Particle effects on friction and wear of aluminium matrix composites

Transcription

1 JOURNAL OF MATERIALS SCIENCE 30 (1995) Particle effects n frictin and wear f aluminium matrix cmpsites Z. F. ZHANG, L. C. ZHANG*, Y.-W. MAI Centre fr Advanced Materials Technlgy, Department f Mechanical and Mechatrnic Engineering, University f Sydney, Sydney 2006, Australia Particle effects n frictin and wear f 6061 aluminium (6061 AI) reinfrced with silicn carbide (SIC) and alumina (AI203) particles were investigated by means f Vickers micrhardness measurements and scratch tests. Unreinfrced 6061 AI matrix ally was als studied fr cmparisn. T explre the effect f heat treatment, materials subjected t three different heat treatment cnditins, i.e. under-aged, ver-aged and T6, were used. Multiplescratch tests using a diamnd and a steel indenter were als carried ut t simulate real abrasive wear prcesses. Vickers micrhardness measurements indicated that T6 heattreated cmpsites had the highest hardness. Single-scratch tests shwed that the variatin f frictin cefficient was similar t that f Vickers hardness and the peak-aged cmpsites exhibited the best wear resistance. The wear rate f fine particle-reinfrced cmpsites was mainly affected by hardness. Hwever, thewear rate f large particle-reinfrced cmpsites was influenced by bth the hardness and fracture f the particles. 1. Intrductin The wear behaviur f particle-reinfrced aluminum matrix cmpsites has been investigated extensively in the past 10 years wing t their prmising wear resistance [-1-7]. Previus studies have shwn that abrasive wear resistance f cmpsites strngly depends n their hardness and als relates t the particle vlume fractin and size [1, 2, 6]. Sme authrs studied the ageing effect n wear f these cmpsites and indicated that different ageing cnditins culd result in varied wear behaviur. Fr example, Wang and Rack [-1], Ma et al. [2] and Lin and Liu [6] shwed that peak-aged aluminium cmpsites had the best wear resistance cmpared t ther ageing cnditins when sliding against abrasive papers. On the cntrary, A1- pas and Embury [7] indicated a decreased wear resistance with increasing hardness when sliding ~tgainst metals, implying that the cunterface culd play an imprtant rle. In additin, wear tests n different heat-treated steels shwed that the wear resistance increased slwly with increasing hardness [-8]. Althugh a series f studies has been dne and several wear mdels prpsed [1, 8-11], the understanding f frictin and wear behaviur is far frm cmplete. This wrk aimed t understand, with the aid f hardness and scratch tests, the behayiur f frictin and wear f metal matrix cmpsites, in terms f particle sizes and ageing cnditins. 2. Experimental prcedure The materials tested were 6061 A1 matrix ally and its cmpsites reinfrced by 10 and 20 vl% angular shaped SiC and A1203 particles (Duralcan). The average diameters were 1.8 pm fr SiC particles and 4.5 pm (10% reinfrcement) and 8.8 gm (20% reinfrcement) fr A1203 particles. The materials were supplied by Cmalc Pty Ltd, Australia. A cmpsite f 20 vl% spherical alumina with an average diameter f 18.7 pm (Cmral-85) was als studied. The matrix ally cmpsitin f Cmral-85 was different t that f Duralcan. T explre the effect f heat treatment, three cnditins f heat treatment, (1) underaged (1.5 h slutin treatment at 530~ quenching int water fllwed by natural ageing fr 20 h and then artificially aged at 175~ fr 2 h), (2) veraged (artificial ageing at 175~ fr 20 h), and (3)T6 cnditin (artificial ageing at 175~ fr 8 h), were investigated fr all the materials used in this study. As the particle sizes and vlume fractins varied ver a wide range, Vickers micrhardness was measured fr all the materials at a lad f 5N. Wear tests were carried ut n a scratch wear machine. Detailed descriptin f the experimental methds can be fund elsewhere [3]. Fr the present study, a pyramidal diamnd indenter with an apex angle 20 equal t 136 ~ was used in bth single and multiple scratches. The rientatin f the indenter was taken with ne leading plane mving frward during scratching. A steel cne indenter with the same apex angle f 136 ~ was als used fr multiple scratches. A velcity f 6rams -1 was used ver a wear track abut 6 mm and the applied lad was ION. * Authr t whm all crrespndence shuld be addressed Chapman & Hall 5999

2 150 'E 145 ~ 140 e ~ 130 f 125 ~, ~ ~ Indentatin lad ( N ) Figure 1 Dependence f Vickers hardness n the indentatin lad fr 20% A1203-A1 cmpsites: (0) spherical, (A) angular. 3. Results and discussin 3.1. Vickers micrhardness tests It is well knwn that the hardness f a material is a majr wear parameter in Archard's wear equatin [12]. Fr a particle-reinfrced cmpsite, hwever, the micrhardness may vary ver a wide range, because the indentr may be placed at either the matrix area r the area f a cluster f particles, depending n the particle size and vlume fractin, and the magnitude f the indentatin lad. T explre the hardness and particle effects n the frictin and wear, different hardness values were achieved by different heat treatments. Because f the large particle sizes, the Vickers hardness f A1203-A1 cmpsites increases with increasing indentatin lad, as shwn in Fig. 1. The hardness f a particle-reinfrced cmpsite is usually determined by averaging a series f measured hardness values under an apprpriate indentatin lad. In general, the Vickers hardness, H, depends n the matrix hardness, Hm, the particle hardness, H v, particle vlume fractin, fv, particle size, d, the indentatin lad, P, and ageing cnditin. Thus, the Vickers hardness is a functin f these parameters, i.e. H = f(hm, fv, d, Hv, P, g) (1) where g is a factr reflecting the effect f ageing cnditins. Based n the results f the hardness test, see Table I and Fig. 1, a practical nn-dimensinal equatin fr the present particle-reinfrced cmpsites can be expressed as _s ~-ro.4 ~0.1 = l + yj~ n v r (2) where H = H/Hm, fftp = Hp/Hm, P = P/(Hmd2), and g is 0.77, 0.58 and 0.50 fr Duralcan cmpsites at T6, veraged and underaged cnditins, respectively. The hardness f SiC is 2480 kgfmm -z and that f A1203 is 2100 kgfmm -2. The Vickers matrix hardness values at T6, ver-aged and under-aged cnditins are 120, 108 and 102 kgfmm -2, respectively, at the same indentatin lad f 5 N. It shuld be nted that the Vickers hardness f SiC-A1 cmpsites is almst independent f indentatin lad, because f the reinfrced SiC particles are s fine (less than 2 gm) that the indenting area always cvers several particles and mre r less the same hardness value can be btained. Therefre, an indentatin lad f 5 N is used fr this kind f fine particle-reinfrced cmpsites. Equatin 2 shws that the hardness f the cmpsites with harder particles is larger than the matrix hardness, Hm, and increases with particle vlume fractin but decreases with particle size. The harder the particle, the higher the cmpsite hardness. It is clear frm the Vickers hardness test that ageing influences prperties f the cmpsites. The T6 cnditin generally resulted in the highest hardness because f precipitate hardening cmpared t the ther ageing cnditins. An ver-aged cmpsite has a higher hardness than an under-aged ne. On the ther hand, it is indicated that the differential thermal cntractins f the matrix and reinfrcements enhanced the interracial bnding and caused a higher dislcatin density f the matrix [9]. In additin, variatin in the type, shape, size and vlume fractin f the particle reinfrcements requires different peak-aged time. This is due t the different precipitatin speeds f the hardening phases f different particles at the same ageing temperature. Equatin 2 plays an imprtant rle in ur wear rate analysis belw Frictin cefficient Single-scratch tests In single scratch tests, the average frictin cefficient f five different materials under the T6 cnditin increased with increasing hardness as shwn in Fig. 2 and Table II. It is clear that the hardness influences frictin significantly. Fig. 3 shws the frictin cefficients f 10% and 20% angular A12Oa-A1 cmpsites under three ageing cnditins. When ther factrs, such as particle size and vlume fractin, are unchanged, the increase f hardness by different heat treatments results in increasing frictin cefficient. This is cnsistent with the frictinal behaviur f mst metals. Hwever, the values f the frictin cefficients f these cmpsites are larger than thse f metals predicted by Kpalinsky and Oxley [10] using slipline field thery because f different micrstructures due t varius particles and ageing cnditins. Zhang et al. [ 11] indicated that the frictin TABLE I Vickers micrhardness f the materials under three ageing cnditins at an indentatin lad f 5 N Materials 6061 A1 ally 6061 A1 +10% SiC 6061 A1 +20% SiC 6061 A1 +10% AlzO A1 +20% A1203 Cmral-85 UA 102 -t _ _+3 OA 108 _ _ _+3 T _ _ _

3 go " g '~ 0.4 U I i I I Vickers hardness ( kgf mm -z ) Figure 2 Variatin f frictin cefficient with the Vickers hardness f five materials under T6 cnditins, at a nrmal lad f 10 N. cefficient was cmpsed f three terms: plughing, adhesin and fracture f the particles. A frictinal mdel was prpsed t explain their relatinships. Enhanced shear strength f the cmpsites by reinfrcements is ne f the reasns fr the resistance t scratching by the indentr. It can als be seen that a higher particle vlume fractin leads t a higher hardness (see als Equatin 2) and thus results in a larger frictin cefficient. The particle size and type (with different hardness), n the ne hand, influence the cmpsite hardness as can be seen frm Equatin 2, in which the cntributin f particle size d t/4 is less imprtant than Hp, and, n the ther hand, affect the frictin cefficient as indicated by Zhang et al. [11]. Hwever, their relatinship is nt direct. Fig. 4 shws that fluctuatins existed althugh the mean frictin cefficient was independent f sliding distance. Scratches n the ther cmpsites exhibited a similar behaviur. Such fluctuatins were als bserved in 6061 A1 ally, in which the grve width was unifrm alng the scratch. It is therefre reasnable t cnclude that the fluctuatins were mainly due t the natural stick-slip characteristics f frictin, but nt the particle reinfrcements Multiple-scratch tests An actual wear prcess is s cmplex that nly a single-scratch study is far frm sufficient [9, 11, 13-16]. Therefre, multiple-pass scratches were arranged and carried ut up t 10 passes by using a hard diamnd indentr and a steel indentr with a Vickers hardness f 450 kgfmm-< A similar ~E O 0.s t- O 4-,.r Under-aged Over-aged Ageing cnditin Figure 3 Frictin cefficients f angular AlzO3-A1 cmpsites under three ageing cnditins. [] 10% A1203; [] 20% A1203. O / T6 [ i i I i Sliding distance ( mrn ) Figure4 Relatinship between frictin cefficient and sliding distance in single-scratch tests n 10% SiC-A1 cmpsite with a diamnd indentr. frictinal behaviur was fund in bth cases. The frictin cefficient was independent f pass number as shwn in Fig. 5. Because f the reciprcating mtin f the indentr, the frictinal frces changed directins whenever the indentr mved backwards. Clearly, there was n nticeable difference f frictin cefficient between single and multiple scratches. This implies that the three terms are still invlved in the frictinal prcess. Nevertheless, the prprtin f each term is nt necessarily the same as that in a single scratch. The adhesin term is expected t increase, and the plughing term t decrease. Hwever, further TABLE II Particle size, Vickers hardness, frictin cefficient and measured wear rate f the materials, at a lad f 10 N under T6 cnditin Materials Diameter (gm) Hardness, Hv (kgfmm-2) Frictin cefficient Wear rate (10-6 mm 2) 6061 A t _ _10 10% SiC _ _ % SiC % AlzO _ % A _+ 15 Cmral _ t _

4 1.0 ~, 0.5 c._e E 0 0 r 0.m '5 " J 1.0 Pass number Figure 5 Variatin f frictin cefficient with pass number in multiple-scratch tests n 20% angular AlzO3-A1 cmpsite, at a nrmal lad f 6 N using a diamnd indentr. cmplex tests (e.g. in the abrasive regimes in a pin-ndisc wear test) presented a much lwer frictin cefficient [17], indicating that the prprtin f three terms changed r ther factrs may be invlved in a real wear system, fr example, the interactin f adjacent grves, real cntact numbers, varius attack angles f the abrasives and the natural undulating surface. Therefre, the establishment f a quantitative relatinship between the simple scratch tests and the cmplex pin-n-disc experiments needs further investigatin. Figure 6 Grve tpgraphies f (a) 20% angular A1203-A1 cmpsite, and (b) 20% SiC-A1 cmpsite. 3.3 Wear rate Single scratch In single-scratch tests, the grve size was cnstant with sliding distance in the tested lad range. Therefre, wear rate, w, is defined as vlumetric wear per sliding distance, i.e. the crss-sectinal area f the grve measured by a laser cnfcal micrscpe. Under a nrmal lad f 10 N, the material underneath the indentr is mainly cut r plastically defrmed and there is little elastic defrmatin. The characteristics f the material alng the scratch grve shuld be cnsidered relative t a real wear situatin. Fr example, are these lsened material r are they merely plastically defrmed ridges? And hw wuld the particles in the ridges behave? The wear rate in a single scratch des nt necessarily describe the wear rate in a real applicatin, because f its simplicity and idealizatin. Nevertheless, it prvides sme fundamental evidence fr the abrasive wear phenmenn. Because f the differences in particle size and particle vlume fractin f different cmpsites, the tpgraphy f the grves is different. Fig. 6a shws a rugh grve ridge and wrn surface f the cmpsite reinfrced with 20 vl% angular alumina particles with an average diameter f 8.8 gin. In this case, the particles beneath the indentr were fractured t debris which culd nt bear any lad in the ensuing passes. In cntrast, a very smth and unifrm grve n the surface f the cmpsite reinfrced with 20 vl % SiC particles with an average diameter f 1.8 ~m was bserved in Fig. 6b. Particle fracture is caused by the shear stress and lateral tensile stress applied n the particle. The crack is always at a certain angle with respect t the sliding directin, and a number f parallel cracks in each fractured particle can be seen in Fig. 6a. The smth tpgraphy f SiC-reinfrced cmpsites in Fig. 6b is similar t that f 6061 A1 ally; this is because these cmpsites cntain such fine reinfrced particles that the pssibility f fracturing and debnding them is very small. The wear vlume f this type f cmpsite and unreinfrced matrix is mainly determined by plastic defrmatin and cutting. Therefre, th e hardness f the material is a dminant parameter affecting the wear rate, which is cnsistent with the bservatin by Rabinwicz [8] and Archard [12]. Archard's wear law is valid fr cmpsites with fine particles and gives the frm PL V = k --ff (3) where V is vlumetric wear, k is a s-called wear cefficient, and L the sliding distance. Cmbining Equatins 2 and 3, the vlumetric wear can be btained as PL V = khm(l+~j~ _*"1.8 ~p f-r0.4 r ~0.1"~ (4) It is clearly seen hw these parameters influence the vlumetric wear f the cmpsites. Fr large particle-reinfrced cmpsites, as discussed abve, besides the hardness effect, particle fracture als plays an imprtant rle in determining the 6002

5 2e P P ~ F d "=I "vge ee e~/ t-e 9 eye ee ~. ~1 h <1 h >1 d ~'- Figure 9 Illustratin f material remval when the rati h/d varies. Figure 7 Schematic illustratin f vlumetric wear f the cmpsite reinfrced with spherical alumina particles EE ' O0 100 I I I I I Pass number Figure 8 Wear f the steel indentr by a 20% angular A1203-A1 cmpsite after ne pass. wear rate. Fig. 7 shws schematically the scratched grve with plughing, cutting r plastic defrmatin and fracture f large particles. The particle effect embraces the fllwing aspects. 1. Increase f particle vlume fractin results in higher hardness thus leading t higher frictin cefficient and less wear rate 2. Large particle can resist the plastic strain efficiently and can reduce wear in cyclic sliding. 3. Particles in the matrix change the stress state under the sliding cnditins and hence influence wear significantly. 4. Particles can be fractured by scratching with either the diamnd r steel indentr. The fractured particle can be develped t a greater area with repeat sliding in the pin-n-disc test [17]. 5. The rubbing pair can be wrn ut if its hardness is lwer than that f the ceramic particle, see Fig. 8. Mrever, the difference f grve edges between cmpsites with fine and carse particles wuld result in different wear behaviur in real wear prcesses. It fllws frm the abve discussin that fr all the cmpsites studied, cutting r plastic defrmatin is always a dminant factr. Hwever, particle fracture plays a significant rle fr cmpsites with large particles. In ther wrds, material hardness wuld mst likely cntrl the wear rate f cmpsites reinfrced with fine particle, but this wuld be altered t a great extent by particle fracture when the particles are relatively large. On the ther hand, a rati f the penetratin depth t particle size is a very imprtant para- Figure 10 Variatin f wear rate f a 20% spherical A1203-A1 cmpsite with pass number. meter. When the rati is less than unity, materials are difficult t be remved by the indentr, but if it is larger than unity, materials tend t be remved mre easily. This is schematically illustrated in Fig. 9. Mrever, a higher vlume fractin f the large reinfrcement will increase the weight f particle fracture and, in turn, lead t a different wear behaviur. Table II demnstrates in detail the crrelatins between particle reinfrcement, Vickers hardness, frictin cefficient and the measured wear rates f 6061 A1 ally, SiC-A1 cmpsites and A1203-A1 cmpsites under the T6 cnditin Multiple scratches T explre crrelatin between scratch test and system wear, multiple scratches were als cnducted. Kplinsky and Oxley [-10] and Jhnsn [18] indicated that it is the accumulatin f large plastic strain under cyclic sliding that causes the frmatin f wear particles. Hwever, relatively large particles in the cmpsites wuld change the stress state under the sliding surface. The larger the particle size, the mre resistant t plastic strain and the lwer the wear rate shuld be. On the ther hand, the particles culd als wear ut the cunterface such as steel. In this-part f the study, a steel indentr with a Vickers hardness f 450 kgfmm -2 was als used t cmpare with a diamnd indentr. The wear rate f the cmpsites in multiple scratches by a diamnd indentr differed t a great extent frm that by a steel indentr, as indicated by the experimental results shwn in Fig. 10. The increase 6003

6 in the cmpsite wear rate by the diamnd indentr is mre prnunced with pass number than that by the steel indentr. This is due t the cntinuus wear f the steel indentr with increasing sliding distance. The wear rate f cmpsites with large particles appeared t be smaller than that with smaller particles. In additin, cmpsites at different ageing cnditins have similar wear behaviur. It is apparent that the mechanical prperties f the indentr materials play an imprtant rle. Because f the lwer hardness f steel relative t the ceramic particles, the steel indentr was wrn ut in the first pass f scratches, Fig. 8. On the cntrary, almst n wear was fund n the hard diamnd indentr even after 10 passes. The steel indentr becmes blunter with increasing pass number and thus its penetratin and cutting ability was weakened. This result can be used t explain the cnsiderable steel wear in a pin-n-disc test. Furthermre, multiple scratches by the steel indentr prduced similar surface features and wear debris t thse prduced by pin-n-disc tests in the regime f abrasive wear [17]. Therefre, the relatinship between scratch and pin-n-disc tests may be established after further systematic investigatins. 4. Cnclusins The micrhardness, frictin cefficient and wear rate f MMCs with different heat-treatment cnditins have been investigated. Based n the experimental bservatins, the fllwing cnclusins can be drawn. 1. The hardness f the cmpsites can be expressed in a nn-dimensinal frm /J 1 + YJv ~v r 2. Increase f hardness by larger vlume fractin f particles r different ageing cnditins increases the frictin cefficient and reduces the wear rate. Cmpared t ther ageing cnditins, peak-aged cmpsites have the best wear resistance but the largest frictin cefficient. 3. The wear rate f a cmpsite with fine particles is mainly affected by its hardness, but that f large particle-reinfrced cmpsites depends n the rati f the penetratin depth t the particle size. 4. Cmpsites with large particles cause remarkable steel wear in a single scratch, thus large particles wuld accelerate the steel wear in a wear system. Acknwledgements The authrs thank the Australian Research Cuncil fr the cntinuing supprt f this wrk, and Cmalc Research Centre in Thmastwn, Victria fr supplying the MMC materials fr testing. The Electrn Micrscpy Unit f the University f Sydney prvided access t its facilities. Z. F. Zhang is supprted by an EMSS schlarship. References 1. A.G. WANG and H. J. RACK, Wear 146 (1991) Z.Y. MA, J. BI, Y.X. LIU, H.W. SHENandY. X. GAO, ibid. 148 (1991) Z.F. ZHANG, Y. X. CHEN, A. K. MUKHOPADHYAY and Y.-W. MAI, in MMC-3, "Prceedings f the 3rd Australian Frum n Metal Matrix Cmpsites", edited by S. Bandypadhyay and A. G. Crsky (University f New Suth Wales, Sydney, 1992) pp C. BADINI, F. MARINO and A. TOMASI, Mater. Sei. Eng. A 136 (1991) H.L. LEE, W. H. LIEU and S. L. CHAN, Wear 159 (1992) S.J. LIN and K. S. LIU, ibid. 121 (1988) A. T. ALPAS and J. D. EMBURY, "Wear f materials" (ASME, New Yrk, 1985) pp E. RABINOWICZ, "Frictin and wear f materials" (Wiley, New Yrk, 1965). 9. K.H. ZUM GAHR, "Micrstructure and wear f materials", Triblgy Series 10 (Elsevier, Amsterdam, 1987). 10. E.M. KOPALINSKY and P. L. B. OXLEY, in "Prceedings f the 4th Internatinal Triblgy Cnference", AUSTRIB'94, edited by G. W. Stachwiak, Vl. 1 (UNIPRINT, University f Western Australia, Perth, 1994) pp Z. H. zhang, L. C. ZHANG and Y-W. MAI, Wear 176 (1994) J.F. ARCHARD, J. Appl. Phys. 24 (1953) A.J. SEDRIKS and T. O. MULHEARN, Wear 6 (1963) C. A. BROOKES, P. GREEN, P. H. HARRISON and B. MOXLEY, J. Phys. D Appl. Phys. 5 (1972) J.A. WILLIAMS and Y. XIE, Wear 155 (1992) B.R. LAWN, ibid. 33 (1975) Z.F. ZHANG, L. C. ZHANG and Y.-W. MAI, J. Mater. Sci. (1994) in press. 18. K. L. JOHNSON, in "Prceedings f the 4th Internatinal Triblgy Cnference", AUSTRIB'94, edited by G. W. Stachwiak, Vl. 1 (UNIPRINT, University f Western Australia, Perth, 1994) pp Received 12 January and accepted 7 June