Synthesis of nano al2o3 dispersion strengthened cu base composite materials by mechanochemical process

Synthesis of nano al2o3 dispersion strengthened cu base composite materials by mechanochemical process

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

SCHOOL OF MATERIALS SCIENCE AND TECHNOLOGY

THESIS OF GRADUATION

strengthened - Cu base composite materials

by mechanochemical process

Student: Phung Anh Tuan

MSE-ATP-K52 Advisor: Dr Nguyen Dang Thuy

Hanoi, June 2012

Table of contents

Preface 5

Chapter I: OVERVIEW 6

1.1 Composite materials 6

1.1.1 Definition 6

1.1.2 Class of composite materials 8

1.2 Metal matrix composites 9

1.2.1 Reinforcements 11

1.2.2 Matrix alloy systems 13

1.3 Partical-reinforced composites 14

1.3.1 Large-particle composites 14

1.3.2 Dispersion-strengthened composites 8

1.4 Bearing materials 19

1.4.1 Structure and properties and applications of bearing materials 20

1.4.2 Conventional bearing materials 22

1.5 Copper alumina composite 29

Chapter II: MECHANICAL ALLOYING 30

2.1 History 30

2.2 Milling 34

2.3 Mechanism of alloying 36

2.3.1 Ductile –Ductile components 38

2.3.2 Ductile – Brittle 39

2.3.3 Britle-Brittle component 40

Chapter III: EXPERIMENTAL PROCEDURE 41

3.1 Milling 41

3.2 Pressing 44

3.3 Sintering 45

Chapter IV: RESULTS AND DISCUSSION 48

4.1 Results after milling 48

4.2 Results after sintering 56

4.3 Porosity 58

4.4 Hardness 61

4.5 Microstructure 62

4.6 Experimental planning and process optimization 64

Chapter V: CONCLUSION AND SUGGESTION 71

5.1 Conclusions 71

5.2 Suggestions 71

References 72

List of figures

1.1 Schematic representations of the various geometrical and

spatial characteristics of particles of the dispersed phase

that may influence the properties of composites

8

1.2 A classification scheme for the various composite types

1.3 Modulus of elasticity versus volume percent tungsten for a

composite of tungsten particles dispersed within a copper

matrix Upper and lower bounds are according to

Equations 16.1 and 16.2; experimental data points are

included

16

1.4 Photomicrograph of a WC–Co cemented carbide Light

areas are the cobalt matrix; dark regions, the particles of

tungsten carbide

17

2.1 Model 1-S attritor and arrangement of rotating arms on a

2.2 Ball-powder-ball collision of powder mixture during

2.3 Scanning electron micrograph depicting the convoluted

lamellar structure obtained during milling of a

ductile-ductile component system (Ag-Cu)

39

2.4 Schematics of microstructure evolution during milling of a

ductile-brittle combination of powders This is typical of an

oxide dispersion strengthened case

40

3.6 Sintering diagram of Cu-Al2O3 46

4.1 SEM images form the initial sample mixture CuO-Cu-Al

4.3 The X-ray diffraction diagram of original mixed powder

4.4 The results of X-ray analysis of powder samples after 4

4.8 The X-ray diffraction diagram of mixed powder material

4.10 SEM images samples Cu- Al2O3 (wt.10%) after sintering at

List of tables

1.1 Properties of typical discontinuous reinforcements for

2.1 Important milestones in the development of mechanical

Preface

As the time elapsed, living standard is continuously increased One of the most important reasons for this is the developing in science and technology The requirement for the new materials is much debated in our social It set new challenges for the materials science and technology In our country, there is a potential market in every fields of the industry The materials nowadays need to have many unique properties Moreover, the prices of synthesis have to be as low

as possible Thus, scientists tend to research to find the simplest method to create the best materials with a proper price That’s a reason why I find interest in the mechanical alloying-the simple method to produce alloys with many advantages

Therefore, I have chosen the project namely “Synthesis of nano Al 2 O 3 dispersion strengthened - Cu base composite materials by mechanochemical process”

In my project I will focus on composite base on Cu with Al2O3 dispersion Cu-Al2O3 composite is one of the newest bearing materials of engines This bearing system is developing in the world However the synthesis method is keep

in secret

I express my deep gratitude to Doctor Nguyen Dang Thuy who helped me to

find enthusiasm in researching, showed me how to think critically and work effectively He is not only my teacher but also my instructor in researching

I send my true thankfulness to every laboratory in School of Materials Science and Technology, Hanoi University of Science and Technology and all technicians, teachers, professors in School of Materials Science and Technology who have already helped me to complete this project

And, thanks to other lovely members in my research group, who have worked with me and helped me a lot

Material property combinations and ranges have been, and are yet being, extended by the development of composite materials Generally speaking, a composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realized According to this principle of combined action, better property combinations are fashioned by the judicious combination of two or more distinct materials Property trade-offs are also made for many composites

Composites of sorts have already been discussed; these include multiphase metal alloys, ceramics, and polymers For example, pearlitic steels have a microstructure consisting of alternating layers of ferrite and cementite The ferrite phase is soft and ductile, whereas cementite is hard and very brittle The combined mechanical characteristics of the pearlite (reasonably high ductility and strength)

are superior to those of either of the constituent phases There are also a number of composites that occur in nature For example, wood consists of strong and flexible cellulose fibers surrounded and held together by a stiffer material called lignin Also, bone is a composite of the strong yet soft protein collagen and the hard, brittle mineral apatite

A composite, in the present context, is a multiphase material that is artificially made, as opposed to one that occurs or forms naturally In addition, the constituent phases must be chemically dissimilar and separated by a distinct interface Thus, most metallic alloys and many ceramics do not fit this definition because their multiple phases are formed as a consequence of natural phenomena

In designing composite materials, scientists and engineers have ingeniously combined various metals, ceramics, and polymers to produce a new generation of extraordinary materials Most composites have been created to improve combinations of mechanical characteristics such as stiffness, toughness, and ambient and high-temperature strength

Many composite materials are composed of just two phases; one is termed the matrix, which is continuous and surrounds the other phase, often called the dispersed phase The properties of composites are a function of the properties of the constituent phases, their relative amounts, and the geometry of the dispersed phase “Dispersed phase geometry” in this context means the shape of the particles and the particle size, distribution, and orientation; these characteristics are represented in Figure 1.1

Figure 1.1 Schematic representations of the various geometrical and spatial characteristics of

particles of the dispersed phase that may influence the properties of composites:

(a) concentration, (b) size, (c) shape, (d) distribution, and (e) orientation

(From Richard A Flinn and Paul K Trojan, Engineering Materials and Their Applications,

4th edition Copyright © 1990 by John Wiley & Sons, Inc Adapted by permission of John

Wiley & Sons, Inc.)

1.1.2 Class of composite materials

One simple scheme for the classification of composite materials is shown in Figure 1.2, which consists of three main divisions: particle-reinforced, fiber-reinforced, and structural composites; also, at least two subdivisions exist for each The dispersed phase for particle-reinforced composites is equiaxed (i.e., particle dimensions are approximately the same in all directions); for fiber-reinforced composites, the dispersed phase has the geometry of a fiber (i.e., a large length-to-diameter ratio) Structural composites are combinations of composites and

homogeneous materials The discussion of the remainder of this chapter will be organized according to this classification scheme

Figure 1.2 A classification scheme for the various composite types discussed in this chapter

1.2 METAL MATRIX COMPOSITES

As the name implies, for metal-matrix composites (MMCs) the matrix is a

ductile metal These materials may be utilized at higher service temperatures than their base metal counterparts; furthermore, the reinforcement may improve specificstiffness, specific strength, abrasion resistance, creep resistance, thermal conductivity, and dimensional stability Some of the advantages of these materials over the polymer-matrix composites include higher operating temperatures, nonflammability, and greater resistance to degradation by organic fluids Metal-matrix composites are much more expensive than PMCs, and, therefore, their (MMC) use is somewhat restricted

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are employed as matrix materials The reinforcement may be in the form

of particulates, both continuous and discontinuous fibers, and whiskers; concentrations normally range between 10 and 60 vol% Continuous fiber

materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals On the other hand, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, and particulates of silicon carbide and aluminum oxide In a sense, the cermets fall within this MMC scheme In Table 16.9 are presented the properties

of several common metal-matrix, continuous and aligned fiber-reinforced composites

Some matrix–reinforcement combinations are highly reactive at elevated temperatures Consequently, composite degradation may be caused by high-temperature processing or by subjecting the MMC to elevated temperatures during service This problem is commonly resolved either by applying a protective surface coating to the reinforcement or by modifying the matrix alloy composition

Normally the processing of MMCs involves at least two steps: consolidation

or synthesis (i.e., introduction of reinforcement into the matrix), followed by a shaping operation A host of consolidation techniques are available, some of which are relatively sophisticated; discontinuous fiber MMCs are amenable to shaping by standard metal-forming operations (e.g., forging, extrusion, rolling)

Automobile manufacturers have recently begun to use MMCs in their products For example, some engine components have been introduced consisting

of an aluminum-alloy matrix that is reinforced with aluminum oxide and carbon fibers; this MMC is light in weight and resists wear and thermal distortion Metal-matrix composites are also employed in driveshafts (that have higher rotational speeds and reduced vibrational noise levels), extruded stabilizer bars, and forged suspension and transmission components

The aerospace industry also uses MMCs Structural applications include advanced aluminum alloy metal-matrix composites; boron fibers are used as the

reinforcement for the Space Shuttle Orbiter, and continuous graphite fibers for the Hubble Telescope

The high-temperature creep and rupture properties of some of the superalloys (Ni- and Co-based alloys) may be enhanced by fiber reinforcement using refractory metals such as tungsten Excellent high-temperature oxidation resistance and impact strength are also maintained Designs incorporating these composites permit higher operating temperatures and better efficiencies for turbine engines

1.2.1 Reinforcements

Reinforcements for metal matrix composites have a manifold demand profile, which is determined by production and processing and by the matrix system of the composite material The following demands are generally applicable:

• High Young’s modulus,

• High compression and tensile strength,

• Good processability,

• Economic efficiency

These demands can be achieved only by using non-metal inorganic reinforcement components For metal reinforcement ceramic particles or, rather, fibers or carbon fibers are often used Due to the high density and the affinity to reaction with the matrix alloy the use of metallic fiber usual fails Which

components are finally used, depends on the selected matrix and on the demand profile of the intended application The information about available particles, short fibers, whiskers and continuous fibers for the reinforcement of metals is given, including data of manufacturing, processing and properties Representative examples are shown in Table 1.1 The production, processing and type of application of various reinforcements depends on the production technique for the composite materials A combined application of various reinforcements is also possible (hybrid technique)

crystal structure δ-Al2O3 hexagonal hexagonal

Table 1.1 Properties of typical discontinuous reinforcements

for aluminium and magnesium reinforcements

Every reinforcement has a typical profile, which is significant for the effect within the composite material and the resulting profile The group of discontinuous reinforced metals offers the best conditions for reaching development targets; the applied production technologies and reinforcement components, like short fibers, particle and whiskers, are cost effective and the production of units in large item numbers is possible The relatively high isotropy

of the properties in comparison to the long-fiber continuous reinforced light metals and the possibility of processing of composites by forming and cutting production engineering are further advantages

1.2.2 Matrix Alloy Systems

The selection of suitable matrix alloys is mainly determined by the intended application of the composite material With the development of light metal composite materials that are mostly easy to process, conventional light metal alloys are applied as matrix materials In the area of powder metallurgy special alloys can be applied due to the advantage of fast solidification during the powder production Those systems are free from segregation problems that arise in conventional solidification Also the application of systems with oversaturated or metastable structures is possible

• Conventional cast alloys

high conductivity or ductility A dispersion hardening to reach the required mechanical characteristics at room or higher temperatures is then an optimal solution

1.3 PARTICLE-REINFORCED COMPOSITES

As noted in Figure 1.2, large-particle and dispersion-strengthened composites are the two sub classifications of particle-reinforced composites The distinction between these is based upon reinforcement or strengthening mechanism The term “large” is used to indicate that particle–matrix interactions cannot be treated on the atomic or molecular level; rather, continuum mechanics is used For most of these composites, the particulate phase is harder and stiffer than the matrix These reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle In essence, the matrix transfers some of the applied stress to the particles, which bear a fraction of the load The degree of reinforcement or improvement of mechanical behavior depends on strong bonding

at the matrix–particle interface

For dispersion-strengthened composites, particles are normally much smaller, with diameters between 0.01 and 0.1 m (10 and 100 nm) Particle–matrix interactions that lead to strengthening occur on the atomic or molecular level The mechanism of strengthening is similar to that for precipitation Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations Thus, plastic deformation is restricted such that yield and tensile strengths, as well as hardness, improve

1.3.1 Large-particle composites

Some polymeric materials to which fillers have been added are really particle composites Again, the fillers modify or improve the properties of the

large-material and/or replace some of the polymer volume with a less expensive large-material the filler

Another familiar large-particle composite is concrete, which is composed of cement (the matrix), and sand and gravel (the particulates) Concrete is the discussion topic of a succeeding section

Particles can have quite a variety of geometries, but they should be of approximately the same dimension in all directions (equiaxed) For effective reinforcement, the particles should be small and evenly distributed throughout the matrix Furthermore, the volume fraction of the two phases influences the behavior; mechanical properties are enhanced with increasing particulate content Two mathematical expressions have been formulated for the dependence of the elastic modulus on the volume fraction of the constituent phases for a two-phase composite These rule of mixtures equations predict that the elastic modulus should fall between an upper bound represented by

Ec(u) = EmVm + EpVp

(For a two-phase composite, modulus of elasticity upper-bound expression)

and a lower bound, or limit,

(For a two-phase composite, modulus of elasticity lower-bound expression)

In these expressions, E and V denote the elastic modulus and volume fraction, respectively, whereas the subscripts c, m, and p represent composite, matrix, and particulate phases Figure 1.3 plots upper- and lower-bound Ec-versus-

Vp curves for a copper–tungsten composite, in which tungsten is the particulate

phase; experimental data points fall between the two curves

Figure 1.3 Modulus of elasticity versus volume percent tungsten for a composite of

tungsten particles dispersed within a copper matrix Upper and lower bounds are according to Equations 16.1 and 16.2; experimental data points are included (From R H Krock, ASTM Proceedings, Vol 63, 1963 Copyright ASTM, 1916 Race Street, Philadelphia, PA 19103 Reprinted with permission.)

Large-particle composites are utilized with all three material types (metals, polymers, and ceramics) The cermets are examples of ceramic–metal composites The most common cermet is the cemented carbide, which is composed of extremely hard particles of a refractory carbide ceramic such as tungsten carbide (WC) or titanium carbide (TiC), embedded in a matrix of a metal such as cobalt or nickel These composites are utilized extensively as cutting tools for hardened steels The hard carbide particles provide the cutting surface but, being extremely brittle, are not themselves capable of withstanding the cutting stresses Toughness

is enhanced by their inclusion in the ductile metal matrix, which isolates the carbide particles from one another and prevents particle-to particle crack

propagation Both matrix and particulate phases are quite refractory, to withstand the high temperatures generated by the cutting action on materials that are extremely hard No single material could possibly provide the combination of properties possessed by a cermet Relatively large volume fractions of the particulate phase may be utilized, often exceeding 90 vol%; thus the abrasive action of the composite is maximized A photomicrograph of a WC Co cemented

carbide is shown in Figure 1.4

Figure 1.4 Photomicrograph of a WC–Co cemented carbide Light areas are the cobalt

matrix; dark regions, the particles of tungsten carbide (Courtesy of Carboloy Systems

Department, General Electric Company.)

Both elastomers and plastics are frequently reinforced with various particulate materials Our use of many of the modern rubbers would be severely restricted with-out reinforcing particulate materials such as carbon black Carbon black consists of very small and essentially spherical particles of carbon, produced

by the combustion of natural gas or oil in an atmosphere that has only a limited air supply When added to vulcanized rubber, this extremely inexpensive material enhances tensile strength, toughness, and tear and abrasion resistance Automobile tires contain on the order of 15 to 30 vol% of carbon black For the carbon black to provide significant reinforcement, the particle size must be extremely small, with diameters between 20 and 50 nm; also, the particles must be evenly distributed throughout the rubber and must form a strong adhesive bond with the rubber matrix Particle reinforcement using other materials (e.g., silica) is much less effective because this special interaction between the rubber molecules and particle surfaces does not exist Figure 1.4 is an electron micrograph of a carbon black-reinforced rubber

1.3.2 Dispersion-strengthened composites

Metals and metal alloys may be strengthened and hardened by the uniform dispersion of several volume percent of fine particles of a very hard and inert material The dispersed phase may be metallic or nonmetallic; oxide materials are often used Again, the strengthening mechanism involves interactions between the particles and dislocations within the matrix, as with precipitation hardening The dispersion strengthening effect is not as pronounced as with precipitation hardening; however, the strengthening is retained at elevated temperatures and for extended time periods be-cause the dispersed particles are chosen to be unreactive with the matrix phase For precipitation-hardened alloys, the increase in strength may disappear upon heat treatment as a consequence of precipitate growth or dissolution of the precipitate phase

The high-temperature strength of nickel alloys may be enhanced significantly by the addition of about 3 vol% of thoria (ThO2) as finely dispersed particles; this material is known as thoria-dispersed (or TD) nickel The same

effect is produced in the aluminum–aluminum oxide system A very thin and adherent alumina coating is caused to form on the surface of extremely small (0.1

to 0.2 µm thick) flakes of aluminum, which are dispersed within an aluminum metal matrix; this material is termed sintered aluminum powder (SAP)

1.4 BEARING MATERIALS

Nowadays, the hydraulic excavators are widely used in many countries Most

of hydraulic excavators have some special bearings between two sliding objects to reduce the abrasion Those bearing have to be changed regularly after some period

of working time Therefore, it needs to be high surface pressure, high offset load, lubrication and low cost material

Figure 1.5 Some bearings are made from copper alloys

1.4.1 Structure and properties and applications of bearing materials

Many millions of bearings operate successfully in the boundary and film modes for their entire service lives The only penalty this entails is an increase

mixed-in friction compared to hydro-dynamically lubricated bearmixed-ings and consequently higher energy expenditure Bearing life, however, will depend very heavily on the choice of bearing material Even hydrodynamic bearings pass through boundary and mixed-film modes during start-up and shut down or when faced with transient upset conditions This means that material selection is an important design consideration for all sleeve bearings, no matter what their operating mode

The general attributes of a good bearing material are:

 A low coefficient of friction versus hard shaft materials,

 Good wear behavior against steel journals (scoring resistance),

 The ability to absorb and discard small contaminant particles (embedibility),

 The ability to adapt and adjust to the shaft roughness and misalignment (conformability),

 High compressive strength,

 High fatigue strength,

 Corrosion resistance,

 Low shear strength (at the bearing-to shaft interface),

 Structural uniformity,

 Reasonable cost and ready availability

A material's inherent frictional characteristics are extremely important during those periods, however brief, when the bearing operates in the boundary mode A low coefficient of friction is one factor in a material's resistance against welding

to, and therefore scoring, steel shafts Frictional coefficients for bronze alloys against steel range between 0.08 and 0.14 During wear, or when there is

absolutely no lubricant present, the frictional coefficient may range from about 0.12 to as high as 0.18 to 0.30

While efforts are normally made to keep bearings and their lubricants clean, some degree of contamination is almost inevitable A good bearing material should be able to compensate for this by embedding small dirt particles in its structure, keeping them away from the steel shaft, which might otherwise be scratched

Likewise, there is always a danger that shafts can be misaligned, or not be perfectly smooth A bearing alloy may therefore be called upon to conform, or

"wear-in" slightly to compensate for the discrepancy This property is called conformability: it is related to the material's hardness and compressive yield strength High yield strength is also related to good fatigue resistance Together, these properties largely define the material's load-carrying capacity

The need for adequate corrosion resistance is especially important in bearings that operate in aggressive environments, or for those bearings which stand idle for long periods of time Good corrosion resistance therefore increases both service life and shelf life

A bearing material should have structural uniformity and its properties should not change as surface layers wear away On the other hand, alloys such as the leaded bronzes are used because they provide a lubricating film of lead at the bearing/ journal interface Lead has a low shear strength, and is able to fill in irregularities in the shaft and act as an emergency lubricant if the oil supply is temporarily interrupted

Finally, a bearing material should be cost-effective and available on short notice No single bearing material excels in all these properties and that is one of the reasons bearing design always involves a compromise However the Cu-Al2O3

alloys provide such a broad selection of material properties that one of them can almost always fit the needs of a particular design

a Bronze bearing materials

Tin Bronzes

Tin's principal function in these bronzes is to strengthen the alloys (Zinc also adds strength, but more than about 4% zinc reduces the anti-frictional properties of the bearings alloy.) The tin bronzes are strong and hard and have very high ductility This combination of properties gives them a high load-carrying capacity, good wear resistance and the ability to withstand pounding The alloys are noted for their corrosion resistance in seawater and brines

The tin bronzes' hardness inhibits them from conforming easily to rough or misaligned shafts Similarly, they do not embed dirt particles well and therefore must be used with clean, reliable lubrication systems They require a shaft hardness between 300-400 BHN Tin bronzes operate better with grease lubrication than other bronzes; they are also well suited to boundary-film operation because of their ability to form polar compounds with small traces of lubricant Differences in mechanical properties among the tin bronzes are not great Some contain zinc as a strengthener in partial replacement for more-expensive tin

Leaded Tin Bronzes

Some tin bronzes contain small amounts of lead In this group of alloys, lead's main function is to improve machinability It is not presented in sufficient concentration to change the alloys' bearing properties appreciably A few of the leaded bronzes also contain zinc, which strengthens the alloys at a lower cost than

tin The leaded bronzes in this family otherwise have similar properties and application as the tin bronzes

High-Leaded Tin Bronzes

The family of high-leaded tin bronzes includes the workhorses of the bearing bronze alloys This alloy has a wider range of applicability, and is more often specified, than all other bearing materials It, and the other high-leaded tin bronzes are used for general utility applications under medium loads and speeds, i.e., those conditions which constitute the bulk of bearing uses Strengths and hardness are somewhat lower than those of the tin bronzes but this group of leaded alloys excels in their antifriction and machining properties High strength is sacrificed for superior lubricity in the bronzes containing 15 and 25 percent lead, These high-leaded tin bronzes embed dirt particles very well and conform easily to irregularities in shaft surfaces and permit use with unhardened shafts As in all leaded bronzes the lead is present as discrete microscopic particles The lead also provides excellent machine ability

Those alloys should not be specified for use under high loads or in applications where impacts can be anticipated They operate best at moderate loads and high speeds, especially where lubrication may be unreliable They conform well and are very tolerant of dirty operating conditions, properties which have found them extensive use in offhighway, earthmoving and heavy industrial equipment

Manganese Bronzes

Manganese bronzes are modifications of the Muntz metal-type alloys (60% copper 40% zinc brasses) containing small additions of manganese, iron and

aluminum, plus lead for lubricity, anti-seizing and embeddibility Like the aluminum bronzes, they combine very high strength with excellent corrosion resistance Manganese bronze bearings can operate at high speeds under heavy loads, but require high shaft hardnesses and nonabrasive operating conditions

Aluminum Bronzes

The aluminum bronzes are the strongest and most complex of the based bearing alloys Their aluminum content provides most of their high strength and makes them the only bearing bronzes capable of being heat treated Their high strength, up to 68,000 psi yield and 120,000 tensile, permits them to be used at unit loads up to 50 percent higher than those for leaded tin bronze Alloy

copper-Because of their high strength, however, they have fairly low ductility and do not conform or embed well They consequently require shafts hardened to 550-600

HB Surfaces must also be extremely smooth Careful attention should be given to lubricant cleanliness and reliability, the latter because these alloys do not have the anti-seizing properties typical of the leaded and tin bearing bronzes On the other hand, the aluminum bronzes have excellent corrosion resistance and are ideally suited for such applications as marine propellers and pump impellers

The aluminum bronzes also have superior elevated temperature strength They are the only bronzes - and the only conventional bearing material able to operate at temperatures exceeding 10oC

Summary

Bearing bronzes offer broad ranges of strength, ductility, hardness, wear resistance, anti-seizing properties, low friction and the ability to conform to

irregularities, tolerate dirty operating environments and contaminated lubricants The corrosion resistance of bearing bronzes is generally superior to other bearing materials, and can be selected to meet particular ambient conditions Bronzes permit easy and economical manufacture, allowing bearings to be made in special and one-of-a-kind configurations simply and at low cost No bearing metals have better machine ability than the aluminum bearing bronzes Almost without exception, a bearing bronze can be selected to satisfy any bearing application that exists

Figure 1.6 The popular designs for Engine bearing structure

One of the most common designs for engine bearing is multilayer The inner layer plays an important role-lubrication The steel back ensure the mechanical properties of the bearing The bonding layer sticks other layers together

b Some non-metallic bearing

Carbon graphite

The self-lubricating properties of carbon bearings, their stability at temperatures up to 750°F, and their resistance to attack by chemicals and solvents, give them important advantages where other bearing materials are unsatisfactory Carbon-graphite bearings are used where contamination by oil or grease is undesirable, as in textile machinery, food-handling machinery, and pharmaceutical processing equipment They are used as bearings in and around ovens, furnaces, boilers, and jet engines where temperatures are too high for conventional lubricants They are also used with low-viscosity and corrosive liquids in such applications as metering devices or pumps for gasoline, kerosene, water, seawater, chemical process streams, acids, alkalis, and solvents

The composition and processing used with carbon bearings can be varied to provide characteristics required for particular applications Carbon graphite has from 5 to 20% porosity These pores can be filled with a phenolic or epoxy resin for improved strength and hardness, or with oil or metals (such as silver, copper, bronze, cadmium, or babbitt) to improve compatibility properties

A PV limit of 15,000 ordinarily can be used for dry operation of carbon bearings This should be reduced for continuous running with a steady load over a long period of time to avoid excessive wear When operating with liquids which permit the development of a supporting fluid film, much higher PV values can be used

A hard, rust-resistant shaft with at least a 10-∞in finish should be used Hardened tool steel or chrome plate is recommended for heavy loads and high-speed applications Steel having hardness over Rockwell C50, bronzes, 18-8 stainless steels, and various carbides and ceramics also can be used

Certain precautions should be observed in applying carbon graphite Since this material is brittle, it is chipped or cracked easily if struck on an edge or a corner or if subjected to high thermal, tensile, or bending stresses Edges should be relieved with a chamfer Sharp corners, thin sections, keyways, and blind holes should be avoided wherever possible Because of brittleness and low coefficient of expansion (about one-fourth that of steel), carbon-graphite bearings are often shrunk into a steel sleeve This minimizes changes in shaft clearance with temperature variations and provides mechanical support for the carbon-graphite elements

Rubber

Elastomeric materials give excellent performance in bearings on propeller shafts and rudders of ships, and in other industrial equipment handling water or slurries The resilience of rubber helps isolate vibration to provide quiet operation, allows running with relatively large clearances, and helps compensate for misalignment

Commercial rubber bearings consist of a fluted structure supported in a solid metal shell This allows the shaft to be carried on a series of rubber strips running the length of the bearing A water flow 2 gpm/in of diameter is normally provided

to cool the bearing and to flush through any dirt collecting in the channels between the rubber strips Maximum load should be limited to 35 to 50 psi There is no limit on speed, as long as operating temperature remains below 150°F

Wood

Lignum vitae and oil-impregnated maple and oak offer self-lubricating properties, low cost, and clean operation Wood bearings are useful at temperatures up to 150°F and at speeds up to several hundred rpm

Ceramics

Silicon nitride has been developed as a high-performance bearing material exhibiting fatigue properties equaling or exceeding that of high-quality bearing steels The primary use for these materials is in such rolling elements as balls and rollers for integration in bearings made with steel races Such bearings are hybrid ceramic and are available in most sizes with standard offerings available in high-performance super-precision ball bearings All-ceramic bearings are available in limited quantities upon special order for special applications

The hardness, rigidity, corrosion resistance, and fine finish of various ceramics, carbides, and cermets have made them of great interest as bearing materials Except for silicon nitride, there is only limited success in applying these materials as bearings

Sapphire and glass

Synthetic industrial sapphire (100% aluminum oxide) is widely used for jewel bearings in low-torque instrument applications Borosilicate glass has been substituted for sapphire in some applications Jewel bearings are available in several configurations, including vee, ring, endstones, cup, and orifice

Summary

Almost non-metallic materials have good properties for engine bearing applications However, the mechanical properties are quite low Therefore, non-metallic bearing materials are hard to be applied for engineering parts

1.5 COPPER ALUMINA COMPOSITE

Dispersion strengthened Cu - Al2O3 composite materials are extensively used as materials for products which require high-strength and electrical properties, such as electrode materials for lead wires, relay blades, contact supports and bearing materials for industry Electrode tips made of this composite material which operating temperature is approximately 800°C demonstrate much higher softening (recrystalization) temperature than tips made of standard high-strength and high conductivity copper alloys Copper-based composites with a fine dispersion of Al2O3 particles produced by high-energy milling has been extensively studied in recent years due to attained better properties than pure copper and precipitation or solid solution hardened copper Further, high energy milled powders are characterized by very fine, nano-scaled grain structure, which may be retained even during compaction This fine-grained structure contributes to copper matrix strengthening together with Al2O3 particles

In this study the copper matrix was strengthened by Al2O3 particles by internal oxidation and mechanical alloying The effect of the various size of copper and Al2O3 powder particles on structure, strengthening, thermal stability and electrical conductivity of Cu-Al2O3 composite was the object of this paper

Chapter II: MECHANICAL ALLOYING

There are many methods to synthesis Cu-Al2O3 such as melting, powder metallurgy and mechanical alloying However, mechanical alloying has some advantages

By choosing the pure metal as sources of the synthesis Cu-Al2O3, with the room temperature therefore, the vaporizing metal does not occur The chemical composition almost unchanged during alloying It ensures the proportional of Cu-

Al2O3 will be 1:1 of the production

Mechanical alloying also save the energy, and is safer than other method for example melting, HIP

Mechanical alloying is easy to be applied in practice

Mechanical alloying is also is smart choice for synthesis bearing materials especially Cu-Al2O3 system It can’t be denied that almost bearing materials need

to be high porosity This property is easily obtained by mechanical alloying Moreover, we can control the alloying carefully

2.1 HISTORY

Mechanical alloying (MA) is a powder processing technique that allows production of homogeneous materials starting from blended elemental powder mixtures John Benjamin and his colleagues at the Paul D Merica Research Laboratory of the International Nickel Company (INCO) developed the process around 1966 The technique was the result of a long search to produce a nickel- base super alloy, for gas turbine applications, that was expected to combine the high-temperature strength of oxide dispersion and the intermediate-temperature strength of gamma-prime precipitate The required corrosion and oxidation

resistance was also included in the alloy by suitable alloying additions Benjamin has summarized the historic origins of the process and the background work that led to the development of the present process

In the early 1960s, INCO had developed a process for manufacturing graphitic aluminum alloys by injecting nickel-coated graphite particles into a molten aluminum bath by argon sparging A modification of the same technique was tried to inoculate nickel-based alloys with a dispersion of nickel-coated, fine refractory oxide particles The purpose of nickel coating was to render the normally unwetted oxide particles wettable by a nickel-chromium alloy The early experiments used metal-coated zirconium oxide and this did not yield the desired result A thorough analysis revealed that the reason for the failure of the experiment was because the vendor had supplied powder that was zirconia-coated nickel rather than nickel-coated zirconia Since the reaction of aluminum with nickel produces a strong exothermic reaction, the heat generated cleansed the surface of the graphite and lowered the surface energy On this basis, it was assumed that coating of the refractory oxide with aluminum would be ideal to produce the exothermic reaction This also did not prove successful When some other attempts also failed to yield the desired result, out of desperation, attention was turned to the ball milling process that had been used earlier to coat hard phases such as tungsten carbide with a soft phase such as cobalt or nickel It was also known that metal powder particles could be fractured by subjecting them to heavy plastic deformation Use of special chemicals could be employed to produce finer particles by preventing cold welding, suggesting that at some stage cold welding could be as rapid as fracturing The reactivity of the element also had to

be considered Taking all these factors into consideration, Benjamin decided to produce composite powder particles by:

- Using a high energy mill to favor plastic deformation required for cold welding and reduces the process times

- Using a mixture of elemental and master alloy powders (the latter to reduce the activity of the element, since it is known that the activity in an alloy or a

compound could be orders of magnitude less than in a pure metal),

- Eliminating the use of surface-active agents which would produce

finepyrophoric powder as well as contaminate the powder

- Relying on a constant interplay between welding and fracturing to yield a powder with a refined internal structure, typical of very fine powders normally produced, but having an overall particle size which was relatively coarse, and therefore stable

This method of making the composite powders reproduced the properties of

TD (thoria dispersed) nickel synthesized by a completely different process Encouraged by this success, experiments were conducted to produce a nickel-chromium- aluminum-titanium alloy containing a thoria dispersoid This was also successfully produced, first in a small high-speed shaker mill and later in a one- gallon stirred ball mill, starting the birth of MA as a method to produce oxide dispersion strengthened (ODS) alloys on an industrial scale

This process, as developed by Benjamin, was referred to as ``milling mixing'', but Mr Ewan C MacQueen, a patent attorney for INCO coined the term mechanical alloying to describe the process in the first patent application, and this term has now come to stay in the literature

Mechanical alloying is normally a dry, high-energy ball milling technique and has been employed to produce a variety of commercially useful and

scientifically interesting materials The formation of an amorphous phase by mechanical grinding of an Y-Co intermetallic compound in 1981 and in the Ni-Nb system by ball milling of blended elemental powder mixtures in 1983 brought about the recognition that MA is a potential non-equilibrium processing technique Beginning from the mid-1980s, a number of investigations have been carried out

to synthesize a variety of stable and metastable phases including supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and amorphous alloys Additionally, it has been recognized that powder mixtures can

be mechanically activated to induce chemical reactions, i.e mechanochemical reactions at room temperature or at least at much lower temperatures than normally required to produce pure metals, nano-composites, and a variety of commercially useful materials Efforts were also under way since the early 1990s

to understand the process fundamentals of MA through modeling studies Because

of all these special attributes, this simple, but effective, processing technique has been applied to metals, ceramics, polymers, and composite materials The attributes of mechanical alloying are listed in Table 2.1 and some important milestones in the development of the field are presented in table:

1966 Development of ODS nickel-base alloys

1981 Amorphization of intermetallics

1982 Disordering of ordered compounds

1983 Amorphization of blended elemental powder mixtures

1987/88 Synthesis of nanocrystalline phases

1989 Occurrence of displacement reactions

Table 2.1 Important milestones in the development of mechanical alloying

2.2 MILLING

Different types of high-energy milling equipment are used to produce mechanically alloyed powders They differ in their capacity, efficiency of milling and additional arrangements for cooling, heating, etc Some common kinds of milling are: SPEX shaker mills, planetary ball mills, attritor mills In this study we deeply consider to attritor mills

Attritor mills

A conventional ball mill consists of a rotating horizontal drum half-rolled with small steel balls The drum rotates the balls drop on the metal powder that is being ground; the rate of grinding increases with the speed of rotation At high speeds, however, the centrifugal force acting on the steel balls exceeds the force of gravity, and the balls are pinned to the wall of the drum At this point the grinding action stops An attritor (a ball mill capable of generating higher energies) consists

of a vertical drum with a series of impellers inside it Set progressively at right angles to each other, the impellers energize the ball charge, causing powder size reduction because of impact between balls, between balls and container wall, and between balls, agitator shaft, and impellers Some size reduction appears to take place by interparticle collisions and by ball sliding A powerful motor rotates the impellers, which in turn agitate the steel balls in the drum

Attritors are the mills in which large quantities of powder (from about 0.5 to 40kg) can be milled at a time Commercial attritors are available from Union Process, Akron, OH The velocity of the grinding medium is much lower (about 0.5 m/s) than in Fritsch or SPEX mills and consequently the energy of the attritors

is low Attritors of different sizes and capacities are available The grinding tanks

or containers are available either in stainless steel or stainless steel coated inside with alumina, silicon carbide, silicon nitride, zirconia, rubber, andpolyurethane A

variety of grinding media also is vailable such as: glass, flint stones,steatite ceramic, mullite, silicon carbide, silicon nitride, sialon, alumina, zirconium silicate, zirconia, stainless steel, carbon steel, chrome steel, and tungsten carbide

Figure 2.1 (a) Model 1-S attritor

The operation of an attritor is simple The powder to be milled is placed in a stationary tank with the grinding media This mixture is then agitated by a shaft

with arms, rotating at a high speed of about 250 rpm (Figure 2.1 (b)) This causes

the media to exert both shearing and impact forces on the material The laboratory attritor works up to 10 times faster than conventional ball mills

Figure 2.1 (b) Arrangement of rotating arms on a shaft in the attrition ball mill Courtesy of

Union Process, Akron, OH.

2.3 MECHANISM OF ALLOYING

During high-energy milling the powder particles are repeatedly flattened, cold-welded, fractured and re-welded Whenever two steel balls collide, some amount of powder is trapped in between them Typically, around 1000 particles with an aggregate weight of about 0.2 mg are trapped during each collision (Figure 2.2) The force of the impact plastically deforms the powder particles leading to work hardening and fracture The new surfaces created enable the particles to weld together and this leads to an increase in particle size Since in the early stages of milling, the particles are soft (if we are using either ductile-ductile or ductile-brittle material combination), their tendency to weld together and form large particles is high A broad range of particle sizes develops, with some as large as three times bigger than the starting particles The composite particles at this stage have a characteristic layered structure consisting of various combinations of the starting constituents With continued deformation, the particles get work hardened