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May 2000

.Materials Letters 43 2000 274280

www.elsevier.comrlocatermatlet

Tribological behaviour of SiC particle-reinforced copper matrixcomposites

S.C. Tjong ), K.C. Lau

Department of Physics and Materials Science, City Uniersity of Hong Kong, Tat Chee Aenue, Kowloon, Hong Kong,

Peoples Republic of China

Received 18 June 1999; received in revised form 26 November 1999; accepted 29 November 1999

Abstract

.Pure copper and its composites reinforced with SiC particles were prepared by hot isostatic pressing HIP process. The

tribological behaviour of copper and composites was studied on a pin-on-disc tester. The pins were slid against a hardened

steel disc under dry ambient conditions. In two-body abrasive wear measurements, the disc surface was bonded with a SiC

abrasive paper of 240 grit size. The abrasive wear measurements showed that soft copper exhibits an extremely high wear

loss. However, additions of SiC particles up to 20 vol.% appeared to improve the abrasive wear resistance of copper

significantly under the applied loads of 1555 N. Dry sliding wear tests also indicated that the composite with 20 vol.% SiC

exhibits a lower wear loss compared to pure copper. This was due to the reinforcing SiC particles being effective to reduce

the extent of wear deformation in the subsurface region during sliding. q2000 Elsevier Science B.V. All rights reserved.

Keywords: Wear; Composite, SiC particle; Copper; Hardness; Abrasive

1. Introduction

Materials with high electrical and thermal conduc-

tivities, and high temperature strength have attracted

considerable interest in recent years. Pure copper

exhibits high electrical and thermal conductivities

but it has some distinct shortcomings such as low

hardness, low tensile and creep strengths. The devel-

opment of Cu-based alloys with high tensile strengthand hardness is of primary importance. The mechani-

cal strength of copper can be increased dramatically

)

Corresponding author. Tel.: q852-2788-7702; fax: q852-

2788-7830. .E-mail address: [email protected] S.C. Tjong .

either by age hardening or by introducing dispersoid

particles in its matrix. The age-hardenable copper

alloys are prone to precipitate coarsening at high

temperatures, thereby reducing their strength drasti-

cally. In this respect, dispersion-strengthened copper

has the ability to retain most of its properties on

exposure to high temperatures. Dispersoid particles

such as oxides, carbides, borides are insoluble in the

copper matrix, and are thermally stable at high tem-peratures. The dispersion-strengthened copper alloys

generally can be classified as the copper-based ma-w xtrix composites 19 . Cu-matrix composites are

promising candidates for applications in electrical

sliding contacts such as those in homopolar machinew xand railway overhead current collection system 11 ,

where high thermal electricalrthermal conductivity

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. .P I I : S 0 1 6 7 - 5 7 7 X 9 9 0 0 2 7 3 - 6

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( )S.C. Tjong, K.C. Lau rMaterials Letters 43 2000 274280 275

and good wear resistant properties are needed. .Among various dispersoids, alumina Al O parti-2 3

cle is commonly used to reinforce copper. The Al O2 3oxide dispersion strengthened copper alloys have

been reported to exhibit superior elevated tempera-

ture strength, increased hardness and improved creepw xresistance compared to pure copper 1 3 . Methods

for the production of dispersion-strengthened copper

matrix composites involve ingot casting and powder .metallurgy PM processes. The PM route consists of

several processes like blending, compacting, and sin-

tering. Blending is one of the crucial processes in

PM where the metallic powders are mixed with the

ceramic reinforced particles. Good blending pro-

duces no agglomeration of both the metallic and

ceramic particle powders. To achieve this, several

parameters such as particle size, blending speed and

duration should be taken into consideration. Conven-

tional melting and casting has distinct limitations dueto the poor wettability between ceramic particles and

molten copper, leading to agglomeration of disper-

soids. Moreover, the difference in the densities and

the reinforcements can also cause segregation in thew xmelts 10,11 . Therefore, the PM route is ideal to

prepare copper matrix composites because of itsw xefficient dispersion of fine particles 1 . In the case

of carbide particle reinforcements, little information

is available in literature concerning with the fabrica-

tion and properties of SiC particle reinforced Cu-

w xbased composites. Recently, Pelleg et al. 12 havefabricated SiC fibre reinforced copper matrix com-

posites by means of induction melting and of PM .process followed by hot isostatic pressing HIP .

They reported that a chemical reaction occurs at the

SiC fibreliquid copper interface during induction

melting. On the contrary, a clean interface is ob-

served at the SiC Cu interface of composites pre-

pared by HIP process. It is expected that hot isostati-

cally pressed SiCCu composites exhibit goodw xstrength, electrical and mechanical properties 12 .

Yih and Chung have fabricated Cu composites con-

taining 3354 vol.% SiC whiskers by hot pressing.

The whiskers were coated with copper prior to press-

ing. They reported that the resulting composites ex-

hibit several good properties such as low porosity,

high hardness, low electrical resistivity and highw xthermal conductivity 13 . It is worth-noting that the

above-mentioned studies were focused on the fabri-

cation of SiCCu composites where the SiC rein-

forcements were in the form of fibres and whiskers.

Ductile copper generally exhibits poor wear resis-

tance because of its low hardness. Materials removal

in the form of wear debris are commonly observed

on the copper surface during sliding against a hard-w xened steel counterpart 14 . Wear debris is formed by

the delamination cracks developed near the subsur-

face region of copper due to the localization ofw xplastic strain 14 . Ceramic reinforcements in the

form of fibres and particulates are widely incorpo-

rated in the soft and ductile materials, like alu-

minium, to relieve the extent of wear deformation in

the subsurface region. Consequently, the wear resis-

tance of Al-based metals is improved considerablyw x1517 . As mentioned above, Cu-based composites

show promise for applications in electrical sliding

contacts. It is of practical importance to understand

the wear behaviour of Cu-based composites. To thebest of our knowledge, no information is available in

the literature concerning the wear properties of SiC

particle-reinforced Cu-based composites, especially

the abrasive wear. In this study, we attempt to

fabricate the Cu composites reinforced with SiC

particles by means of the HIP process, and to study

the abrasive as well as dry sliding wear properties of

resulting composites. Particular attention is paid on

the abrasive wear behavior of the SiCCu compos-

ite. The HIP process consisting of sintering the

blended powders under the applications of high pres-sure and high temperature. This process involves the

simultaneous application of a high-pressure usually.inert gas and an elevated temperature in a specially

constructed vessel. The pressure applied is isostatic

because it is developed with a gas, so that no alter-

ation in component geometry occurs. Under these

conditions of heat and pressure, internal pores of thew xmaterials are eliminated 18 . Several workers have

successfully fabricated the Al-, Fe-, Ni-, and Ti-basedw xMMCs using HIP process 1922 .

2. Experimental and results

.Copper powder 99.9 percentage pct purity, Cu,

Pb-2000 ppm, O-2000 ppm ; 50 mm in diame-. ter and SiC particles 99 pct purity; 75 mm in

.diameter were purchased from Goodfellow Cam-

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( )S.C. Tjong, K.C. Lau rMaterials Letters 43 2000 274280276

Fig. 1. Variation of density with SiC content for the CuSiC

composites. The dash line is drawn based on the equation D sc .fD q 1yf D where D , D and D are the density of com-p m c p m

posite, particle and metal matrix, respectively; and f is the

volume fraction of reinforcing particles.

bridge. The composites with 5, 10, 15 and 20 vol.%SiC were fabricated by HIP method. The Cu and SiC

powders were mixed mechanically, and the blended

powders were filled into copper tubes. The tubes

were sealed by welding, and placed into an ABB .HIP equipment model QIH-3 . The samples were

hot isostatically pressed at 8808C and 100 MPa for

1.5 h. The density of copper composites was deter-

mined according to Archimedes method. In this

technique, density is determined by measuring the

difference between the specimens weight in air and

when it was suspended in distilled water at roomtemperature. The results showed that the density of

composites is closed to that predicted from the theo- .retical calculation Fig. 1 , and there is no cavities

observed in the composites after hipping. Moreover,

Fig. 1 reveals that the density of composites tends to

decrease with increasing SiC content. This is due to

the density of SiC particles being much smaller than

that of copper. The yield strength and percentage

Fig. 2. Fractograph of composite with 20 vol.% SiC after tensile

test.

elongation of specimens were determined using an .Instron tensile tester model 4206 . Microhardness of

both pure copper and composites were determinedusing an MXT-CX7 Vickers tester under an applied

load of 10 g. Microhardness tests for composites

were carried out in the matrix phase. The density and

mechanical properties of specimens investigated are

summarized in Table 1. Apparently, the hardness of

copper generally improves considerably with the ad-

ditions of SiC particles at the expense of its ductility.

There is no improvement in the yield strength of

composites associated with the SiC additions. This is

possibly due to a large sized of SiC particles used,

i.e., 75 mm. Fig. 2 shows a typical fractograph of thecomposite with 20 vol.% SiC after tensile tests. The

nature of failure appears to be ductile and intergranu-

lar. It is generally known that the yield strength of

metal-matrix composites is dependent on the size

of particle reinforcement. The yield strength of com-

posites tends to decrease with increasing size of

reinforcing particles. On the contrary, the wear resis-

tance of metal-matrix composites is known to im-

Table 1

Mechanical properties of Cu and SiCCu composites

Specimen Yield strength, Percentage Vickers Microhardness,

MPa elongation, % HV

Cu 154 36 82"2

Cu5 vol.% SiC 113 19 99"4

Cu10 vol.% SiC 108 12 113"2

Cu15 vol.% SiC 98 7.9 100"5

Cu20 vol.% SiC 97 6.5 101"5

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prove considerably with increasing the size of rein-

forcing particles. As will be discussed later, the

abrasive wear resistance of copper improves dramati-

cally by reinforcing with SiC particles of 75 mm, and

the abrasive wear resistance of CuSiC composites

increases markedly with increasing SiC content. In

this context, a compromise can be reached in the

yield strength and wear resistance of CuSiC com-

posites by properly controlling the size of reinforcingw xparticles. Indeed, Tjong and Lau 23 reported that

the yield strength of Cu TiB composites increases2with increasing TiB content due to the size of TiB2 2reinforcing particles is smaller, i.e., 45 mm.

Cylindrical pin specimens 5 mm in diameter and

15 mm in length were prepared from the rods pro-

duced by HIP. They were used for the abrasive and

dry sliding wear measurements. Two-body abrasive

wear tests were performed with a pin-on-disc tester

.Plint Tribology model TE 67 . The pin was loadedagainst a rotating disc, which carried a bonded abra-

.sive SiC paper of 240 grit ;60 mm . The applied

normal loads used were 15, 35 and 55 N. The sliding

velocity employed was 1 m sy1. To ensure fresh

supply of abrasive particles to the pins, the worn SiC

abrasive paper was replaced with the new one for

every sliding distance of 20 m. The weight loss of

the pin was measured at various intervals in an

analytical balance of 0.0001=g precision. The

weight loss was converted to volume loss values.

Dry sliding wear measurements were performedon pure copper and Cu20 vol.% SiC composite

using the pin-on-disc tester. The pins were slid

against a hardened steel disc with a hardness of HRC

60. The disc was rotated at 1 m sy1, and the normal

load varied from 15 to 55 N. The worn surfaces of

pins subjected to both abrasive and sliding wear tests

were observed in a scanning electron microscope .SEM .

3. Discussion

We first consider the results of abrasive wear

measurements. Fig. 3 shows the variation of volume

loss with sliding distance for pure copper and com-

posite specimens tested under an applied load of

15N, and a sliding velocity of 1 m sy1. Pure copper

exhibits an extremely high weight loss during abra-

Fig. 3. Variation of volume loss with sliding distance for all

specimens tested under an applied load of 15 N and a sliding

velocity of 1 m sy1 .

sive sliding as expected. And an increase in wear

volume with increasing sliding distance is observed.Moreover, the addition of only 5 vol.% SiC particle

appears to reduce its volume loss remarkably due to

the incorporation of SiC particles leads to an in- .crease in the hardness of specimen Table 1 . Further

increasing SiC content results in a considerable re-

duction in volume loss, especially for composite

containing 20 vol.% SiC. Fig. 4 shows the variation

of abrasive wear resistance with SiC content for the

specimens tested at different applied normal loads.

The abrasive wear resistance is defined as the in-

verse of volume loss. Evidently, the abrasive wearresistance of copper composites increases with in-

creasing SiC volume content, particularly at low

applied normal load of 15 N. This is because SiC

abrasives can penetrate easily to soft copper during

sliding, resulting in excessive material removal from

the worn surface of pure copper. However, the mate-

rial removal is reduced markedly in composites due

to the SiC reinforcing particles can resist the micro-

cutting action of abrasives.

Wear behaviour of materials is a complicated

phenomenon due to many variables such as sliding

parameters, materials properties, abrasive effects and

lubricating conditions, etc., governing it. For two-w xbody abrasive wear, Rabinowicz 24 has attempted

to relate the volume loss with the hardness of materi-

als and operational parameters. The volume loss is

considered to be resulted from the removal of mate-

rial chips from the specimen due to the microcutting

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Fig. 4. Variation of abrasive wear resistance with SiC content for

all specimens tested under an applied load of 15 N, a sliding

velocity of 1 m sy1 , and a sliding distance of 40 m.

of abrasive particles. Mathematically, it is expressedas follows,

Vs kPLrH 1 .

where V is the volume loss, P the applied load, L

the sliding distance, H the hardness of specimen and .k the wear coefficient. Eq. 1 indicates that the

volume loss of a material is inversely proportional to

its hardness. Hence, the composites with higher SiC .contents exhibit a better wear resistance Fig. 3 due

to SiC additions improve the hardness of specimens

as indicated in Table 1. .Fig. 5 a shows the longitudinal cross-section

SEM micrograph of the composite with 20 vol.%

SiC after sliding against the disc bonded with SiC

abrasive paper under an applied load of 15 N, a

sliding velocity of 1 m sy1, and a sliding distance of

40 m. Some SiC particles are exposed to the worn

surface of this composite specimen. The reason for

this is explained as follows. When the surface of

composite initially comes in contact with SiC abra-

sive paper, adhesive contact occurs. The SiC abra-

sive particles with sharp edges then cause mi-

croploughing and grooving in the surface of copper

matrix. Therefore, materials in the form of chips are

removed from the grooves, thereby exposing SiC

particles. In this case, the SiC abrasives come in

direct contact and slide against with the SiC reinforc-

ing particles. The larger size of SiC reinforcing

particles can offer protection to the Cu matrix during

sliding. Once the reinforcing SiC particles fracture or

loosen from the copper matrix, they can be removed

easily from the matrix, resulting in a certain amount ..of material loss Fig. 5 b .

Finally, we discuss the dry sliding wear behaviour .of pure copper and composite specimens. Fig. 6 a

.and b summarize the results of dry sliding wear

measurements for pure copper and composite with .20 vol.% SiC. Fig. 6 a clearly indicates that the

composite specimen experiences a lower wear loss

compared to pure copper. It can be seen from Fig. .6 b that increasing sliding velocity leads to a further

reduction in volume wear. In the case of pure cop-

per, the asperities of the counterpart steel disc can

deform, penetrate and cut into the copper surface

during dry sliding wear. This is because pure copper

is much softer than the steel counterpart. This results

.Fig. 5. a SEM longitudinal cross-section micrograph of the

composite with 20 vol.% SiC after abrasion wear tests. The

composite was subjected to an applied load of 15 N, a slidingy1 .velocity of 1 m s , and a sliding distance of 40 m; b other

region of composite tested under similar conditions.

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. .Fig. 6. a Volume loss vs. sliding distance and b volume loss

vs. applied load for pure copper and composite with 20 vol.% SiC

subjected to dry sliding wear tests.

in plastic strain localization in the subsurface region,leading to the formation of delamination cracks Fig.

Fig. 7. SEM plan micrograph showing the formation of delamina-

tion surface cracks in copper after sliding against a hardened disc

at an applied load of 35 N, sliding velocity of 1 m s y1 and a

sliding distance of 2500 m.

Fig. 8. Vickers microhardness depth profiles for pure copper and

composite with 20 vol.% SiC after sliding against a hardened disc

at an applied load of 35 N, sliding velocity of 1 m s y1 and a

sliding distance of 2500 m.

.7 . The excessive delamination of surface layers of

copper leads to a high wear loss, which increases

with increasing the sliding distance as shown in Fig. .6 a . Addition of 20 vol.% SiC particles to copper

matrix considerably increased the hardness of com-

posite and resulted in a reduction of the extent of

plastic deformation of matrix. In this case, the wear

loss of the composite is reduced considerably. Fig. 8

shows the microhardrness depth profiles of the worn

surfaces of pure copper and composite with 20 vol.%

SiC after sliding against a hardened disc at a slidingvelocity of 1 m sy1 , an applied normal load of 35 N

and a sliding distance of 2500 m. Apparently, pure

copper shows the evidence of work-hardening due to

plastic strain localization in the subsurface region on

sliding against a hardened disc. Thus, the microhard-

ness of subsurface region of pure copper is higher

than the underlying material. It is noticed that the

microhardness of copper matrix of the composite

near the subsurface region is reduced considerably

due to the SiC addition. This is because the incorpo-

ration of SiC particles to copper is very effective in

reducing the extent of strain localization in the sub-w xsurface region. More recently, Tjong and Lau 23

also reported that the copper composites reinforced

with TiB particles also exhibit superior wear resis-2tance compared to pure copper. And the wear resis-

tance of CuTiB composites is comparable to that2of CuSiC composites in this study.

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