Kinetic mechanism and microstructure during the formation of Si3N4 matrix in Si3N4–MoSi2 composites

The silicon nitridation reaction of powders formed by MoSi2 and Si has been studied in the temperature range 1350–1450 °C. Below the melting point of Si, a gas–solid reaction occurs, forming a Si3N4 film on the Si particles. The film reaches a maximum thickness of 250–300 nm, when the degree of nitridation β equals 15%. For temperatures higher than the Si melting point, the reactions 3Si(s)=3Si(l) and 3Si(l)+2N2(g)=Si3N4(s) account for the formation of an interconnecting percolate microstructure of Si3N4 and pores. The temperature dependence of this process has been explored in the range 1410–1450 °C and the total apparent enthalpy of activation is 681±12 kJ mol−1. The rate-determining step has been identified with the liquid-gaseous silicon nitridation reaction. Needle-like structures of Si3N4 are observed in the composite matrix where the MoSi2 clusters are spread all over with pores of 340 nm in diameter and total porosity equal to 37vol.%.