Friction and wear of aluminium-silicon alloys

Friction and wear of aluminium-silicon alloys

157 Wear, 61(1980) 157 - 167 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands FRICTION AND WEAR OF ALUMINIUM-SILICON ALLOYS A. D. SA...

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157

Wear, 61(1980) 157 - 167 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

FRICTION AND WEAR OF ALUMINIUM-SILICON

ALLOYS

A. D. SARKAR and J. CLARKE Department of Metallurgy, Manchester Polytechnic, (Gt. Britain)

Chester Street, Manchester MI 5GD

(Received July 17,1979)

summary The friction and wear of aluminium-silicon alloys have been studied using a pin-on-disk machine together with metallographic investigations, microhardness testing and scanning electron microscopy. Silicon content does not appear to be a dominant factor in wear resistance. The eutectic alloy was the most wear resistant of the alloys studied. The predominant wear mode appears to be by separation of all or part of the work-hardened layers from random patches on the wear track. Nucleation sites for wear debris are probably created below the sliding surface as a result of Hertzian stressing.

1. Introduction Al-Si alloys find applications in such tribological components as clutches, cylinder liners and pistons in the automotive industry. A well-tried material used for pistons is the hypoeutectic age-hardening material. The requirements of a satisfactory piston material are exacting, but two main advantages of an aluminium base alloy are its relative lightness and good thermal conductivity. Silicon, in addition to reducing the coefficient of thermal expansion of the aluminium, produces an alloy with good casting, machining and corrosion characteristics. A number of alloys are in use but these can be described collectively as complex Al-Si alloys of neareutectic composition containing additions of copper, nickel and magnesium.

2. Application and previous experiments Favourable tribological properties combined with a low coefficient of thermal expansion have resulted in a fairly widespread use of hypereutectic Al-Si alloys in such applications as cylinder liners, cylinder blocks, pistons, brake drums and pulleys. A particularly good example is the Vega automobile engine [1] where a 16 - 18% silicon alloy is used to die-cast a

158

cylinder block which requires no steel or cast iron liner in the cylinder bore. The alloy shows good wear resistance, which is attributed to the presence of primary silicon particles distributed throughout the aluminium matrix. The cast cylinder bores are machined, honed and then etched by electrochemical machining which removes the aluminium phase but not the silicon particles so that the latter stand proud of the matrix, Thus only the hard silicon phase comes into sliding contact with the counterface, the effect being to provide a scuff- and wear-resistant surface. Ad~tion~ly, the depressions left by the lost aluminium phase act as emergency reservoirs for liquid lubricants in a manner analogous to that of cast iron where the cavities left by the lost graphite flakes hold the oil under boundary lubrication. The hypereutectic alloy is difficult to cast but the large quantity of the hard silicon phase resists wear as it is assumed that the harder a surface, the better are its tribological properties. The bulk hardness of an aluminium alloy may not increase with the amount of silicon in the alloy, but the interfacial interaction responsible for wear takes place at asperity level. In that case it is a fair speculation that individual microconstituents may influence wear rather than the bulk hardness as measured by, say, the Vickers hardness test, If the hardness of the mi~roconstituent is a criterion, a large amount of silicon should impart wear resistance to the alloy provided the silicon particles are well distributed. The hardness of the silicon phase is about 1450 HV but laboratory studies do not confirm the beneficial effect expected of well-distributed silicon particles in hypereu~ctic alloys. To date there is no confirmation of the positive role of modification in Al-Si alloys from the point of view of wear resistance. Okabayashi and Kawamoto [ 21 carried out unidirectional sliding experiments on an alloy containing 21.6% silicon running against steel and itself respectively. In both cases the size of the primary silicon particles was varied by modifying the melt with phosphor copper during the casting stage of the wear samples. The important conclusion from their experiments was that increasing the silicon particle size resulted in a slight increase in the tensile strength but the wear rate was not influenced. Colligan [3] supports this conclusion, and Stonebrook [4] from his observations on clutches, cylinder liners and pistons in service has stated that it is the quantity rather than the size of the silicon particles which controls the wear rate, a high silicon content being desirable. In contrast work by Vandelli [ 51 on three different hypereutectic alloys with silicon contents of 14.5%, 17% and 25% respectively showed that when each alloy was slid under reciprocating conditions on cast iron the alloy with the intermediate amount of silicon was the most wear resistant. He attributed this to the fine particle size of the silicon phase uniformly distributed in the matrix. Sarkar [6] has studied two commercial alloys after age-hardening ~eatmen~ and found that the near-eutectic alloy was more wear resistant than the material containing about 20% silicon. The controversy regarding the role of silicon remains, but it should be noted that the difference in wear rates between the hypoeutectic and hypereutectic alloys was small, although consistent 161. The friction and wear modes of

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binary Al-Si alloys with a range of silicon contents have been examined and some of the results are reported.

3. Friction Running aluminium alloy pins on steel produces very large fluctuations in the values of frictional resistance. In the current experiments most of the traces showed an initial rise followed by a fall and then an increase in the value of the mean coefficient of friction (Fig. 1). The mean frictional resistance then settled to an almost steady value. It is difficult to explain the initial rise and fall shown in Fig. 1 but a further study of the variation of friction with time of these alloys may elucidate mechanisms of sliding interaction. However, the present rather detailed experimentation highlights certain points regarding the frictional resistances of various binary Al-Si alloys.

15

30

15

Sliding distance

60

( x 10’

75

90

cm)

Fig. 1. Coefficient of friction of an Al-13%Si alloy sliding against hard steel: load, 2 kgf; pin diameter, 6.25 mm; sliding speed, 200 cm s-l.

A typical friction trace shows fluctuations to various degrees as sliding continues and all that can be done is to obtain an average coefficient of friction for a particular alloy at chosen loads and speeds. It does not follow that a wildly fluctuating friction trace will give a low average friction. For example a 6.3% silicon alloy showed very rapid changes of friction with time but the average coefficient of friction was 0.34. In contrast, a 13% silicon alloy showed a high coefficient of friction of about 0.6 although the sliding was quiescent, producing a smooth friction trace. Another interesting observation was that a high degree of surface damage did not necessarily mean a high coefficient of friction. For example at a load of about 2 kgf on a pin of diameter 6.25 mm running on a steel counterface a 1.8% silicon alloy scuffed whereas a 13% silicon alloy did not, but both gave a coefficient of friction of 0.5. These observations create anomalies in the theories of friction which postulate that as the opposing asperities make contact and form cold welds resistance to shear is experienced. It is intuitively correct to think that the more consolidated the cold welds, the greater the surface damage due to tangential traction. Strong welds should also mean a high value of friction.

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4. Wear The quasi-empirical laws of wear suggest that the rate of wear is given by $( W/H) where W is the applied normal load and H is the hardness of the metal. Unfortunately experiments [6, 71 show that the wear rate of these alloys does not show a linear variation with load but follows a curvilinear pattern. There is no general agreement on the role of silicon in wear but it seems clear [6] that a hypereutectic alloy wears more than a hypoeutectic alloy under conditions of dry sliding. The difference in the wear rates between these alloys, although consistent, is not great, but by plotting [ 71 isowear lines at various combinations of load and silicon content for a particular wear rate it is shown that the maximum load-bearing capacity is obtained with a hypoeutectic alloy. This is fortuitous because a hypoeutectic alloy has good castability.

5. Counterface The first effect of running an aluminium alloy pin on a steel or cast iron counterface is a deposit of a layer of the former onto the latter. If small bushes are used on a pin-bush machine where the bush is the counterface of cast iron, the cast iron initially picks up enough aluminium alloy to be detectable by a weighing balance but a linear loss of weight is soon established with continued sliding (Fig. 2). When Fig. 2 is compared with Fig. 3 which

4

I

I 0.2

0.4

Sliding

distance I [email protected]

0.6

0.6

cm)

Sliding

distance

[email protected]‘cml

Fig. 2. Weight loss/gain of a grey iron bush sliding against aged Al-ll%Si alloy pins: pin 3.6; V,4.1. diameter, 6.26 mm; speed, 196 cm s-l. Loads (kgf) 0,0.6; *, 1.6; q,2.6;*, Fig. 3. Weight loss with sliding distance of aged hypoeutectic Al-Si alloy pins at various loads: pin diameter, 6.26 mm; grey iron counterface; speed, 196 cm s-l. Loads (kgf): 0, 0.6;A,

1.6;

0, 2.6;*,

3.6;

V, 4.1.

161

shows the weight loss of the aluminium alloy pins, it is seen that the bushes start wearing approximately at the same time as the pins begin the regime of steady state wear. When the load is unrealistically high the pin wears out rapidly before it reaches the steady state. The bush still wears and does show a linear pattern with sliding distance. Work with steel bushes seems to show that at realistically light loads of, say, up to about 1.5 kgf a high silicon alloy is associated with a high wear rate of the counterface. If the amount of interfacial deformation is excessive, silicon is ineffective as shown in Fig. 4 where the wear of the bush appears to be independent of the silicon content of the pin and is decided by load only. The unresolved question is: How does the iron or steel counterface wear? Figure 5 shows the wear rate of both an as-cast Al-ll.l%Si alloy and a steel bush of hardness 900 HV. After a running-in period the pin shows a steady state wear. During the running-in stage of the pin the bush appears to receive an increasing amount of aluminium alloy deposit but starts wearing progressively as the steady state regime of the pin is realized. Although the bush wears an amount of deposit also remains on this counterface. If this deposit is removed by immersing the bush in caustic soda it is seen (Fig. 5) that both the pin and the bush continue to lose weight and the former does not change slope.

1

2

3

L

Load (kg1

2 L

6 8 IO I2 IL 16 I8 20 22

Sliding

distance

I km I

Fig. 4. Wear rate against load of grey iron bushes sliding against Al-Si alloys at 196 ems -l. The pins were solution treated at 510 “C for 30 min (pin diameter, 6.25 mm): 0, hypoeutectic pin aged at 100 “C for 30 min; A, hypoeutectic pin aged at 275 “C for 90 min; 0, hypereutectic pin aged at 200 “C for 5 h. Fig. 5. Weight losses of steel bush and an Al-B

alloy pin: 0, pin; 0, bush.

A cleaned bush receives a fresh deposit from the opposite member of the couple with average weights varying as shown in Table 1. The first row of Table 1 shows the weight loss of the pin after running the machine for 8 min after weighing the pin from the previous run. The deposit, however, is the cumulative effect after running the friction couple for about 15 km (Fig. 5).

162 TABLE 1 Weight of deposit on a hard steel bush after cleaning and running for the periods of time shown Time of running after cleaning (min)

Deposit weight (9)

Pin weight loss (9)

8

0.0064

10 20

0.0007 0.0010

0.0043 (deposit removed) 0.0140 0.0154

Pin, Al-ll.l%Si rev min-l.

alloy 6.25 mm in diameter; bush, 25 mm in diameter; speed, 1500

It can be said that the bush achieves an equilibrium layer of the opposing material, but as the deposit is removed and the machine run the pin weight loss is always greater than the deposit. Thus an amount of material is lost from the pin by deposition on the counterface and the remainder is lost by some other mechanism. The possible mechanism of wear of the aluminium alloy pins is discussed later but there is scope for a programme of work to examine the mechanism of wear of cast iron and steel counterfaces. Figure 5 shows that the deposit does not appear to affect the wear rates of the pins. This is important as there is a tendency to assume that, since a deposit forms on the counterface, sliding is between like materials. A coating of aluminium alloy on the bush behaves in a very different way to running a friction couple comprising like materials. In the latter case the aluminium base pin wears out at an accelerated rate, possibly because of a solid solution effect.

6. Work hardening Longitudinal sections of worn AI-Si alloys show a work-hardened layer in which the silicon particles are finer than those in the bulk material (Fig. 6). This hard layer can be as thin as 200 pm and the cold-worked state can be confirmed by X-ray analysis. Suitable age-hardening treatment gives a rounded morphology to the silicon particles, the average diameter of which in the deformed layer is about 1 pm. A similar size range due to polygonization by migration of vacancies created during mechanical working to edge dislocations has been observed in heavily cold-worked pure aluminium. The wear debris incorporate AIZOs and microprobe analysis shows the presence of iron in the wear scar. A typical microhardness trace avoiding the silicon particles for an 11% silicon alloy is shown in Fig. 7 which shows that the wear track is some four times harder than the parent material. It is expected that the bulk of the wear debris is produced from this work-hardened layer. A particle size analysis of the debris [ 73 has shown that the finest particles

163

Wear track

Stratified primary silicon

Fig. 6. Longitudinal section through the worn surface of an Al-lG%Si alloy pin of diameter 6.25 mm: steel counterface; load, 2 kgf; speed, 200 cm s-l. Note the primary silicon stratified within a work-hardened layer containing a fine dispersion of eutectic silicon. Magnification 225~.

were the most numerous irrespective of the silicon content or the load. This is surprising but a possible reason is that whatever the amount of silicon or the nature of the microstructure a hard deformed layer is in direct confrontation with the counterface so that the wear process should be controlled by this layer. The distribution pattern of the wear debris should therefore be independent of the prior nature of the aluminium alloy. This will be a fruitful area of further study. The percentage N of particles in a given size range for a complete wear run can be related to the mean particle diameter d for a given particle size range. Thus a plot of In N against d is linear (Fig. 8) and the relation can be expressed as N = A exp(-Kd)

(1)

where A and K are constants. The values of A and K were obtained from a large number of results for various Al-Si alloys running against steel at different loads and hence N = 27.2 exp(-0.000473d) 7. Wear mechanism Sarkar [ 61 has suggested that owing to sliding under load an initially plastic surface of an Al-Si alloy becomes work hardened so that subsequent

(2)

164

600 p Oxidized layer o Lighter cdoured a

work- hardened

layer

450’

+ 2 9 .u 300 E o rl d 150-\ p obLL_.-

I Distance

o-_-o20

from

CO

the worn track (xlO-*mm)

-61 0

300 Men

600 particle

900 size

I200

d tx IO-'mml

Fig. 7. Variation of microhardness from the wear track towards the centre of an Al13%Si pin of diameter 6.25 mm: load, 2 kgf; sliding speed, 200 cm s-l. Fig. 8. In N us. mean particle size of wear debris where N is the percentage of particles: pin diameter, 6.25 mm; load, 1 kgf; speed, 200 cm s-l ; silicon content, 11%.

encounters are elastic. Considering the asperities of the counterface, the stresses at asperity level are now Hertzian so that cracks nucleate at regions below the surface. As sliding continues the cracks enlarge owing to alternate stressing and wear debris emanates from these sites. Transverse cracks (Fig. 9) are not uncommon features in Al-Si alloys under unidirectional sliding but it has yet to be established whether these originate at or below the wear track. Metallographic examinations of longitudinal sections show cleavage of the transition boundary between the workhardened layer and the bulk material (Fig. 10). Detachment of a workhardened layer can be seen at both the leading (Fig. 11) and the trailing edge (Fig. 12) of pins and the transverse crack shown in Fig. 9 is from the middle portion of a wear track. Such layer type debris probably constitutes the bulk of the weight loss of the pm, another mode being deposition of material on the counterface. Although the location of nucleation sites for wear debris has not been established, there is evidence that the silicon particles fracture. This is shown up well with hypereutectic alloys (Fig. 13) where the primary silicon particles distort in the direction of sliding and fracture. The silicon particles have been shown to fracture by general and localized plastic deformation of the matrix under conditions of tensile loading [9] . The orientation of the cracks was perpendicular to the direction of maximum tensile strain but Fig. 13 shows that the cracks have random inclinations to the shear direction.

X65

&tion of sliding Fig. 9. Scanning electron micrograph of the wear scar of an aged A&22%Si alloy pin sliding on a similarly treated Al-ll! Si alloy bush: load, 1.1 kgf; pin diameter 6.25 mm; speed, 196 cm s-l, Magnification, 4162.6X.

8. Concludingremarks Experimentalevidence to date suggests#at the effect of silicon on the wear rates of Al-Si alloys is not highly significantalthoughthe eutectic alloy is superior.Wear occurs to a small extent by deposition onto the counterface. There may be other ways by which loss of materialoccurs owing to slidingunder load but the predominant mode seems to be the separationof all or part of the work-hardenedlayers from random patches on the wear track, Nucleation sites for wear debris are probably created below the sliding surface as a result of Hertzianstressbut more experimentsare necessaryto confirm this postulation.

Fig. 10. Cleavage of the transition boundary between the work-hardened layer and the bulk material of a pure aluminium pin: pin diameter, 6.25 mm; load, 2 kgf; speed, 200 cm s-l. Magnification 614~. Fig. 11. The leading edge of an Al-lG%Si alloy pin showing the formation of some of the larger fractions of wear debris. Magnification 139.5x.

z&ion Fig. 12. Trailing edge of an Al-2lkSi

of sliding

alloy pin. Magnification

t

Wear track

99x.

Fig. 13. Longitudinal section through an Al-Pl%Si alloy pin. Distorted and fractured primary silicon particles can be seen in a coarse eutectic matrix. Pin diameter, 6.25 mm; hard steel counterface; load, 2 kgf; speed, 200 cm s-l. Magnification 429x.

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References 1 F. J. Kneieler, D. A. Martens and R. W. Midgley, SAE (Sot. Automot. Eng.) Pap. 710147, 1971, pp. 1 - 26. 2 K. Okabayasbi and M. Kawamoto, Bull. Univ. Osaka Prefect., Ser. A, 17 (1) (1967) 199. 3 G. A. Colligan, Trans. Am. Foundrymen’s Sot., 81 (1973) 359. 4 E. E. Stonebrook, Mod. Cast., 38 (1960) 111. 5 G. Vandelli,Alluminio, 37 (3) (1968) 121. 6 A. D. Sarkar, Wear, 31 (1975) 331. 7 J. Clarke and A. D. Sarkar, Wear, 54 (1979) 7. 8 R. D. Heidenreich, J. Appl. Phys., 20 (1949) 943. 9 A. Gangulee and J. Garland, Trans. Metall. Sot. AIME, 239 (1967) 269.