Study of Dry Sliding Wear Characteristics of Boron Carbide and Ilmenite Mineral Reinforced AMCs

Abstract

Dry sliding wear behaviour of stir-cast aluminium matrix composites (AMCs) containing LM13 alloy as matrix and synthetic ceramic particles and mineral particles as reinforcements was investigated at room temperature as well as at high temperature operating conditions. Particles were reinforced in three different ways viz. (a) only synthetic ceramic particles (boron carbide; B4C), (b) only mineral particles (ilmenite; FeTiO3), and (c) combination of synthetic and mineral particles in various mixing proportions. Composites processed using these reinforcements were designated as ‘BR’ composites, ‘IR’ composites, and ‘BI’ composites respectively. Particle size for both types of reinforcements was categorized into two classes viz. (a) ‘fine’ and (b)‘coarse’ whose range was 20–32 μm and 106–125 μm respectively. For each particle size, the weight percentage of reinforcement was varied from 0–15 wt.%, with a step size of 5 wt.%. For combination of synthetic and mineral particles, reinforcements with same particle size were mixed in three different weight proportions of 1:3, 1:1, and 3:1. For processing of AMCs, the base alloy (LM13) was melted in the stir casting set-up using an electric furnace maintained at 700 °C. Ceramic particles to be reinforced were pre-heated in another electric furnace maintained at 450 °C to eliminate moisture or volatile matter (if present) from the particles. Before addition of pre-heated particles, a vortex was developed in the molten mass by using a three-blade graphite stirrer. Uniform stirring at 625 rpm was maintained for 10 min. Thereafter, the required amount of particles (wt.%) was added to the molten mass by reducing the stirring speed to 250 rpm. Again, stirrer speed was maintained at 625 rpm for 10 min. Finally, molten mass was poured in a cast iron mould to allow cooling till room temperature. SEM images of ball milled reinforced particles were used to obtain the average particle size whereas EDS was used to obtain the elemental composition of reinforcements. For boron carbide particles, the average particle size of ‘fine’ and ‘coarse’ categories was 25.52±3.48 μm and 115.64±5.70 respectively. For ilmenite particles, the average particle size for ‘fine’ and ‘coarse’ categories was 25.24±2.85 μm and 116.60±5.72 μm respectively. EDS analysis confirmed 100% purity of both the reinforcements after ball milling process. XRD analysis of

‘IR’ composites showed presence of silicon oxide (SiO2), iron oxide (FeO), and titanium- silicon (TiSi2) phases. These phases were formed due to interfacial reaction between ilmenite

and silicon of base alloy. In hybrid composites, ilmenite particles caused a transition in phases in the presence of boron carbide particles leading to formation of iron oxide (Fe3O4), silicon

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oxide (SiO2), and titanium oxide (TiO). The formation of interfacial products resulted in strong interfacial bonding between ilmenite and matrix material. Reinforced particles were also responsible for reduction in crystallite size and rise in crystallinity. Relatively lower crystallite size and higher crystallinity were obtained on addition of boron carbide particles in comparison to ilmenite particles. Similar results were obtained with reduction in particle size from ‘coarse’ to ‘fine’. Optical micrographs of ‘BR’ composites, ‘IR’ composites, and ‘BI’ composites showed uniform dispersion of reinforced particles in the matrix material. Reinforced particles refined the eutectic silicon grain size (of matrix material) and changed its morphology from acicular to globular type. Increase in reinforcement level and reduction in particle size decreased the grain size for all the composites. Out of ‘BR’ and ‘IR’ composites, lower grain size was obtained for ‘BR’ composites. Further, for hybrid AMCs, increase in concentration of boron carbide in the reinforcement mixture resulted in lower grain size (‘15FBI-31’ composite showed the lowest grain size). For room temperature conditions, hardness of AMCs showed an increasing trend with rise in reinforcement level and reduction in particle size. For the case of individual reinforcements (single particle reinforced AMCs), maximum increase in hardness was obtained for ‘15FBR’ composite and ‘15FIR’ composite which was 41.07% and 34.77% higher than the base alloy. In case of ‘BI’ hybrid composites (dual particle reinforced composites) also, rise in reinforcement level resulted in higher hardness. At 15 wt.%, the hardness of ‘15FBI-31’ composite showed an increase of 39.87% over the base alloy. Increase in hardness of AMCs was attributed to increase in plastic deformation of the matrix material and rise in dislocation density in AMCs in the presence of reinforced particles. Coefficient of thermal expansion (CTE) showed reduction with increase in reinforcement level, both for single particle as well as hybrid AMCs. For any increase in temperature over the entire range (50–300 °C), CTE values showed an increasing trend for ‘IR’ and ‘BI’ composites. For ‘BR’ composites, CTE values showed an increasing trend till 100 °C beyond which values started decreasing. This variation in CTE was attributed to increase in solubility of silicon in aluminium of base alloy under the presence of boron carbide particles. For a particular combination of reinforcements in hybrid AMCs, highest reduction in CTE value was obtained at higher level of reinforcement (15 wt.%), with ‘fine’ particles of reinforcement, and with higher content of boron carbide in the reinforcement mixture (B4C:FeTiO3 wt.% equal to 3:1). Maximum improvement in CTE of ‘15FBR’, ‘15FIR’, and ‘15FBI-31’ composites was 49.67%, 60.02%, and 55.18%, respectively (over the base alloy). Dry sliding wear behaviour of ‘BR’, ‘IR’, and ‘BI’ composites significantly improved over the base alloy under room temperature conditions. In the context

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of individual reinforcements, addition of boron carbide particles helped in providing increased stability to the mechanical mixed layer. The low thermal conductivity of ilmenite particles resulted in early oxidation and provided a protective layer on the surface of composites. So, both types of reinforcements led to improvement in wear characteristics of AMCs, though the mechanisms involved were very different. The wear rate of ‘15FBR’, ‘15FIR’, and ‘15FBI-31’ composites showed a maximum improvement of 61%, 52%, and 90% over the base alloy. Comparison of ‘15FBI-31’ composite with the commercially used brake rotor material under room temperature conditions showed superior wear behaviour (14% lower wear rate), lower processing cost, and lower materials cost. These characteristics make the developed composites a suitable substitute for commercial material being used for brake rotor applications (under ambient conditions). With regards to coefficient of friction (COF) values, lower values were obtained on addition of ilmenite particles (i.e. ‘IR’ composites). COF values reduced with lower particle size, lower applied load, higher reinforcement level, and higher concentration of

boron carbide particles in the reinforcement mixture. COF of ‘15FIR’, ‘15FBR’ and ‘15FBI- 31’ composites resulted in a maximum reduction of 35.18%, 27.45%, and 46.21% over the

base alloy. These improvements were caused by (a) early oxidation of sliding surface and formation of strong interfacial bonding by ilmenite particles, and (b) increased stability provided to mechanical mixed layer by boron carbide particles. SEM images of wear tracks and worn debris revealed increase in plastic deformation with rise in applied load conditions. This was attributed to rise in formation of craters (delamination wear). EDS analysis signified the formation of mechanical mixed layer (MML) on sliding surfaces. MML was relatively more stable for boron carbide reinforced ‘BR’ composites. For high temperature conditions, wear rate of AMCs showed an increasing trend with increase in applied load and operating temperature. For all the samples, transition temperature was 200 °C after which wear rate showed a sharp increase. Improvement in wear characteristics of AMCs (compared to base alloy) at high operating temperatures conditions was attributed to higher restriction to micro-cracks by refined grain structure, higher pinning effect causing restriction to thermal softening, strong interfacial bonding between matrix-reinforced particles, and early formation of oxide layer. At transition temperature of 200 °C, the maximum improvement in steady state wear rate of ‘15FBR’, ‘15FIR’, and ‘15FBI-31’ composites was 82%, 75%, and 87% respectively (over base alloy). The composite having lowest wear rate (‘15FBI-31’ composite) showed comparable wear rate behaviour with commercially used brake rotor material (only 23% higher). COF of AMCs showed an increasing trend with rise in applied load and operating temperature. However, at a given testing condition, a reduction in

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COF values was observed with increase in reinforcement level and proportion of boron carbide particles. Similar observations were noted with reduction in particle size. Maximum reduction in COF values was obtained for ‘15FBI-31’ composite (with reduction of 46.15% at 100 °C), which was followed by ‘15FIR’ composite (with reduction of 37.16% at 200 °C) and ‘15FBR’ composite (with reduction of 32.33% at 100 °C). SEM analysis of wear tracks and debris at 200 °C revealed the change in mechanism of wear with increase in applied load conditions. At 200 °C-9.8 N, abrasive wear was dominant mechanism for removal of material which changed to delamination wear for 49.0 N at 200 °C. For each property under a given testing condition, the hybrid composites showed superior results than the single particle reinforced AMCs. This signified that each type of reinforcement in the hybrid AMCs played a significant and diverse role in improving the properties. Boron carbide particles were responsible for higher stability of MML, higher refinement of eutectic silicon and increased resistance to softening of matrix material at high temperatures. On the other hand, ilmenite particles helped in early oxidation of the pin surface and formed strong interfacial bonding between matrix-reinforced ilmenite particles.