Title: Tribologicalevaluation of Hybrid Aluminum matrix composites (HAMCs) for
high temperature applications.
Author: *SuyashY.Pawar 1&Yashwant R.Kharde1
Affiliations: 1Department of Mechanical Engineering, Pravra Rural Engineering College, Loni, Ahmednager 413736, Maharashtra, India.
*Corresponding author:
Mr.SuyashY.Pawar,Department of Mechanical Engineering, Pravra Rural Engineering College, Loni, Ahmednager 413736, Maharashtra, India
Ph No: +91-9284720898
Email: suyashyp@gmail.com
Abstract:
The current study explores the Tribological behavior of Hybrid Aluminum matrix composites (HAMCs) reinforced with Silicon Carbide (SiC) and Nickel coated Graphite (Ni.-Gr.) at varioustemperatureconditions for high temperature applications.TheHAMCs were developedby stir castingmethod by addition of reinforcing particles like SiC (5% & 10%) and Ni.-Gr.(2% fixed) into LM26 aluminum alloy. Further friction and wear properties were determined with the help of pin on disc apparatus.The L9 orthogonal arraywas selected for experimentation to understand effects of process parameters like weight percentage of hybrid reinforcements, load, sliding distance and temperature. Analysis of Variance (ANOVA) technique was used to find out optimumslidingconditionforreducedwearlossandCoefficient of friction (COF)for HAMCs.It was observed that the temperature and load predominantly affects the tribological behavior of HAMCs. The wear and COF properties were affected by varying hybrid percentage (%) and the results were supported with worn surface SEManalysis.
Keywords:Hybrid Aluminium matrix composites, Wear, COF, Taguchi technique, ANOVA.
Abbreviations:
Aluminium matrix composites :AMCs
Hybrid Aluminium Matrix Composites: HAMCs
Silicon Carbide :SiC
Nickel coated Graphite :Ni.-Gr.
Coefficient of friction: COF
SEM: Scanning electron microscope
- Introduction
Modern engineering systems require materials with an extensive range of properties, which are quite difficult to achieve using singlematerial alone. In the present era, the need for low cost, high performance materials is greatly increased. Researchers across the world aiming to substitute the metals and alloys with the advanced materials. Metal matrix composites (MMCs) have been well known to offer such suitable property combinations required in a wide range of applications. The Aluminium matrix composites (AMCs) are in great demand for their strength, stiffness and light weight, when reinforced with ceramic particles [1]. During adversative conditions like high temperature, lack of lubrication and so on, the aluminium alloys show poor mechanical and tribological properties. To succeed in dealing with this, new materials with greater resistance to wear with expected tribological and mechanical properties support the development of AMCs[2]. Nowadays a new class, Hybrid Aluminium Matrix Composites (HAMCs) become the primary substitute material in many engineering applications like automobile, aerospace, defence and other related sectors due to their smart properties. The use of HAMCs can provide salient features like low cost and weight, in addition to this use of solid lubricant particles, such as graphite in HAMCs can reduce friction coefficient and wear rate of composites.It has been reported by several investigators that,HAMCs comprises primary and secondary reinforcements which includes use of ceramic particulates like Titanium carbide(TiC), alumina (Al2O3), Silicon carbide (SiC), Boron carbide (B4C), Silica (SiO2), Aluminium nitride (AlN) and Industry wastages such as red mud,fly ash with secondary reinforcements like graphite,MOS2 [3]. There are several techniques available for making HAMCs from powder metallurgy to casting methods. The use of stir casting technique for fabrication of HAMCs reduces the cost of composites, as it is economical, simple to perform and highly productive method [4].
The AMCs reinforced with SiC particulates are known for high strength and wear resistance compared to traditional aluminiumalloys[5].Increaseinsize and particulates volumefraction ofSiCdecreases the specific wear rate of AA6061/SiC composites [6]. Improved properties like wear resistance and mechanicalstrengthwere observed withan increaseinSiCparticulates volume fraction but at the same time increasein hardnesswas also found which affects machinability of composite material [7].If solid lubricant particles likeGraphite (Gr.) and Molybdenum disulphide (MoS2)were added along with hard ceramic particles like TiB2,B4C wear properties of composites were improved [8-10]. Further, properties like wear resistance and mechanicalstrength were improved by addition of hard particles like SiCwith reduction of coefficient of friction due to the formation of lubrication film were reported[12-13].For friction stir welded Al/Mg2/Si metal matrix cast composite rise in Gr. level decreases both fracture toughness and wear resistance of the composites [14].In some studies increase in Gr. percentage reduces hardness of the material with porosity problems limits add on level of solid lubricants [15].Further adverse effect of additional Gr. level were eliminated by the alumina particle addition were also reported [16].Hard particles like carbon and mica reinforced AMCs with improved hardness were testified [17]. AMCs like Al-Mg/ Al2O3 and B4C shown improved wear resistance than pure Al-Mg alloy alone[18]. Effect of influencing parameters like applied load, sliding distance and reinforcement percentage on friction and wear behaviour using statistical techniques were also described[19-20].Hence, it can be stated that optimum level of solid lubricant addition is preferred and it can be combined with other ceramic particles such asSiC,B4C,TiCand TiB2 etc. Further, in order to improve theproperties like wear rate and friction, most of the cases graphite is added as reinforcementbecause of its self-lubricating nature. But very few attempts only have been made on Nickel coated Graphite (Ni.-Gr.) as a consolidation component to aluminium matrix composites which also have better self-lubricatingand wettability properties.
Therefore, present research in HAMCs is intended to develop stable material for high temperature applications which may offer higher wear resistance and low coefficient of friction, which would be inexpensive, lighter, nontoxic (lead free) and probably self-lubricating (minimal lubrication) as compared to other metals and alloys.
2 Materials & Methods
2.1Preparation of Material
The Aluminium alloy (LM26) is selected as base material and SiCparticles of size41μm and Ni.-Gr. of size 93μmwere selected as a reinforcement material. TheLM 26 alloy ingots were placedin a graphite crucible and heated up to 725°C in anmuffle furnace and then a measured quantity of preheatedSiC powder is added to molten metal with a fixed amount of Ni.-Gr.(2%)as a wettability agentfollowed by stirring at the speed of 380 rpm for 10 minutes in closed chamber with nitrogen gas purging as shown in Fig. 1. Then the composite mix is discharged into the die to get the required samples.Same procedure is repeated to fabricate other composites by varying weight percentage of SiC (5%, 10%)with a fixed amount of Ni.-Gr.(2%) . The wear test samples were prepared with the help of CNC machine. Then, end surface of the samples were polished with emery paper of different grit size to attain a uniform surface. The developed samples were eviscerated by using acetone in order to remove impurities and foreign particles present in the surface.The matrix materials used in this experiment was LM-26 alloy whose chemical composition is shown in Table 1.
2.2 Wear and Friction Test
Wear tests were conducted using Pin on Disc apparatus (DUCOM TR-20LE-PHM 600). The specimens were produced according to ASTM G99−95 standards with size of the specimen 30 mm in length and 10 mm in diameter.The wear tests were performed under three different loads (10N, 20N and 30N), temperatures (1000C, 1500Cand 2000C), sliding distances (400m, 600m and 800m), three combinations of reinforcement percentages 0% wt, 5%wtand 10%wt of SiCwith 2%Ni.-Gr. constant.For overall trials,sliding velocity was kept 1.57m/s constant.The prepared pinswere held against a rotating disc made of EN32 steel disc hardened to 65 HRC. For all the set of experiments, each trial was repeated three times to get exact wear loss values. After each trial, the disc surface was cleaned with acetone to remove contamination and wear fragments fused on it. Weight lossof the specimen was measured with the help of a weighing machine of 0.001g accuracyand the coefficient of friction (COF) was calculated by the ratio of tangential and normal force (µ = Ft / Fn).
2.3 Design of Experiments
The present study explores the effect of applied load,weight percentage of hybrid reinforcement, temperature and sliding distance on wear loss and coefficient of friction of the prepared hybrid composites.Taguchi designof experiments technique wasselected and L9 orthogonal array was selected with the help of Minitab 18 software. The array selection depends on the number of factors, levels and their responses involved.All the considered factors are shown in Table 2with their levels.Corresponding L9 orthogonal array designed based on this data is as shown in Table 3.The S/N ratio combines various data and assesses them based on the characteristics of the data.Further three types of evaluation criteria was selected as “smaller the better”, “larger the better” and “nominal the best”. Out of these three criteria,“Smaller the better” characteristic of S/N ratio was considered for minimum wear rate and COF. Further Analysis of Variance(ANOVA) technique was used to determine the percentage effect of each parameter on wear and COF.
3 Results and Discussion
3.1Microstructure Analysis:
Microstructure analysis of the LM 26 alloy and developed composites were carried out byScanning electron microscope (SEM). Figure 3a&3b shows microstructure of LM26 alloy and LM26+10% SiC+2%Ni.-Gr.hybrid composite respectively.The distribution of reinforcement particles in LM26 alloy withSiCand constant weight percentage ofNi.-Gr.clearly indicates their uniform distribution(Fig.3c) in matrix alloy.
3.2 Density and Hardness
The hardness and density of pure LM26 alloy and differenthybrid composites prepared by varying weightfraction of SiC&Ni.Gr were shown in Table 4. Density of the composites is found to be increasingwith increase in reinforcement,amongst all HAMCs developed, the compositereinforced with 10% SiC has superiorhardness value. Increase in hardness of the HAMCs with reinforcement addition may be attributed to higher hardnessvalue of reinforced SiC.
3.3Wear Loss and Coefficient of Friction
The values of wear loss and COF for the different combination of control factors are shown in Table 5. The response variables were converted into SN ratio based on three criteria namely“smaller the better”, “larger the better” and “nominal the best” as per Taguchi method. SN ratio value is calculated by considering “smaller the better” criterion since the objective is to decrease the wear lossand COF. Main effect plots wereconstructed (Fig. 2 & 3) and the effect of each factor over responsevariableswas identifiedusing the calculated SN ratio. Effect of control factors on wear loss is shown in Table6.Wear loss of the HAMCs decreases with an increase in wt.% of hybrid reinforcement this isbecause harder materials with refined microstructurealways showshigher wear resistance than softer materials.Also, as hybrid reinforcement percentage is increased, the amount of soft matrix (pin)asperities that is in contact with hard counterface (disc) reduces, which results in increased wear resistance [21]. The distribution of SiC and Ni.-Gr. in the aluminium matrix forms a strong interfacial bond which minimizes the wear loss of the hybrid composite. The wear rate is much lower for the HAMCs at all temperatures in comparison to the pure LM26 alloy because of improved strength and grain refinement along with precipitation and dispersion strengtheningof the HAMCs [22].As load and sliding distance increases, the wear loss also increases because contact between hybrid composite and counter body also increases. Further increases in wear loss was seen as increase in temperature because of material softening[23].Wear loss is low for 10% SiC+2 %Ni.-Gr. composite, so the developed hybrid composite can be used for the high temperature wear resistant applications. From the main effect plot the optimum condition for reduced wear lossis LM26+10% SiC +2 %Ni.-Gr., 10N load, 100 °C temperatures and 800m sliding distance.
3.4 ANOVAforWear Loss
The experimental results were analysed using ANOVA technique to investigate the effect of the wear parameters namely; applied load, sliding distance, weight percentage of reinforcement, and sliding distance. The analysis was carried out at 95% confidence level and 5% significance level and the results are shown in Table 6. The value of significance factor alpha is 0.05Pr. indicates percentage of contribution and degree of influence of each variable on wear loss. From ANOVA, most significant factor was denoted on the basis of P value. It should be noted that P value for load is approximately zero so it has greater influence on wear loss (45.51%) followed by temperature, percentage of reinforcement and sliding distance that contributes 29.79%,17.9% and 0.29% respectively.In S/N ratio graph the factors which lies far away from horizontal line has most significant effect on the response variables and the factor which lies closer to horizontal line has less significant effect.The main effect plot indicates that applied load and temperature lies far away from the horizontal line and hence most influencing factor in wear loss. The response table for S/N ratio was calculated in MINITAB 18 software. Ranking of each factor was formulated on the basis of delta value, which is the difference between maximum and minimum value of signal to noise ratio. As load parameter having highest value of delta so it’s the most significant factor for wear loss as shown in Table 7.
3.5Effect of Control Factors on COF
Effect of control factors on COF is shown in Figure 4&5and Table 8. The COF of the LM26 alloy increased with increase in load and temperature due to the severe transfer of pin material to the counter face [23]. Further, it was observed that at higher loads the COF is high due to formation of rough surface with reinforcement lumps [24].COF of the composite decreases with increase in wt. percentage of hybrid reinforcementbecause hard abrasive reinforcement particles present on the composite surface delays the severe contact between the composite pin and counter disc. It clearlyindicates presence of thermal stability and formation of oxide layer in between mating surfaces of hybrid composites. Therefore less energy is required to break off asperities while sliding for HAMCs as compared to the pure LM26 alloy [25,26].
3.6 ANOVAfor COF
The experimental results were analysed using ANOVA technique to investigate the effect of the various parameters namely; applied load, sliding distance, weight percentage of reinforcement, and sliding distance on COF.From ANOVA most significant factor were denoted on the basis of P value. It should be noted that P value for percentage of reinforcement approximately zero so it has greater influence on wear loss (59.12%) followed by sliding distance, load and temperature that contributes 16.70%,10.06% and 2.27% respectively. The response table for S/N ratio was calculated in MINITAB 18 software. Ranking of each factor was formulated on the basis of delta value which is the difference between maximum and minimum value of signal to noise ratio. As percentage of reinforcement parameter having highest value of delta so it’s the most significant factor for COF as shown in Table 9 and Figure 6&7.
3.7Analysis of Regression Equation and Prediction
To establish relationship between control factor and response factor a mathematical model for wear loss and COF was developed using regression method.The Equations 1and 2are the developed mathematical expression for wear loss and co-efficient of friction based on weight percentage of hybrid reinforcement , temperature, load, sliding distance.
Wear loss = 0.013048 + 0.0000450 (Load) +0.0000007 (Temperature) – 0.000057 (% Reinforcement) + 0.000000176 (Sliding Distance) …….. (Eq.1)
COF = 0.4183 + 0.000983 (Load) + 0.000093 (Temperature) -0.00477 (% Reinforcement) + 0.000063 (Sliding Distance) …………. (Eq.2)
Negative value of correlation coefficient corresponding to percentage of reinforcement indicates that wear loss and COF are inversely proportional. Correlation coefficient having positive value shows there is direct relationship between control factor and response factor. As the control factors such as applied load, temperature and sliding distance increases wear loss and coefficient of friction also increases. Sliding distance has a negligible effect on wear loss whereas applied load and temperature has negligible effect on coefficient of friction.
3.8Confirmation Experiment
The final step in experiment is confirmation test conducted for sample materials by selecting set of variables as shown in Table 10. In confirmation experiment values of wear loss and COF obtained from regression equation are compared with experimental value and corresponding error is determined. From the result we observed that, wear value for wear loss is varies from 2.28% to 7.73% (Table 11) whereas for coefficient of friction error value varies from 3.55% to 6.68%. (Table12).Thus, the calculated values for wear loss and COF closely resembles with actual data with minimum error, thus design of experiment (DOE) by Taguchi technique was successfully implemented for calculating wear loss and COF for HAMCs.
3.9 Wear mechanism
The SEM micrograph of worn surface of LM26 alloy matrix tested at 30N load with room temperature (R.T.) and 200◦C (Fig. 8a &8b) clearly shows the presence of parallel grooves on worn surface due to ploughing, micro cutting and grain pull outs which may have caused the increase of wear loss.This morphology of LM26 alloy indicates that at room temperature, wear mechanism observed is abrasion whereas at high temperature(200◦C)severe mode of wear with adhesion and metal flow is observed.However, the worn surfaces of the LM26+10% SiC +2 %Ni.-Gr. HAMCstested at 30N load with temperature 100◦C and 200◦C (Fig. 8c&8d) show finer grooves and slight plastic deformation. As the SiC weight percentage increases the surface morphologies also have been altered. The surfaces also appear to be smooth because of the Ni.-Gr. reinforcement addition. The worn surface of hybrid compositeis smooth with fewer grooves due to the lubrication effect of Ni.-Gr.The general wear pattern appears to be ploughing, fine grooves with less plastic deformation. It forms an adherent film over the contact surface which leads to decrease of plastic deformation of HAMCs. The fine Ni.-Gr. grains weremixed properly with a smooth graphite rich tribolayeris formed on the worn surface. Therefore, the probability ofsevere wear in the HAMCs is low and abrasionand delamination are the dominant wear mechanisms.
- Conclusion
The LM26/SiC/Ni.-Gr. HAMCs weresuccessfully fabricated through stir casting method. TheTaguchi method could be employed to find the optimum factors for better wear and friction performance of LM26/SiC/Ni.-Gr. HAMCs.The ANOVA results specified that, wear loss applied load (45.51%) is most dominating factor followed by temperature (29.79%) and percentage of reinforcement (17.94%). For COF percentage of reinforcement (59.12%) is most dominating factor followed by sliding distance (16.70%) and load (10.06%). Signal to noise ratio is used for optimization of operating condition. Maximum value of signal to noise ratio in S/N ratio plot gives us optimum condition. The results shows that applied 10N load, 1000 C temperature, 10% hybrid reinforcement and 400 m sliding distance gives optimum condition. Confirmation of experiment is done which shows error for wear loss is varies from 2.28% to 7.73% and for coefficient of friction (COF) it varies from 3.55% to 6.68%, resulting in conclusion that design of experiment (DOE) by Taguchi method is successful for calculating wear loss and COF with least error.The microstructure results shows that,at room temperature and elevated temperature wear resistance of HAMCs is increased with the addition of SiC and Ni.-Gr. particles. Coefficients of friction of these HAMCs are lower than the base alloy. The wear rate of pure LM26 alloy and HAMCs increasedwith increase in applied load and sliding temperature.The mild to severe wear mode transition in HAMCs is dependent on theapplied load, temperature and reinforcement content.Normalized wear rate decreases with increase in temperature for the given amount of reinforcing particles, which clearly indicates that the SiC and Ni.-Gr. particles in the LM26 alloy are more effective in resisting the wear at high temperaturethan at room temperature.