Study of dry sliding wear characteristics of sillimanite reinforced aluminium alloy matrix composites

Abstract

The aim of the present study was to develop light-weight, economical, and high temperature resistant aluminium matrix composites for brake rotor applications. Aluminium matrix composites (AMCs) containing LM30 aluminium alloy as matrix and sillimanite mineral particles as reinforcement were processed using stir casting. Sillimanite particles were taken in three different size range viz. coarse (75–106 µm), medium (32–50 µm), and fine (1–20 µm). Two main categories of AMCs were processed. The first category included single particle size AMCs (SPS) where the matrix material was reinforced with sillimanite particles of one specific size range (coarse, or medium, or fine). The second category included dual particle size AMCs (DPS) where the matrix was reinforced with sillimanite particles of two specific size ranges. For both type of AMCs, the maximum reinforcement level (with uniform distribution of particles with nearly no agglomeration) was 15 wt.% (0–15 wt.%; in step size of 3 wt.%). Further, for DPS composites, a mix of fine and coarse sized particles were reinforced with fine:coarse weight ratio as 1:3, 1:1, or 3:1. For processing of composites, the base alloy was melted and stirred at 630 rpm for 4–5 minutes at 750 . Pre-heated sillimanite particles (heated to 350 ) were added to molten mass at a reduced stirrer speed of 250 rpm. The mixture was stirred at 630 rpm for 10–12 minutes and poured into a cast iron mould. For wear testing, pins of 8 mm diameter were machined as per ASTM G99 standard. EDS analysis of sillimanite particles showed the constitution as SiAl1.8O6.1 which confirmed the purity of sillimanite (Al2SiO5). XRD analysis of base alloy showed presence of aluminium, silicon, and aluminium-copper phases. For AMCs, in addition to these phases, sillimanite and aluminium silicate (Al2Si4O10) phases were also observed. Optical micrographs of base alloy showed presence of eutectic aluminium, eutectic silicon, and large facets of proeutectic (primary) silicon phases. For AMCs, in addition to these phases, sillimanite particles were also observed. Micrographs revealed that sillimanite particles were uniformly distributed in AMCs till 15 wt.% reinforcement. X-ray line profile of AMCs revealed high concentration of silicon phase in the proximity of reinforced particles. Further, X-ray dot mapping showed significant refinement of silicon phase in AMCs compared to base alloy. High nanohardness values were obtained at the particle-matrix interface of AMCs indicating that the processing methodology was effective. Further, Brinell hardness values of AMCs showed increase with increase in reinforcement level and decrease in particle size. For SPS composites, maximum hardness was obtained for composite reinforced with 15 wt.% of fine sized sillimanite particles. The overall maximum hardness was obtained for 15 wt.% reinforced DPS composite containing large proportion of fine sized particles (fine:coarse ratio of 3:1). The hardness of this DPS composite was comparable (only 4.5% lower) to commercial grade cast iron material used in brake rotor applications. Wear rate of AMCs under room temperature conditions was significantly lower than the base alloy for any given contact pressure and sliding distance condition. Further, wear rate of AMCs showed a continuous decrease with increase in reinforcement level and decrease in particle size. For SPS composites, maximum reduction in wear rate (55% lesser than base alloy) was obtained for composite reinforced with 15 wt.% of fine sized sillimanite particles. The overall best wear results (59% lesser than the base alloy) were obtained for 15 wt.% reinforced DPS composites containing large proportion of fine sized particles (fine:coarse ratio of 3:1). The maximum wear rate of this DPS composite was comparable (only 8% higher) to cast iron specimen. Mathematical modelling of wear rate showed good agreement with the experimental results. Coefficient of friction (COF) of AMCs were also significantly vi lower than the base alloy with maximum reduction of 42% (over the base alloy) achieved for the 15 wt.% DPS composites. Finally, for room temperature testing conditions, SEM-EDS analysis of wear tracks and wear debris revealed that wear mechanisms causing material loss were mainly dependent on contact pressure and sliding distance. At low contact pressure and smaller sliding distances, abrasive wear mechanism was predominant. However, at higher contact pressure and larger sliding distances, adhesive/delamination wear mainly caused material loss. Next, the properties of processed AMCs were evaluated under elevated temperature conditions. DTA-TGA analysis revealed that sillimanite particles were thermally stable till 900 . Coefficient of thermal expansion (CTE) values of AMCs were significantly lower than the base alloy. For SPS composites, maximum reduction in CTE value (25% less than base alloy) was obtained for composites reinforced with 15 wt.% of fine sized sillimanite particles. The overall maximum reduction in COF value (28% lower than base alloy) was obtained for 15 wt.% reinforced DPS composite (with fine:coarse ratio of 3:1). For all elevated temperature conditions (50–300 ), sillimanite reinforcement decreased the wear rate of AMCs significantly over the base alloy. At any given elevated temperature, wear rate of AMCs decreased with increase in reinforcement level and decrease in particle size. The transition from mild-to-severe wear for base alloy was observed at an operating temperature of 150 . Sillimanite reinforcement in AMCs raised the transition temperature to 200 . At this transition temperature of 200 , maximum reduction in wear rate of 80% (over the base alloy) was shown by 15 wt.% DPS composite containing large proportion of fine particles (fine:coarse ratio of 3:1). The mean steady-state-wear rate of this DPS composite at the transition temperature (200 ) was even lower (1.5% less) than the cast iron specimen. This DPS composite also showed maximum reduction of 54% in COF value over the base alloy. The results of XRD analysis of wear track and wear debris of AMCs showed presence of various oxides (Al2O3, SiO2, Fe2O3, and FeO etc.), sillimanite particles, and various other phases like aluminium, silicon and aluminium-copper etc. and confirmed the formation of mechanically mixed layer. SEM-EDS analysis of wear track and wear debris at the transition temperature of 200 indicated that at contact pressure of 0.2 MPa, abrasive wear was predominant whereas at 1 MPa condition, adhesive/delamination wear mainly causing material loss. The present research showed that the best wear and friction results (both at room temperature and elevated temperature conditions) were observed for 15 wt.% DPS composite containing fine particles in larger proportion (fine:coarse ratio of 3:1). Wear rate and COF values of SPS AMCs at a given reinforcement level decreased with decrease in particle size. However, it was noted that at a given reinforcement level, the wear rate and COF values reduced further for DPS composites containing larger proportion of finer particles (fine:coarse ratio of 3:1). The fine particles provide large interfacial sites for effective load transfer and hence reduced wear rate/COF values whereas the coarse particles carry a major proportion of applied load and shield the finer particles from ploughing action. Further, the wear rate and COF values of 15 wt.% DPS composites with higher concentration of fine particles i.e. fine:coarse in the ratio of 3:1 was almost comparable with grey cast iron specimen. Also, the brake rotors fabricated using composites provide a weight reduction of 60% as compared to cast iron brake rotors. Considering these facts, 15 wt.% DPS composites with higher portion of finer particles can act as a substitute for brake rotor materials in light motor vehicles.