oil-immersed circuit breakers, heavy-duty relays, automotive starters, and switches Lower properties can be obtained by pressing and sintering

In a material made by infiltration, the function of the infiltrant (silver or copper) is twofold First, because silver or copper does not alloy with tungsten, molybdenum, or carbides, the conductivity of the composite depends strictly on the volume percentage of infiltrant Second, during arcing, the high temperature melts the infiltrant; consequently, the heat of fusion absorbs (quenches) a portion of the heat generated by the arc Theoretically, the skeleton, which is made of a high-melting element, will not begin to melt until all the low-melting component evaporates The refractory skeleton also prevents molten infiltrant from flowing by capillary action Because of this, erosion loss of the contact is low Properties (such as the erosion data in Fig 4 and 6) of the contact vary with the composition of the composite A composite with high skeletal composition has high hardness and better wear resistance, but lower current-carrying capacity On the other hand,

a high-silver composite possesses high electrical and thermal conductivities and undergoes lower temperature rise, but is softer

Compositions of Refractory Component. There is a lower limit for the composition of the skeleton material Generally, when the amount of refractory or carbide is less than about 30 vol%, it is difficult to form a sound and uniform skeleton to accommodate the amount of silver For practical purposes, the skeleton material should amount to a minimum

of 50 wt% for tungsten and molybdenum and 35 wt% for tungsten carbide Any composite containing lesser amounts than these limits should be made by the press-sinter-re-press method and should be considered a silver-base composite in which the function of the refractory material is to reinforce the silver matrix For compounds with 60% or less tungsten, the classical method of mixing the powders, pressing, sintering (generally below the copper melting point), and re-pressing might also be used Materials with 60 to 80% W are generally produced by infiltration, either of loose tungsten powder or of a pressed-and-sintered tungsten compact

Tungsten, tungsten carbide, and molybdenum powders are the most commonly used materials for making skeletons for infiltrated contacts Composites with tungsten skeletons have the best arc-interrupting and arc-resisting characteristics and the best arc-erosion resistance Their antiwelding properties are moderate (Fig 5) High-energy devices usually use silver-infiltrated composites having a tungsten skeleton

Composites with tungsten carbide skeletons have better resistance to welding, better anticorrosion properties, and more stable contact resistance compared with other infiltrated composites Devices that handle switching arcs usually use composites based on tungsten carbide skeletons

For a combination of properties, or sometimes for a special requirement, a skeleton made of a mixture of tungsten and tungsten carbide is used The blended powder contains either the mixture of tungsten and tungsten carbide or a mixture of tungsten and graphite In the latter case, the graphite and part of the tungsten react to form tungsten carbide during sintering

Composites with molybdenum skeletons have relatively low contact resistance and behave well in interrupting devices For the same current-carrying capacity, a molybdenum-base composite costs less than the other two, but the antiwelding and anticorrosion properties of molybdenum-base composites are inferior to those with tungsten or tungsten carbide skeletons Figure 6 compares the erosion characteristics of a molybdenum composite with tungsten and tungsten carbide composites

circuit-Silver-Base Composites

The main advantage of a silver composite over a silver alloy is that the bulk conductivity of a silver composite depends generally on the percentage of silver by volume An alloying element in solution greatly decreases the conductivity of silver For instance, the volume of silver in Ag-15CdO composite is less than that in Ag-15Cd alloy, yet the electrical conductivity of the former (65% IACS) is much greater than that of the latter (35% IACS)

In silver composites, the second phase forms discrete particles that are dispersed in the silver matrix The dispersed phase improves the matrix in two ways First, it increases the hardness of the composite material in a manner similar to dispersion hardening Second, in the region where two mating contacts touch upon closure, the second phase particles reduce the surface area of silver-to-silver contact This greatly reduces the tendency to stick or weld In cases where the contacts do weld, the second-phase oxide particles (which are weaker and more brittle than silver) behave as slag inclusions and reduce the strength of the weld, allowing the device contact-separating force to pull the contacts apart

Silver-base composites can be divided into two types: type 1 uses a pure element or carbide as the dispersed phase; type 2 uses oxides as the dispersed phase In both types, the hardness increases and the conductivities decrease as the volume fraction of dispersed phase increases, and vice versa

Silver-Base Composites with a Pure Element or Carbide. In type 1, the dispersed phase functions as a hardener and improves the mechanical properties of the silver matrix The dispersed phase also promotes improved electrical performance such as antiwelding properties Elements used include tungsten, tungsten carbide, molybdenum, nickel, iron, graphite, and mixtures of these materials

Silver-Tungsten and Silver-Molybdenum Composites. Silver composites (made by the press-sinter-re-press method using tungsten, tungsten carbide, and molybdenum as the dispersed phases) show electrical conductivities similar

to those of infiltrated composites of the same components However, their mechanical properties are inferior because the dispersed phases do not form a refractory skeleton

Silver-Nickel Composites. One of the elements typically combined with silver by P/M processes is nickel Nickel is more effective as a hardening agent than copper; consequently, silver nickel is considerably harder than coin silver At the same time, nickel does not increase contact resistance appreciably, particularly in combinations that include 15 wt% Ni or less Silver nickel is combined in proportions ranging to about 40 wt%

Composites with nickel as the dispersed phase resist mechanical deformation or peening under impact and possess good antiwelding properties Silver-nickel composite contacts can be used as both members of a contact pair Sometimes, a silver-nickel composite is used as the moving contact operating against a stationary contact of a different composite such

as silver-graphite

The combinations most widely used are 60Ag-40Ni and 85Ag-15Ni These materials are very ductile and can be formed

in all of the shapes in which silver contacts are used, including very thin sheets for facing large contact areas This material is ideal for use under heavy sliding pressures It does not gall like fine silver and coin silver, but instead takes on

a smooth polish It is therefore suitable for sliding contact purposes, as well as for make-break contacts Silver nickel can handle much higher currents than fine silver before it begins to weld It has a tendency to weld when operated against itself Therefore, it is frequently used against silver graphite

The 60Ag-40Ni composite is the hardest material in the silver-nickel series It is the most suitable for sliding contact in which pressure is high This alloy also has the lowest rate of wear under sliding action It is less ductile than silver-nickel materials containing less nickel, but it is still sufficiently ductile for all conventional manufacturing processes

The 85Ag-15Ni composite is the most widely used material in the silver-nickel series Because of its ideal mechanical properties, 85Ag-15Ni is an ideal material for motor-starting contactors and is superior in this type of application to fine silver, coin silver, and copper It is also suitable as a general-purpose contact for various types of relays and switches

The contact resistance of clean 85Ag-15Ni contacts that have not operated under load tend to be slightly lower for fine silver However, in make-break circuits, silver tends to gradually increase contact resistance This increase is not necessarily permanent, as contact resistance varies with the effects of arcing on the contacts Generally, average resistance

is higher than the initial resistance before the contacts operate The contact resistance of 85Ag-15Ni is similar, except that

it usually varies within a narrower range Exhibiting nearly constant contact resistance is more important than possessing low contact resistance

85Ag-15Ni exhibits a lower contact resistance and is also harder than coin silver

Another advantage of 85Ag-15Ni is its low flammability; that is, it makes a smaller arc than other materials In testing of more than 40 contact materials, 85Ag-15Ni exhibited the lowest arc energy Low arc energy is important in that the ability to break a circuit with as little flame as possible is desirable This characteristic was primarily responsible for the adoption of 85Ag-15Ni for relays in aircraft electrical systems

Silver-Graphite Composites. Graphite is also combined with silver by P/M techniques Graphite in silver-base

composites serves as a good lubricant, reducing the damage caused by frictional forces Silver-graphite composites are used chiefly as sliding or brush contacts These materials have high resistance to welding and are also used as make-break contacts In circuit breakers, they are usually paired with silver-nickel composites

The most frequently used composition is 95Ag-5C, although graphite compositions ranging from 0.25 to 90% with the remainder silver have been used This material was developed as a circuit breaker contact material The addition of graphite prevents welding Frequently, 95Ag-5C is used in combination with silver-nickel or silver-tungsten contacts It is also used in combination with pure nickel contacts and with fine silver contacts Silver graphite is soft compared to other types of contact materials, and electrical and mechanical erosion is more rapid

95Ag-5C has been widely used as a material for contacts in molded-case circuit breakers, sliding contacts, and contact brushes This material is only moderately ductile and can be rolled into sheets and punched into contacts of various shapes However, it cannot be headed to make solid rivets or bent to any great extent without cracking It can be coined to

a moderate extent 95Ag-5C contacts can be individually molded Depending on size, shape, and quantity, contacts of this material are either punched from rolled slabs, extruded, or individually molded from powders Copper is combined with graphite as a substitute for silver in certain applications

A modified form of silver graphite is silver-nickel-graphite Typical compositions are 88Ag-10Ni-2C and 77Ag-20Ni-3C These materials are substantially harder than 95Ag-5C and exhibit superior wear resistance, but offer less protection against welding Like 95Ag-5C, they can be manufactured from slabs or by molding individually

Composites of silver iron exhibit good antiwelding and good wear characteristics when used in creep-type thermostat devices These materials have poor corrosion resistance

Silver-Base Composites with Dispersed Oxides. Type 2 silver-base composites use semirefractory oxides as the dispersed phase These silver-base composites are produced by a variety of methods such as internal oxidation, preoxidation, and conventional P/M processes (see the section "Composite Manufacturing Methods" in this article)

The semirefractory component of type 2 silver-base composites includes metal oxides such as CdO, SnO2, or ZnO In general, the semirefractory constituents promote nonsticking qualities or provide increased resistance to wear

The Ag-CdO group of electrical contact materials is the most widely used of all the silver semirefractory contact materials The addition of 5 to 15% CdO to silver imparts excellent nonsticking and arc quenching qualities

Because of its resistance against arc erosion and its low contact resistance, which does not increase even after switching, Ag-CdO has proved to be a universally good contact material for many switching devices Ag-CdO contact materials are well suited for contactors and motor starters, but are also used in circuit breakers, relays, and switches with medium to low currents

Ag-CdO material has antiwelding and antierosion properties united with constant resistance, examples of its main advantage of well-combined properties Another favorable quality is that it has good workability It can be fabricated by either the internal oxidation (least costly), preoxidation, or P/M methods The Ag-CdO material can also be cold reduced

or rolled quite easily For instance, Ag-15CdO material can endure more than 70% cold reduction

Ag-SnO 2 , which is used widely in Europe as a contact material, is a class of composite materials that has the potential to replace Ag-CdO composites in many electrical contact applications However, general comparisons of Ag-SnO2 contacts with Ag-CdO contacts are difficult because results may depend on the specific conditions of testing Previous concerns on the toxicity of CdO, which was one of the motivations for using Ag-SnO2 contacts, have also been relaxed in Japan and Europe The toxicity of CdO must be distinguished from the highly toxic nature of cadmium

Like Ag-CdO contacts, Ag-SnO2 contacts can be produced by internal oxidation or P/M techniques One drawback of the Ag-SnO2 composite is that a third element (such as indium) must be added to achieve internal oxidation when the silver alloys contain more than 4% Sn The oxidized material also does not allow a high level of cold reduction because of its brittleness Therefore, a press-sinter-re-press method or extruded method is the most feasible way to fabricate Ag-SnO2

although extruded products are more brittle than extruded Ag-CdO powder of similar compositions For example, extended Ag-10SnO2 can be subject to a maximum of 30% cold reduction compared to more than 60% for Ag-12CdO

Another drawback is the higher-temperature rise of Ag-SnO2 contacts (as compared to Ag-CdO) after arcing This troublesome characteristic has, however, been eliminated with Ag-SnO2 materials made by P/M methods

Table 2 lists three grades of commercially available Ag-SnO2 composite contact materials Ag-SnO2 contact materials cannot be easily brazed or welded To be able to braze Ag-SnO2 contacts, they are made with at least two layers, the contact layer and brazable or weldable fine silver layer The brazing alloy can be applied separately in the shape of paste, wire, or foil, or it is already clad onto the semifinished product

Ag-ZnO is another composite material that has been tested and marketed for contact applications Ag-ZnO, like Ag-SnO2

composite, cannot take high cold reduction because of the brittleness of the oxidized material When internal oxidization

is used, the maximum zinc content cannot exceed 6% for good oxidation Typical applications of a commercially available Ag-ZnO composite are listed in Table 2

Multiple-Component Composites. There is no ideal material to meet all conditions for contact applications If required by manufacturers of switching devices, contact manufacturers can offer composite materials consisting of as many as four or five components Most of these composites serve only special purposes They are not universally accepted and generally cost more Two common three-component composites are listed in Table 2

Powder Metallurgy Electrical Contact Materials

Composite Manufacturing Methods

The methods used to manufacture composite contact materials can be classified into three major categories:

• Standard P/M processes, for producing composites from materials that cannot be conventionally alloyed

• Internal oxidation processes, for producing silver-base composites with dispersed oxides

• Hybrid consolidation, which is a combination of the internal oxidation and P/M consolidation processes

Powder Metallurgy Methods

Infiltration is used exclusively for making refractory metal and carbide-base composite contact materials Metal powder

or carbide powder is first blended to the desired composition with or without a small amount of binder to impart green strength, then is pressed and sintered into a skeleton of the required shape Silver or copper is then infiltrated into the pores of the skeleton This method produces the most densified composites, generally 97% or more of theoretical density Complete densification is not possible because of the presence of some closed pores in the sintered skeleton After infiltration, the contact is sometimes chemically or electrochemically etched so that only pure silver appears on the surface The contact thus treated has better corrosion resistance and performs better in the early stages of use

Press-Sinter. For small refractory metal contacts (not exceeding about 25 mm, or 1 in., in diameter), a high-density material can be obtained by pressing a blended powder of exact final composition into shape and then sintering it at the melting temperature of the low-melting-point component (liquid-phase sintering) In some cases, an activating agent such

as nickel, cobalt, or iron is added to improve the sintering effect on the refractory metal particles For this process, powders of much finer particle size are required so that more bonding surface exists However, the skeleton formed by this process is weaker than that formed by the infiltration process Formation of the skeleton usually shrinks the apparent volume of the refractory portion of the composition, thus bleeding out the molten component onto the surface of the finished contact

Press-Sinter-Repress. The press-sinter-repress process is used for all categories of contact materials, especially those

in the silver-base category Blended powders of the correct composition are compacted to the required shape and then sintered Afterward, the material is further densified by a second pressing (re-pressing) Sometimes the properties can be modified by a second sintering or annealing The versatility of this process makes it applicable for contacts of any configuration and of any material However, it is difficult to obtain material with as high a density as is obtained with other processes Material thus produced also may have weak bonding between particles

Press-Sinter-Extrude. Blended powder of final composition is pressed into an ingot and sintered The ingot is then extruded into wires, slabs, or other desired shapes The extruded material may be subsequently worked by rolling, swaging, or drawing Material made by this method is usually fully dense

The press-sinter-extrude process is used mostly for base composites Other processes used for manufacturing base composite contacts are direct extrusion or direct rolling of loose powder Although they appear to be uncommon, they are economically feasible if the equipment is properly designed and built

silver-Internal Oxidation

Silver-base composites with dispersed metal oxides can be produced by internal oxidation In this process, a silver alloy (such as a silver-cadmium alloy) is first cast into ingots, which are rolled into strips or fabricated further into the finished product form The silver alloy material is then heated in air or oxygen, so that the oxygen diffuses into the alloy and forms metal oxide particles (such as CdO in the case of a silver-cadmium alloy) dispersed in the silver matrix

Internal oxidation is used in the production of a substantial portion of Ag-CdO composites The initial silver-cadmium alloy can be internally oxidized either in strip or finished product form The silver-cadmium alloy is heated between 800

to 900 °C (1470 to 1650 °F) in a furnace with air, oxygen-enriched air, or pressurized oxygen Under this condition, the oxygen species diffuse into the silver-cadmium alloy and oxidize the cadmium species Upon the completion of the

oxidation, the cross section of the material will display a microstructure of CdO particles embedded in a silver matrix Contact parts are punched from the strip and then coined into required shapes

The size of the CdO particles and the uniformity of their dispersion are dependent on the temperature and the partial pressure of oxygen Reduced temperature decreases coalescence of cadmium prior to being oxidized and thereby causes a finer dispersion of CdO Increasing the partial pressure of oxygen in the furnace increases the diffusion rate of oxygen into the silver This also causes a finer CdO dispersion by reducing the time available for the cadmium to coalesce

During the internal oxidation of a silver-cadmium alloy, the cadmium species become depleted in zones when the oxygen front moves into the silver-cadmium alloy The cadmium atoms before the oxygen front immediately diffuse into the zone against the oxygen front As the oxidation front moves from the surfaces of the strip toward the center, the concentration

of the cadmium species becomes increasingly dilute as compared to the original composition Hence, after the oxidation is completed, the cross section will display a significant oxide-deficient or oxide-depleted zone in the center of the contact body

For some applications the presence of the depletion zone is detrimental, requiring its removal or displacement from the center There are two common methods to achieve this result In the first, an oxidation barrier, such as ceramic glaze, is applied to one surface so that the oxidation can proceed from only one side The second method is to laminate two silver-cadmium sheets of the same size and to form a package by welding along the four edges After oxidation, the sheets are separated The oxide-deficient zone will appear on one side (the inner side of the package) of each sheet

Package rolling is another technique for reducing the size of the depletion region In this method, very thin cadmium sheets are first oxidized Then a number of sheets (for example, 16 sheets) are stacked together and hot-bond rolled into one slab The cross section of the final product displays very thin depleted zones equal to the number of sheets

silver-Hybrid Consolidation

Various hybrid techniques use a combination of internal oxidation and P/M methods These methods are used to produce

a finer average oxide size and/or a more uniform distribution of cadmium oxides in the matrix of a Ag-CdO composite Hybrid consolidation methods include preoxidized-press-sinter-extrude and coprecipitation Table 3 compares Ag-CdO composites manufactured by different methods

Table 3 Comparison of Ag-CdO material made by different methods

Low contact resistance and temperature

Note: 1 indicates that under most conditions this is the preferred material; 2 indicates that under most conditions the material is preferable to 3, but not as good as 1; 3 indicates that the material may be acceptable, but under typical operating conditions it is not as good as 1 or 2

The preoxidized-press-sinter-extrude process combines the oxidation process and the press-sinter-extrude process Commercially, it is called "preoxidized process." The purpose of this method is to redistribute the oxide-deficient center of Ag-CdO composites

The preoxidized process is used exclusively for making Ag-CdO material Alloys are reduced to small particles in the shape of flakes, slugs, or shredded foil These particles are oxidized and then consolidated with the press-sinter-extrude process Material made by this method is more uniform than the same material made by conventional internal oxidation Mechanical properties are superior to those of the same material made by the press-sinter-re-press method

The Ag-CdO particulates are made by one of four methods and then are pressed into ingots, sintered, and extruded according to standard metallurgical method There are four processes to prepare the particulates:

• Granulated wire: Silver-cadmium alloy is first made into wire and oxidized The oxidized wire is then

chopped into granules with a length of about 3 mm ( in.)

• Low-pressure water atomization: The molten silver-cadmium alloy is atomized by water at a pressure of

100 to 200 kPa (15 to 30 psi) The approximately quarter-inch particulates are in the form of thin twisted flakes Then the flakes are oxidized for consolidation

• High-pressure water atomization: The molten silver-cadmium alloy is atomized with high water

pressure, usually higher than 2750 kPa (400 psi) The powder sizes range between 40 mesh (420 m) and 270 mesh (53 m) Then the alloy powders are oxidized before consolidation

Coprecipitation. Conventional blending or mechanical mixing of silver and CdO powders begins by dissolving the proper amounts of silver and cadmium metals in nitric acid Compounds of silver and cadmium coprecipitate from the solution when the pH value of the solution is changed by adding either hydroxide or carbonate solutions During subsequent calcination at about 500 °C (930 °F), the compound mixture decomposes to form a mixture of silver and CdO Alkali metal content can be controlled in the ppm range by adequate washing Controlled amounts of sodium, potassium, and lithium may enhance electrical life Excessive amounts of these elements can lead to rapid erosion, restrike, and generally poor electrical life Depending on device design, the range may be from 10 to 300 ppm Contacts are consolidated from this mixture by conventional P/M methods The microstructure of contacts made by this method displays a finer particle size and a more uniformly dispersed CdO phase than material made by conventional blending

Powder Metallurgy Electrical Contact Materials

Availability

Silver, gold, platinum, palladium, and most of their ductile alloys are available as stamped contacts Except for material from which contact disks are produced, a variety of stock sizes are not maintained because, in general, no two applications are identical For disks, material in strip form, varying in thickness from 0.25 mm to 1 mm (0.010 to 0.040 in.) is available

Except for tungsten and molybdenum, contact materials are ductile enough so that they can be produced in all contact forms Tungsten and molybdenum and some of the P/M materials have lower ductility and are available in fewer forms The commercially available forms of common electrical contact materials are listed in Table 4

Table 4 Commercially available forms of electrical contact materials

method Alloy

Solid rivet

Wire Strip Tape Disks Attached (a) Composite

weld disks (b)

Clad (c) Rings Brushes Melting P/M

(a) Contact disks attached to screws, rivets, blades, and bars

(b) Composite welding-type buttons produced for resistance welding attachment Backings of nickel,

Monel, and steel

(c) Clad materials including overlay, throughlay, edgelay, strip of precious metal on (or in) base metal

Silver, gold, platinum, palladium, and nearly all the alloys of these metals, as well as tungsten, molybdenum, and the various sintered products of silver and the refractory metals, can be used to produce steel-back contacts Steel-back contacts have been made in the form of screws, rivets, or buttons for projection welding

Powder metallurgy materials are available with final densities up to 99% of theoretical and with high-conductivity surfaces as inserts or overlays Both P/M and wrought materials may be attached to appropriate carriers by brazing, welding, or diffusion bonding, even though they contain CdO For percussion welding of Ag-CdO contacts, backing is not required For resistance welding, a fine silver backing is needed

Powder metallurgy contacts are available with a silver matrix for air and oil applications, and with a copper matrix for oil applications only The second phase may be tungsten, molybdenum, graphite, tungsten carbide, CdO, or ZnO

Materials with a high content of refractory metal (50% or more) are usually made by infiltration In this process, the refractory metal powder is pressed into the desired size and shape with a controlled porosity The compact is sintered at high temperature in a reducing atmosphere, and then molten silver or copper is infiltrated into the porous sintered compact

Compositions of a lower refractory content are made by blending powders, pressing them to a desired size and shape, sintering the pressed compact at a high temperature, and re-pressing to size the parts and to increase the density of the compact

Although parts usually are molded to final shape, they are sometimes finished by machining or grinding to obtain special shapes or unusually close tolerances When only a few parts are needed, they may be machined from bars to save the cost

of expensive dies for pressing a P/M compact

Sintered materials are available in sizes ranging from rectangles or disks about 0.8 mm ( in.) thick by 3 mm ( in.) square or 3 mm ( in.) in diameter to bars 200 mm (8 in.) long Most are available in widths up to 75 mm (3 in.) and thicknesses up to 12.5 mm ( in.) For some materials, these dimensions may be exceeded

In small sizes (up to about 12.5 mm, or in., square), the higher refractory compositions are frequently made with serrated surfaces coated with excess silver or silver-base brazing alloy so that they can be attached easily to a backing by welding or by resistance brazing in a welding machine Larger sizes are generally attached to backing by silver alloy brazing The materials are often pretinned with silver-base brazing alloy in a controlled atmosphere for good wetting of the contact material

Many of the low-refractory contact materials are fabricated as disks, rectangles, and special-contour facings that also are attached to backing by silver alloy brazing Most materials with 90% or more silver are ductile enough to allow fabrication as rivets and to allow assembly by staking or spinning Among silver-graphite materials, those containing more than 0.5% graphite cannot be satisfactorily cold headed into rivets

Ag-CdO mixtures are available as round, rectangular, or special-shape facings, and also as rivets

Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

Introduction

THE TECHNOLOGY of fabricating lower density, porous P/M materials can fulfill a wide variety of applications A rigid, permeable structure can be created using P/M technology by forming a network of sintered powder particles and interconnected pore channels Using similar manufacturing equipment and technology as structural P/M components, porous P/M materials are normally sintered to densities between 25 and 85% of theoretical mean density (TMD) Structural P/M components are typically 85 to 99.9% of TMD These unique engineered materials provide specialized products for applications such as filtration, fluid flow control, self-lubricating bearings, spargers, and battery electrodes

Porous P/M materials are specified when special characteristics are required such as good mechanical properties, rigidity, corrosion resistance, uniform porosity, and controlled permeability For example, porous bronze, stainless steel, or nickel alloys are often selected for service in elevated-temperature and pressure environments Porous materials such as papers, synthetic fibers, and plastics are commonly selected for service in less severe conditions due to their lower cost (Ref 1) However, in applications where disposal costs are significant due to the nature of the service environment, renewable porous P/M materials offer excellent cleanability for economical reuse Other porous metal materials made from sintered fibers, foam, perforated sheet, and screen may be selected as alternatives to porous P/M materials, depending on the application (Ref 2) Metal fiber/screen composites, foam/powder composites, screen/powder composites, and fiber/powder composites are also sintered to form porous media (Ref 3, 4, 5) Porous ceramics are utilized in applications

that require higher-temperature service or more extreme corrosion resistance than porous metals However, porous ceramics are limited in their application because they have substantially lower mechanical properties than do metals and because of fabrication difficulties that result when joining to metal hardware (Ref 6)

Porous P/M materials accounted for about 2% of the overall P/M market based on market share by application Approximately 70% of the overall P/M market is comprised of structural components for the automotive industry (Ref 7) Porous P/M materials are also estimated to account for less than 2% of the overall filtration and fluid control markets that are currently dominated by lower cost, disposable porous media Despite the small market size currently served by porous P/M materials, this material science is an emerging technology driven by increased environmental concerns, improved product quality demands, and more complex processing of industrial products The unique materials, characteristics, fabrication methods, and applications of porous P/M technology are summarized in this article

References

1 C Dickenson, Filters and Filtration Handbook, 3rd ed., Elsevier Science, 1994, p 66-82

2 V Tracey, Porous Materials: Current and Future Trends, Int J Powder Metall Powder Technol., Vol 12

(No 1), 1976, p 25-35

3 V Tracey and N Williams, The Production and Properties of Porous Nickel for Alkaline Battery and Fuel

Cell Electrodes, Electrochem Technol., Vol 3 (No 1-2), 1965, p 17-25

4 M Eisenmann, A Fischer, H Leismann, and R Sicken, P/M Composite Structures for Porous Applications,

Proc 1988 Int P/M Conf., Metal Powder Industries Federation, 1988, p 637-652

5 P Koehler, Porous Metal Article and Method of Making, U.S Patent No 4,613,369, 1986

6 C Dickenson, Filters and Filtration Handbook, 3rd ed., Elsevier Science, 1994, p 184-188

7 D White, Challenges for the 21st Century, Int J Powder Metall., Vol 33 (No 5), 1997, p 45-54

Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

Materials Selection

Materials can be selected from a wide variety of P/M materials, depending on the combination of application requirements and economics The porosity is determined by the powder particle shape, the powder size distribution, the powder surface texture, and other powder characteristics that are dependent on the material processing method Because the powder characteristics are one of the major factors in determining the porous material properties of the finished component, reproducible powder characteristics and manufacturing methods are critical to production of a consistent product

The four most common porous P/M materials are bronze, stainless steel, nickel, and nickel-base alloys Other materials such as titanium, aluminum, copper, platinum, gold, silver, iron, iron aluminide (Fe3Al), niobium, tantalum, and zirconium are fabricated into porous materials from powder (Ref 8) The size distribution of the particles can be a direct result of the powder manufacturing process or can be altered by sieving or air classification techniques The characteristics, sintering conditions, and commercial availability of a powder determine whether the desired material can

be manufactured into a porous structure

Bronze powders consisting of 89 to 90% Cu, 10 to 11% Sn, and 0.1 to 0.5% P are the most common material for porous bronze P/M components such as filters and self-lubricating bearings Small amounts of phosphorous can be added to improve mechanical strength (Ref 9) Bearings can be produced from preblended elemental powder mixes or prealloyed powders These 89/11 or 90/10 bronze powders are available from three main processes Gas or air atomization of prealloyed bronze yields fine, spherical particles that can be processed to low densities by gravity sintering, as shown in Fig 1 Spherical bronze particles can also be obtained by tin coating of gas-atomized copper powder Rounded particles

are made from cutting tin-coated copper wire with a composition of 93 to 97% Cu and 3 to 7% Sn (Ref 10) as shown in Fig 2 Bronze powder particle sizes range from 10 to 850 m in diameter depending on the powder type

Fig 1 Bronze particles made by tin coating atomized copper powder with a size range of 45 to 100 m that are

gravity sintered to 64% dense in order to yield a 10 m filter grade disk 100×

Fig 2 Bronze particles made from tin-coated, cut copper wire with a size range of 250 to 425 m that are

gravity sintered to 55% dense in order to yield a 40 m filter grade disk 100×

Stainless steel and nickel-base alloys are often specified for increased service life when more corrosion resistance, temperature resistance, or strength is required Due to economic considerations, the most common stainless steel (SS) materials chosen are the extra-low-carbon, austenitic grade, type 316L, or from the nonstandard, 2 to 3% Si modified grade, known as 316B The additional silicon serves as a sintering aid by lowering the sintering temperature and controlling the porosity and mechanical properties Other austenitic, 300 series grades, such as 304L and 347, along with the ferritic grades such as 430 are used in a few specialized applications where the required corrosion resistance or mechanical properties are superior to 316L SS Nickel-base alloys such as Hastelloy C22, Inconel 600, Monel 400, and Hastelloy X are utilized for extreme corrosion or thermal resistance

Stainless steel and nickel-base alloy powders are produced by water or gas atomization (refer to Sections on production of stainless steel and high-alloy powders and production of nickel-base powders in this Volume) Water atomization produces powders with rounded, irregular shapes that can be processed by compaction and sintering, as shown in Fig 3 and 4 These particles interlock when compacted to form a part with sufficient green strength for handling The water-atomization process yields a wide variation in particle size distribution that must be separated into much narrower size ranges for subsequent processing into porous components As the powder size decreases, the particle shape becomes more

rounded and higher compaction forces are required to produce components with adequate green strength Some base alloys such as Hastelloy C22, tend to atomize into more rounded particles than similar size distributions of 316L SS and thus require increased compaction force to obtain similar green strength Material characteristics such as hardness can also influence the compressibility of the powder Water atomization typically produces particle sizes that range from as fine as 100 nm to as coarse as 1000 m The particles less than 10 m in diameter are normally more spherical in shape and tend to have limited cold compaction green strength unless combined with an additive

nickel-Fig 3 Water-atomized, 316L SS powder particles with a size range of 300 to 600 m that are compacted and

sintered to 45% dense in order to yield a 40 m filter grade sheet 100×

Fig 4 Water-atomized, 316L SS powder particles with a size range of 45 m and less that are compacted and

sintered to 75% dense in order to yield a 0.5 m filter grade sheet 100×

Stainless steel and nickel alloy powders are also produced by the gas-atomization process, which typically yields particles with spherical shapes This powder cannot be readily compacted without additives Components can be fabricated by vibratory filling of molds and gravity sintering The mechanical properties of gravity sintered, spherical powders are normally much lower than other fabrication methods that use compaction Therefore, gravity sintering is only used in special applications that require higher porosity or unique shapes In addition, irregular-shaped powders, and powders with binders, can be processed using gravity sintering to obtain low-density products

Carbonyl or water-atomized nickel powders are rapidly growing in acceptance for use as porous P/M products The thermal decomposition of nickel carbonyl produces very fine (0.5 to 10 m) particles attached in a filamentary chain-type structure Very narrow particle size distributions are possible using this process, which can result in extremely uniform pore size materials after processing, as shown in Fig 5 Although these powders do not flow well, components with higher porosity and finer pore sizes are made using very low compaction pressures less than 6.9 MPa (1000 psi) (Ref 11)

For nickel components with larger pore sizes, water-atomized, irregular-shaped particles with diameters greater than 20

m are normally selected instead of carbonyl nickel because the latter process produces more rounded particles as the particle size increases (Ref 12)

Fig 5 Carbonyl nickel powder particles with a size range of 2 to 4 m that are compacted and sintered to 35%

dense in order to yield a 0.1 m filter grade cup 2000×

The atomization process can be varied to produce particles with other shapes designed to optimize porosity and permeability Potato-shaped powders consist of elongated, rounded particles with smooth surfaces Coarser grades of potato-shaped powders can be compacted directly to form shaped parts while finer grades are normally combined with a binder to achieve adequate green strength Other processes have compacted and sintered mixtures of irregular-shaped particles with up to 25% spherical particles The smoother surface and packing characteristics of potato-shaped powders and spherical powders can result in significant permeability increases, especially in the coarser filter grades (Ref 13)

The characteristics of the four most commonly used porous metal powder materials are briefly summarized to illustrate the different powder particle characteristics available to create porous structures Characteristics of P/M materials are more completely described in previous sections of this Volume and other references (Ref 14, 15)

References cited in this section

8 Porous Metal Products for OEM Applications, Mott Technical Handbook, Mott Corporation, 1996, Sections

1000-9000

9 H Neubing, Properties and Sintering Behavior of Spherical Tin Bronze Powders for the Manufacture of

Filters, Int J Powder Metall., Vol 18 (No 4), 1986, p 4

10 Bronze Filter Powders, ACu Powder International Technical Brochure, Acu Powder International, 1997

11 N Bagshaw, M Barnes, and J Evans, The Properties of Porous Nickel Produced by Pressing and Sintering,

Powder Metall., Vol 10 (No 19), 1967, p 13-31

12 R German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 96

13 G Hoffman and D Kapoor, Properties of Stainless Steel P/M Filters, Int J Powder Metall., Vol 12 (No 4),

1976, p 371-386

14 R German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 28-81

15 M Phillips and J Porter, Comp., Advances in Powder Metallurgy and Particulate Materials, Part 1, Metal

Powder Industries Federation, 1995

Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

Processing Methods

After the material is selected, the P/M processing method is critical for determining the final mechanical properties and porous characteristics of the porous component Selection of the fabrication method is dependent on the powder characteristics and the type of porosity required by the application The preparation of the powders, the use of additives, the compaction methods, and the sintering conditions must be carefully controlled to produce uniform and repeatable porous characteristics Secondary operations are normally required to enhance metallurgical properties and to allow easier adaptation to the intended application

Powder preparation begins with the separation of the powder particles in the desired size distribution Depending on the amount of secondary processing, commercial powders are available with either wide or narrow particle size distributions For example, a wide particle size distribution such as -150 m (-100 mesh screen) can be generated depending on the powder manufacturing process However, secondary sieving or screening can classify a much narrower particle size distribution such as between 75 and 150 m (-100/+200 mesh) Screening must be carefully monitored because there are significant variations in the screen openings or because the screen openings are blocked with fine particles Therefore, significant amounts of particles that fall outside of the desired particle size range are not removed by screening The porous part manufacturers process the commercially available powders into even narrower size distributions by removing oversize and undersize particles or by adding another screen to produce a tighter cut, such as between 106 and 150 m (-100/+140 mesh) Vibratory or ultrasonic screening methods are normally used for particles greater than 20 m (600 mesh screen) and other separation methods such as air classification are used for particles less than 20 m Porous P/M components manufactured from narrow particle size distributions have excellent reproducibility and uniformity of the final porosity

Once the desired particle size distribution is obtained, the powder must be properly blended prior to use in order to avoid segregation and to maximize uniformity The apparent and tap densities of the powder cuts are normally measured to meet tight specifications because these characteristics control further processing steps such as powder flow and die filling Excessive blending can change the powder characteristics by generating additional fine powder particles or by changing the particle surface Additives such as lubricants, pore formers, or binders are precisely mixed with the powder if required

by the processing method (Ref 16, 17) Significant amounts of powder with controlled particle size distributions are used

as loose pack beds for applications such as polymer filtration However, most applications require further processing to develop a more rigid porous structure The powder cleanliness and chemical analysis must also be carefully monitored to maintain the chemistry requirements of the material and to avoid contamination with other materials being processed Because these materials are often used in aggressive environments, contamination from iron, carbon, aluminum, sulfur, and other elements could cause significant reduction in mechanical properties or corrosion resistance Dedicated powder processing equipment and manufacturing facilities are the best defense against contamination Other trace impurities such

as oxygen, alumina, silica, and nitrogen levels from the powder manufacturing process can also affect the final mechanical properties

Compaction methods and sintering are normally required to produce porous materials from stainless steel, nickel-base alloys, and nickel materials in order to achieve the best combination of mechanical strength, metallurgical properties, and porosity Lower-density, higher-permeability parts from these alloys can also result from using a gravity-sintering process without compaction of the powder when the application does not require higher strength levels of compacted and sintered parts Die compaction, isostatic pressing, and roll compaction increase the green strength of the part by cold welding of the particle to particle contact areas due to plastic deformation As compaction force is increased, the density and green strength of the porous part increases, which results in finer porosity and lower permeability than methods that do not use compaction prior to sintering

However, when compared to compacted structural P/M parts, the lower density and green strength limit the shape complexity of a porous component For porous materials, uniform density is critical to the function of the parts because the largest porosity and greatest permeability are in the lowest-density region Density variations of more than ±2.5% within the same component cause significant differences in performance

Die compaction and sintering are the most common methods of improving mechanical properties for porous parts with length-to-diameter ratios of less than 5 to 1 Disks, cups, bushings, and other shapes can be readily compacted by filling the powder into dies and using hydraulic or mechanical P/M presses Typically, porous materials are manufactured to achieve the lowest-density part in order to maximize porosity and permeability

Minimum green handling strength can be obtained by using lower compaction forces in the range of 700 to 2100 kg/cm2(5 to 15 tons/in.2) By comparison, higher-density structural P/M components require compaction pressures of 4200 to

8400 kg/cm2 (30 to 60 tons/in.2) (Ref 18) Porous parts with 20 to 50% green densities usually have minimal green strength and require careful handling; they often have to be manually picked off the press and placed in a sintering tray Higher-green-density parts in the 50 to 80% range have sufficient green strength to allow the press feed shoe to eject the part off the die table and into a container For low-density parts made from coarse powders, press rates can be less than 5 parts/min and part handling is critical to final yield rates With proper sintering, die compaction can hold dimensional tolerances within ±1%

Die compaction with lower tooling forces can be accomplished with die-wall lubrication or adding less than 0.5% lubricant to the powder In the case of coarser water-atomized stainless steel powders, only 0.1 to 0.3% lubricant addition

to the powder is generally used Because smaller particle sizes of this same material are generally more rounded and have better particle packing (smaller pores), slightly higher pressing forces and lubricant additions are required for adequate green strength Waxes and stearates are commonly added to the powder to reduce die-wall friction and tool wear However, because increasing the lubricant percentage also reduces green strength, minimizing lubricant additions is often

a better alternative than obtaining maximum tool life service Tooling must be kept in a highly polished condition with tight tolerances to avoid smearing of the part surface during ejection If smearing or galling of the part surface occurs, significant reductions in the final surface porosity size and in the overall part permeability occur Improper monitoring of lubricant additions and tooling conditions can lead to problems with the uniformity and the reproducibility of the porosity

Isostatic compaction is a common method of compaction for producing components such as tubes that have a to-diameter ratio greater than 3 to 1 Isostatic compaction can produce porous components with more uniform density than can die compaction (Ref 19) The wet bag cold isostatic pressing (CIP) process uses a hydraulic fluid to apply pressure ranging from 3.5 to 550 MPa (500 to 80,000 psi) to the tooling, which is sealed after powder filling As the hydraulic fluid is gradually pressurized, the soft polyurethane outer liner compresses the powder against the metal core rod to obtain sufficient green strength This "outside-in" pressing process results in a green part with a smooth inner surface and a rougher outer surface If the pressing direction is reversed by using an expandable polyurethane inner bladder to compress the powder against a metal outer sleeve, a tube with a smooth outer surface can be obtained Other near-net shapes such as cones, funnels, and tubes with splines can be manufactured using the CIP process (Ref 20) For example, a 25 mm (1 in.) outside diameter (OD) by 19 mm ( in.) inside diameter (ID) by 76 mm (3 in.) long tube would have uniform density along the entire length, whereas a similar-sized, die-compacted bushing would have high-density ends and a lower-density center Hot isostatic pressing (HIP) combines the advantages of CIP and sintering into one process and is utilized in applications where complex shapes are needed or when a material has low green strength for handling

length-Roll compaction of porous nickel sheet materials have been commonly used in the production of battery materials (Ref

21, 22) Nickel and other powders are rolled to a thickness ranging from 0.13 to 3.8 mm (0.005 to 0.150 in.) and widths

up to 1 m (39.37 in.), depending on the green strength and requirements of the application Controlling the powder feed is critical to the process to obtain the best uniformity of the final product Attempting to roll sheet with larger cross-sectional thickness can lead to delamination or low-density center regions that cause poor mechanical properties Thin cross sections or powders with low green strength limit the handling of the porous sheet prior to sintering The rolled sheet can

be directly fed into a sintering furnace to minimize handling problems in certain high-volume applications Roll compaction is used to produce an economical, uniform density material with good mechanical properties and tight dimensional tolerances (±2% on thickness) Width and length tolerances are determined by secondary machining, cutting,

or shearing operations Shapes or patterns can often be designed into the rolls, or parts can be stamped from the finished porous sheet

Other forming methods used to manufacture porous metal components include centrifugal slurry casting (Ref 23, 24), blow molding (Ref 25), metal injection molding (Ref 25), extrusion (Ref 26), and gravity filling of shaped molds Sheet materials can also be made using other processes such as a liquid lay-down method, which uses a powder and binder slurry (Ref 27) Complicated shapes and lower-density components are formed using additives or pore formers such as water and/or thermoplastic binders that allow sufficient handling strength Removal of the additives prior to sintering is normally accomplished in the preheat zone of the furnace or in a separate controlled bake-out process Metal

spraying (Ref 28) can create a controlled porous structure with or without additives by spraying molten metal onto a base material to combine compaction and sintering into one process

Sintering of porous metal is a critical balance between maximizing material properties and maximizing the open porosity and permeability However, because permeability and material properties such as strength and ductility are generally inversely related, the desired balance of these characteristics normally occurs in a very small processing window (Ref 29, 30)

Sintering requires the proper compromise of temperature, time at temperature, and atmosphere to arrive at the desired porosity characteristics Porous components that are not adequately sintered exhibit poor mechanical properties due to lower density and insufficient interparticle neck growth Porous components that are exposed to excessive sintering conditions have lower permeability and higher densities than desired

The preheat and cooling portions of the sintering cycle must also be closely controlled to achieve the proper metallurgical properties Controlling the preheat conditions ensures adequate burnoff of additives and lubricants as well as minimizing the distortion of the parts The cooling conditions must be designed to provide maximum corrosion resistance and to avoid oxidation

Sintering temperature must be selected by considering the material, the powder shape, and the powder particle size distribution Sintering is normally accomplished at 70 to 90% of the material melting temperature Finer powder particles require a lower sintering temperature because the surface energy driving force to initiate bond growth is much higher than for a coarser particle Sintering at too high a temperature also causes the formation of very large pores and nonuniform porosity just prior to melting Controlling the furnace temperature within ±1% of the optimal sintering temperature achieves the best porosity uniformity and reproducible properties Furnace design, furnace loading, and furnace hot-zone uniformity are critical to the production of porous metal components Even slight lot-to-lot variations due to differences in the powder shape or the powder particle size distribution can often require adjustment of the sintering temperature to maintain the balance of properties and permeability

Sintering time must be monitored to allow for a minimum exposure time at the desired sintering temperature Sintering for at least 30 to 60 min at the maximum sintering temperature is recommended for most materials for sufficient bond formation and growth Inadequate sintering time can lead to large variations in part shrinkage and final density, causing porosity and permeability variations The sintering time at temperature must allow for the temperature of the furnace load

to stabilize at the desired temperature, especially when batch size and furnace recovery can widely vary Furnace profiles and recovery rates for various load sizes for each furnace must be considered when determining appropriate sintering time Shrinkage and density measurements can be used to control sintering time and temperature if the furnace atmosphere is consistent Excessive sintering unnecessarily reduces permeability due to pore size reduction and pore closure without significantly improving mechanical properties

Sintering atmosphere selection is critical for determining the metallurgical properties of the porous metal product Because porous materials have much higher surface area than a similar-size structural part, the atmosphere has more contact with surfaces throughout the part rather than just near the surface Porous parts also contain relatively large amounts of trapped air in the pores that must be removed by purging or good atmosphere circulation in the furnace Often, higher flow rates of an atmosphere or longer vacuum pump down times are required to displace the air prior to sintering The sintering cycle can be subdivided into three main processes:preheating, sintering, and cooling Each process of the sintering cycle has unique atmosphere requirements that provide the optimal properties of the porous metal product just as with structural P/M components (Ref 31, 32)

The preheating cycle must remove any lubricants or binders from the green compact and normally requires an oxidizing atmosphere A separate furnace is often used to burn off these additives in order to avoid contamination of the sintering furnace Preheating in the same furnace with a reducing atmosphere is often sufficient to remove low levels of additives because the open porous network allows more complete removal of the thermal decomposition products than does a structural part Specifically, mechanical strength and corrosion resistance properties are highly dependent on the interaction with the sintering atmosphere Porous materials are commonly sintered either in reducing atmospheres such as nitrogen-hydrogen mixtures, hydrogen, dissociated ammonia, or vacuum Inert atmospheres such as dry argon can be used

to sinter some of the specialty porous materials such as aluminum, refractory metals, and precious metals Reactive materials such as titanium and zirconium require good vacuum sintering with a high-purity, inert backfill gas Nitrogen-containing atmospheres can have a nitriding effect on some materials such as porous stainless steels Series 300 stainless steels can benefit from increased tensile strength with slightly lower ductility in a reactive nitrogen-sintering atmosphere such as dissociated ammonia However, reduced corrosion resistance may result unless the cooling rate is carefully

controlled to minimize the formation of chromium nitrides (Ref 33) A hydrogen atmosphere with a low dew point or vacuum sintering with a hydrogen backfill during cooling produces the best combination of corrosion resistance and mechanical properties for porous stainless steels The cooling process cycle usually requires a reducing or oxygen-free, inert gas atmosphere for best metallurgical properties When heavy oxidation or nitriding is prevented during cooling, a protective passive surface layer will form and good mechanical properties will result Carbon-containing atmospheres are not normally utilized in processing porous materials because higher carbon levels are usually detrimental to corrosion resistance

Gravity sintering refers to one of the simplest methods of sintering to improve the mechanical properties of a porous component Diffusion or liquid-phase bonds form between the particles when loose (noncompacted) powders are heated

to a temperature near the solidus temperature (Ref 34) Localized melting of the particle surfaces form necks between the powder particle contact surfaces, and the amount of shrinkage during sintering controls the characteristics of the interconnected porosity As an example, spherical porous bronze powders are poured and vibrated into a graphite or an oxidized stainless steel mold without compaction The bronze or tin-coated copper powder is then sintered at 800 to 1000

°C (1472 to 1832 °F) for 20 min in a reducing atmosphere to form a liquid phase between the particles (Ref 9) Upon cooling, a strong metallurgical bond is formed at the contact points between the powder particles Disks, cups, bushings,

or other simple shapes can be produced using this method Typically, parts have a 1° draft angle to facilitate part release from the mold and have dimensional tolerances of ±3%

Secondary operations are widely used to modify the part shape, the porosity, and the surface characteristics of a porous part Porous parts can often be formed to net shapes for use without secondary operations However, the lower green strength and density of porous parts can limit the complexity of the green part and subsequent handling prior to sintering The part shape complexity can be enhanced by sizing, machining, and forming Secondary sizing processes can improve the roundness, flatness, and other dimensional tolerances that cannot be held during sintering Porous P/M materials must not be overstressed during sizing because cracking or closure of the pores will occur Sizing operations increase the density of the part and reduce the surface porosity in the tool contact areas

Secondary machining operations are used to produce features such as threads or tighter dimensional tolerances that cannot

be readily controlled during compaction Conventional machining of porous materials closes off the majority of the surface porosity by metal smearing Machining can be utilized to blind off the porosity and fluid flow in selected areas, depending on the application Controlled chemical etching of a machined surface can restore the surface porosity and still retain the tighter dimensional tolerances produced by machining Care must be taken to avoid contamination of the porous material during machining or chemical reactivating Machining methods without coolants and special cleaning processes are often used to remove the contamination after processing Other machining methods such as electrical discharge machining (EDM) are also used to avoid closing off the surface porosity However, removal of any machining fluids again requires special cleaning methods Conventional machining, EDM, and laser methods are used to cut porous parts from larger parts or sheets

Forming of porous materials by cold working using dies or calendering rolls is used to change the part shape Flat sheet can be formed into tubes and seam welded Cold working during forming increases the density, hardness, and tensile strength of the porous material, with a corresponding decrease in ductility Forming is also used to make special shapes and assemblies Light sizing and swaging operations alter the surface layer to produce parts with tighter dimensional tolerances than as-pressed components The surface porosity is flattened during forming, and the permeability and the surface pore size are decreased However, porous P/M materials are difficult to straighten or flatten because of their elastic properties, which result in significant "spring back" behavior just prior to crack initiation

Joining methods such as brazing, welding, gluing, sinterbonding, and mechanical interlocking are used to create porous metal assemblies The conditions for joining lower-density materials to other porous materials or to solid materials are very different from joining solid materials For example, brazing materials and processing conditions must be selected to avoid wicking of the braze material away from the joint area Often a braze material is selected to form a higher melting point eutectic upon alloying with the porous base material in order to solidify the braze material in the joint area

Welding of porous materials to other porous materials or to solid materials becomes much more complex than welding solid materials The addition of filler metal, the reduction of weld heat input and the use of special welding gases are required to overcome welding problems associated with higher porosity and gases trapped in the pores Good weld joint design allow proper heat distribution to prevent leaks and large holes Weld purge gases and material cleanliness are important to avoid contamination and to provide good corrosion resistance Tungsten inert gas, electron beam, and laser welding processes are used to obtain full penetration of weld joints with relatively narrow heat-affected zones Postweld heat treatment processes such as annealing or stress relieving are recommended for good metallurgical properties

Heat treatment or thermal cycles in reducing atmospheres can also be used for sinterbonding porous parts by processing at

10 to 65 °C (50 to 150 °F) below the sintering temperature in order to maintain the porosity and permeability Sinterbonding is a thermal processing method used to form a metallurgical diffusion bond between two parts such as between a porous part and assembly hardware The hardware is generally the same material as the porous component and allows the sinterbonded assembly to be more easily adapted to the application than if only a porous part was used The diffusion bonds formed are significantly lower than welded or brazed joints because the sinterbonds are only formed at the local contact points However, the sinterbonds are significantly stronger than just an interference fit alone and can ensure that the porous component cannot be easily removed Sinterbonding can be accomplished during a sintering cycle

if proper allowances for shrinkage are considered The sintered porous part must have adequate strength to allow for a press-fit assembly without developing cracks The press-fit interference for parts less than 25.4 mm (1 in.) OD is between 0.025 and 0.1 mm (0.001 and 0.004 in.) The hardware surface should have at least a 0.4 m (16 in.) surface finish so that the assembly interface does not have any gaps that may form openings larger than the pores in the porous metal component The assemblies are normally checked for joint integrity after sinterbonding

Thermal processing can also be used for removal of oxidation, burnoff of certain contaminants introduced by secondary operations, and for passivation of the metal surfaces Oxidation color from the heat-affected zone of welds or from the heat of a cutting operation must often be removed by heat treatment in a reducing atmosphere for corrosion resistance or aesthetic reasons Organic contaminants that are captured in the pores from prior processing steps or improper handling can be vaporized and removed from the pores by thermal processing Heat treatment in a reducing atmosphere can also restore a uniform oxide layer upon cooling that forms a passive surface on the porous metal in order to increase corrosion resistance

Porous sintered metals can be coated with a material such as silicone to alter the surface characteristics Silicone treatment

of the pores changes the hydrophilic nature of the surfaces to a hydrophobic surface that still maintains some permeability Metal oxide coatings have been successfully formed on the porous surfaces to increase corrosion resistance

or to decrease the pore size by applying a thin layer of submicron particles of a metal oxide such as silica, titanium dioxide, or alumina (Ref 35, 36) These fine metal oxide particles can be bonded to a coarser porous metal substrate to produce a dual-porosity structure with good permeability and excellent corrosion resistance

References cited in this section

9 H Neubing, Properties and Sintering Behavior of Spherical Tin Bronze Powders for the Manufacture of

Filters, Int J Powder Metall., Vol 18 (No 4), 1986, p 4

16 C Helliker and T O'Sullivan, Process for Fabricating Porous Nickel Bodies, U.S Patent No 4,255,346,

1980

17 Porous Metal Council, Porous Metal Design Guidebook, Metal Powder Industries Federation, 1980

18 R German, Particle Packing Characteristics, Metal Powder Industries Federation, 1989, p 219-252

19 L Albano-Muller, Filter Elements of Highly Porous Sintered Metals, Powder Metall Int., Vol 14, 1982, p

73-79

20 F Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, 1980, p

143-154

21 V Tracey, The Roll-Compaction of Metal Powders, Powder Metall., Vol 12 (No 24), 1969, p 598-612

22 N Williams and V Tracey, Porous Nickel for Alkaline Battery and Fuel Cell Electrodes: Production by

Roll-Compaction, Int J Powder Metall., Vol 4 (No 2), 1968, p 47-62

23 P Koehler, Seamless Porous Metal Article and Method of Making, U.S Patent No 4,822,692, 1989

24 P Koehler, Seamless Porous Metal Article and Method of Making, U.S Patent No 4,828,930, 1989

25 P Neumann, V Arnhold, K Heiburg, and R Rohlig, Porous Metal Products with Special Properties,

Powder Metal Materials Colloquium, Soc Franc de Metal et de Mater., April 1992, p 4-1 to 4-6

26 L Mott, Process for Making Porous Metallic Bodies, U.S Patent No 2,792,302, 1957

27 P Koehler, S Geibel, and M Whitlock, Liquid Laydown Process and Metal Filter, U.S Patent No 5,149,360, 1992

28 Porous Metal Council, From Powder to Porous Metal Parts, Design Engineering, Metal Powder Industries

Federation, 1981, p 56-89

29 V Tracey, Sintering of Porous Nickel Theoretical and Practical Considerations, Modern Developments in

Powder Metallurgy, Vol 12, Metal Powder Industries Federation, 1980, p 423-438

30 V Morgan, Sintered Metal Filters, Hydraulic Pneumatic Power, Trade and Technical Press, 1974, p

323-324

31 H Nayar, Production Sintering Atmospheres, Powder Metallurgy, Vol 7, Metals Handbook, American

Society for Metals, 1984, p 340-350

32 D Garg, K Berger, D Bowe, and J Marsden, Effect of Atmosphere Composition on Sintering of Bronze,

Gas Interactions in Non Ferrous Metal Processing, Minerals, Metals and Materials Society, 1996, p 17-26

33 H Nayar, R German, and W Johnson, Effect of Sintering on the Corrosion Resistance of 316L Stainless

Steel, Proc 1981 P/M Conf., Metal Powder Industries Federation, 1981, p 653-667

34 V Tracey, Sintering of Porous Nickel, Powder Metall., Vol 26 (No 2), 1983, p 89-92

35 Graver Separations Brochure Number 5-106, Graver Chemical Company, 1996

36 A Mulder et al., SiO2-Membranes on Porous Sintered Metal Substrates, Starting from Silicone Solutions,

Key Eng Mater., Vol 61-62, 1991, p 411-414

Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

Porous Material Characteristics

Design engineering information is readily available for many of the porous material characteristics such as density, pore size and distribution, bubble point, permeability, and mechanical properties (Ref 8, 10, 17) However, many of the unique characteristics of porous P/M materials such as elevated service temperature and corrosion resistance properties in specialized environments require additional design engineering considerations The density, pore size, pore size distribution, and permeability determine the overall performance of a porous material in an application The interrelationship of these four characteristics often allows the design engineer to specify only one or two of these factors For example, testing has shown that the permeability of a porous part can be accurately predicted if the density and the initial powder particle characteristics are known for a particular processing method (Ref 37) The initial powder particle size controls the pore size and distribution when sintered to a specified density The permeability is related to the pore size and pore distribution Other porous material characteristics can be estimated from wrought (or full-density P/M) products because higher-density products have more extensive test data available in the literature Limited availability of design engineering data for porous materials often results in the need for actual testing in the intended application prior to use

Material density is an important characteristic for predicting mechanical properties and permeability As the density decreases (and porosity increases), the mechanical properties are reduced and the permeability is increased Porosity consists of interconnected channels and isolated pores Isolated pores that are either closed or dead-end channels do not contribute to the overall permeability, and the percentage of isolated pores must be considered if density comparisons are used to specify a component Nearly all of the porosity in a component with an overall density less than 75% of TMD is interconnected to the surface pores and is usable for filtration and fluid-flow paths (Ref 38) As the overall density increases from 75 to 92% of TMD, the percentage of isolated pores increases rapidly and the functional interconnected porosity is depleted

Density measurement methods for porous materials are described by International Organization for Standardization (ISO) Document 2738 (Ref 39), which includes the use of Archimedes principle The porosity must be fully sealed prior to immersion in water using this method Overall density can be estimated by weighing and volume calculation for simple part geometry Density can also be measured by optical analysis of carefully prepared surfaces The cutting, polishing, and etching techniques during metallographic preparation must be monitored to avoid smearing or excessive removal of the surface porosity Density uniformity throughout a porous product must be considered when measuring overall density because the forming process may introduce large variations For example, a coaxial pressed cup could have a density variation of more than 5 to 10% when comparing the lower-density side walls versus the higher-density end cap

Pore characteristics such as size, shape, and distribution can be determined using many different measurement techniques A comparison of the various pore characterization methods reveals that while the test method is usually consistent to show relative differences between various porous material grades, the test methods do not always agree (Ref 40) Because it is normally possible to correlate the various test methods, technical information regarding pore characteristics must be carefully compared to determine if the pore characteristics are equivalent

One of the most common pore size characterization methods is a "bubble-point" test, which measures the pressure required to release the first bubble from the surface of a porous material submerged in a liquid and pressurized from one side as shown in Fig 6 The liquid selected for this testing must have good wetting characteristics with relatively low viscosity and surface tension, such as isopropyl alcohol Several standard test methods, such as ASTM E 128 (Ref 41) and ISO 4003 (Ref 42), have been developed to measure the pressure at which the first bubbles form and break away from one area The maximum interconnected pore diameter can be estimated using Poiseuille's capillary law and an additional pore shape correction factor for noncylindrical pores:

d = 4KR cos B/P

where d is the pore diameter, K is the shape correction factor, R is the surface tension of the wetting liquid, B is the contact angle of liquid and porous material, and P is the pressure at first bubble point (Ref 43) The shape correction factor, K, is largely dependent on the shape of the pores and the porous material The shape correction factor can vary

from about 0.2 for more spherical pores (e.g., bronze) to 0.4 for more irregular-shaped pores (e.g., 316L SS) Therefore, the bubble-point test is not a highly accurate measurement of pore size, but serves only as a reproducible, nondestructive test for ranking porous material grades by the largest pore size The maximum pore size of the porous media calculated from the bubble-point test is not a direct indication of filtration performance, but the results can be correlated to more complex filter-retention testing The filter industry often refers to the diameter of the largest pore determined by the bubble-point method as a micron rating This terminology should not be confused with the diameter of a particle retained

by a filter or the absolute filtration rating The bubble-point test is not a good indicator of the gas or liquid filtration rating because filtration is dependent on many other variables, such as the uniformity and distribution of the pores, the media thickness, and the type of solids being filtered The bubble-point method is also an excellent integrity test of a material that has been subjected to secondary operations such as welding or forming in order to determine if any larger pores were created

Fig 6 Bubble-point testing apparatus

Mercury porosimetry and a mean flow pore size distribution measurement technique developed by Wenman and Miller use the Washburn Equation to calculate the pore diameter as the pressure is gradually increased (Ref 44, 45) Mercury porosimetry calculates the interconnected pore size and pore distribution of material by measuring the pressure required

to force mercury into the pores and the volume of mercury in the pores The other method determines the pore size

distribution using a fluid that fully prewets the porous sample and then measures the pressure differential as the air flow is gradually increased As the pressure increases, the fluid is forced from the larger pores first, and the air flow permeability begins to increase until the sample is dried A comparison of the wet and dry permeability curves allows the calculation of the pore size and distribution ASTM F 316 (Ref 46) details the test method for determining the maximum pore size by bubble-point testing and the pore size distribution by mean flow pore testing The capillary action of the porosity is dependent on pore size and is an important property for applications that require fluid wicking and storage

Permeability is a measure of the flow resistance through a porous material and is one of the most important characteristics for design engineers specifying porous materials The fluid flow through a porous material is normally expressed as a volumetric flow rate at a specified pressure differential for a given surface area (e.g., standard liters per

minute per square centimeter at dP = 1000 kg/cm2 of nitrogen) The nature of the fluid flow can be characterized as diffusional, laminar, or turbulent depending on flow velocity For very low flow rates and small pores or low absolute pressures, diffusional flow is the predominant mechanism and is not dependent on the fluid viscosity because the mean free path of the molecular collisions is larger than the pores Fluid flow that is unidirectional along parallel flow paths with uniform velocity is considered laminar or viscous flow Turbulent flow occurs when flow is high enough to cause irregular, random flow paths Other types of flow mechanisms termed slip and inertial flow occur in the transition regions among diffusion, viscous, and turbulent flow (Ref 47) For example, inertial flow mechanisms must be considered to account for the energy losses and increased flow resistance of the gas at higher flow rates as it changes direction in the

tortuous pore network flow path For laminar flow of liquids, the volumetric flow rate, Q, can be predicted by Darcy's

Law:

Q = dPACv/tV

where dP is the differential pressure, A is the surface area, Cv is the coefficient of viscous permeability, t is the material thickness, and V is the dynamic viscosity of the fluid In the case of compressible gases, Darcy's law is modified to allow

for the gas volume change with pressure as shown below:

where P1 is the upstream pressure and P2 is the downstream pressure (Ref 48) For higher pressures and gas velocities, the flow rate through a porous material can be predicted by a modified version of Darcy's law, which includes correction factors for inertial and slip flow (Ref 48) Although the permeability can often be accurately modeled, the complex gas flow physics of the porous structures limits the accuracy of the theoretical calculation of fluid flow, especially in the extreme low- and high-flow regions As the gas density increases due to an increase in pressure or a decrease in temperature, the permeability decreases As the viscosity of a gas increases due to an increase in pressure or temperature, the permeability also decreases Pressure increases result in only a slight increase in the gas viscosity, but significantly increase the gas density Temperature increases typically result in an increase in the gas viscosity and a reduction in the gas density When comparing different porous materials, the accuracy of theoretical calculations is often reduced due to the interactions of these variables along with the various flow mechanisms operating in the microchannels of the porous structure

As shown above, the flow rate is also dependent on the surface area and the thickness of a porous structure As surface area is increased, the differential pressure required to produce the same flow rate is decreased Also, as the thickness of a material is decreased, the pressure differential is reduced to obtain the same flow rate When comparing two porous materials, higher permeability indicates that a lower pressure differential was required to produce the same flow Therefore, less surface area of a material with a higher permeability is required to produce the same fluid flow rate as a porous material with lower permeability The economics become more favorable as the porous material surface area is reduced to meet the desired flow rate The pressure differential and permeability are important design characteristics for determining overall flow rates for applications such as filter systems

Permeability testing is one of the best methods for specifying the characteristics of a porous material by measuring the fluid flow rate and pressure drop Permeability if a function of the gas or liquid testing fluid and requires close control

of the fluid test conditions Gas permeability is dependent on many variables such as density, viscosity, temperature, and pressure conditions Many references have investigated gas-flow dynamics and have proposed models for predicting flow rates as conditions are varied (Ref 30, 48, 49, 50) The permeability of a fluid through a rigid porous material can be determined by standard test methods as described by ISO 4022 (Ref 51) and ASTM 128 (Ref 41) A typical gas permeability test schematic diagram is shown in Fig 7 The flow path position of the volumetric flow measuring device is

critical in order to avoid inaccurate measurements if an excessive back pressure or vacuum is created The flow rate is measured by controlling the pressure differential and correcting the measured flow to standard temperature and pressure conditions using the ideal gas law to correct for gas volume changes The atmospheric pressure and gas temperature can

be measured in order to correct the measured gas flow rate back to standard temperature (21.1 °C, 70 °F, or 294.1 K) and pressure (760 mm Hg, or 29.92 in Hg) Accurate flow rate comparisons can then be made between porous parts tested with nonstandard conditions A general formula that accounts for volume changes and corrects the measured gas flow rate back to standard conditions is given below:

Qs = Qm(Pm/Ps)(Ts/Tm)

where Qs is the standard flow rate, Qm is the measured flow rate, Pm is the measured pressure in mm Hg, Ps is the standard

pressure, Tm is the measured temperature, and Ts is the standard temperature Corrections to standard conditions are significant because a small pressure change of 25.4 mm Hg (1 in Hg) or a small temperature change of 2 °C (4 °F) can result in approximately, a 1% difference in the flow rate

Fig 7 Schematic of gas permeability testing apparatus

Additional correction factors can further improve the accuracy of permeability testing by accounting for changes in the gas viscosity and the interactions of the gas with the pore structure as temperature and pressure are varied As temperature increases, the gas viscosity increases and the gas density decreases, which in turn increases the measured gas flow rate relative to standard conditions as shown in Fig 8 Also, as the gas contracts to enter or expands to exit the pore structure, energy losses occur to reduce the flow (Ref 49) When measuring the flow of one gas and calculating the flow rate equivalent of a different gas, the flow rate is adjusted for differences in gas viscosity or specific gravity Gas flow conversions are important when testing of the actual gas is not practical due to safety or handling reasons For example, the flow rate of nitrogen is often measured and used to estimate the actual flow rate of a toxic or corrosive gas at the same temperature and pressure conditions

Fig 8 Typical gas flow versus pressure drop through porous sintered metal restrictors

The volumetric flow rate can also be calculated by dividing the mass flow rate by the fluid density The mass flow rate of gases can be measured with an accuracy of about ±2% by devices that correlate the gas flow rate to the cooling rate of the gas as it flows past a hot wire These devices are based on calculating the gas flow rate by measuring the amount of energy required to keep the wire at a reference temperature and knowing the thermal conductivity of the gas Mass flow rates do not need to be corrected back to standard conditions Volumetric flow rates that are corrected back to standard conditions can achieve an accuracy of better than ±1% by collecting the fluid in a known volume as a function of time Volumetric flow rate devices such as positive displacement piston meters are highly accurate primary standards Mass flow rate devices are slightly less accurate and are secondary standard devices

Liquid permeability is also sensitive to variations in temperature, pressure, viscosity, and density Liquids flowing through porous materials are highly dependent on the viscosity and the ability of the fluid to wet the pore structure Liquid flow rates are often difficult to reproduce if the fluid does not have consistent wetting properties For example, isopropyl alcohol is preferred for liquid flow testing of porous metal materials, whereas water does not wet well Water can also have large differences in the mineral content depending on the source, and these quality differences can make flow rate comparisons difficult, especially for tighter porous structures The permeability of gases and liquids through a rigid porous material can be determined by specialized testing as described by ISO 4022 (Ref 51) and ASTM E 128 (Ref 41)

Mechanical properties such as ductility, tensile strength, shear strength, collapse strength, burst strength, and fatigue life of porous materials are highly dependent on the porosity and the processing method (Ref 52, 53) Table 1 illustrates typical values for tensile strength, elongation, and shear strength of 90% Cu/10% tin bronze discs and 316L stainless steel sheet as a function of density and filter grade as rated by the bubble-point method (Ref 8, 10, 54) Mechanical properties increase significantly as the pore size and the percentage of porosity decreases (Ref 54, 56, 57, 58) Alternatively, permeability decreases as the pore size and the percentage of porosity decreases Therefore, an optimal balance of mechanical properties and permeability must be achieved to meet the application Processing methods and materials can usually be selected to create a porous P/M material that meets the minimum mechanical properties and provides the maximum permeability In some more severe applications involving high-pressure differentials, corrosive environments, and/or high-temperature service, mechanical properties are maximized at the expense of permeability in order to

maximize service life Enhanced mechanical properties also extend the number of cleaning and reuse cycles in some applications

Table 1 Typical mechanical properties of bronze and 316L SS filters

(a) For comparison purposes, wrought bronze (90% Cu/10% Sn) has a

theoretical mean density of 8.8 g/cm3 with a minimum UTS of 300 MPa (43 ksi) and 20% elongation 316L SS bar stock (cold finished and annealed per ASTM A 276) has a theoretical mean density of 8.0 g/cm3 with a minimum UTS of 482 MPa (70 ksi) and 30%

elongation Data are for commercial bronze filter disks and 316L SS sheet

(b) Filter grades in micrometers as estimated by bubble-point test

method

(c) Shear strength is the punching force divided by the sheared edge

area per DIN standard V 30910 (Ref 55)

Other material properties such as thermal conductivity, thermal expansion, fatigue, electrical conductivity, and magnetic properties are also highly dependent on porosity and generally decrease as porosity increases (Ref 59, 60) For example, thermal expansion can be directly related to the porosity by the equation (Ref 59):

where Ct is the effective thermal expansion of the porous material, Co is the bulk thermal expansion of the wrought

material, and Df is the fractional density Other theoretical relationships have been proposed to cover the mechanical properties over a broad range of porous materials and porosity levels, but actual testing is recommended to determine the properties because slight variations in porosity and processing methods can produce significant changes (Ref 61, 62)

Elevated-temperature and corrosion properties are often important criteria for selection of porous P/M materials versus other porous materials such as plastics and papers The maximum service temperatures for several P/M materials are listed in Table 2 The application environment is also critical to obtaining maximum service life because the atmosphere may be oxidizing, reducing, or inert in nature Mechanical properties at an elevated temperature may be significantly reduced by accelerated corrosion reactions in atmospheres that contain ammonia, carbon, chlorine, fluorine, hydrogen, moisture, sulfur, or other reactive materials For example, catastrophic oxidation of 316L SS can occur at a lower temperature than expected due to the formation of low-melting-point eutectic such as vanadium pentaoxide if vanadium is used as a reaction catalyst The presence of lead and molybdenum can also result in catastrophic oxidation (Ref 63) The corrosion resistance of 316L SS is also reduced in ammonia service at 540 °C (1004 °F) due to the formation of a brittle nitride layer and in the presence of sulfur due the formation of iron and nickel sulfides Temperature fluctuations and localized hot spots can cause accelerated oxidation due to spalling or cracking of the passive layer

Table 2 Elevated temperature service for selected porous filter materials

Maximizing temperature

oxidizing atmosphere

Maximum temperature

reducing atmosphere Material

in the pores to create a localized concentration cell Inadequate cleaning or prolonged storage without drying after removal from service can cause pitting failure

There are many methods to enhance the corrosion resistance of porous materials Proper alloy selection results in the best protection against corrosion although the economic considerations of the more expensive alloys may be prohibitive One advantage of the P/M process is that special alloys or custom blends can be made For example, more nickel, chromium, and manganese can be added to improve corrosion resistance of a 300 series stainless steel rather than being limited to standard alloys Bronze filters can be plated with nickel or tin to improve corrosion resistance While it is difficult to predict actual service life in a particular application, common corrosion tests can give some indication of corrosion reactions Evaluating corrosion resistance using standard immersion (weight-loss) testing and salt-spray tests can often predict the behavior of porous materials (Ref 69) One of the best indicators of corrosion resistance uses porous tensile bar samples suspended in the actual solution While the fluid-flow velocity effects are not considered in this test, mechanical testing and optical analysis can detect the effects of corrosion Electrochemical testing can also be used to indicate the potential of a sample to exhibit passive behavior in a solution (Ref 66, 69) For stainless steels, indicator tests

to determine the presence of unalloyed iron or carbide precipitation at grain boundaries determines proper processing Sensitization of 316L SS due to slow cooling in nitrogen-containing atmospheres during sintering or carbide precipitation during welding can lead to intergranular corrosion (Ref 70) Welding should be followed by an annealing or stress relieving heat treatment to increase corrosion resistance and mechanical properties Proper handling to minimize contamination and corrosion initiation sites (e.g., fingerprints, scratches, dents, foreign material) can also improve corrosion resistance

Filtration properties are dependent on the porous material characteristics, the surface area available for filtration, and the process conditions of the application Filtration and separation are intricate technologies that can be classified into six general areas: gas-solid, liquid-solid, gas-liquid, gas-gas, solid-solid, and liquid-liquid separation Porous P/M parts are normally used for solid particle separation from a process stream, but have significantly different filtration properties in gas or liquid service (Ref 71) The filter efficiency or filter rating in a gas stream is higher than in liquids There are additional gas-liquid separations possible with porous P/M parts such as bronze filters, which separate water from air due

to surface tension differences Filters are specified and compared by their desired performance characteristics, which include high permeability, low-pressure drop, retention efficiency of specified particle sizes, particle-loading capacity and resistance to blinding The desire to have high permeability must be balanced with the minimum mechanical properties

required for filtration As the porosity is reduced, the density of a filter increases, resulting in higher strength and lower permeability Predicting filter performance in an application is difficult because there are many process variables such as the nature of the particles (size, shape, compressibility, and composition), particle concentration, fluid flow rate, viscosity, vibration, service temperature, and pressure

Surface- and depth-type filter mechanisms perform the separation of solids from a process stream with porous P/M media Surface filtration is characterized by the formation of a layer of particles (cake filtration) on the upstream surface of porous media due to sieving, direct interception, and bridging of the particles As the filter cake forms, the layer of particles begins to filter the process stream, and the porous media act more as a support Wire mesh screens are examples

of surface-type filters that are generally used to retain coarser particles (>140 m) Finer filtration retention can be accomplished with screens, but the higher pressures required to drive the fluid often pushes the finer particles into the screen openings to cause blinding Thin layers of fiber metal bonded to the surface of a screen can provide even finer levels of filtration with improved permeability (Ref 72) Surface filters can be easily cleaned by reversing the flow direction if properly sized Depth filtration is a more complex form of filtration than surface filtration because it is characterized by the retention of particles within the interconnected pore structure Particles are retained by a variety of mechanisms including direct interception, inertial impaction, diffusion, adsorption, electrostatic attraction, and gravitational settling (Ref 73) In gas-solid separation, depth filtration depends primarily on capturing particles by interception, diffusion, electrostatic deposition, and impaction mechanisms (Ref 74) These mechanisms are less important in liquid-solid separation because the electrostatic repulsion and the hydrodynamic fluid flow forces allow certain particle sizes to navigate the pore channels and not be captured Hydrophobic and electrostatic adsorption can also

be important in liquid filters, depending on the nature of the electrical charge interactions of the filter media, the contaminants, and the liquid Sieving mechanisms also occur in depth filters depending on the size and distribution of the contaminants

Porous P/M materials offer a combination of surface and depth filtration for filtration requirements less than 140 m Porous P/M materials are fabricated in relatively thin cross sections as low as 0.12 mm (0.005 in.) for use as surface filters Typically, thicker wall cross sections are fabricated to obtain a strong, rigid media that can withstand the higher operating pressures associated with depth-type filters The permeability decreases, and the corresponding drop in pressure across the filter increases as the thickness of the media is increased Filtration ratings can be determined from standardized testing under tightly controlled conditions in order to compare various filter grades and materials Standardized filter efficiency tests using monosized glass beads, test dusts, or polydisperse salt particles are used to more accurately determine filter ratings (Ref 58, 75, 76, 77) Other common industrial tests such as the Beta Rating Test use a known contaminant in a multipass filter challenge method (Ref 78) Filter challenge testing eliminates some of the confusion when filters are described by absolute, nominal, or mean filter ratings based on a correlation of the pore size determined by the bubble-point test These descriptions can only offer a reference point for the actual filter efficiency at a given particle size because a "10 micron" rated filter may retain anywhere from 50 to 99.9999999% of 10 m particles, depending on the application and test method In addition, this filter rating does not indicate the filtration efficiency of the most penetrating particle (Ref 74) Particles that are smaller or larger than 10 m can be more difficult to retain, depending on the test conditions and filter media Testing the filter in the application is recommended to determine the actual performance and efficiency by measuring the downstream contaminants

Cleaning and subsequent reuse of porous P/M materials is a major advantage over disposable media that generally have low strength, limited temperature capability, and corrosion resistance and cannot be cleaned practically The economic justification for using porous metals often depends on extending the service life with numerous cleaning cycles Regeneration of porous metal media with blocked porosity can be accomplished with a variety of cleaning techniques and solutions depending on the nature of the foulant Cleaning methods include thermal removal, ultrasonic immersion, reverse flow methods, high-velocity waterjets, and chemical processing with soaking and rinsing baths Cleaning solutions can range from mild detergents to harsher caustic solutions or diluted acids, depending on the type of blockage The cleaning solution and technique must be selected to avoid damage or corrosion of the base metal Combinations of methods and solutions can be selected for optimal cleaning Some of the reverse flow, gas, and liquid blowback methods can be performed in situ while other methods such as furnace cleaning, salt baths, and ultrasonic baths require removal of the porous part Cleaning service organizations determine the best method to clean the part without damage based on testing and experience After cleaning the part, the cleaning solutions must be thoroughly flushed and dried Visual inspection, bubble-point testing, and permeability testing are performed to determine the effectiveness of the cleaning method to return the porous part to the original manufacturer specifications Measuring the weight before and after cleaning can also be used to evaluate cleaning performance as long as corrosion is not an issue Gathering data on each serialized part prior to service provides a baseline for measuring cleaning effectiveness Mechanical cleaning methods such as wire brushing, scraping, sand blasting, or glass bead blasting must be avoided to prevent pore smearing and

further entrapment of the contaminants The porous part cannot always be fully cleaned without damaging the base material, but the part can be sufficiently cleaned to be returned to service

References cited in this section

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1000-9000

10 Bronze Filter Powders, ACu Powder International Technical Brochure, Acu Powder International, 1997

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30 V Morgan, Sintered Metal Filters, Hydraulic Pneumatic Power, Trade and Technical Press, 1974, p

323-324

37 D Smith, J Smugeresky, and B Meyers, "The Dependence of Permeability and Filtration on Pore Morphology in Consolidated Particulate Media," Report SAND87-8227, Sandia National Laboratories,

1987, p 1-74

38 D Smith, E Brown, J Smugeresky, and T McCabe, Characterization of Controlled Density P/M Structures

for Filtration Applications, Proc 1985 P/M Conf., Metal Powder Industries Federation, 1985, p 653-667

39 "Permeable Sintered Metal Materials Determination of Density, Oil Content, and Open Porosity," Standard

2738, International Standards Organization, 1987

40 R Iacocca and R German, A Comparison of Powder Particle Size Measuring Instruments, Int J Powder

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43 V Morgan, Filter Elements by Powder Metallurgy, Symposium on Powder Metallurgy, The Iron and Steel

Institute, 1956, p 81-89

44 R Lines, A Pore Man's Guide, Filtration News, Eagle Publications, Vol 10 (No 2), 1992, p 40-44

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47 R German, Particle Packing Characteristics, Metal Powder Industries Federation, 1989, p 353-390

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49 "Flow of Fluids through Valves, Fittings and Pipe," Technical paper 410, Crane Company, 1979

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2, 1979, p 45-48

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Materials, Poroshk Metall., 1989, p 668-673

61 "Determination of Properties of Sintered Bronze P/M Filter Powders," Standard 39, Metal Powder Industries Federation, 1983

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63 J Davis, Ed., ASM Specialty Handbook: Stainless Steels, ASM International, 1994, p 211-212

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Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

Table 3 Major application areas of porous P/M materials

o Condensate water polishing

o Fossil-fuel waste streams

o Radioactive material refining

• Semiconductor

o Particle removal process gas

o Bulk gas delivery systems

o Purifier media retainers

o Process gases and liquids

o Fluid-bed reactor products

• Mineral processing

o Coal, silica, metal oxides

o Calciner and incinerator off-gas

o Catalyst manufacturing

• Fluid power

o Protect hydraulic valves

o Pneumatic equipment

o Water removal in air lines

• Food and beverage

o Removal of yeast from beer

o Acids, solvents, and inks

o Adhesives and greases

o Precipitates, salts, and carbon

• Other filter areas

o Oil burners

o Aircraft and marine fuel

o Paper and pulp

• Gas and liquid metering

• Flow-rate timing devices

• Fuses, Vacuum delay valves

Surface area devices

• Liquid wicks, evaporation

• Printer ink reservoirs

• Turning bars for film/webs

• Fluidizer plates for fluid beds

o Steam injectors/heat transfer

• Spargers

o Chlorine and oxygen bleaching

o Oxygen and volatile stripping

of particle shedding, and cleanability are critical to the filter system operation Porous metals can also be used to support finer filter membranes or used as filter septum when they are coated with ion-exchange resins

Laminar flow control devices utilizing porous P/M flow restrictors (Ref 2, 8, 80, 81, 82) are more accurate and reliable than other volumetric-flow-limiting products such as orifices and micrometering valves that operate at higher fluid velocities Orifice technology normally relies on controlling gas flow in the choked flow regime by requiring that the inlet pressure be at least double the outlet pressure Porous P/M materials are more resistant to contamination and plugging because there are hundreds of small flow paths available instead of a single orifice Flow restrictors are used to provide a constant flow for a given set of conditions (pressure, temperature, and fluid) and are mechanically strong devices that do not have mechanical or electrical components that can wear or require calibration

A few applications of porous P/M materials rely on the extremely high internal surface area and porosity available (Ref 2,

22, 83) For example, the conductivity of the electrodes for alkaline batteries and for fuel cells can be significantly increased by large contact area with the reaction fluid Biomedical implants use the large surface area to allow tissue growth into the porous structure to allow good joint attachment

Reservoirs and capillary-attraction devices are other major application areas (Ref 2, 8, 81, 83) Self-lubricating bearings that store lubricants in the pores and protect the surface from wear are one of the oldest commercial applications of porous P/M materials Heat removal devices for microelectronics packaging and for cooling devices used in satellites are some of the newest application areas

Sound, pressure, and vibration dampening components are also common industrial applications (Ref 2, 80, 81, 82) Porosity acts as an acoustical impedance and dampens certain sound frequencies while others pass through A pressure surge in a fluid delivery line is also dampened by reducing the fluid velocity as it passes through the small interconnected channels Many pressure gages have porous metal pressure snubbers to dampen the vibration or pressure change rate and

to provide smoother operation of the indicating needle

Boundary layer control devices such as air flotation bars and vacuum hold-down plates are highly specialized applications (Ref 2, 8, 81, 82, 84) A boundary layer of air for noncontact turning of thin films, tapes, or webs can be formed on the outside diameter of a fine-porosity tube when the inside cavity is pressurized

In gas-liquid contacting applications (Ref 84, 85), flowing gas through a fine-grade, porous P/M material with high surface area produces small bubbles that result in high-efficiency transfer contact of the gas to the liquid Finer pore size materials are used to prevent the liquid from penetrating back into the pores at lower internal pressures Conventional drilled pipe spargers produce larger bubbles that do not dissolve and react as well as the finer bubbles generated by porous metals

Overall, the application of porous P/M technology is rapidly emerging as a cost-effective, renewable, industrial resource that offers diverse solutions to engineering challenges

References cited in this section

2 V Tracey, Porous Materials: Current and Future Trends, Int J Powder Metall Powder Technol., Vol 12

(No 1), 1976, p 25-35

8 Porous Metal Products for OEM Applications, Mott Technical Handbook, Mott Corporation, 1996, Sections

1000-9000

22 N Williams and V Tracey, Porous Nickel for Alkaline Battery and Fuel Cell Electrodes: Production by

Roll-Compaction, Int J Powder Metall., Vol 4 (No 2), 1968, p 47-62

77 C Moreland and B Williams, "Selecting Polymer Filtration Media," presented at the Fiber Producer Conf., PTI Technologies, 1980

78 J Eleftherakis and R Webb, Industrial Filtration Test Methods, Filtrat News, Sept-Oct 1995, p 46-49

79 "The Pall Gas Solid Separation System for the Chemical Process, Refining and Mineral Industries," Product Bulletin GSS-1, Pall Corporation, 1987, p 3-5

80 M Busche, Porous Metals Filters Liquids, Cut Noise, Dampen Vibration, Materials in Design Engineering,

Vol 65 (No 2), 1966, p 80-83

81 J Snyder, P/M Porous Parts, Powder Metallurgy, Vol 7, ASM Handbook, American Society for Metals,

1984, p 696-700

82 W Mossner, "Applications and Properties of Controlled Porosity P/M Parts," SSI Sintered Specialties, 1986

83 F Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, 1980, p

359-380

84 W Johnson and M Shenuski, Controlling Fluid Flow with Porous Metals, Mach Design., Jan 1987, p 3

85 Porous Metal Products for OEM Applications A Guide to Advanced Gas Sparging and Gas-Liquid

Contacting, Mott Technical Handbook, Mott Corporation, 1996, Section 11000

Porous Powder Metallurgy Technology

Mark Eisenmann, Mott Corporation

References

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8 Porous Metal Products for OEM Applications, Mott Technical Handbook, Mott Corporation, 1996, Sections

1000-9000

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12 R German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 96

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1976, p 371-386

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17 Porous Metal Council, Porous Metal Design Guidebook, Metal Powder Industries Federation, 1980

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21 V Tracey, The Roll-Compaction of Metal Powders, Powder Metall., Vol 12 (No 24), 1969, p 598-612

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Roll-Compaction, Int J Powder Metall., Vol 4 (No 2), 1968, p 47-62

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24 P Koehler, Seamless Porous Metal Article and Method of Making, U.S Patent No 4,828,930, 1989

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26 L Mott, Process for Making Porous Metallic Bodies, U.S Patent No 2,792,302, 1957

27 P Koehler, S Geibel, and M Whitlock, Liquid Laydown Process and Metal Filter, U.S Patent No 5,149,360, 1992

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47 R German, Particle Packing Characteristics, Metal Powder Industries Federation, 1989, p 353-390

48 R German, Gas Flow Physics in Porous Metals, Int J Powder Metall., Vol 15 (No 1), 1979, p 23-30

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80 M Busche, Porous Metals Filters Liquids, Cut Noise, Dampen Vibration, Materials in Design Engineering,

Vol 65 (No 2), 1966, p 80-83

81 J Snyder, P/M Porous Parts, Powder Metallurgy, Vol 7, ASM Handbook, American Society for Metals,

1984, p 696-700

82 W Mossner, "Applications and Properties of Controlled Porosity P/M Parts," SSI Sintered Specialties, 1986

83 F Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, 1980, p

359-380

84 W Johnson and M Shenuski, Controlling Fluid Flow with Porous Metals, Mach Design., Jan 1987, p 3

85 Porous Metal Products for OEM Applications A Guide to Advanced Gas Sparging and Gas-Liquid

Contacting, Mott Technical Handbook, Mott Corporation, 1996, Section 11000

Even pure metals and metallic alloys can be produced as cellular solids or metal foams In the past, metal foams were prepared by adding a foaming agent to a molten metal after properly adjusting the viscosity of the melt (Ref 1, 2) The foaming agent is usually a powdered metal hydride, for example, TiH2, which releases hydrogen gas when heated to temperatures above approximately 400 °C As soon as the foaming agent comes into contact with the molten metal, it decomposes such that there is little time to achieve a homogeneous distribution of the gas-releasing powder Because this process is difficult to control, more widespread application could not be achieved with this technology

A new P/M process for production of metal foams has been developed at the Fraunhofer-Institute for Applied Materials Research (Bremen, Germany) (Ref 3, 4) This enlarges the application range of cellular materials with the advantageous mechanical and thermal properties of metal foams and fewer ecological problems in comparison to polymer foams The process has been developed for aluminum foams and is currently being extended to other metals and alloys such as tin- and zinc-base foams (Fig 1) These nonferrous foams are the subject of this article

Fig 1 Optical micrographs of (a) aluminum (4 to 1), zinc (6 to 1), and lead (4 to 1) foams

For the production of steel foams, the type of foaming agent must be changed to prevent excessive oxidation The foaming process for steel also has to take place in an inert atmosphere or in a vacuum Using steel foams, the applicable temperature range could be extended As an example, the exhaust manifold of car engines could be manufactured from this material Due to the strongly reduced thermal conductivity of the manifold, it would require less time to reach the normal operating temperature of the exhaust catalyst, leading to a reduction in emissions

References

1 J.A Ridgeway, "Cellarized Metal and Method of Producing the Same," U.S Patent 3,297,431, 1967

2 S Akiyama et al., "Foamed Metal and Method for Producing the Same," European Patent Application EP 0

210 803 Al, 1986

3 J Baumeister, "Method for Producing Porous Metal Bodies," German Patent DE 40 18 360, 1990

4 J Baumeister and H Schrader, "Methods for Manufacturing Formable Metal Bodies," German Patent DE 41

01 630, 1991