Chemical interaction silicon nitride ceramics and iron alloys

Metal/ceramic diffusion experiments are helpful to study bonding mechanisms or the effect of metal composition on the chemical wear of ceramic cutting tools. The reaction kinetics of Fe alloys/Si3N4 ceramic diffusion couples was investigated in the temperature range 1050 degreesC-1250 degreesC, for 0.5h to 80h, under inert atmosphere. Optical microscopy, SEM and EPMA were carried out in cross sections of the reacted pairs. Si3N4 decomposes into S i and N that dissolve and diffuse through the metal. Both the diffusion zon e on the metal side and the reaction zone on the ceramic side obey paraboli c growth laws of time, with activation energies in the range Q=310-460kJmol (-1). The amount of dissolved Si, the length of the diffusion zone and thus the reactivity of the ceramic increase as the alloy carbon content decreas es. Due to Si accumulation, the alpha -Fe solid solution is stabilised at t he reaction temperature and a steep decrease in the Si concentration is obs erved beyond the diffusion zone. The reinforcement of the Si3N4 composites with Al2O3 platelets enhances the chemical resistance of the ceramic due to the inertness of this oxide and to the partial crystallisation of the inte rgranular phase. Other dispersoids such as HfN, BN and TiN do not improve t he chemical resistance of the matrix by iron attack.