Sintering behaviour of glass-silicon nitride composites for thermal spray precursor powders

Silicon nitride composite powders have been prepared in which each individual powder particle contains submicron silicon nitride particles completely encapsulated in a glass matrix. This necessitated raising the sintering temperature as the silicon nitride content increased in order to reduce the viscosity of the glass and enhance the Darcian flow. However, these higher sintering temperatures caused porosity due to thermal degradation of the glass, and as a consequence, the density of the composites decreased monotonically with increasing silicon nitride content. The hardness results revealed two conflicting effects of increasing the silicon nitride content and as a result, an optimum silicon nitride content of 30 vol.-% giving a hardness of over 680 HV was found. The silicon nitride was also found to have a greater effect on scratch resistance than hardness due to its greater ability for inhibiting crack development than shear banding.