tribological property of aluminium matrix hybrid composte

62 ALUMINIUM · XX/201162 ALUMINIUM · XX/2011 ALUMINIUM · XX/2011 63

Tribological characteristics of aluminium matrix hybrid composites reinforced with SiC and Al2O3

Md AI Mehedi, K. M. H. Bhadhon and Prof. Dr. M.N. Haque, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

Metal Matrix Composites (MMC) have become very popular for extensive physi-cal, mechanical and tribological property. Among many, aluminium is the most used material due to its lightweight property which is reinforced with Al2O3, SiC, TiB2 to produce good tribological property for automotive and machine parts. In this study Al-3.73Mg alloy is reinforced with different ratios of hybrid particulate mix-ture of SiC and Al2O3 and the tribological property is investigated. The investigation reveals that with the increase of % vol-ume SiC, the wear rate decreases in case of hybrid composite at different loads and sliding velocity. The wear mechanism was also investigated from the worn surface micrographs.

1. Introduction

The use of MMC is increasing every year due to their better physical, mechanical and tribo-logical properties. Many materials based on Al, Zn and Mg are extensively used in various industries due to their lightweight property (1, 2, 3). MMCs are generally manufactured with high strength and high modulus ceramic particles, whiskers or fibre reinforcement. The reinforcement improves the strength, stiffness and hardness with the expense of ductility.

Aluminium alloys are generally used in automotive industries for their lightweight property (3). The use of ceramic particulates imparts high wear resistance to the aluminium alloy. Aluminium alloys are usually reinforced with particles, whiskers or fibres of Al2O3, SiC, TiB2 etc. These reinforcements increase the wear resistance and reduces coefficient of thermal expansion (1).

Metal matrix composites are manufactured using different techniques. These techniques are liquid phase (casting) processes (4), vacu-um infiltration (5), pressureless infiltration (6) and dispersion methods. Solid state processes are manufactured by powder metallurgy (PM). Liquid– solid processes are manufactured by compocasting and stir casting. Stir casting is extensively used in the manufacture of particle MMC.

The light weight of aluminium allows the

production of high strength to weight ratio materials. The application of SiC or Al2O3 reinforced aluminium alloy composites in aerospace and automotive industries has been gradually increased for pistons cylinder. How-ever, much of the attention in wear studies has concentrated on the dry sliding wear behav-iour of MMC including various reinforcements such as SiC and Al2O3 particles.

The fact is that tribological properties are the one that defines the possible application of material far more than their mechanical properties, since they are in better correlation with behaviour in practice. Numerous authors have investigated friction and wear properties of aluminium matrix composites (AMC) and have analysed different influences:• Matrix and the reinforcement material (7)• Reinforcement type, size and % vol. (8, 9)• Variables like load, speed, temperature, atmosphere and distance (10, 11).Al2O3 and SiC helps in resisting the surface deformation in dry sliding condition with abra-sive wear mechanism (12). Ramesh and his as-sociates (13) stated that SiC reduced the wear rate of Cu, where the use of graphite increases the wear rate because of its softer particles. Na-plocha and Granat (14) and Daoud et al. (15) suggested Al2O3 for better wear resistance. Al-Al2O3 composites having good mechanical and tribological properties are used at crank bearings and mo-tor blocks in order to improve wear resistance (16).

The soft struc-ture of aluminium has very low wear resistance, which makes it impossi-ble to use in ma-chine parts. Using hard phases like SiC or Al2O3 could impart hardness and wear resist-ance in aluminium or its alloy. But a very few stud-ies investigate the possibility of

SiC and Al2O3 hybrid composite. In this case SiC will definitely work as harder phase than Al2O3 as SiC has hardness of 2,800 kg/mm2

relative to 1,400 kg/mm2 of Al2O3. Al2O3 has lower price than SiC, so investigation of this new hybrid composite can bring an economi-cal solution of the MMC in automotive and aerospace uses.

In this study Al-Mg alloy is reinforced with 6% particulate hybrid (Al2O3 and SiC) and two different compositions are analysed with different ratios of SiC and Al2O3 (1:1 and 2:1). The tribological property is investigated at dif-ferent ratios of hybrid composites.

2. Experimental

2.1 Materials: Pure aluminium ingot (> 99.5% Al) and magnesium ribbons were used to pro-duce the master alloy. This master alloy was used as the matrix of the composites. Alumina (Al2O3) particles and silicon carbide (SiC) par-ticles sizing 106 µm were the reinforcement phase. These reinforcement phases were add-ed in various proportion in order observe their effect on wear behaviour of the composites.

2.2 Master Alloy: The master alloy was pro-duced by using pure aluminium ingot (> 99.5% Al) and pure magnesium ribbons. Magnesium ribbons were sized and weighted. Then these ribbons were rapped with aluminium foils in

Fig. 1: Stir casting process of Al-Mg alloy with alumina and SiC particulate rein-forcement


order to minimise the loss of magnesium in form of magnesium oxide which forms due to high reactivity of the magnesium in contact with the air at high temperature.

A pure aluminium block weighing 5,455 gm was melted in a crucible in a pit furnace. The temperature of the melt was raised to 750°C. After that the magnesium ribbons rapped with aluminium foils were added gradually by using a stainless steel holder. As magnesium is very reactive the stainless steel holder was used to ensure that the magnesium ribbons does not come in contact with the air and burn away.

Finally after adding all the magnesium rib-bons, the molten alloy was poured into a per-manent metallic mould. Afterwards the master alloy was analysed by optical emission micro-

were conducted in air and dry sliding condi-tion. The disk was of cast iron with hardness HRC 60. An arm was used to hold and load the pin specimen vertically on the cast iron disk. The arm can move freely in both vertical and horizontal direction. The wear specimens were prepared from the casted bars by using a Lathe machine. ASTM G99-05 wear speci-men dimensions were followed in preparing

all the test specimens. Two different parameters (Load and sliding

speed) were selected and their effects were observed in two different composites. The weight loss of the wear specimens were cal-culated for three different loads (17.21 KN, 17.79 KN and 20.43 KN) at 2.54 m/s and for three different sliding speeds (1.89 m/s, 2.10 m/s and 2.54 m/s) at 17795.34 N. After that the worn surfaces were carefully analysed in a scanning electron microscope in order to de-termine the wear mechanism operating at the sliding interfaces.

3 Result and Discussion:

3.1 Effect of Sliding speed on mass loss (wear rate)From Fig. 3 it can be seen that the mass loss has increased as the sliding speed increased in all the specimens. But when the master alloy was reinforced the mass loss gradually decreased for sliding speed 1.89 m/s and 2.10

Composition %

Al 94.42Mg 3.73Fe 0.73Si 0.16Ni 0.88Ti 0.08Zn 0.07

scope and it revealed the master alloy contain 3.73% magnesium.

The following table states the chemical composition of master alloy:2.3 Composites: The desired composites were synthesized by the stir casting method. At first the master alloy was cut into several pieces for two different composites. SiC and Al2O3 particle were mixed and added in two differ-ent ratios, 1:1 and 2:1 respectively (total 6% wt of master alloy). The particle mixtures were preheated at about 800°C for 2 hours in order to improve its wettability.

The master alloy was melted in a holding furnace and temperature was raised to about 720°C. Then the previously mentioned pre-heated particle mixture for particular compos-ite was gradually added into the molten alloy which was continuously stirred at 550 rpm (rotations per minute). The particle was added in the vortex which was produced due to stirring. Then after adding all the particle mix-ture, the stirring was continued for 4-5 min-utes in order to get a homogeneous distribu-tion of the reinforced powder mixture. Final- ly the composite was poured into two perma-nent metallic moulds at a pouring temperature of 625°C. Permanent metallic moulds were used in order to minimise the porosities which are one of the main disadvantages in conven-tional sand moulds. 2.4 Wear test: Wear tests were conducted by using the Pin-on-disk method. All the tests

Fig. 2: Schematic diagram of pin on disc method of wear test (17)

Fig. 3: Mass loss of the matrix and composites at 17.21 KN load at different sliding speeds

Fig. 4: Mass loss of the matrix and composites at 17.79 KN load and 2.54 m/s sliding speed


m/s. Profound reduction of mass loss was ob-served in case of 2.54 m/s sliding speed. In this case mass loss went from 0.05 gm to 0.01 gm as reinforcement phase was introduced. Especially when the percentage of SiC in the powder mixture was increased, i.e. doubled the mass loss reduced drastically. This mass loss is equal to the same mass loss observed at lower sliding speed (2.10 m/s) for the same composite. This indicates that at higher sliding speed wear decreases very rapidly as the SiC

particles increase in the composite. SiC par-ticles were found to be more wear resistant and have more effect on the sliding speed than the alumina (Al2O3) particles. During sliding at different speed under same load 17.79534 KN the ceramic reinforcement particles sup-pressed the operating wear mechanism and hence reduce mass loss. The ceramic reinforce-ment particles have more strength and hard-ness than the Al-Mg alloy matrix. As a result less wear occurred where the sliding interface encountered these hard ceramic particles in the composites. 3.2 Effect of reinforcement on mass loss (or wear rate)Reinforcement in the master alloy certainly

reduced the amount of mass loss during the dry sliding wear tests as shown in Fig. 4. An unreinforced Al-Mg alloy shows a mass loss of 0.02 gm. As this alloy was further reinfoeced with 1:1 alumina (Al2O3) and SiC particles mass loss decreased substantially by 0.01 gm. As the ratio of SiC particles in the powder mix-ture was increased it reduced the mass loss by reducing the wear rate at normal condi-tion but not to the same extent as the previ-ous case. This can be confirmed by the earlier

figure (Fig. 3) where profound effect of SiC particles were only observed at higher sliding speed (2.54 m/s). As the amount of reinforce-ments were same in both the cases, in normal condition their wear rate was also almost the same. The slight reduction observed was due to increase in the percentage of SiC in the re-inforcement mixture because of the fact that hardness of SiC particles. Hardness of SiC par-ticle is 2,800 kg/mm2 where Al2O3 has hard-ness of only 1,400 kg/mm2. Thus the hard SiC particle takes the load of the matrix phase as well the relatively soft powder phase(Al2O3) helps to reduce the stiffness. 3.3 Effect of load on mass loss (or wear rate)The effect of load on the wear rate of the sam-

ples was investigated by calculating the mass loss at same dry sliding speed (2.54 m/s). Fig. 5 reveals that the effect of load is almost re-ciprocal to that of the effect of sliding speed shown in Fig. 3. In each cases the mass loss decreased as reinforcement phases changed it’s ratio. Gradual reduction of mass loss was observed in between 17.79534 KN and 17.21616 KN load. But in 2:1 SiC and alu-mina (Al2O3) AMC specimens, the decrease in mass loss was very low compared to the other results. This indicates that at higher load further decrease in mass loss i.e. wear resist-ance increased remarkably. Mass loss of higher ratio of SiC content wear specimen has almost the equal value as the specimens from the 1:1 SiC and alumina (Al2O3) AMC. This is because of the fact that higher load causes the sliding interface to come more and more closer to each other and have better sliding contact. On the other hand with higher specific loads tem-perature at the sliding contact rises and thus plastic deformation occurs. As a result more friction occurs causing higher wear rate and mass loss. 3.4 Examination of worn surfaceIn Fig. 6 three worn surfaces of composite un-der three different loads are examined with SEM. Fig. 6c reveals a parallel and continu-ous ploughing grooves on the worn surface of the composite material. The abrasion wear mechanism is observed with the case of low load in Fig. 6b and 6c. In these cases, there are some patches in the worn surface from where the materials were removed from the surface during the course of wear. The parallel grooves suggest abrasive wear as characterised by hard particles into the softer surface, which is the key concept of Al-Mg alloy-particulate composites. The grooves may be formed due to deposition of wear-hardened deposits in disc track.

At high loads local deformation and frac-ture take place which is observed in Fig. 6a. At high loads it is impossible for the reinforcing particles to maintain the stability of protective layer under the ploughing action. Materials

Fig. 5: Mass loss of the matrix and composites at 2.54 m/s sliding speeds at different loads

Fig. 6: SEM micrograph of worn surface of Al-Mg alloy composite (with 2:1 SiC: alumina reinforcement) at 2.54 m/s sliding speed at a)20.43 KN b)17.79 KN c)17.21 KN

a) b) c)


Fig. 7: SEM micrograph of worn surface of Al-Mg alloy composite a) unreinforced, b) with 1:1 (SiC:Al2O3) reinforcement and c) with 2:1 (SiC:Al2O3) reinforcement at 17.19 KN load and 2.54 m/s sliding speed

removal is in the form of flake type debris. After the surface material is removed, the cracks get nearer to surface and then shear strain increased. This causes the surface re-moval by delamination process (18). The ma-jor mechanism at high load is delamination wear which causes excessive fracture of the reinforcements and the matrix.

From Fig. 7 we see that abrasive wear take place with unreinforced alloy where parallel grooves continuly dominates the wear process. The proughing grooves in Fig. 7 a and in some extent Fig. 7b shows the plastic deformaion with no or less reinforcemnt with SiC. But af-ter the reinforcemnt with hard SiC is incresed, the delamination process replaces ploughing and thus subsurface fracture dominates in composites with high SiC than Al2O3.

4. Conclusion

Effects of reinforcement with SiC and Al2O3 on the tribological property of Al-Mg alloy under dry sliding condition have been investigated. The composites with different ratios of hybrid particulates were prepared with stir casting method. It was found that with the increase in SiC particle in reinforced alloy, the wear resistance is increased. In the same way, rein-forced alloy has higher wear resistance than unreinforced one. The increase of load reduces the wear and increase of sliding velocity in-creases the wear rate. The topographical image of worn surface of composites concludes that the predominant wear mechanism is abrasion and delamination.The increase in load moves the wear mechanism from abrasion to subsur-

face crack and delamination.

References

1. Aluminium matrix composites: challenges and opportunities. Surappa, M.K. 1-2,319-334, s.l.: Sad-hana, 2003, Vol. 28.2. Aluminium metal matrix composite for auto-motive application: tribological consideration. S.V.Prasad, R.Asthana. 3,445-453, s.l.: Tribology Letters, 2004, Vol. 17.3. Aluminium matrix composites in automotive uses. Mehedi, Md AI. 7/8,55-57, s.l.: International ALUMINIUM Journal, 7/8-2011, Vol. 87.4. Particle reinforced metal matrix composites, a review. Ibrahim IA, Mohamed FA, Lavernia EJ. s.l.: Mater Sci , 1991, Vols. 26:1137-56.5. Characterisation of MMC’s fabricated by vacuum infiltration of a liquid metal under and inert gas pres-sure. Cgiou JM, Chung DDL. s.l.: J Mater Sci, 1991, Vol. 26:2583.6. Analysis and performance of fiber composites. Agarwal BD, Braatman L. s.l.: John Willey & Sons Ltd, 1990.7. The tribological charecterstices of the Al-20Si-3Cu-1Mg alloy reinforced with Al2O3 particles in relation to the harness of a matring steel. D.Bialo, J.Zhou,J.Duszczyk. 21,5497-5501, s.l.: Journal of Material Science, 2000, Vol. 35.8. Effect of articulate reinforcement on wear be-haviour of aluminium matrix composites. Korkut, M.H. 1,73-81, s.l.: Materials Science and Technol-ogy, 2004, Vol. 20.9. Sliding wear behaviour of Al-Si-Cu composite re-inforced with SiC particles. X.G. Zou, H. Miyahara, K.Yamamoto, K.Ogi. 11,1519-1526, s.l.: Materials Science and Technology, 2003, Vol. 19.10. The effect of sliding speed and microstructure on dry wear properties of metal matrix composites. Y.Sahin, S.Murphy. 1,98-106, s.l.: Wear, 1998, Vol. 214.11. Effect of temperature on dry sliding wear per-formance of Al alloys and Al matrix composites.

S.Wilson, A.T. Alpas. 1-2,270-278, s.l.: Wear, 1996, Vol. 196.12. Effect of SiC particulate reinforcement on the dry sliding wear of Al-Si alloys(A356). A.T.Alpas, J.Zhang. 155, 83-104, s.l.: Wear, 1992.13. Development and performance analysis of novel cast copper-SiC-graphite hybrid composite. C.S.Ramesh, R.Noor Ahmed,M.A. Mujeebu and M.Z. Abdullah. 1957-1965, s.l.: Matter. Des., 2009, Vol. 30.14. Dry sliding wear of Al/Suffil/C hybrid metal ma-trix composites. Granat, K.Naplocha and K. 1734-1740, s.l.: Wear, 2008, Vol. 265.15. Tensile and wear properies of rolled Al5Mg-Al2O3 or C particulate composites. A.Daoud, T.El Bitar and A.A El Azim. 390-397, s.l.: J.Mater. Eng. perform, 2003, Vol. 12.16. Wear resistance of d-type alumina fibre rein-forced Al-4% Cu matrix composite. Moustafa SF, Soliman FA. 311-5, s.l.: Tribol Lett., 1997, Vol. 3.17. Effects of reinforcements on sliding wear behav-iour of aluminium matrix composites. T. Miyajima, Y. Iwai. 606-616, s.l.: Wear, 2003, Vol. 255.18. Wear behaviour of Al-Mg/SiCp MMCs. Dutt. I, Allan S. M. and Hafley J.L. 2553, s.l.: Journal of Met-allurgical and Material Transections, 1991, Vol. 22.

Authors

Md AI Mehedi1 and K. M. H. Bhadhon2 are Dean’s award winner undergraduate students of Bangla-desh University of Engineering and Technology (BUET), Dhaka, Bangladesh.Prof. Dr. M.N. Haque3 is the current Dean of Engi-neering Faculty and former head of the Department of Materials and Metallurgical Engineering, Bang-ladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh Contact:[email protected], [email protected], [email protected]

a) b) c)

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