Volume 18 - Friction, Lubrication, and Wear Technology Part 22 pdf

Application areas of the process can be categorized as Ref 1: • Special composite materials All thermal spray processes rely on three basic operational mechanisms: • Heating a coating m

There are no clear-cut trends for the effects of lubricants on the coefficients of friction PTFE lubricants demonstrate the lowest static coefficients of friction at all temperatures when compared to metallic lubricants (intercalated graphite, SbSbS4 and CaF2) However, graphite powder can offer a lower static coefficient of friction than PTFE (Fig 10 and 11)

Fig 10 Effect of lubricant and test temperature on the coefficients of friction of carbon fiber/PEEK composites

(a) Static coefficient of friction (b) Dynamic coefficient of friction

Fig 11 Effect of lubricant and test temperature on the coefficients of friction of carbon fiber/PEEK composites

(a) Static coefficient of friction (b) Dynamic coefficient of friction

Coefficients of friction are expected to decrease with increasing thermal stability of the resin (for example, melt point, Tg,

or continuous-use temperature) This trend is not well demonstrated by 30% carbon-fiber-reinforced PEEK, PEK, or HTX

(Fig 12)

Fig 12 Effect of test temperature on the coefficients of friction of 30% carbon-fiber-reinforced PEEK, PEK, and

HTX composites (a) Static coefficient of friction (b) Dynamic coefficient of friction

The coefficient of friction increased for the PTFE-lubricated, carbon-fiber-reinforced PEEK composite with increasing temperature (Fig 13)

Fig 13 Plot of coefficient of friction versus test temperature for LCL-4033

The reinforced, lubricated PPS composite demonstrates a clear trend for the dynamic coefficient of friction to increase with temperature The static coefficient of friction reaches a maximum at approximately 150 °C (300 °F) and then rapidly diminishes with further increases in temperature (Fig 14)

Fig 14 Plot of coefficient of friction versus test temperature for Lubricomp O-BG

Conclusions. The results of the tests conducted on the composites listed in Table 2 can be summarized as follows:

• PEEK composites formulated with lower molecular weight resin demonstrated lower wear factors than analogs using higher molecular weight resin at elevated temperatures This is probably due to superior wet-out of the resin and fiber in the low-molecular-weight composites

• Intercalated graphite is a very effective (and expensive) high-temperature lubricant for reinforced high-molecular-weight PEEK

carbon-fiber-• For service to 205 °C (400 °F), Lubricomp O-BG (PPS) and LCL-4033 (PEEK) composites are the materials of choice for wear and friction applications

• For service at 260 °C (500 °F), lubricated PEEK composites are the materials of choice for wear and friction applications

• For service at temperatures in excess of 260 °C (500 °F), composites based on PEK and HTX will be required for wear and friction applications

Thermal Spray Coatings

Burton A Kushner and Edward R Novinski, Perkin-Elmer Corporation, Metco Division

Introduction

THE THERMAL SPRAY COATING PROCESS is by far the most versatile modern surfacing method with regard to economics, range of materials, and scope of applications The thermal spray process permits rapid application of high-performance materials in thicknesses from a few mils to more than 25 mm (1 in.) on parts of a variety of sizes and geometries Thermal spraying requires minimal base-metal preparation, can be applied in the field, and is a low-temperature (>95 °C, or >200 °F) method compared with techniques such as weld overlay Typical part configurations include piston rings, journals, conveyors, shifter forks, extrusion dies, transformer cases, ship hulls, ship tanker compartments, and suspension bridges

Thermal spraying reduces wear and corrosion and greatly prolongs part service life by allowing use of a performance coating material over a low-cost base metal Application areas of the process can be categorized as (Ref 1):

• Special composite materials

All thermal spray processes rely on three basic operational mechanisms:

• Heating a coating material in either wire or powder form to a molten or plastic state

• Propulsion of particles of the heated material

• Impact of the material onto a workpiece whereby the particles rapidly solidify and adhere both to one another and to the substrate to form a dense, functional, protective coating

The particles bond to the substrate mechanically and, in some cases, metallurgically Particle velocity, substrate roughness, particle size, material chemistry, particle temperature, and substrate temperature influence the bond strength of the coating material The process was originally referred to as flame spraying, metal spraying, flame plating, or metallizing when it was limited to the oxygen-fuel (oxyfuel) wire spray method

Thermal Spray Processes

Currently, five different commercially available thermal spray methods are in use:

• Oxyfuel wire (OFW) spray

• Electric arc wire (EAW) spray

• Oxyfuel powder (OFP) spray

• Plasma arc (PA) powder spray

• High-velocity oxyfuel (HVOF) powder spray

Selection of the appropriate thermal spray method is typically determined by:

• Desired coating material

• Coating performance requirements

• Economics

• Part size and portability

The oxyfuel wire spray process (also called wire flame spraying or the combustion wire process) is the oldest of the thermal spray coating methods and among the lowest in capital investment The process utilizes an oxygen-fuel gas flame

as a heating source and coating material in wire form Solid rod feed stock has also been used During operation, the wire

is drawn into the flame by drive rolls that are powered by an adjustable air turbine or electric motor (Fig 1) The tip of the wire is melted as it enters the flame and is atomized into particles by a surrounding jet of compressed air and propelled to the workpiece

Fig 1 Atomization of wire feedstock from the nozzle of an OFW spray gun

Spray rates for this process range from 2.3 to 55 kg/h (5 to 120 lb/h) and are dictated by the melting point of the material and the choice of fuel gas The wire spray gun is most commonly used as a hand-held device for on-site application, although an electric-motor-driven gun is recommended for fixed-mounted use in high-volume, repetitive production work

The OFW process is widely used for corrosion protection of large outdoor structures, such as bridges and storage tanks, and for restoration of dimension to worn machinery components It is a good choice for all-purpose spraying Coatings can be applied rapidly and at low cost, and a wide variety of metal coating materials are available Typical spray materials include austenitic and martensitic stainless steels, nickel aluminide, nickel chromium alloy, bronze, Monel, babbitt, aluminum, zinc and molybdenum

Electric arc wire spraying also applies coatings of selected metals in wire form Push-pull motors feed two electrically charged wires through the arc gun to contact tips at the gun head (Fig 2) An arc is created that metals the wires at temperatures above 5500 °C (10,000 °F) Compressed air atomizes the molten metal and projects it onto a prepared surface

Fig 2 Spray pattern generated by two electrically charged wires melted at the nozzle of an EAW spray gun

The EAW process is excellent for applications that require heavy coating buildup or that have large surfaces to be sprayed The arc system can produce a spray pattern ranging from 50 to 300 mm (2 to 12 in.) and can spray at high speeds It has built-in flexibility, allowing coating characteristics, such as hardness or surface texture, to be tailored to specific applications

The EAW method is characterized by strong coating adhesion because of the high particle temperatures produced Because the process uses only electricity and compressed air, it allows equipment to be moved relatively easily from one installation to another, and eliminates the need to stock oxygen and fuel gas supplies Materials applied by the EAW method are similar to those used in the OFW process

The oxyfuel powder spray method extends the range of available coatings and subsequent applications to include ceramics, cermets, carbides, and fusible hardfacing coatings Using either gravity flow or pressurized feed, powder is fed into the gun and carried to the gun nozzle (Fig 3), where it is melted and projected by the gas stream onto a prepared surface For general-purpose spraying, the gravity-flow system is used When exacting coating consistency and/or high spray rates are desired, the pressurized feed system is used Oxyfuel powder guns are the lowest cost thermal spray equipment and are easiest to set up and change coating materials The OFP method finds widest use in short-run machinery maintenance work and in the production spraying of abradable clearance-control seals for gas turbine engines

Fig 3 Cross-sectional view of an OFP spray system showing powder feed material being transported by the

carrier gas and then melted by the oxy-fuel mixture

Plasma arc powder is one of the most sophisticated and versatile thermal spray methods Temperatures that can be

obtained with commercial plasma equipment have been calculated to be greater than 11,000 °C (20,000 °F) and are far above the melting or even the vaporization point of any known material Decomposition of materials during spraying is minimized because of the high gas velocities produced by the plasma, resulting in extremely short residence time in the thermal environment The plasma process also provides a controlled atmosphere for melting and transport of the coating material, thus minimizing oxidation, and the high gas velocities produce coatings of high density

The plasma gun operates on the principle of raising the energy state of a gas by passing it through an electric arc The release of energy in returning the gas to its ground state results in exceedingly high temperatures A gas such as nitrogen

or argon enters a direct-current arc between a tungsten cathode and a copper anode that make up the nozzle (Fig 4) Both components are cooled by a constant flow of water through internal passages Here the plasma gas first dissociates (in the case of nitrogen, into two atoms), followed by ionization that releases free electrons The electrons recombine outside the electric arc, and energy is released as heat and light In addition, frequent collisions transfer energy from the electrons to the positive ions, accelerating them until the plasma reaches a state of equilibrium The result is a thermal plasma, in which the energy of the electrons has been turned into enthalpy, or heat content (Ref 2) At this point, powdered coating material suspended in a gas is injected into the plasma and is subsequently melted and propelled at high velocity to the workpiece In practice, a small amount of a secondary gas, such s hydrogen or helium, is mixed with the primary plasma gas to increase operating voltage and thermal energy

Fig 4 Schematic of a plasma arc powder spray system showing routing of plasma gas and powder material at

the output nozzle

The high temperatures and high gas velocities produced by the plasma process result in coatings that are superior in mechanical and metallurgical properties to low-velocity OFW or OFP coatings The plasma process is particularly efficient for spraying high-quality coatings of ceramic materials, such as zirconium oxide for turbine engine combustors and chromium oxide for printing rolls The plasma process is also readily field-portable and is thus used for large on-site applications such as power utility plant boiler tubes

Current plasma spray technology permits fully automatic start/stop operation and closed-loop computer control for power level, plasma gas flow, and powder feed rate System problems can be diagnosed via computer modem to the equipment manufacturer, and system performance can be documented and stored on a digital recorder or data logger A variation of the plasma spray process is to conduct spraying within a vacuum or low-pressure chamber Although this significantly adds to cost, the quality of metallic coatings is improved by minimizing oxides within the coating, reducing porosity, and increasing coating adhesion

With any thermal spray powder method, the degree to which a given flame effectively melts and accelerates the powder depends on the type of coating material and the size and shape of the particles Each particular coating material and gun combination has an optimum particle size Particles much smaller than ideal will overheat and vaporize; much larger particles will not melt and may fall from the flame or rebound from the target (Ref 2)

The high velocity oxyfuel powder spray process (also known as the hypervelocity oxy-fuel powder spray

process, the oxyfuel detonation [OFD] process, and the D-gun process) represents the state of the art for thermal spray metallic coatings The HVOF process uses extremely high kinetic energy and controlled thermal energy output to produce very-low-porosity coatings that exhibit high bond strength, fine as-sprayed surface finish, and low residual stresses

The HVOF process with an oxygen-fuel mixture consisting of oxygen and either acetylene, propylene, propane, or hydrogen fuel gas, depending on coating requirements The fuel gas flows through a siphon system, where it is thoroughly mixed with oxygen (Fig 5a) In one design, the mixed gases are ejected from the gun nozzle and are ignited The high-velocity gases produce uniquely characteristic multiple shock diamond patterns, which are visible in the flame (Fig 5b) Combustion temperatures approach 2750 °C (5000 °F) and form a circular flame configuration Powder is injected into the flame axially to provide uniform heating, and powder particles are accelerated by the high-velocity gases, which typically approach a speed of 1350 m/s (4500 ft/s)

Fig 5 Schematics showing input and output of an HVOF powder spray device (a) Key components of an HVOF

system (b) Close-up view of HVOF spray system output

The low residual coating stress produced in the HVOF process allows significantly greater thickness capability than the plasma method, while providing lower porosity, lower oxide content, and higher coating adhesion Coatings produced by the HVOF process also have much better machinability compared with other methods, and coating porosity has closely approached wrought materials, as verified by recent gas permeability testing HVOF systems are available with closed-loop computer control and robotics capability

Process Parameters

Several important factors must be considered when selecting an appropriate thermal spray method:

• Surface preparation

• Deposition rate

• Coating thickness limitations

• Bond coat materials

• Coating finishing method

Surface Preparation. Bonding of thermal spray coatings relies primarily on mechanical interlocking with the substrate material Good surface preparation cannot be overemphasized; most coating failures can be traced to poor practice at this first step of the operation

Nonporous surfaces should be cleaned to remove organic contamination by vapor degreasing or by washing with hot detergent solutions or steam cleaning Porous castings may require heat treatment to pyrolize contamination at approximately 200 to 300 °C (400 to 600 °F) Cleaned parts should be uniformly abrasive blasted to achieve a white metal condition and a minimum of 6 m (250 in.) Ra (arithmetic average surface roughness) finish using either aluminum oxide or chilled, angular iron steel grit The grit and air supply must be oil-free so that the cleaned surfaces are not recontaminated Blasted parts should be handled with clean gloves and protected from shop soil until the coating operation Spraying should be done within 2 h of blasting Parts should be reblasted if this time interval is exceeded Machined undercuts should have at least a 45° taper and must be cleaned and grit blasted as described above

Deposition Rate. The coating deposition rate is limited by the method used and the melting point of the coating material Other considerations include the deposit efficiency and target efficiency Deposit efficiency is the quantity of coating material deposited relative to the quantity being sprayed Target efficiency relates to the area of the spray pattern relative to the size of the part or target Both factors influence cost

Coating Thickness Limitations. All thermal spray coatings exhibit a degree of internal stress as a result of shrinkage from a molten state to a solid state These stresses accumulate as the coating thickness increases and result in a shear force

at the substrate interface Ductile coating materials tend to exhibit low stress; the opposite is true for hard coating materials, such as carbides or ceramics Also, very porous coatings exhibit lower stress than denser coatings When the internal stress exceeds the adhesion, the coating can delaminate from the substrate or crack Equipment manufacturers usually provide the practical thickness limit for each spray material

Bond Coat Materials. Bond coatings form a metallurgical bond with the substrate They include materials that react with the substrate exothermically, such as nickel-aluminum alloys, and materials that melt at high temperatures, such as molybdenum

These materials are frequently applied as a thin coating under a poor-bonding or high-stress topcoat to enhance the adhesion provided by grit blasting alone Bond coat materials are typically low in internal stress, and single coats are sufficient if the physical properties meet the design specifications Selection of a particular bond coating depends on its compatibility with the spray method being used in terms of parameters such as bond strength, thermal expansion properties related to the base metal, corrosion resistance, and oxidation resistance

Coating Finishing Method. Thermal spray coatings are finished to dimension either by single-point machining or by grinding Hard surfacing materials such as ceramics and carbides are restricted to grind finishing, typically with diamond wheels Thermal spray coatings are sufficiently different from the same material in wrought form that different grinding wheel and finishing tool recommendations are almost always required Therefore, tools and wheels should not be chosen based on experience with the parent material in wrought or cast form (Ref 3)

Most thermal spray coatings exhibit some degree of porosity However, the HVOF method produces coatings closest in machining and grinding behavior to wrought materials In general, thermal spray coatings can be machined to 1 to 2 m

(40 to 80 in.) Ra and ground to 0.25 to 0.5 m (10 to 20 in.) Ra Many plasma spray ceramic coatings can be lapped

down to 0.025 to 0.05 m (1 to 2 in.) Ra

Coatings for Friction and Wear Applications

The most outstanding feature of thermal spray coatings is probably their diverse applicability There are two primary reasons for this (Ref 4) First, materials selection is almost unlimited Second, the thermal spray process, properly controlled, imparts very little heat to the substrate (100 to 260 °C, or 200 to 500 °F), avoiding metallurgical change, distortion, and oxidation This allows thermal spray coatings to be applied to practically any substrate (metals, plastics, composites, etc.) Also, such coatings can often be applied to the finish machined part, a practice unthinkable with welding, heat treating, or other high-temperature processes

Thermal spray coatings provide the design engineer with many options They offer material properties not available in wrought metals Coatings can be applied to selected areas rather than treating the entire part Manufacturing costs can be reduced by eliminating unnecessary processing steps Extraordinary performance characteristics can be designed into a part to extend its useful life, creating a new, marketable product in the process

Thermal spray coatings provide the solution to many mechanical, electrical, and corrosion resistance problems involving metal parts and assemblies However, there are certain applications where such coatings should not be used Before a thermal spray coating is specified, its suitability can usually be determined according to these criteria (Ref 5):

• No strength is imparted to the base material by a sprayed deposit The component to be sprayed must, in its prepared form, be able to withstand any mechanical loading that will be experienced in service (In a few applications, some strength can be added by a thermal spray coating; however, such uses are unusual and should be carefully tested)

• If the area on a part to be sprayed or any section of the total area will be subjected to shear loading in service, the part is not a suitable candidate for thermal spraying Gear teeth, splines, and threads are examples

• Point loading with line contact on a sprayed metal deposit will eventually spread the deposit, causing detachment If the deposit is on a moving component with such loading, it will fail rapidly For example, needle and roller bearing seats, where the bearing elements are in direct contact with the sprayed deposit, may not be good thermal spray candidates

• If the base metal of a component to be treated has been nitrided, thermal spraying is not recommended unless the nitriding is first removed Other forms of substrate hardening require special treatment, such

as intensive grit blasting

Wear applications for thermal spray coatings can be categorized as adhesive wear, abrasive wear, or surface fatigue

Adhesive Wear

Adhesive wear occurs when two surfaces slide against each other with intended motion, producing fragments from one surface that adhere to the other (Ref 6) It arises from the strong adhesive forces set up when two materials come into intimate contact It generally occurs when lubrication is inadequate and results in metal transfer, usually called galling

Soft bearing coatings allow the embedding of abrasive particles and also permit deformation for alignment of bearing surfaces Adequate lubrication is required Coatings of this type are generally low in cost, because they wear in preference

to the mating surface (Fig 6a)

Fig 6 Cutaway views of thermal spray coated bearings showing areas that exhibit adhesive wear (a) Soft

bearing material (b) Hard bearing material

The following performance factors apply to soft bearing coatings:

• Good lubrication must be provided or the wear rate will be excessive

• The coating must be soft enough to trap the many abrasive particles that will be carried by the lubricant

• These coatings generally have poor abrasive wear resistance

• The inherent nature of thermal spray coatings enhances their usefulness as bearing coatings Pores act as reservoirs for lubricant; with reduced particle junctions, there is less tendency for adhesive wear

Applications and recommended materials for soft bearing coatings are listed in Table 1

Table 1 Thermal spray coatings for friction and wear applications

Type of wear Coating material Coating

process (a)

Applications Adhesive and abrasive wear

Soft bearing coatings:

Aluminum bronze

OFW, EAW, OFP,

PA, HVOF Tobin bronze OFW, EAW Babbitt OFW,

steel

OFW, EAW

Alumina/titania

OFP, PA Tungsten

carbide

OFP, PA, HVOF Co-Mo-Cr-Si PA, HVOF

Adhesive wear

Fe-Mo-C PA

Bumper crankshafts for punch press, sugar cane grinding roll journals, antigalling sleeves, rudder bearings, impeller shafts, pinion gear journals, piston rings (internal combustion); fuel pump rotors

Aluminum oxide PA Chromium oxide PA Tungsten carbide PA, HVOF Chromium

carbide

PA, HVOF Ni-Cr-B-SiC/WC

(fused)

OFP, HVOF Ni-Cr-B-SiC

(fused)

OFP, HVOF

Abrasive wear

Ni-Cr-B-SiC (unfused)

HVOF

Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry); concrete mixer screw conveyors; grinding hammers (tobacco industry); core mandrels (dry-cell batteries); buffing and polishing fixtures; fuel-rod mandrels

Surface fatigue wear

Molybdenum OFW, PA Mo/Ni-Cr-B-SiC PA

Fretting: Intended motion

applications

Co-Mo-Cr-Si PA, HVOF

Servomotor shafts, lathe and grinder dead centers, cam followers, rocker arms, piston rings (internal combustion), cylinder liners

Aluminum bronze OFW,

EAW, PA, HVOF Cu-Ni-In PA, HVOF

Fretting: Small-amplitude

oscillatory displacement

applications at low temperature

(<540 °C, or 1000 °F)

Cu-Ni PA, HVOF

Aircraft flat tracks (air-frame component); expansion joints and midspan supports (jet engine components)

Co-Cr-Ni-W PA, HVOF

PA, HVOF

Compressor air seals, compressor stators, fan duct segments and stiffeners (all jet engine components)

Chromium carbide

PA, HVOF Tungsten carbide PA, HVOF WC/Ni-Cr-B-SiC

(fused)

OFP, HVOF WC/Ni-Cr-B-SiC

PA Wear rings (hydraulic turbines), water turbine buckets, water

turbine nozzles, diesel engine cylinder liners, pumps

SiC

Ni-Al/Ni-Cr-B-PA Type 316 stainless steel

PA Ni-Cr-B-SiC

(fused)

OFP, HVOF Ni-Cr-B-SiC

(unfused)

HVOF Aluminum bronze PA, HVOF Cu-Ni PA, HVOF

(a) OFW, oxyfuel wire spray; EAW, electric arc wire spray; OFP, oxyfuel powder spray; PA,

plasma arc spray; HVOF, high-velocity oxyfuel powder spray

Hard bearing coatings are highly resistant to adhesive wear They are used where embeddability and self-alignment are not important and where lubrication is marginal (Fig 6b)

The following performance factors apply to hard bearing coatings:

• Lubrication should be good, but is not as important as for soft bearing coatings because the high wear resistance of these materials allows them to withstand momentary unlubricated service

• Applications that require hard bearing coatings are usually characterized by high load and low speed

• Surfaces should generally be of equal hardness

• Although like coatings can be used for sliding against each other, unlike combinations are frequently used for example, a coating running against a wrought metal This reduces seizing and scuffing

• Wear rate generally increases with temperature

Applications for hard bearing coatings are listed in Table 1 Recommended coatings include nickel-, iron-, cobalt-, and molybdenum-base alloys, ceramics, and tungsten carbides (see Table 1)

Abrasive Wear

Abrasive wear occurs when hard foreign particles, such as metal debris, metallic oxides, and dust from the environment, are present between rubbing surfaces (Fig 7) These particles abrade material off both surfaces Selection of coating materials for this application should generally be based on operating temperature and surface finish requirements The following performance factors must be considered:

• The coating must be hard In particular, surface hardness should exceed the hardness of the abrasive grains present

• The most common abrasive is silica (sand), with a hardness of approximately 820 HK (For comparison, tungsten carbide/cobalt composite is 1400 to 1800 HK; Al2O3 is approximately 2100 HK)

• Information about the abrasive how often it is replenished, whether it is sharp and brittle, how it breaks down is important in selecting the coating and estimating its performance

• If the system is closed, debris created by the wear process will also contribute to the wear rate and thus must also be considered

• The coating must exhibit oxidation resistance at the service temperature

Applications and recommended materials for coatings resistant to abrasive grains at low and high temperatures are listed

in Table 1

Fig 7 Abrasive wear on the flight surfaces of a helical screw used in spiral conveyor applications

Surface Fatigue Wear

Repeated loading and unloading cause cyclic stress on a surface, eventually resulting in the formation of surface or subsurface cracks The surface ultimately fractures and large fragments are lost, leaving pits This phenomenon can occur only in systems where abrasive and adhesive wear are not present for example, in systems with high surface contact loads An area of surface must be stressed repeatedly, without constant removal of particles, to fail in fatigue Fretting, erosion, and cavitation are typical examples of this type of wear

Fretting. Some fretting-resistant coatings resist wear caused by repeated sliding, rolling, or impacting over a track (Fig 8a) The repeated loading and unloading cause cyclic stresses inducing surface or subsurface cracks Other coatings resist wear caused when contacting surfaces undergo oscillatory displacement of small amplitude (Fig 8b) This type of wear is difficult to anticipate, because no intended motion is designed into the system Vibration is a common cause of fretting The following performance factors apply to coatings for fretting resistance:

• The coating must be resistant to oxidation at the service temperature If an oxide forms, it must be tough and tenacious; a loosely adherent oxide will cause severe abrasive wear

• A surface that is free of stress, particularly tensile stress, is desirable High-shrink coatings tend to have high surface stress and do not perform as well as low-stress coatings

• Brittle coatings fail rapidly Tough coatings tend to perform better

• Coatings with hard particles distributed in a soft matrix are generally the most durable

Applications and recommended materials for fretting-resistant coatings are listed in Table 1

Fig 8 Two types of fretting wear (a) Stem and seat wear caused by the intended cyclic up-and-down motion

of an engine valve (b) Wear caused by the unplanned but unavoidable oscillatory motion of a press-fitted shaft

on the inner ring of a bearing

Erosion is caused when a gas or a liquid that ordinarily carries entrained particles impinges on a surface with velocity (Fig 9) When the angle of impingement is small, the wear-producing mechanism is closely analogous to abrasion When the angle of impingement is normal to the surface, material is displaced by plastic flow or is dislodged by brittle failure The following performance factors apply to erosion-resistant coatings:

• If the angle of particle impact is less than 45°, the coating selected should be harder and more abrasion resistant

• If the angle of particle impact is greater than 45°, the coating should be softer and tougher

• At high service temperatures, coatings should have high hot hardness and oxidation resistance at temperatures and environments ranging from 540 to 815 °C(1000 to 1500 °F)

• When the carrier is liquid, the corrosion resistance of the coating must be considered

Applications and recommended materials for coatings used to resist particle erosion are listed in Table 1

Fig 9 Typical erosive wear of a fan blade assembly generated by the high-velocity impingement of a gas or

liquid on the blade surface

Cavitation is caused by mechanical shock that is induced by bubble collapse in liquid flow (Fig 10) Materials that resist fretting-type surface fatigue are resistant to cavitation The most effective coating properties are toughness, high wear resistance, and corrosion resistance The following performance factors apply to cavitation-resistant coatings:

• Relative motion between a liquid and metal surface, including bubble generation and bubble collapse, must exist for cavitation to occur

• Liquids will penetrate sprayed coatings unless fused; therefore, all coatings should be sealed

• Selection of a coating must be influenced by its resistance to the liquid used in a particular application

• Hardness is an important factor, but coatings must also be tough Brittle coatings fail quickly

• Coatings that work harden are especially resistant to the repeated pounding of cavitation

Applications and recommended materials for coatings resistant to cavitation are listed in Table 1

Fig 10 Typical surface fatigue wear produced by cavitation in a pump impeller component

References

1 E.R Novinski, Application of Thermal Spray Processes to Conserve Critical Materials, Workshop on Conservation and Substitution Technology for Critical Materials, Conference Proceedings, Vanderbilt

University, 16 June 1981

2 H Herman, Plasma Sprayed Coatings, Sci Am., Sept 1988

3 H.S Ingham and A.P Shepard, Flame Spray Handbook, Vol III, Perkin-Elmer Corp., Metco Division, 1965

4 G.L Kutner, Thermal Spray by Design, Adv Mater Proc Met Prog., Oct 1988, p 67

5 M.L Thorpe, Thermal Spraying Becomes a Design Tool, Mach Des., 24 Nov 1983

6 F.N Longo, Handbook of Coating Recommendations, Perkin-Elmer Corp., Metco Division, 1972

Electroplated Coatings

Rolf Weil and Keith Sheppard, Stevens Institute of Technology

Introduction

IT IS OFTEN NECESSARY to coat a material that is subject to friction and, possibly, wear The coating can be tailored

to the tribological demands of the environment and can provide a wider choice in selecting a base material to meet special requirements, such as strength or low cost

Like other of surfaces used in tribological applications, electroplated coatings have two primary categories Hard coatings are normally used to resist many forms of wear, such as those that involve abrasive, adhesive, and erosion process Some degree of toughness in these coatings is often desirable, in order to resist cracking Soft coatings are sometimes used on bearing surfaces to provide low shear strength They are typically used at ambient temperatures and low loads

An additional consideration in coating selection is a requirement to provide corrosion protection Corrosive wear places particularly severe demands on the protective abilities of such a coating, because the thin surface film that is primarily responsible for limiting the corrosion kinetics is continually being worn away

Coatings that are used to control friction and wear can be electrochemically deposited either with or without an externally applied current Deposition without an external current is called electroless plating For many wear applications, electrochemical deposition is the most rapid and economical means to apply coatings ranging from 10 to 500 m (0.4 to

20 mils) in thickness (Ref 1)

Adhesion to the substrate is a very important requirement of all coatings Because adhesion primarily depends on the cleanliness of the substrate surface to be coated, proper pretreatment of the substrate is usually necessary

Deposition Fundamentals

Electroplating is the coating of an electrically conducting surface by application of an electrical potential in a suitable solution that contains the ions of the metals to be deposited The electrode to be coated is the cathode The counter-electrode, the anode, can be of the soluble type, so that it supplies metal ions to the solution Alternatively, the anode can

be insoluble, in which case the ions of the metals to be deposited must be continuously or periodically added to the plating solution to compensate for the depletion

The deposition rate depends primarily on the current density If all supplied electrons reduce the metal ions, then the deposition rate can be readily calculated from Faraday's law, which states that 96,500 coulombs (1 Faraday) deposit 1 gram (0.035 oz) equivalent weight (atomic weight divided by valence) However, hydrogen evolution or other secondary reactions may use some of the current supplied Thus, only a fraction of the supplied electrons reduces the metal ions In this case, the plating efficiency is less than 100% (The plating efficiency is the ratio of the metal yield to that calculated from Faraday's law, which assumes that there are no other reactions.)

The microstructures of electrodeposits, which to a large extent determine their properties, depend on a number of factors These factors include the microstructure of the surface to be coated, the plating conditions (that is, the current density), the temperature and composition of the plating solution, as well as the degree and type of agitation The composition and the pH of the plating solution in the vicinity of the cathode (which generally differs from that of the bulk) can have a large effect on the structure and properties of the deposit In the absence of significant surface inhibition, it is possible for the deposit to reproduce the structure of the substrate surface However, even minute quantities of certain substances can greatly affect the structure These substances may be intentionally added to the plating solution or be present as impurities from the water, chemicals, or secondary reactions Foreign substances can alter the grain orientation by being preferentially adsorbed on certain crystal planes Then, grains with other planes exposed on their surfaces can grow preferentially until they essentially compose the entire deposit, thereby producing a fiber texture, that is, most of the grains have same crystallographic direction normal to the surface

The adsorption of some foreign material can greatly reduce the grain size These materials impede grain growth, thereby requiring almost continuous renucleation Some addition agents are present in plating solutions to level the deposit surface, that is, to make it smoother than the substrate They do so by being preferentially adsorbed on asperities By blocking growth on the asperities, the recesses receive most of the depositing metal, thereby producing a smoother surface

High internal stresses, particularly of the tensile type, can adversely affect the wear properties of electrodeposits They can arise from the lattice misfit between the substrate and an epitaxial deposit, from the coalescence of crystallites, or from the diffusion of hydrogen out of the surface layers These stresses can be large enough to cause cracking, as is the case in some chromium deposits Most electrodeposits are also characterized by a high dislocation density, which results

in strength, hardness, and ductility values that are similar to those of the same metal in the cold-worked condition

Pulse plating has been used to enhance the tribological properties of electrodeposits Generally, a square-wave current pulse is employed The current is on for a certain time period and off for another The ratio of the time to the sum of the

on and off times is called the duty cycle By varying the pulse frequency, the duty cycle, and the current density, the structure and properties of electrodeposits can be beneficially changed

Electroless plating is an autocatalytic reaction in which a reducing agent supplies an internal current The advantages of electroless plating are that nonmetallic substrates can be coated and the deposit thickness is uniform The autocatalytic

reaction has to be activated on some surfaces, particularly those that are nonmetallic Thus, the areas to be plated can be controlled by specific activation The disadvantages are a slow deposition rate and a higher cost It is also necessary to maintain the composition of the plating solution by continuous or frequent additions of the depleted chemicals and removal of the impurities

Wear-resistant composite can be produced by the codeposition of fine particulate matter Hard particles, such as diamond, silicon carbide, and aluminum oxide, are kept in suspension by agitation and become occluded in the deposit Solid lubricants, such as polytetrafluoroethylene (PTFE) particles, have also been codeposited As much as 30 vol% of the particles can be attained in both electrodes and electroplated coatings (Ref 2)

In some applications, only limited areas of a component are subjected to wear If the deposition of precious metals is involved, then it may be economically desirable to coat these areas selectively Such limited coverage can be achieved by jet plating, laser-enhanced and laser-jet plating, and by physically masking off areas not to be plated, as in photolithography (Ref 3)

In jet plating, a fine jet of the plating solution is directed onto the areas to be coated Because the current is constrained within the jet, only the areas where it is applied become coated The jet also provides rapid ion transport to the depositing surface and therefore permits to the deposition rates In laser-enhanced plating, the heating effect of the laser enhances mass transport locally It can also influence the deposition kinetics such that metal is not plated outside of the heated area Laser-jet plating, which is a combination of jet and laser-enhanced plating, can provide improved deposit characteristics The jet then acts as a waveguide for the laser

The principal electrochemically deposited materials used in tribological applications are chromium, nickel, and both precious and soft metals The characteristics of each type of deposit are described in this article References 4, 5, 6, 7, 8,

9, 10, 11 provide additional information relevant to electrochemically deposited metals and alloys used in tribological applications

Chromium

Hard chromium coatings are widely used because of their low coefficient of friction and good wear properties They are deposited at higher temperatures and current densities than decorative chromium The plating solution for hard chromium has a lower ratio of chromic oxide to sulfuric acid (the main constituents of the solution) than that used for decorative coatings The thickness of hard chromium deposits varies from about 0.1 to 100 m (0.004 to 4 mils), whereas decorative thicknesses usually range from about 0.1 to 0.2 m (4 to 8 in.) Very strict antipollution to regulations govern the discharge of solutions containing hexavalent chromium ions, which are used for hard-chrome plating Therefore, plating solutions that contain mostly trivalent chromium ions are of interest These solutions generally contain formic acid or one

of its salts Carbon will deposit in these solutions The deposits can therefore be heat treated to precipitate a chromium carbide

The hardness of hard chromium varies from about 900 to 1100 on the Knoop and Vickers hardness scales These values are considerably higher than the hardness of bulk chromium Deposits from trivalent solutions are softer than those that are plated from hexavalent chromium solutions However, after heat treating at about 700 °C (1290 °F), a hardness comparable to that of hard chromium can be achieved (Ref 12)

Chromium deposits are characterized by high internal tensile stresses that can reach 1000 MPa (145 ksi) These stresses can reduce the fatigue properties of coated components Hydrogen is also codeposited with chromium and can diffuse into components, causing hydrogen embrittlement Heat treatments are typically required to relieve the stresses and hydrogen effects, but can reduce the hardness

The coefficients of friction of hard chromium against hard materials are generally the lowest of any electrochemically deposited coatings The actual values vary considerably, depending on the test method, the mating surfaces of the materials, and the degree of lubrication Some values of static and sliding coefficients of friction are listed in Table 1 The static coefficient is calculated from the force to initiate movement of one component of a couple against the other The force to maintain movement enters into the calculation of the sliding coefficient It is important to note that only coefficients of friction obtained under the same conditions can be compared The values should not be considered as absolute

Table 1 Coefficients of friction

coefficient

Sliding

coefficient

Chromium-plated steel versus itself 0.14 0.12

Chromium-plated steel versus steel 0.15 0.13

Steel versus steel 0.30 0.20

Source: Ref 13

The wear rates of hard chromium can vary greatly, depending, again, on the type of test The rates also vary with the mating material and whether adhesive or abrasive wear predominates Some dry abrasive wear data from a Taber abrasion test for three chromium deposits labeled CrA, CrB, and CrC are shown in Fig 1 The Taber test measures, for a certain number of cycles, the weight loss that results from abrasion with resilient, abrasive wheels at a load of 9.8 N (11 kgf) Figure 1 shows that deposit CrC had less wear than deposit CrA

Fig 1 Effect of number of cycles on mass loss in the Taber abrasion test for uncoated steel substrate (Fe),

three chromium deposits (CrA, CrB, CrC), and three electroless nickel deposits: as-plated nickel (EN), heat treated at 400 °C (750 °F) (EN400), and heat treated at 600 °C (1110 °F) (EN600) Source: Ref 1

Figure 2 represents data obtained in a Falex wear test for the same three chromium deposits In this test, a pin is rotated between two V-shaped blocks Deposit CrA showed less wear than deposit CrC, illustrating the effect of a lubrication test method on the results Figure 1 also shows that the chromium coating improved the abrasion resistance of the steel substrate The hardest chromium deposits do not necessarily exhibit the least wear The low friction coefficients and good wear properties of chromium have been attributed to a self-healing Cr2O3 film that forms on the surface In general, hard chromium has a lower wear rate than either electroplated or electroless nickel, which are the two competing materials This effect is also illustrated in Fig 1 and 2

Fig 2 Effect of number of cycles on mass loss of plated pin versus steel blocks in a Falex test for same three

chromium deposits (CrA, CrB, CrC) and same three electrodes nickel deposits (EN, EN400, EN600) shown in Fig 1 Effects on two electroplated nickles from a sulfamate solution (EP-S) and a Watts solution (EP-W) are shown Source: Ref 1

Under corrosive wear conditions, chromium coatings do not protect substrates if they crack in response to high internal stresses Pulse plating can reduce the internal stresses and the resulting cracking of the deposit The effects of pulse plating and deposition temperature on the wear rate of chromium deposits are shown in Fig 3 The improve wear resistance at higher pulse frequencies and temperatures corresponds to increases in hardness The wear resistance in some applications can also be improved by the inclusion in the deposit of hard particles or those of a solid lubricant (Ref 15)

Fig 3 Effect of pulse frequency and solution temperature on mass loss of chromium deposits in a Taber

abrasion test dc, direct current Source: Ref 14

Chromium is widely used for wear resistance in automotive and aircraft components, such as pistons and shock absorbers (Ref 13) Other applications include coatings on drills, taps, dies, extrusion screws, and rolls The wear resistance of gun barrels can also be improved by chrome plating Salvaging of worn parts by chromium electrodeposition is an important industrial application

Most of these applications require relatively thick chromium deposits Thick deposits have a nodular surface structure, which is shown in Fig 4 Cracks that are due to high internal stresses are also visible The nodular structure is too rough for some applications and therefore requires a mechanical finishing operation Machining tools are usually plated with a thin chromium deposit, which is smooth and does not need to be finished (Ref 16)

Fig 4 Optical micrograph of electrodeposited chromium showing nodular structure and cracks

Electroplated Nickel

The most widely used solution for plating nickel for wear applications is the Watts solution Its main components are nickel sulfate, nickel chloride, and boric acid Organic addition agents in the plating solution can increase the hardness and wear resistance mainly by decreasing the grain size Nickel usually is deposited with a tensile internal stress Some sulfur compounds can cause the stress to become compressive, but also make the deposit more brittle, especially under elevated-temperature conditions The nickel sulfamate plating solution produces low-stress deposits It is possible to codeposit such metals as tungsten and molybdenum with nickel, even though they cannot be plated alone in aqueous solutions Inclusion of hard particles or those of solid lubricants can also improve the wear or friction properties of electroplated nickel

Improved wear resistance resulting from the incorporation of SiC particles is shown in Fig 5 In the test on which the data

of Fig 5 are based, a plated block was pressed against a lubricated steel ring Further improvement that was done to phosphide precipitates in heat-treated Ni-P-SiC coatings is also seen in this figure

Fig 5 Effects of codeposited SiC particles and phosphide precipitates on wear of electrodeposited nickel heat

treated for 1 h at 400 °C (750 °F) Source: Ref 17

The hardness of nickel deposits can vary from about 150 to 500 on the Vickers scale The hardness depends on the plating conditions, that is, current density, solution pH and temperature, and composition Pulse plating can increase the hardness The coefficient of friction and wear rates of electroplated nickel are generally greater than those of chromium The lower wear rates of electroless, compared with electroplated, nickel can be seen in Fig 1 and 2 The widest use of electroplated nickel for wear applications is as an undercoat for chromium If thick deposits are needed, for example, in building up heavily worn parts, it is usually not practical to do so using only chrome plating, because of its low current efficiency ad high internal stress In such cases, most of the deposit thickness is composed of nickel, with chromium constituting only a thin outer layer

Under corrosive conditions, chromium may not protect the substrate, because of its cracks, but a nickel undercoat will There are some applications of electrodeposited Ni-W, Ni-Mo, and Ni-Cr alloys that are hard and wear resistant These alloys also offer good corrosion protection

Electroless Nickel

The tribological and associated properties of electroless nickel were extensively discussed by Weil and Parker (Ref 18) The reducing agents for electroless nickel are sodium hypophosphite, sodium borohydride, or organic aminoboranes When sodium hypophosphite is the reducing agent, the deposit generally contains between 3 and 11 wt% phosphorus The boron contents of electroless nickel range from 0.2 to 4 wt% and from 4 to 7 wt% when the reducing agents are an aminoborane and sodium borohydride, respectively

As-deposited electroless nickel is a supersaturated solid solution because the equilibrium solid solubility of boron or phosphorus in nickel is essentially zero However, the second phase cannot form, because the time interval between the

deposition of successive layers is too short for the necessary diffusion The structure of the deposit changes from microcrystalline to amorphous, with increasing alloy content The second phase, which is Ni3P Ni3B, can form on annealing and result in precipitation hardening

Electroless nickel can be heat treated to hardnesses comparable to those of electrodeposited chromium The maximum hardness can be attained in 1 h at about 400 °C (750 °F) or 10 h at 260 °C (500 °F) The hardness of as-plated Ni-P alloys varies from 500 to 650 on the Vickers scale As-plated Ni-B deposits are generally harder than the Ni-P ones The ability

of electroless nickel deposits to maintain their hardness under elevated-temperature service conditions increases with increasing phosphorus or boron content, but decreases rapidly above 385 °C (725 °F) Nickel-boron coatings tend to better withstand wear at elevated temperatures and are therefore more widely used under these conditions

Coefficients of friction of electroless nickel in the as-deposited condition (EN) and heat treated at 400 °C (750 °F) (EN400) and at 600 °C (1110 °F) (EN600) are listed in Table 2 They are compared to the three chromium alloys depicted

in Fig 1 and 2 The counter surfaces were diamond and plain carbon steel The coefficients friction of the electroless nickels are higher than those of the chromium deposits It should be noted that the values for the chromium deposits against steel are considerably higher than those listed in Table 1, again highlighting the fact that only data obtained under identical conditions can be compared

Table 2 Coefficients of friction of chromium versus electroless nickel

°C (1110 °F) showed the least wear However, the corrosion resistance of as-plated Ni-P coatings, which is one of the main reasons for their use, deteriorates upon heat treating to the maximum hardness, because of cracking Therefore, under corrosive wear applications, it may be prudent for forgo the higher hardness

The widest use of electroless nickel for both corrosion and wear resistance is in valves that control the flow of either liquids or gases Other uses (Ref 20) of electroless nickel in wear applications include aluminum piston heads, aircraft engine shafts, components of gas turbines and engine mounts in the aircraft industry, and such automotive parts as differential pinion ball shafts, fuel injectors, ball studs, disk brake pistons, transmission thrust washers, knuckle pins, and hose couplings

In mining applications and associated material-handling equipment, where abrasive wear conditions prevail, electroless nickel has been substituted for hard chromium A very important application of electroless nickel is the salvage of worn surfaces, especially because it is possible to coat only specific areas Thus, little or no subsequent machining or grinding

is required The inclusion in electroless-nickel deposits of hard or solid-lubricant particles can also improve their tribological properties The effect of these particles on the wear of electroless nickel is similar to that shown in Fig 5

Precious Metal Deposits

Separable electrical connectors are not only subjected to wear, but must maintain low contact resistance Electrodeposited gold from a cyanide plating solution is most widely used, because it does not form on oxide surface film of high electrical resistivity Soft gold deposits wear poorly The addition of cobalt ions to the plating solution is the most frequent means

used to increase the hardness of gold However, cobalt additions reduce the ductility of the deposits These effects appear

to be due primarily to grain-size reduction The addition of cobalt is accompanied by the codeposition of fine polymer particles that are cobalt-cyanide compounds

Gold plate from solutions that contain nickel ions is also used for separable connectors An additive-free, gold-plating solution based on phosphoric acid results in deposits that have properties comparable to those of the cobalt-hardened deposits (Ref 21) To prevent interdiffusion of gold and the substrate, which is usually copper, a layer of nickel is often deposited as a barrier

Palladium, Pd-Ag, and Pd-Ni have been substituted for gold (Ref 22), primarily in an effort to reduce costs Polymerization of organic air pollutants can occur on the catalytically active palladium surface under tribological conditions and result in a film of high electrical contact resistance Alloying reduces polymer formation Other measures can also be taken, including coating one or both mating surfaces with a thin gold layer (250 m, or 10 mils) or using higher contact forces Fretting corrosion, which results from the small-amplitude movements of the mating surfaces that are often due to vibrations or different thermal expansion rates, can cause a thin gold deposit to wear through and the polymer film to form

The Knoop hardness is about 200 for cobalt-hardened gold and about 180 for gold from the additive-free phosphate solution On the same scale, 25-karat gold has a hardness of only about 50 Table 3 compares the characteristics of various precious metal contacts (Ref 23) A low ductility, that is, less than 1% elongation in a tension test, is desirable for gold deposits to reduce prow formation, which is the build-up of a lump of work-hardened metal Antler (Ref 24) has reviewed the tribology of electronic connectors

Table 3 Plated contact metallurgical properties

Gold Properties

Wear resistance P G VG E(a)

Cost (b) (equal thickness) 30% of Au 20% of Au

E, excellent; VG, very good, G, good; P poor

(a) With gold flash

(b) Approximate values

Soft Metals

Electrodeposited soft metals such as tin, lead-tin, and silver alloys are used as bearing coatings Tin and silver deposits prevent galling, especially during start-up The codeposition of 3.4 to 3.6% Pb with silver provides excellent antiseizure coatings for bearings (Ref 25) Silver alloys are used for elevated-temperature solid lubrication Separable electrical contacts have also been coated with electrodeposited Pb-Sn In this case, friction results in the removal of a high-resistivity surface film An alloy of lead with from 4 to 10% tin, and sometimes containing copper, is used for sleeve-bearing overlays (Ref 26) Lead alloyed with indium is an excellent bearing material that also possesses good corrosion resistance However, it is more expensive than alloys of lead and tin Electrodeposits for bearings are reviewed in greater detail by Eastham and Crooks (Ref 27)

Magnetic Materials

Electroless and electrodeposited magnetic alloys, such as Permalloy (Ni-Fe) and various cobalt alloys (Co-P and P), are used in advanced magnetic storage technologies, such as hard memory disks for computer systems Electroless nickel that has a high enough phosphorus content (greater than 7 wt%) to be amorphous and therefore nonmagnetic is commonly used as an underlayer for magnetic coatings These storage systems represent a demanding tribological environment, because of the need to maintain minimal separation between the read/write heads and the storage media This close proximity is necessary to maximize information storage density It has been suggested that the resources lost to friction and wear at the head-media interfaces in magnetic storage amount to about 1% of the U.S gross national product (Ref 28) Even if this figure were an overstatement, it does indicate the scale of the problem

Co-Ni-Because it is necessary to limit the wear of the head and disk materials, the very thin magnetic layer on storage disks is normally coated with a thin wear-resistant and/or lubricating layer The coating also affords corrosion resistance Electroplated rhodium, which has properties similar to hard chromium, has been used for this purpose However, it is now more typical to employ such coatings as silicates or the so-called diamond-like hydrogenated carbon (Ref 3) In addition

to the coating, a lubricating layer may also be necessary

Moisture and ionic contaminants, such as salts, can lead to corrosion Porosity in the protective coating can allow corrosion of the magnetic layer and undercoat The primary strategy is to minimize corrosion by producing pore-free protective layers However, corrosion resistance can be improved by incorporating elements such as chromium into the magnetic layer, as long as potentially detrimental effects on the magnetic properties can be ameliorated In practice, it is possible to codeposit chromium by vapor phase methods Presently, the codeposition of chromium in cobalt magnetic films represents a challenge to electrodeposition technology, along with the provision of a self-lubricating surface (Ref 3)

It is also apparent that the need to maintain submicron spacing between the magnetic layer and the heads in advanced storage systems requires much better control of the electrochemical plating processes than is generally exercised

References

1 D.T Gawne and U Ma, Friction and Wear of Chromium and Nickel Coatings, Wear, Vol 129, 1989, p 123

2 N Feldstein, Composite Electroless Plating, Electroless Plating: Fundamentals and Applications, G.O

Mallory and J.B Haydu, Ed., American Electroplaters and Surface Finishers Society, 1990, p 269-288

3 L.T Romankiw and T.A Palumbo, Electrodeposition in the Electronic Industry, Electrodeposition Technology, Theory and Practice, L.T Romankiw and D.R Turner, Ed., Proceedings, Vol 87-17, The

Electrochemical Society, 1987, p 13-41

4 Metals Handbook, 9th ed., Surface Cleaning, Finishing and Coating, Vol 5, American Society for Metals,

1982

5 F.A Lowenheim, Ed., Modern Electroplating, 3rd ed., John Wiley and Sons, 1974

6 L Durney, Ed., Electroplating Engineering Handbook, 4th ed., Van Nostrand Reinhold, 1984

7 J.K Dennis and T.E Such, Nickel and Chromium Plating, Butterworth, 1972

8 G.O Mallory and J.B Haydu, Ed., Electroless Plating: Fundamentals and Applications, American

Electroplaters and Surface Finishers Society, 1990

9 J.Cl Puippe and F Leaman, Ed., Theory and Practice of Pulse Plating, American Electroplaters and

Surface Finishers Society, 1986

10 W.H Safranek, Ed., The Properties of Electrodeposited Metals and Alloys: A Handbook, 2nd ed., American

Electroplaters and Surface Finishers Society, 1986

11 Electrodeposited Coatings Database, American Electroplaters and Surface Finishers Society

12 M Takaya, M Matsunaga, and T Otaka, Hardness of Deposits from Trivalent Chromium

Sulfate/Potassium Formate Baths, Plat Surf Finish., Vol 74 (No 6), 1987, p 90

13 G Dubpernell, Chromium, Chapter 5, Modern Electroplating, 3rd ed., John Wiley and Sons, 1974, p

87-151

14 J.H Chai, D.Y Chang, and S.C Kwon, The Properties of Chromium Electroplated with Pulse Current,

Plat Surf Finish., Vol 76 (No 6), 1989, p 80

15 J Zahavi and H Kerbel, Properties of Electrodeposited Composite Coatings, Plat Surf Finish., Vol 69

(No 1), 1987, p 76

16 K.G Budinski, Wear Characteristic of Industrial Plating, Selection and Use of Wear Tests for Coatings,

STP 769, ASTM, 1982, p 118-131

17 X Changgeng, D Zonggeng, and Z Lijun, The Properties of Electrodeposited Ni-P-SiC Composite

Coatings, Plat Surf Finish., Vol 75 (No 10), 1988, p 54

18 R Weil and K Parker, Chapter 4, Properties of Electroless Nickel Plating, Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, 1990, p 111-138

19 K Parker, Hardness and Wear Resistance Tests of Electroless Nickel Deposits, Plat Surf Finish., Vol 61

(No 9), 1974, p 834

20 J Colaruotolo and D Tramontana, Chapter 8, Engineering Applications of Electroless Nickel, Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, 1990, p

207-228

21 F.B Kock, Y Okinaka, C Wolowodiuk, and D.R Blessington, Additive-Free Hard Gold Plating for

Electronic Applications, Plat Surf Finish., Vol 67 (No 6), 1980, p 50

22 G.L Ide and J.B Vanhumbeeck, Palladium and Palladium Alloy Electroplating for Contact Applications,

Electrodeposition Technology, Theory and Practice, L.T Romankiw and D.R Turner, Ed., Proceedings,

Vol 87-17, The Electrochemical Society, 1987, p 179-190

23 A.D Knight and S Levine, High Performance Signal Connectors and Contact Interface Metallurgy,

Electrochemical Technology in Electronics, L.T Romankiw and T Osaka, Ed., Proceedings, Vol 88-23,

The Electrochemical Society, 1988, p 161-170

24 M Antler, The Tribology of Contact Finishes: Mechanisms of Friction and Wear, Plat Surf Finish., Vol 75

(No 10) 1988, p 46

25 B.M Luce and D.G Foulke, Silver, Chapter 14, Modern Electroplating, 3rd ed., John Wiley and Sons,

1974, p 358-376

26 H.J Wiesner, Lead, Chapter 11, Modern Electroplating, 3rd ed., John Wiley and Sons, 1974, p 266-286

27 D.R Eastham and C.S Crooks, Plating for Bearing Applications, Trans Inst Metal Finish., Vol 60, 1982,

VAPOR DEPOSITION PROCESSES can principally be divided into two types (Ref 1):

• Physical vapor deposition (PVD) processes, which require creation of material vapors (via evaporation,

sputtering, or laser ablation) and their subsequent condensation onto a substrate to form the film

• Chemical vapor deposition (CVD) processes, which are generally defined as the deposition of a solid

material from the vapor phase onto a (usually) heated substrate as a result of numerous chemical reactions

PVD and CVD processes can be classified as shown in Table 1

Table 1 Classification of physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes

on the basis of material deposited on substrate

Process

Physical vapor deposition

Evaporation deposition Ion plating

Conventional

Sputter deposition Activated reactive evaporation (ARE) Reactive ion plating (RIP)

Table 2 Selected parameters of basic PVD and CVD processes

Physical vapor deposition Parameter

Thermal energy Momentum transfer Chemical reaction

Deposition rate Can be very high (up to 7.5 ×

105 /min)

Low except for pure metals (for example, copper, 104 /min)

Moderate (200-2500 /min)

Throwing power Poor line-of-sight coverage

except by gas scattering

Good, but nonuniform thickness distribution

Good

Refractory compound

deposition

Energy of deposited species Low (1.6 × 10-20 to 8.0 × 10-20 J,

Growth interface perturbation Not normally Yes Yes (by rubbing)

Substrate heating (by external Yes, normally Not generally Yes

As with all processes, the user is concerned with the process itself as well as the resulting microstructures and properties

of the product In order to understand various vapor deposition processes, one has to model them in terms of three steps (Ref 1):

• Step 1: creation of vapor phase specie (a material can be put into the vapor phase by evaporation,

sputtering, laser ablation, electrolyte, chemicals, plasma spray, D-gun, gases, vapors, and so on)

• Step 2: transport from source to substrate (the vapor species are transported from the source to the

substrate with or without collisions between atoms and molecules; during transport, some of the vapor species can be ionized by creating a plasma)

• Step 3: film growth on the substrate (this process involves the condensation of the vapor species onto

the substrate and subsequent formation of the film by nucleation and growth processes; the nucleation and growth processes can be strongly influenced by bombardment of the growing film by ionic species, resulting in a change in microstructure, composition, impurities, and residual stress)

The degree of independent control of these three steps determines the versatility or flexibility of the deposition process For example, these three steps can be independently controlled in PVD processes and therefore give a greater degree of flexibility in controlling the structure, properties, and deposition rate, whereas all of the three steps take place simultaneously at the substrate and cannot be independently controlled in the CVD process Thus, if a process parameter such as the substrate temperature that governs the deposition rate in chemical vapor deposition is initially chosen, the user

is limited to the resultant microstructure and properties obtained

Physical Vapor Deposition Processes

The basic PVD processes fall into two general categories: (1) sputtering and (2) evaporation PVD techniques are used for

a wide variety of applications, from decorative uses to deposition of high-temperature superconducting films The thickness of the deposits can vary from angstroms to millimeters A very high deposition rate (25 m/min, or 1000 in./min) has been achieved with the advent of electron beam (EB) heated sources A very large number of inorganic materials (metals, alloys, compounds, and mixtures) as well as some organic materials can be deposited using PVD technologies Ion plating is a hybrid PVD process because it is defined as an atomistic film deposition process in which the substrate surface and the depositing film is subjected to a flux of high-energy particles that is sufficient to cause changes in the interfacial region between the film and the substrate in addition to changes in the properties of the deposited film as compared to a nonbombarded film These changes may be in the adhesion of the film to the substrate or

in morphology, density, or stress The source of the depositing species can be evaporation, sputtering, gases, or vapors

Sputter Deposition

Sputtering is the phenomena of momentum transfer from an indirect energetic projectile to a solid or liquid target resulting in the ejection of surface atoms or molecules In the sputter deposition process, the target (a source of coating material) and the substrate are placed in the vacuum chamber and evacuated to a pressure typically in the range of 13 to 0.013 mPa (0.1 to 10-4 mtorr) A diagram of the sputter coating process is shown in Fig 1 The target (also called a cathode) is connected to a negative voltage supply, and the substrate generally faces the target A glow discharge is initiated after an inert gas (usually argon gas) is introduced into the evacuated chamber Typical working pressure of the

argon is in the range of 3 to 20 Pa (20 to 150 mtorr) The sputter target erosion rate, R, in /min, is given by:

(Eq 1)

where J is the ion current density in mA/cm2; S is the sputtering yield in atoms/ion; MA is the atomic weight in grams; and

is the density of the target material in g/cm3 Sputtering yield, S, assuming perpendicular ion incidence onto a target

consisting of a random array of atoms, can be expressed as:

(Eq 2)

where Mi is the mass of the incident atom, Mt is the mass of the target atom, Ei is the kinetic energy of the incident ion,

and U is the heat of sublimation of the target material

Fig 1 Schematic showing primary components of a sputter deposition process system

Planar diode glow discharge deposition is the simplest sputtering system This configuration consists of the cathode (the target) and anode facing each other The substrates are placed on the anode The target (which is usually water cooled) performs two functions during the process: one as the source of coating material, and the other as the electrode sustaining the glow discharge The distance between the cathode and anode is usually about 50 to 100 mm (2 to

4 in.)

The discharge current increases with the applied voltage, thus increasing the sputtering rate However, the discharge current does not increase linearly with the applied voltage as the voltage increases above 1.6 × 10-17 J (100 eV) because the ionization cross section decreases with increasing electron energy The sputtering rate can be increased if the working gas pressure increases at a given voltage due to an increase in ion collection by the cathode (Ref 2) However, the deposition rate starts decreasing at high gas pressures due to gas scattering of the sputtered atoms

The deposition rate is mainly determined by the power density at the target surface, the size of the erosion area, the source-to-substrate distance, the source material, and the working gas pressure Some of these factors, such as pressure and power density, are interrelated Therefore, the optimum operating condition is obtained by controlling the parameters

to get the maximum power flux that can be applied to the target without causing cracking, sublimation, or melting The maximum power limit can be increased if the cooling rate of the target is increased by designing the coolant flow channels properly and improving the thermal conductance between the target and the target backing plate

Even though planar diode glow discharge sputter deposition techniques are widely used due to their simplicity and the relative ease of fabrication of targets for a wide range of materials, they have several disadvantages Among the disadvantages of this deposition technique are the low deposition rate, the substrate heating due to the bombardment of high-energy particles, and the relatively small deposition surface areas

Magnetron Deposition. By employing magnetic fields in the diode sputtering process, the ionization efficiency can be greatly increased In the conventional planar diode process, ions are generated relatively far from the target and the probability that ions will lose their energy to the chamber walls is great Furthermore, the number of primary electrons hitting the anode at high energies without experiencing collisions is increased as the pressure decreases, thus reducing ionization efficiency These electron losses are not offset by impact-induced secondary emission Therefore, ionization efficiencies are low and the discharge cannot be sustained in planar diodes at pressures <1.3 Pa (<10 mtorr)

In the magnetron sputter deposition process, an applied magnetic field parallel to the cathode surface forms electron traps and restricts the primary electron motion to the vicinity of the cathode The magnetic field strength is in the range of a few hundred gauss: therefore, it can influence the plasma electrons but not the ions The electrons trapped on a given field line can advance across the magnetic field to an anode or walls by making collisions (mostly with gas atoms) Therefore, their chances of being lost to the walls or anode without collisions are very small Because of the higher efficiency of this ionization mechanism, the process can be operated at pressures of 130 mPa ( 1 mtorr) with high current densities at low voltages, thus providing high sputtering rates

There are several configurations of magnetron sputter deposition technologies Figure 2 shows key components of the cylindrical magnetron, the planar magnetron, the S-gun magnetron, and the unbalanced magnetron

Fig 2 Magnetron sputter deposition process units that use a variety of sources to supply the magnetic field (a)

Cylindrical magnetron source (b) Planar magnetron source (c) S-gun magnetron source

Cylindrical Magnetron. The cylindrical magnetron technique is very useful for preparing uniform coatings over large areas, because it employs long cathodes Furthermore, the cylindrical-hollow magnetron technique is effective for coating objects with complex shapes The cylindrical-post magnetron can be used to avoid substrate bombardment by energetic particles, thus preventing heating of the substrate

Planar Magnetron. Metallic films and dielectric films can be deposited with high deposition rates using planar magnetron sputtering (as compared with those that can be achieved with diode sputtering

S-Gun Magnetron. To deposit films on thermally sensitive substrate (such as electronic devices), the S-gun magnetron can be used because this technique allows good isolation of the substrate from the glow discharge plasma

Unbalanced Magnetron. A very recent development is the unbalanced magnetron, in which some of the electrons from the target undergo collisions with gas atoms and form a plasma around the substrate from which ions are attracted to the substrate and bombard the growing film to produce changes in the structure and properties of the deposited film In the normal balanced magnetron, the ion flux at the substrate is limited

Magnetron Limitations. Even though the magnetron sputtering techniques have the advantages of high sputtering rates and low bombardment rates of energetic particles onto the substrate, the utilization of these techniques is impeded

by the limitations in the choice of target materials and by the difficulties in fabricating the target For example, if ferromagnetic materials are used as the sputtering target, they should be thin enough to ensure that the material is saturated by the magnetic field Because high power is localized in a selected area in the magnetron sputtering process, targets should be prepared without voids or bubbles to avoid local melting and spitting

Radio Frequency Deposition. The development of the radio frequency (rf) sputter deposition technique made it possible to deposit films from nonconducting sputtering targets, which cannot be sputtered by the direct current (dc) methods because of the charge accumulation on the target surface

Most ions are almost immobile relative to electrons, which can follow the temporal variations in the applied potential at the typical rf frequencies used for sputtering (5 to 30 MHz) When the electrode is coupled to an rf generator, a negative

voltage is developed on the electrode due to the difference in mobility between the electrons and the ions Because the insulating target electrode simulates a capacitor in the electrical circuit, there should be no dc component to the current flow Therefore, the voltage on the electrode surface is required to be self-biased negatively to compensate for the difference in mobility of electrons and ions and to satisfy the condition of net zero current (averaged over each cycle) The magnitude of the resulting negative bias is almost the same as the zero-to-peak voltage of the rf signal The period for the electrode to act as an anode is very short, and the electrode mostly acts as a cathode during the rf cycle Therefore, the target can be expected to be sputtered as in the dc case Significant numbers of ions are not accumulated on the target surface while the electrode acts as a cathode because of the high frequency employed in rf sputtering (normally 13.56 MHz)

Deposition can be performed at considerably lower pressures (typically, 0.7 to 2.0 Pa, or 5 to 15 mtorr) in rf sputtering as compared to the planar dc discharge This is possible because electrons oscillating at high frequency can obtain sufficient energy to cause ionizing collisions and the number of electrons lost (without making collisions) can be reduced

Radio frequency sputtering is widely used to deposit various kinds of materials (for example, conducting, semiconducting, and insulating coatings) despite the complexity of the rf power source This technique can also be applied to magnetron sputtering sources

Ion Beam Deposition. A relatively recent development called ion beam sputtering (Fig 3) provides:

• Excellent adhesion

• A high-purity deposit resulting from the use of low operational pressures ( 13 mPa, or 0.1 mtorr)

• Very low substrate hearing effects because the substrate is not in contact with the plasma

In this technique, an ion beam of high energy (hundreds to thousands of electron volts) extracted from the ion source is directed at a sputtering target of the desired material An inert or reactive gas is used for the ion beam source The substrate is suitably located to collect the sputtered species from the sputtering target (Fig 3) Two primary ion sources are used in practical thin-film deposition: (1) the Kaufman source and (2) the duoplasmatron No further discussion of these sources will be presented in this article; additional information is available in the book by Wilson and Brewer (Ref 3)

Fig 3 Key components of an ion beam sputter deposition apparatus

Because the substrate can be isolated from the plasma generation source, ion beam deposition permits independent control over the substrate temperature, gas pressure, and the type of particle bombardment of the growing film In addition, it is possible to control the energy and the target current density independently in this technique, whereas it can be done only

by varying the working gas pressure in conventional glow discharge sputtering technology The new unbalanced magnetron sputtering systems can also independently vary the ion flux and energy

In general, the deposition rate in ion beam sputtering (50 nm/min, or 500 /min) is lower than that in conventional sputtering The reason for this low rate is mainly due to the low beam current in the conventional (dual-grid) ion beam

sputtering system This low beam current can be greatly increased by using a single-grid system Nishimura et al (Ref 4)

obtained a very high deposition rate (>90 nm/min, or 900 /min, for selected materials such as aluminum, copper, and so on) using single-grid ion beam sputtering A small coverage of deposition area due to the small ion beam size ( 10 mm,

or 0.4 in.) is another drawback of the ion beam sputtering The beam size can be increased to 100 mm ( 4 in.) by adaptation of space-type ion engine technology

Evaporation Deposition

In evaporation processes, vapors are produced from a material located in a source that is heated by various methods (Fig 4) The process uses an evaporation source to vaporize the desired material; the substrates are located at an appropriate distance facing the evaporation source Resistance, induction, arc, electron beam, or lasers are the possible heat sources for evaporation The substrate can be heated and/or biased to the desired potential using a dc/rf power supply Evaporation is carried out in vacuum in a pressure range of 1.3 × 10-3 to 1.3 × 10-8 Pa (10-5 to 10-10 torr) In this pressure range, the mean free path (MFP) is very large (5 to 105 m, or 16 to 3 × 105 ft) as compared to the source-to-substrate distance Hence, the evaporated atoms essentially undergo a collisionless line of sight transport prior to condensation on the substrate, leading to thickness buildup directly above the source that decreases steeply away from it Planetary substrate holders are therefore used in some cases to even out the vapor flux on multiple substrates In some cases, an appropriate gas such as argon at pressures of 0.7 to 30 Pa (5 to 200 mtorr) is introduced into the chamber to reduce the mean free path so that vapor species undergo multiple collisions during transport from the source to substrate, thus producing reasonably uniform thickness coatings on the substrate This technique is called gas scattering evaporation or pressure plating (Ref 5)

Fig 4 Typical evaporation deposition process apparatus incorporating an electron beam as a heat source for

evaporation

The transition of solids or liquids into the vapor phase is an atomistic phenomenon It is based on thermodynamics and results in an understanding of evaporation rates, source-container reactions, and the accompanying effect of impurity introduction into the vapor state, changes in composition during alloy evaporation, and stability of compounds An excellent detailed treatment of the thermodynamic and kinetic bases of evaporation processes is given by Glang (Ref 6)

The rate of evaporation is given by the well-known Hertz-Knudsen equation,

where v is the evaporation coefficient; dNe/Aedt is the number of molecules evaporating from a surface area, Ae in time,

dt; p* is the equilibrium vapor pressure at the evaporant surface; p is the hydrostatic pressure acting on the surface; m is the molecular weight; k is Boltzmann's constant; and T is the absolute temperature Evaporation coefficient, v, is very dependent on the cleanliness of the evaporant surface and can range from very low values for dirty surfaces to unity for clean surfaces

For reasonable deposition rates (100 to 1000 nm/min, or 1000 to 105 /min) at a source-to-substrate distance of 200 mm (8 in.), the vapor pressure should be about 1.3 Pa (10-2 torr) The source temperature should be adjusted to give this value

of the vapor pressure

The directionality of evaporating molecules from an evaporation source is given by the cosine law Holland (Ref 7) and others (Ref 8) have thoroughly discussed the theoretical distribution of vapor from a point, a wire, a small surface, an extended tip, and cylindrical and ring types of sources For the ideal case of deposition from a clean, uniformly emitting

point source onto a plane receiver, the rate of deposition varies as (cos )r2 (Knudsen's cosine law), where r is the radial

distance of the receiver from the source and is the angle between the radial vector and the normal to the receiver direction

PVD Techniques for Deposition of Metals, Alloys, and Compounds

The great versatility of the PVD processes is their ability to deposit a very large number of materials (for example, metals, alloys, semiconductors, superconductors, and polymers) and to fabricate composites of various types (for example, particulate, fibrous, or laminate)

Single-Element Specie Deposition

Single-element specie deposition can be carried out by evaporation or sputter deposition processes The deposition rate depends on the process and the process parameters

• Coevaporation or Cosputtering Using Multiple Sources This technique involves simulneous

coevaporation of the constitutive elements of the alloy The composition of the deposited film is controlled by adjusting the evaporation/sputtering rate of the respective elements In elaborate systems, separate deposition rate monitors are used with appropriate feedback networks to control the deposition rate from each individual source independently Near-stoichiometric films of many binary alloys have been successfully deposited using this technique Dispersion-strengthened alloys such as Ni-ThO2 have also been successfully deposited by this technique

• Evaporation from a Single Source This technique involves evaporation of an alloy using a rod-fed

electron beam source Evaporation operates under steady-state conditions where the composition and volume of the liquid pool on the top of a solid rod are kept constant (Fig 5) A detailed description of the process is given in Ref 9

• Flash Evaporation In this process, pellets of the alloy are dropped onto a very hot strip and are

vaporized completely, thus maintaining the composition of the alloy in the deposit This technique works very well for elements with high vapor pressures

• Sputter deposition from an alloy target

• Sputter deposition from a segmented target where the segments consist of each of the two

components of the alloy and the ratio of the target sample area of each element is inversely proportional

to the sputtering yields

• Laser ablation from an alloy target

Fig 5 Schematic showing direct evaporation of an alloy from a single-rod-fed electron beam source

Steady-state conditions: Vapor pressures, = 10 ; in feed rod composition A1B1; molten pool composition A10B1; vapor and deposit composition A 1 B 1

Deposition of Compounds

Deposition of compounds can be performed in two ways:

1 Direct evaporation using conventional heating methods or laser ablation, where the composition of the

evaporant is the same as that of the compound that is to be deposited

2 Reactive evaporation, where the elements of the compound are evaporated and react with the gas to

form the compound

Plasma-assisted reactive evaporation processes are often used because they activate the reactions leading to compound formation Reactive evaporation and plasma-assisted reactive evaporation are discussed in the section "Reactive Evaporation Process" in this article

Direct Evaporation. When a compound is heated, evaporation can occur with or without dissociation of the compound into fragments A small number of compounds are evaporated without dissociation (for example, SiO2, MgF2, B2O3, GaF, and other Group IV divalent oxides)

Conventional Heating Methods. In the more general case, when a compound is evaporated, the material is not transformed to the vapor state as compound molecules but as fragments thereof Subsequently, the fragments have to recombine on the substrate to reconstitute the compound Therefore, the stoichiometry (anion-to-cation ratio) of the deposit depends on factors such as:

• Vaporization rate

• Ratios of the various molecular fragments in the vapor

• Impingement of other gases present in the environment on the film

• Surface mobility of the fragments (which in turn depends on their kinetic energy and substrate temperature) on the substrate

• Mean residence time of the fragments on the substrate

• Reaction rate of the fragments on the substrate to reconstitute the compound

• Impurities present on the substrate

For example, it was found that direct evaporation of Al2O3 resulted in a deposit that was deficient in oxygen In another example, the deposit from direct evaporation of TiB2 contained both the monoboride and diboride phases (Ref 10)

and insulating thin films Very recently, these techniques have been applied successfully for the deposition of

high-critical-temperature (high-Tc) superconducting films (Ref 11, 12)

In the laser ablation technique, material is vaporized and ejected from the surface of a target as it is irradiated by a laser beam Films are formed by condensing material ablated from the target onto a solid substrate Absorption characteristics

of the material to be evaporated determine the laser wavelength to be used To obtain the high-power density required in many cases, pulsed laser beams are generally employed Pulse width, repetition rate, and pulse intensity are selected for specific applications

Although laser ablation is an attractive approach for the synthesis of high-purity metal, alloy, and compound films, it suffers from the following limitations:

• Complex transmitting and focusing systems need to be employed to direct the beam from the laser located outside the vacuum chamber onto the evaporant placed inside the system This involves special designs and increases the cost of the setup In addition, a window material that efficiently transmits the wavelength band of the laser must be found and mounted in such a way that it is not rapidly covered up

by the evaporant flux

• It is not always possible to find a laser with a wavelength that is compatible with the absorption characteristics of the material to be evaporated

• Energy conversion efficiency is very low

• The size of the deposited film is small (10 to 20 mm, or 0.4 to 0.8 in., diameter), resulting from the small size of the laser impact spot

Various compounds have been synthesized from metal targets with reactive gases:

• Oxides with air, O2, or H2O

• Nitrides with N2 or NH3

• Oxynitrides with O2 + N2

• Sulfides with H2S

There are several advantages to these processes For example, various kinds of compounds can be prepared using relatively easy-to-fabricate metallic targets, insulating compound films can be prepared using direct current (dc) power supplies (reactive sputtering), and graded composition films can be formed

the gas phase Reactions can occur on the cathode surface, at the substrate, and in the gas phase However, reactions in the vapor phase are precluded by considerations of momentum and energy conservation unless the process is performed at high pressures to allow multiple-body collisions in the gas phase

The target is a nominally pure metal The compound film is synthesized by sputtering in a pure reactive gas or mixture of inert gas and reactive gas Usually, the inert-reactive gas mixture is preferred because of sputtering rate considerations A compound target also can be used in the RS technique In this case, the target is chemically decomposed by inert gas ion bombardment It is usually necessary to add the reactive gas to compensate for the loss of the reactive component by dissociation

The main problem in reactive sputtering is target poisoning As the reactive gas partial pressure increases, the rate of compound formation exceeds the removal rate of compounds on the target surface, resulting in a decrease in deposition rate due to the low sputtering yield of the compound formed on the cathode surface and to the fact that compounds have higher secondary electron emission yield than pure metals The increase in secondary electron emission results in a reduction in both the discharge voltage and ion component in the cathode current at constant voltage In other words, more of the energy of incoming ions is consumed to produce and accelerate secondary electrons Various solutions have been found to reduce the effect of target poisoning (Ref 13)

Reactive Evaporation Process. The difficulties involved in direct evaporation processes due to fragmentation of the vaporized compounds are overcome in reactive evaporation, where a metal is evaporated in the presence of the reactive gas The compound is formed by reaction of the evaporating metal species with the molecules of the reactive gas Even though this technique has been extensively used to deposit a variety of oxide films for optical applications, it is generally observed that the films are deficient in oxygen It is also observed in some cases (for example, the synthesis of carbide films) that the deposition rate becomes a limiting factor governing the growth of the films In such cases, stoichiometric titanium carbide films could be deposited only at very low rates ( 0.15 nm/s, or 1.5 /s) This deposition rate limitation

is due to the reaction kinetics of the compound formation in this process The presence of a plasma in the activated reactive evaporation (ARE) process influences the reaction kinetics by providing activation energy to the reactive species, thereby making it possible to synthesize compound films at considerably higher rates (Ref 14) and lower temperatures

presence of the plasma of a reactive gas (Ref 15, 16) For example, titanium carbide and titanium nitride coatings are deposited with this process by evaporating titanium in the presence of C2H2 and N2 plasmas, respectively The two basic variants of the ARE process (one version incorporates an electron beam to evaporate the target material, whereas the other version utilizes resistance elements to provide the heat required to evaporate the target material) are shown in Fig 6 Detailed information on other modifications of the ARE process is available in the article by Bunshah and Deshpandey (Ref 17)

Fig 6 Activated reactive evaporation (ARE) process equipment incorporating different heat sources to

evaporate target materials (a) Electron beam source (b) Resistance element heating source

The role of the plasma is two-fold:

1 To enhance the reactions that are necessary for deposition of compound films

2 To modify the growth kinetics and, hence, the structure/morphology of the deposits

Hybrid PVD Processes. Ion plating and reactive ion plating (RIP) are two deposition processes classified as hybrid PVD processes

evaporation) or momentum transfer (that is, sputtering) or supplied as a vapor (very similar to CVD processes) In this technique, the vaporized (or supplied) coating materials pass through a gaseous glow discharge on their way to the substrate, thus ionizing some typically 1% of the vaporized atoms

The glow discharge is produced by biasing the substrate to a high negative potential (-2 to -5 kV) and admitting a gas (usually argon) at a pressure of 0.7 to 30 Pa (5 to 200 mtorr) into the chamber (Fig 7) In this simple mode, which is known as diode ion plating, the substrate is bombarded by high-energy gas ions that sputter off the material present on the surface This results in a constant cleaning of the substrate (that is, a removal of surface impurities by sputtering), which

is desirable for producing better adhesion and lower impurity content The ion bombardment also causes a modification in the microstructure and residual stress in the deposit On the other hand, it produces the undesirable effects of decreasing the deposition rate because some of the deposit is sputtered off; it also causes a considerable (and often undesired for microelectronic applications) heating of the substrate by the intense ion bombardment The latter problem can be alleviated by using the supported discharge ion-plating process, in which the substrate is no longer at the high negative potential because the electrons necessary for supporting the discharge come from an auxiliary heated tungsten filament The high gas pressure used during deposition causes a reasonably uniform deposition on all surfaces due to gas scattering

Fig 7 Typical ion-plating process system that uses a dc diode discharge to ionize the vaporized atoms

Reactive ion plating is very similar to the reactive evaporation process in that the metal atoms and reactive gases, aided by the presence of a plasma, react to form a compound Because the partial pressure of the gases in reactive ion plating is much higher (>1.3 Pa, or 10-2 torr) than in the ARE process (>0.013 Pa, or 10-4 torr), the deposits can become porous or sooty The plasma cannot be supported at lower pressures in the simple diode ion-plating process Therefore, Kobayashi and Doi (Ref 19) introduced an auxiliary electrode biased to a positive low voltage (as originally conceived for the ARE process) to initiate and sustain the plasma at lower pressure ( 0.13 Pa, or 1 mtorr) (Fig 8) This is no different from the ARE process with a negative bias on the substrate reported much earlier by Bunshah (Ref 20) and designated by him as the biased ARE (BARE) process