Spark plasma-sintered MoSi2-reinforced Y−α−SiAlON ceramics: mechanical and high temperature tribological properties

Silicon aluminum oxy-nitride commonly abbreviated as SiAlON is indeed one of the most promising non-oxide engineering ceramics due to its ease of formation compared to extremely covalent silicon nitride ceramic and scope of tailoring the material properties as per application demand. To make it more suitable for high-performance applications, composites containing various secondary phases have been attempted so far to improve the mechanical performance over its monolithic counterpart. In the present work, reinforcement of particulate molybdenum disilicide (MoSi2) in Y-α-SiAlON matrix has been undertaken under spark plasma sintering (1750 °C, 10 min, 50 MPa die pressure) followed by Vickers hardness (HV), fracture toughness (KIC), and high-temperature tribological properties under different conditions of the formed composites containing 10 and 20 wt.% secondary phase were investigated. Sintered specimens were ≥ 97.5% dense with negligible porosity. Addition of MoSi2 in Y−α−SiAlON resulted in reduced HV. The 20 wt.% MoSi2/Y−α−SiAlON composite showed ~ 11% lower HV1 compared to the monolith. On the contrary, KIC of the 20 wt.% MoSi2/Y−α−SiAlON composite was found to be around 24% higher compared to pure Y−α−SiAlON (KIC ≈ 3.9 MPa-m0.5). Additional fracture energy dissipation through crack deflection and bridging by the dispersed MoSi2 particulates was the primary reason behind obtaining higher fracture toughness for the composites. Unlubricated reciprocating ball-on-disc experiments against dense silicon nitride ball revealed a significant effect of MoSi2 oxidation in achieving improved wear resistance of the composites over the monolith, especially, beyond 300 °C in ambient air under applied normal loads up to 90 N and sliding distance up to 70 m.