HARDNESS VALUES AND FRICTION BEHAVIOR OF AL-SI-FE ALLOY/COCONUT SHELL ASH PARTICULATE METAL MATRIX COMPOSITES (PMMCS)

A.APASIA, D.S.YAWASDepartment of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria.

ABSTRACTHardness values and friction behavior of aluminium alloy (Al-Si-Fe) reinforced with coconut shell ash particulate (CSAp) was produced by stir-casting process. The effect of the increase in percent coconut shell ash particle additions on the hardness values and the friction behavior of the composites were investigated. The hardness values were determined (ASTME 18-79,200) using the Rock Well hardness tester on “B” scale (Frankwell test, Rockwell Hardness Tester, model 38506). The friction behavior was determined by the variation of coefficient of friction during tests of dry sliding wear characteristics under different loads were determined. The results show that the hardness values increased from 63.50 HRB at 0wt% to 78.60 HRB at 15 wt%, and the coefficient of friction during test show that at higher load, frictional force increases with greater dissipation of energy resulting to higher coefficient of friction with increasing load.

Keywords: Al-Si-Fe Alloy; Stir-casting; Hardness; Friction

INTRODUCTIONParticulate reinforced metal matrix composites (PMMCs) are currently being used as structural components in aerospace, automotive and industrial applications. Discontinuously reinforced metal matrix composites have received much attention because of their improved specific strength, good wear resistance and modified thermal properties unattainable in either of the starting (monolithic) materials (Ritnner, (2000), Wahab et al, 2009). These materials have emerged as the important class of advanced materials giving engineers the opportunity to tailor material properties according to their needs. Essentially these materials differ from the conventional engineering materials from the view point of homogeneity (Varajappa and Ghandrmohan, 2005).

PMMCs combine the ductility and toughness of the metal matrices with the high strength and stiffness of the ceramic reinforcement to achieve properties unattainable in either of the starting materials. PMMCs often have high strength to weight ratios, which is an important consideration in weight sensitive applications. Other distinctive properties of PMMCs include good thermal stability and excellent wear resistance (Aigbodion, 2007).

Dispersing small particulates (less than 1µm) in a metal increases its strength, typically by Orowan type strengthening mechanisms. Traditionally high modulus ceramic particulates such as silicon carbide (SiC) and alumina (Al2O3) have been used as reinforcements purposely for

stiffness enhancement, plus strengthening. It is however, known that other property benefits can be achieved by carefully controlling the matrix properties, the reinforcement properties, and the interface formed between them.

Apart from the emerging economical processing techniques that combine quality and ease of operation researchers are at the same time turning to particle reinforced aluminum-metal matrix composites (AMMCs) not only because of their relatively low cost but also in their isotropic properties especially in those applications not requiring extreme loading or high thermal condition e.g. automotive components (Ejiofor and Reddy, 1997). Particle reinforced MMCS are produced through various routes and can also be achieved by standard metallurgical processing methods such as powder metallurgy, direct casting, rolling, forging and extrusion. The products can be machined by using conventional machining facilities. However one of the factors limiting the use of metal matrix composites MMCS for engineering components according to Aigbodion and Hassan, (2006) is lack of property characterization in relation to the unreinforced alloys. The lack of data according to them extends from processing parameters to final mechanical properties, and understanding the factors that influence the physical and mechanical properties of these materials is very important in the sense that these properties are sensitive to the type of reinforcement, the mode of production and the details of post-production processing (Aigbodion and Hassan, 2006).EXPERIMENTAL PROCEDURESMETHODSThe coconut shell was ground to form coconut shell powder, the powder was packed in a graphite crucible and fired in an electric resistance furnace at a temperature of 1300oC to form coconut shell ash (CSAp)(see Photos 2.1 (a,b,c,& d). The particle size analysis of the coconut shell ash was carried out in accordance with BS1377:1990(Bienia et al, 2003) .About 100g of the coconut shell ash particles was placed unto a set of sieves arranged in descending order of fineness and shaken for 15minutes which is the recommended time to achieve complete classification.

(a) Coconut Shell (b) Crushed Coconut Shell

(c) Coconut Shell Powder (d) Coconut Shell Ash

Photo 2.1: Photograph of Coconut shell ash particles

Specimen preparationThe metal matrix composite that was used in this study was produced using double stir-casting technique at the foundry shop of the Department of Metallurgical Engineering Ahmadu Bellow University, Zaria, Nigeria. The specimens were produced by keeping the percentage of iron and silicon constant and varying the reinforcing material (Coconut shell ash) particles in the range 0, 3, 6, 9, 12, 15% SIC. High purity aluminum electrical wires obtained from Northern Cable Company NOCACO Kaduna, Nigeria, free from dust, impurities and other contaminats. It was charged in a graphite crucible kept in an electric resistance furnace and heated to about 7500c till the entire alloy in the crucible was melted and 2wt% iron powder was added, the reinforcement particle (CSA) were preheated to 8000c for 1 hour before incorporation into the melt. Preheating of the reinforcement particle (CSA) is necessary to promote wettability and harmonize the melt. After the molten metal was fully melted degassing tablets (hexachloroethane) was added to reduce porosity, simultaneously, 1wt% magnesium was added to enhance wettability between coconut shell ash particles and the alloy melt. It was noticed that without the addition of magnesium, the particle of coconut shell ash were rejected.

The stirrer (Fig.1) made up of stainless steel was lowered into the melt slowly to stir the molten metal at the speed of 500-700 rpm. The preheated CSA particles were added into melt at a constant rate during stirring time. The stirring was continued for another 5 minutes even after completion of particle feeding. The mixture was poured into the mould which was also preheated to 5000c for 30 minutes to obtain uniform solidification using the process 0,3,6,9,12,and 15% by weight CSA particle reinforced composites were produced.

Determination of hardness valuesThe hardness values of the samples were determined (ASTM E18-79, 2000) using the Rockwell hardness tester on “B” scale (Frank Well test Rockwell Hardness Tester, model 38506) with 1.56mm steel ball indenter, minor load of 10kg, and major load of 100kg and hardness value of 101.2HRB on the standard block. Before the test, the mating surface of the indenter, plunger rod and test samples were thoroughly cleaned by removing dirt, scratches and oil and calibration of the testing machine using the standard block.

Photo 2.2: Photograph of the samplesMicrostructural examination Metallographic specimens were cut from the unreinforced and reinforced specimen of the Al-Si-Fe/SiC particulate composites. The cut specimens were then mounted in bakelite, and mechanically ground progressively on grades of Sic impregnated emery paper (80-600grits) sizes using water as the coolant. The ground specimens were then polished using 1-um size alumina polishing powder suspended in distilled water. Final polishing was done using 0.5um alumina polishing powder suspended in distilled water. Following the polishing operation etching of the polished specimen was done using Keller’s reagent. The structure obtained was photographically recorded using an optical microscope with built in camera.

RESULTS AND DISCUSSION ResultThe various microstructures developed for different Sic additions are shown in Micrographs 3.1-3.5The result of the hardness for different values of coconut shell ash addition is showing in Fig.2 while Al-Si-Fe alloy and the composites containing CSAp have been examined for their friction behaviour determined by the variation of coefficient of friction with wt% of coconut shell particles during tests of dry sliding wear under different loads as shown in Fig

DISCUSSIONMacro structural observation revealed a reasonable uniform distribution of coconut shell ash particles. The distribution of coconut shell ash particles is influenced by good wettability of the coconut shell particles beyond 15wt% made the composites slurry too thick and the fluidity of the molten composite was reduced.The microstructure of the aluminum alloy is shown in micrograph (4.2). The structure reveals the eutectic phase containing Fez Si, Al6Fe in and aluminum matrix

Micrograph 3.1: SEM/EDS spectrum of the aluminum alloy with 3wt%CSAp

Lower magnification b) Higher magnification

Micrograph 3.2: SEM spectrum of the aluminum alloy with 6wt%CSAp

Lower magnification b) Higher magnification

Micrograph 3.3: SEM spectrum of the aluminum alloy with 9wt%CSAp

(a) Lower magnification b) Higher magnification

(a) Lower magnification b) Higher magnification

Micrograph 3.4: SEM spectrum of the aluminum alloy with 12wt%CSAp

(a) Lower magnification b) Higher magnification Micrograph 3.5: SEM spectrum of the aluminum alloy with 15wt%CSAp

Fig. 2: Variation of hardness with wt% of coconut shell ashHardness of the developed composi tes inc reased w i th increase in percentage of coconut shell ash particle additions. It is noteworthy that the hard value of coconut shell ash is 95.05HB and the presence of the hard ceramic phase in the ductile matrix has resulted in the increase in hardness of the composite (see Figure 2. For example the hardness values increased from 63.50HRB at 0wt% to 78.60HRB at 15wt%coconut shell ash particles. These increments are attributed to increase of the weight percentage of the hard and brittle phase of the coconut shell ash particles in the aluminium alloy. This hardness of the coconut shell ash particles is obtained from the SiC, Al2O3, Fe2O3 and SiO2 in the chemical makeup of the particles. Also the presence of coconut shell ash particles in the alloy increase the dislocation density at the particles-matrix interfaces. This is as a result of differences in coefficient of thermal expansion (CTE) between the hard and brittle reinforced particles and soft and ductile metal matrix which results to elastic and plastic incompatibility between the matrix and the reinforcement(Rajan et al, 2007, Aigbodion and Hassan,2010,Mohan and Srivastava,Sahin,2003).

Figures 3: Variation of Coefficient of Friction with wt% of Coconut shell ash particles

The friction force rises in the initial period and then fluctuates around a mean during dry sliding. The mean has been determined from the individual values of coefficients of friction excluding the initial rising part, and it has been observed that the mean coefficient of friction increases with increasing load for the Al-Si-Fe alloy and the composites containing CSAp as shown in Figure 3. At a higher load, the frictional force increases with greater dissipation of energy leading to a higher temperature at contact which results in a higher of coefficient of friction with increasing load. But higher coefficient of friction with increasing CSAp content in the composite is surprising as these particles are supposed to create weak junctions with the asperities of the counter face because of low oxide-metal interfacial energy and also, contribute in the formation of a greater cover of transfer layer which should make relatively weak junctions. Therefore, the only inescapable conclusion is that ploughing of the sliding surface and micro-cutting during the wear test contributes to higher frictional forces in composites containing more hard oxide particles.

ConclusionThe present study is centered on the effect of coconut shell ash as reinforcement on the hardness values and friction behavior of particulate metal matrix composites (PMMCs). Considerable success was recorded in the synthesis of Al-Si-Fe alloy with silicon carbide addition using stir-casting method. From the results obtained in this research, the following conclusion can be drawn:

1. The hardness values of the developed composites increased with increase in percentage of coconut shell ash additions. It is noteworthy that the hard value of coconut shell ash is 95.05HB and the presence of hard ceramic phase in the ductile matrix has resulted in the increase in the hardness of the composites.

2. It has been observed that the mean coefficient of friction increases with increasing load for the Al-Si-Fe alloy and the composites containing CSAp. At a higher load, the frictional force increases with greater dissipation of energy leading to a higher temperature at contact which results in a higher coefficient of friction with increasing load.

REFRENCESAigbodion V.S and Hassan S.B; Effects of silicon carbide reinforcement on microstructure and

properties of cast al-Si-Fe/Sic particulate composites materials science and engineering A.447(2007) Pp 355-360.

Aigbodion V.S and Hassan S.B: Experimental correlation between wear rate and wear parameter of Al-Cu-Mg/BAgasse Ash particulate composite. Journal of Materials and Design. Materials and design 31 (2010) 2177-578.

Ejiofpr, J.U and Reddy R.G; Developments in the processing and properties of particulate Al-Si Composite, Jom Publication of minerals, metals and materials society 49 (11) (1997) pp31-37).

Haryn M.B Shamsudin S.R, Yazid H, Selamut, Sattar M.S and Jalil M. effect of Fly Ash Particle Reinforcment on Microstructure, Porosity and Hardness in Al- (Si-Mg) Cast Composites, AJSTD Vol. 23 Issues 1&2 (2006) pp 113-122.

Ikechukwuka N. A Oguocha: Characterization of aluminum alloy 261 x and its composites containing alumina particles, unpublished PhD Thesis, Department of Mechanical Engineering, University of Saskaton (1997), pp1-2000.

Ibrahim I.A, Mohammed F.A and Lavernia E.J Journal of Materials Science USA 26, (1991) pp 1137-1156.

Surapa M.K Rohatgi P.K Preparations and properties of aluminum alloy ceramic particle composites journal of material science 16: (1981) pp983-993

Sarki J. Hassan S.B Aigbodion V.S, and Oghenvweta J.E; Potential of using