Effects of Si and Oxide Additives on Phase Evolution, Microstructure and Mechanical Properties of Si3N4–Mo Cermet Fabricated by Conventional Powder Metallurgy

Abstract

Silicon nitride (Si3N4) based cermets are very popular due to their versatile applications like high temperature engine parts, cutting tools, refractory and structural applications. Here, in this work, Si3N4–Mo (Si3N4 as matrix and molybdenum as reinforcement) and Si3N4–Mo–Si (Si3N4 as matrix, Mo and Si as reinforcement) cermets are fabricated through powder metallurgy route and phase evolution, micro-structure and mechanical properties are analyzed. Three initial powder compositions of 90Si3N4–10Mo (S90M10), 75Si3N4–25Mo (S75M25) and 75Si3N4–15Mo–10Si (SMS) (all in wt%) are prepared through planetary milling and total additive amount of 5, 10 and 15 wt% of Y2O3 and Al2O3 are mixed (equal wt ratio of each) with milled base powder, cold compacted and followed by conventional pressure-less sintering at 1500 °C for 1 h. The effects of additives and reinforcements on phase evolution, microstructure and mechanical properties are discussed. From XRD and SEM analysis of sintered pellets, the presence of molybdenum silicide (i.e. Mo3Si, Mo5Si3 and MoSi2) in all the cermets is confirmed. It has been observed that the relative density and hardness values are increased with the increase in additive wt%. Comparing all the cermets, 75Si3N4–15Mo–10Si with 10 wt% additives has shown the maximum compression strength and fracture toughness of 134.2 MPa and 8.89 ± 0.34 MPa.m1/2 respectively. The maximum relative density of 75.24% is observed in S90M10 and hardness of 8.81 ± 0.13 GPa is found in S75M25 both cermets containing 15 wt% of additives. The transformation from α- to β- Si3N4 is observed in all the cermets after sintering.