synthesis and characterisation of aluminium-silicon...

Indian Journal of Engineering & Materials Sciences Vol. 13, June; 2006, pp. 238-246

Synthesis and characterisation of aluminium-silicon-silicon carbide composite J P Pathak, J K Singh & S Mohan

Centre of Advanced Study, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India

Received 9 February 2005; accepted 20 April 2006

Aluminium-silicon-silicon carbide composite was prepared by mechanical mixing (300 rpm) of silicon carbide powder (−300 mesh) in solidifying solid-liquid slurry of hypoeutectic, eutectic and hypereutectic compositions of aluminium-silicon alloy. Requisite amount of hot (300°C) silicon carbide powder was charged into agitating metallic melt of aluminium-silicon matrix alloy and mixing continued while dropping the temperature of the system. The mixing period was 3-4 min during which temperature of the system dropped to 15±5°C below the liquidus. On completion of mixing, plumbago crucible containing the composite slurry was taken out from the furnace and finally solidified composite was removed from the crucible for the further study. X-ray diffraction analysis indicated the presence of silicon carbide in the composite. Silicon carbide occurred in the grain boundry regions associated with silicon and fragmented dendrites together with eutectic mixture. Strength, hardness and wear resistance of composite increased with increase of silicon carbide content. Coefficient of friction of composite having higher amount of silicon carbide was always lower than the composite with lower amount of silicon carbide, whether the tests were conducted in lubrication, semi-dry and dry sliding conditions.

IPC Code: C22C47/00, C22C49/00

Development of metal matrix composite (MMC) with improved tribological property has been one of the major requirements in the field of materials science and technology. The high strength and modulus with low friction and superior wear resistance at low cost of production have paved the way for commercial production and exploitation of particulate composites1. An understanding of the reinforcement and method of processing that influence the characteristics of the composite is uniquely important because mechanical and tribological properties are sensitive to the type of reinforcement and method of processing2,3. Today, a number of techniques involving solid, liquid and vapour state routes are employed for the production of metal matrix composite4-16. Out of these techniques, foundry technique is cost-effective and simple with respect to other routes processes. By foundry technique one can produce MMC by introducing dispersoid particles from outside in the liquid metal without restraints of the phase diagram. T present a variety of foundry processes are available for introducing and dispersing solid particles of different kinds, grades and sizes in metallic melts/slurry of different compositions. Hard (Al2O3, SiC, MgO, etc.) and soft (graphite, mica, talc etc.) ceramic particles and even soft metallic phases are incorporated in aluminium-silicon matrix to

generate suitable mechanical and tribological properties. In the present investigation, silicon carbide particles (50 μm size) were incorporated in solidifying metallic slurry of hypoeutectic, eutectic and hypereutectic compositions of aluminium-silicon alloy. As cast structure mechanical and tribological properties were assessed.

Experimental Procedure Raw materials Commercially pure aluminium (98.2 wt.% purity), Al-20wt.% Si (Master alloy), wt.%, 2.0 wt.% and 3.0 wt.% silicon carbide powder (50 μm size) were charged as dispersoid in different runs. Melting procedure Various Al-Si-SiC composites were prepared by melting the constituent metals and master alloy in an electric resistance muffle furnace and incorporating the silicon carbide in the solidifying metallic slurry of aluminium-silicon matrix alloy. Hypoeutectic (Al-4.6 wt.% Si) eutectic (Al-11.8 wt.% Si) and hyper eutectic (Al-14.4 wt.% Si) aluminium-silicon matrix alloys were selected and silicon carbide powder was dispersed simply by an experimental arrangement shown in Fig. 1. A weighed amount (900 g) of charge consisting of commercially pure aluminium and aluminium-silicon

PATHAK et al.: ALUMINIUM-SILICON-SILICON CARBIDE COMPOSITE

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master alloy was charged into a plumbago crucible. This crucible was kept rigidly in electrical resistance heating muffle furnace. Before connecting the furnace with the electric power supply through control panel, the furnace mouth was closed tightly with an asbestos sheet cover specially fabricated for this purpose. There were suitable openings in the furnace cover for the thermocouple and mixing paddle. During heating and melting, the opening for the mixer paddle was kept closed. One end of the thermocouple was connected to the temperature controller attached to the control panel and the other end was inserted in the furnace through the cover opening. The temperature of the melt was regulated continuously, with another thermocouple of which one end was sheathed in alumina, touching the melt by passing through the cover opening and other end was connected to the furnace temperature indicator. The charge was melted with the help of electric power and a superheat of nearly 200°C was provided. Metallic melt was left at this temperature for about 5 min for homogenization. Molten alloy temperature was lowered down and brought within its solidification range (hypoeutectic 640±5°C, eutectic 587±5°C, hypereutectic 590±5°C) where some amount of melt was in solid and some in liquid state. The stirring of liquid-solid phases was started mechanically (300 rpm speed) with the help of a graphite paddle attached to the graphite rod, during which hot (300°C) silicon carbide powder of required amount was introduced into the solid liquid slurry through the opening provided in the furnace cover. While charging and mixing silicon carbide, temperature of the solidifying slurry was lowered down (640±5°C to 610±5°C for hypoeutectic, 587±5°C to 582±5°C for eutectic and 590±5°C to 580±5°C for hypereutectic compositions) by regulating the furnace electric power supply. Total mixing period for each heat was 3-4 min. On completion of mixing electric power was off. Plumbago crucible containing the composite slurry was taken out from the furnace and placed on sand bed. The composite slurry was allowed to solidify in the plumbago crucible itself. After solidification composite was removed from the crucible for characterisation. Density measurement Specimens for density measurement having 30 mm diameter and 40 mm length were cut from the various portions of the casting of aluminium-silicon-silicon carbide composite and machined for the purpose. Each specimen was weighed in air and water with the

help of a single pan electric balance. By these two weights density of a specimen was calculated. X-ray analysis Machine turnings from various sections of the aluminium-silicon carbide composite were collected and brought in powder form by further crushing and grinding. X-rays, by using Cu-Kα radiation, was passed through this powder and corresponding diffraction patterns were obtained and matched with the standard diffraction pattern for the constituents present in the powder and hence in the composite. Metallographic examination Casting of composites were sectioned in the transverse and longitudinal directions and suitable pieces were machined for metallographic study. These pieces were mechanically polished and etched with Keller’s reagent and after removing excess etchant and drying, the specimen were examined under optical microscope. Evaluation of tensile and compressive properties Specimen of standard dimensions, 16 mm gauge length and 4.5 mm dia for tensile test and 10 mm length and 8 mm dia for compression test, required for various mechanical tests machined from the as cast aluminium-silicon-silicon carbide composites. Tensile and compression tests of all the specimens were performed by using Instron testing machine. Cross-head speed of machine was 2 mm/min and chart speed 20 cm/min during tensile test but under compression load cross-head speed was 0.2 mm/min with same chart speed. Brinell hardness tests were performed on all the specimen using 10 mm hardened steel ball indenter under a load of 500 kg for a period of 30 s.

Fig. 1—Schematic diagram of experimental set-up.

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Wear resistance evaluation

A suitable modified version of pin-on-disc wear test machine17,18 was used to assess the wear resistance of aluminium-silicon-silicon carbide composite. Cylindrical test pin 8 mm diameter and 35 mm length were machined from the composite. The flat end of test pin and hardened steel disc (hardened to 60-62 Rc) were ground to a surface finish of 0.45 μm, before bringing them in contact. The test pin was gripped with load pan and brought in contact with steel disc. A suitable load was applied on the load pan and the steel disc was kept under rotation with the help of variable speed D.C. motor connected with the steel disc by a belt and pulley system. After wear run the pin was weighed and by the difference of initial and final weights, weight loss was known. The wear rate (wear volume per unit sliding distance) as a function of load was computed from this wear resistance (reciprocal of wear rate) of aluminium silicon-silicon carbide composite was evaluated. Frictional resistance test

Friction of sliding surfaces is assessed while creating various frictional states between shaft and specimen bush sliding surfaces of a rig (Fig. 1). On varying applied load, speed and viscosity of lubricant frictional states of mating surfaces change. A relation between applied load and coefficient of friction is set-up which indicates the prevailing condition of the mating surfaces. For this purpose a rig was designed, fabricated and employed for the frictional resistance test Fig. 2. Cylindrical bush specimen (40.8 mm OD × 35.5 mm ID × 19.6 mm long) of matrix and composites and steel shaft (50 mm length) mating surfaces were ground to a surface roughness of 40 μm and the clearance between shaft and bush was maintained within the limit of 0.0010 per mm of shaft diameter as normally recommended19. Shaft was push fitted on the steel rod. Bush was inserted in bearing housing and gripped by two halves of housing by bolts and nuts. Complete bush and housing assembly was mounted on the steel shaft and clamped by threaded grooves and bolts, via collar disc. Supply of lubricant to the mating surfaces started and the motor was connected to the power supply. A running period of 6 h with shaft speed of 30 rpm and 15 kg of applied load was allowed to develop smooth contact between bearing and shaft mating surfaces. At the end of running period 15 kg load was removed and 5 kg load

was employed for 30 min with a shaft speed of 30 rpm corresponds to this load and speed, coefficient of friction was measured. Similarly for every 5 kg increase of load, coefficient of friction was measured. While increasing the load in 5 kg step bearing test was performed, coefficient of friction achieved minimum value and on further increase of load coefficient of friction also increased. During semi-dry test running in period was 6 h with a shaft speed of 30 rpm under a load of 15 kg. Lubrication was continued only for making the mating surfaces smooth enough that is only during running in period. At the end of this period motor and lubrication were stopped. Bearing housing was opened, bush and shaft mating surfaces were cleaned. The bearing was then tested without lubrication and coefficient of friction was measured as a function of stepwise increase of load of 5 kg with a shaft speed of 30 rpm for a period of 30 min. Further, under dry test running in period was 2 h with a load of 5 kg and 30 rpm shaft speed. At the end of running in period actual experiment was started and frictional force was measured for each 5 kg increase in load with shaft speed of 30 rpm and 30 min run time. In all the above cases seizure load was noted whenever seizing occurred.

Fig. 2—Line diagram of frictional resistance test machine.(A-movable tie rod, B-spring balance, C-oil container, D-spirit level, E-nut-bolt system, F-floating bearing housing, G-oil hole, H-bearing bush, I-shaft, J-guide pin, K-thermocouple, L-bottom plate, M-load shaft, N-slotted weight, O-weight pan)


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Results and Discussion Density Table 1 presents the as cast density of various aluminium-silicon-silicon carbide composites prepared in this investigation. It is found that the density values are very close to theoretical ones that indicate the soundness of the composites but marginal difference could be due to the presence of entrapped gases and shrinkage porosity. Microstructure Microstructures of Al-Si-SiC composites Figure 3a shows the optical microstructure of hypoeutectic Al-4.6%Si-1.5%SiC composite. It is seen that aluminium rich dendrites are more or less rounded. Basic dendrites arms are broken due to mixing of SiC in the solidifying matrix alloys. Silicon carbide particles have been dispersed in the grain boundary regions as coagulated gray particles associated with black silicon particles and fragmented dendrites. Aluminium silicon dendrites appear light nearly round in shape in hypoeutectic eutectic and hypereutectic compositions. There are primary dendrites (Fig. 3b, Al-11.8%Si-1.5%SiC) along with the eutectic silicon which are crystallized due to eutectic composition shift to a high silicon content and late nucleation of primary silicon phase over the dendrites which may be consequence of a large under cooling from the high rate of heat extraction when the plumbego crucible was taken out from the furnace and kept on the sand bed. Silicon carbide particles are associated with the eutectic mixture and fragmented dendrites more or less in coagulated form. Primary silicon also appeared in the composites. Figures 3c shows the optical microstructure of Al-14.4%Si-1.5%SiC composite. It is seen, that there are primary

Table 1—Composition, density values and mechanical properties of aluminium-silicon-silicon carbide composites

Matrix alloy and composite composition (wt.%)

Density kgm-1 × 103 (measured)

UTS (MPa) 0.2% offset tensile-proof stress (MPa)

50% compressive stress (MPa)

Elongation (%)

Hardness (BHN)

Al-4.6%Si 2.54 185.0 147.2 335.3 4.6 60.6 Al-4.6%Si-0.6%SiC 2.60 205.2 168.8 367.2 3.3 74.6 Al-4.6%Si-1.5%SiC 2.70 228.4 179.7 383.0 3.1 85.3 Al-4.6%Si-2.2%SiC 2.81 237.3 188.0 394.5 2.8 93.2 Al-11.8%Si 2.30 190.4 166.6 341.5 5.0 69.4 Al-11.8%Si-0.6%SiC 2.42 209.3 172.1 349.2 3.9 75.0 Al-11.8%Si-1.5%SiC 2.59 227.8 178.3 356.4 3.2 29.3 Al-11.8%Si-2.2%SiC 2.61 229.7 186.4 362.1 2.7 83.8 Al-14.4%Si 2.21 196.4 165.7 394.2 2.1 74.2 Al-14.4%Si-0.6%SiC 2.43 234.5 179.1 426.1 1.9 77.0 Al-14.4%Si-1.5%SiC 2.57 254.7 155.6 441.4 1.7 89.0 Al-14.4%Si-2.2%SiC 2.69 261.2 192.3 463.3 1.2 96.3

Fig. 3—Optical micrographs of aluminium-silicon-silicon carbide composite (a) Al-4.6%Si-1.5%SiC composite, (b) Al-11.8%Si-1.5%SiC composite and (c) Al-14.4%Si-1.5%SiC composite.


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polyhedral silicon, of different sizes. Growth of primary silicon is hindered due to mixing of silicon carbide with solidifying aluminium-silicon alloy. In the hypereutectic composition also silicon carbide particles are trapped in the inter-dendritic region in the form of fine particles associated with silicon and fragmented dendrites. Mechanical property Mechanical properties are very important and special requirement of a composite. A composite should have sufficient tensile and compressive strengths along with hardness and ductility. Mechanical properties of composites understudy were evaluated at room temperature. Table 1 presents the value of various mechanical properties of the prepared composite. There is variation of UTS, proof stress, %elongation, compression strength and hardness. It may be seen that with increasing amount of silicon carbide, the UTS, 0.2% offset proof stress compression strength and hardness of the composite increased similar to data reported by Embury20. It is well-known that reinforcement constituents are one of the important factors controlling the strength of Al-Si-SiC composites. The increase in strength is probably due to closer packing of the reinforcement and a small inter-particles spacing in the matrix. It has been observed21 that the strength increased with increasing reinforcement content, as long as composite was able to exhibit enough ductility to attain full strength. It is also seen in Table 1 that elongation percentage has decreased and hardness increased with increasing the amount of SiC in the matrix. It may be due to straining of composites with silicon carbide particles. Silicon carbide particles being hard and brittle lead to dispersion hardening of matrix. These particles act as second phase in the matrix and resist the movement of dislocations and hence harden the composite. On increasing the amount of silicon carbide inter-particle spacing is decreased and hence movement of dislocation becomes critical. Compression strength increased with increasing silicon carbide particles. Compression of composite became tougher with the increased amount of silicon carbide and applied load. This happened till the matrix can accommodate the particles without distortion. X-ray analysis of composites The Al-Si-SiC composites prepared under this investigation were subjected to X-ray analysis for the constituents present and the results obtained are

shown in Fig. 4a for Al-4.6%Si-0.6%SiC and Fig. 4b for Al-14.4%Si-2.2%SiC composites. Powder samples for the test were drilled from different sections of the composite. X-ray (Cu Kα radiation), was passed through these powder samples and X-ray patterns were obtained. Different constituents like aluminium, silicon and silicon carbide were identified after matching the experimental d values with those of the theoretical d values. X-ray diffraction results reveal the presence of silicon carbide. Wear behaviour of composites The wear resistance of Al-Si-SiC composites under dry sliding under for different loads is tabulated in Table 2. It is seen that wear resistance increases with the amount of silicon carbide under the load applied in this study. It is also found that wear resistance decreases when the applied load increases. It may be considered that during the start of wear run, asperities of the specimen pin sliding surface come in contact with the steel disc and under the influence of load and sliding velocity junctions are formed. These junctions

Fig. 4—X-ray diffraction patterns of (a) Al-4.6%Si-0.6%SiC composite and (b) Al-14.4%Si-2.2%SiC composite.


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start to get work hardened during sliding. A stage arrives where junctions are distorted, broken and plucked out and fill up the adjacent valley of the pin surface together with extruded SiC. Wear resistance increases both due to the presence of SiC particles and oxides built-up on the sliding surface. In the next few runs over the entire pin surface a uniform film of smeared oxide and silicon carbide is formed causing wear resistance to be steady17. Removed of material from the pin surface becomes lesser and lesser with the increasing hardness of the composites. Junction hardening and removal of particles are decelerated with increased hardness of the composites. This occurs when silicon carbide is incorporated in the aluminium silicon alloy. Strength and hardness of aluminium-silicon alloy increased with increased silicon amount as well as silicon carbide. There is less distortion, rupture and cavity formation on the pin mating worn surface of harder composite (Fig. 5a, Al-4.6%Si-2.2%SiC) than to less harder one (Fig. 5b, Al-4.6%Si-1.5%SiC). Oxidation of metallic particles occurs in the working atmosphere and oxides are formed which smear and form a layer on the pin surface. Once the oxide layers are formed, distortion, spalling and fracture of the layers take place over the pin mating surface in course of sliding. All the oxides are not completely dislodged from the mating surface. Some amounts present between the mating surfaces, wear resistance somewhat increases due to dilution of metallic contact of the surface. But as the load exceeded a certain value a significant decrease in wear resistance is observed, so at higher load (5 kg) gross damage occurs to the pin material (Fig. 5c, Al-4.6%Si-2.2%SiC). The hardness and strength of composite are more than the respective matrix alloy, and hence composite possesses higher wear resistance. It is also found that damage of sliding surface of higher amount of silicon carbide composite (Fig. 5a) is less than the lower amount of silicon carbide composite (Fig. 5b) for the same matrix alloy. Frictional behaviour It is found that bushes made of Al-Si-SiC composites do not seize under oil lubrication test at 5 to 50 kg load with 30 rpm. Increasing the load coefficient of friction (frictional force per unit applied load) decreases attains a minimum value and then increases again. In the beginning of test a oil film is formed between the shaft and bush surfaces. This continuous oil film prevents metallic contact. The film thickness decreases with increase in applied load

and hence coefficient of friction also decreases. A minimum coefficient of friction is achieved corresponds to the minimum thickness of the film. On further increase of the load this film breaks and

Fig. 5—Optical micrographs of worn surface of Al-Si-SiC composite (a) Al-4.6%Si-2.2%SiC (2 kg/load, 0.5 m/s sliding speed, 60 min run time), (b) Al-4.6%Si-1.5%SiC (2 kg/load, 0.5 m/s sliding speed, 60 min run time) and (c) Al-4.6%Si-2.2%SiC (5 kg/load, 0.5 m/s sliding speed, 60 min run time).


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metallic contact between mating surfaces occurs which increases the coefficient of friction and this increase continues with load. It is found that the coefficient of friction Table 3 of composite is some extent less than the respective matrix alloy. A mixed film of debris (generated during running) and oil is formed at the bush and shaft mating surface. This film is more adherent, durable, reduces frictional force, and protects bush and shaft from seizing. Formation of mixed film is more pronounced in case of higher amount of silicon carbide composites, therefore coefficient of friction (Table 3) decreases with silicon carbide content of composite. Table 3 also presents the coefficient of friction values of composites tested under semi-dry and dry sliding conditions. In both the cases it is found that neither matrix nor composites seize even up to 40 kg of load. It may be inferred that strength and hardness of the matrix and composite are such that they do not seize with the shaft. There was

ploughing distortion and dislodging of material from the bush mating surfaces. Fig. 6a (Al-11.8%Si-1.5%SiC) and Fig. 6b (Al-11.8%Si-2.2%SiC) even in semi-dry test condition, when the load exceeded to 20 kg but there was no seizing. Ploughing and distortion of harder bush mating surface (Fig. 6b) is less than to less harder bush mating surface (Fig. 6a). In dry test, friction coefficient is higher than in semi-dry test. Under semi-dry test bush has run under oil in running in period so it has absorbed and retained some oil in its pores which on further running creeped under pressure, to the bush mating surface and reduced wear and friction. It is also seen that degree of distortion of bush mating surface is less for higher SiC content than to low SiC content of composite. Bushes seize due to higher wear rate associated with higher friction under high unit load. It has been reported22-24 that under dry condition a weak material undergoes sub-layer plastic flow and surface rupture which result in

Table 2—Wear resistance (reciprocal of wear rate that is reciprocal of volume loss per unit sliding distance,m/m3 × 1011) of Al-Si-SiC composites under dry sliding, sliding speed 50.26 m/s × 10-2, and sliding distance 1809.5 m

Matrix and composite (wt.%) Wear resistance, under 2 kg

load

Wear resistance, under 3 kg load

Wear resistance, under 5 kg load

Al-4.6%Si 1.66 1.52 0.82 Al-4.6%Si-0.6%SiC 2.35 2.26 2.21 Al-4.6%Si-1.5%SiC 2.58 2.41 2.37 Al-4.6%Si-2.2%SiC 3.10 2.87 2.72 Al-11.8%Si 2.68 2.51 2.32 Al-11.8%Si-0.6%SiC 3.15 3.10 2.87 Al-11.8%Si-1.5%SiC 3.62 3.41 3.22 Al-11.8%Si-2.2%SiC 3.85 3.60 3.52 Al-14.4%Si 3.47 3.44 3.37 Al-14.4%Si-0.6%SiC 3.89 3.71 3.56 Al-14.4%Si-1.5%SiC 4.42 4.30 4.16 Al-14.4%Si-2.2%SiC 4.91 4.82 4.65

Table 3—Frictional characteristics of Al-Si-SiC composites under various test conditions shaft speed 30 rpm, test duration 30 min

Matrix alloy composite

wt.% Test under oil lubrication coefficient of friction (μ)

under 30 kg load

Test under semi-dry condition coefficient of friction (μ) under

25 kg load

Test under dry condition coefficient of friction μ

under 25 kg load Al-4.6%Si 0.047 0.42 0.62 Al-4.6%Si-0.6%SiC 0.046 0.41 0.62 Al-4.6%Si-1.5%SiC 0.040 0.37 0.58 Al-4.6%Si-2.2%SiC 0.039 0.36 0.57 Al-11.8%Si 0.037 0.36 0.59 Al-11.8%Si-0.6%SiC 0.037 0.34 0.57 Al-11.8%Si-1.5%SiC 0.034 0.31 0.55 Al-11.8%Si-2.2%SiC 0.030 0.29 0.54 Al-14.4%Si 0.036 0.35 0.56 Al-14.4%Si-0.6%SiC 0.036 0.33 0.56 Al-14.4%Si-1.5%SiC 0.031 0.28 0.55 Al-14.4%Si-2.2%SiC 0.028 0.26 0.51


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rapid arise in friction and wear. In the present case higher content of silicon carbide makes the composite more stronger and harder and hence there is less plastic flow of sub-layer which results in less friction and wear. Figures 7a and 7b show the optical microtopographs of bush surfaces of Al-14.4%Si-1.5%SiC and Al-14.4%Si-2.2%SiC composites respectively, slided for 30 min under 20 kg load and 30 rpm speed under dry sliding. There is less ploughing and distortion of harder and strong mating surface (Fig. 7b) than to less harder and weak mating surface (Fig. 7a). Conclusions Based on the results of this investigation following conclusions were drawn. 1. Aluminium-silicon-silicon carbide composites

were prepared by incorporating and mechanical mixing of hot (300°C) silicon carbide powder

(50 μm size) into the solidifying solid-liquid slurry of Al-Si matrix alloys.

2. Silicon carbide particles were entrapped in grain boundary regions associated with silicon particles and fragmented dendrites.

3. Tensile yield stress, UTS and hardness of matrix alloys increased with incorporation of silicon carbide powder. There was some decrease in percentage elongation with silicon carbide powders. Higher amount of silicon carbide shows more influence on properties.

4. Wear resistance of composite under dry sliding increased with increased amount of silicon carbide.

5. Frictional resistance of composite sliding under lubrication, semi-dry or dry condition is always lower than to respective matrix alloys. Composite having higher amount of SiC possesses lower frictional resistance than to lower amount of SiC composite.

Fig. 6— Optical photographs of bushes showing sliding mating surfaces (semi-dry test, 30 kg load, 30 rpm speed, 30 min run time) (a) Al-11.8%Si-1.5%SiC composite and (b) Al-11.8%Si-2.2%SiC composite.

Fig. 7⎯Optical microphotographs of worn surfaces of bush tested under dry condition, (20 kg load, 30 rpm and 30 min run time) (a) Al-14.4%Si-1.5%SiC composite and (b) Al-14.4%Si-2.2%SiC composite.


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