Gr–Al composite reinforced with Si3N4 and SiC particles for enhanced microhardness and reduced thermal expansion

Abstract

Thermal management remains an important consideration for many applications such as automobile pistons, dies in furnaces and heat sinks in electronics, where high thermal conductivity and low coefficient of thermal expansion are desired. In this paper, we report an improved microhardness value and negative coefficient of thermal expansion (CTE) for Gr–Al composites by incorporating silicon nitride (Si3N4) and silicon carbide (SiC) into Gr–Al matrix. Si3N4 and SiC particles (average size of 72 nm) were obtained after 30 and 50 h of ball-milling, respectively. Gr–Al composite were reinforced with Si3N4 and SiC particles (4, 8, 12 and 16 wt% each), and then sintered at a temperature of 560 °C, pressure of 50 MPa, heating rate of 50 °C/min and dwelling time of 10 min using spark plasma sintering system. The sintered samples were characterized for morphology, microhardness, relative density, coefficient of thermal expansion and wear resistance. Addition of 16 wt% Si3N4 reduced relative density from 99.9 to 89.47% indicating lower densification. CTE decreased with increased Si3N4 fraction reaching minimum value of 12.37 × 10–6 °C (which is 42.5% reduction) for Gr–Al + 12 wt% Si3N4. However, Gr–Al + 8 wt% SiC + 8 wt% Si3N4 composite has the minimum negative CTE (− 21 × 10–6 °C), maximum displacement (2.48 mm), maximum displacement rate (1.12 mm/s) and minimum hardness value (26.8 GPa). While the hardness was marginally increased with Si3N4, addition of 12 wt% SiC + 12 wt% Si3N4 raised the hardness by 37%. Also, the wear area increased with applied force. These results indicate that Gr–Al + 8 wt% SiC + 8 wt% Si3N4 composite is suitable for applications requiring combination of low (negative) thermal expansion, high microhardness value and maximum densification. Effectively, the synergic effect of combined SiC and Si3N4 particles has been brought to the fore.