The reaction rate at Si3N4/steel interfaces as a function of sintering aids

The solid-state interaction between Si3N4 and steels can be appraised with high temperature (1000–1300 °C) static diffusion pairs. The reaction zone thickness of the ceramic side of these couples distinguishes the chemical resistance of different Si3N4 based compositions to the attack by iron alloys. A careful selection of the amount and composition of sintering aids for Si3N4 sintering may contribute to increase the chemical/diffusive wear resistance during cutting of Fe-based alloys. The reaction interfaces in diffusion couples were analysed with SEM/EDS and optical microscopy and the attack path within the ceramic was investigated by TEM on reaction interfaces of selected samples. Parabolic reaction rate constants at 1423 K vary between 7×10−17 m2 s−1 for the least reactive system and 486×10−17 m2 s−1 for the most reactive ceramic. The former is a Y2O3–SiO2 containing Si3N4 ceramic while the latter is from the less refractory AlN–CeO2 additive system. TEM analysis proved that intergranular amorphous phases are the preferential path for Fe penetration. A compilation of published kinetic data in Si3N4/steel interaction is thoroughly compared with the present experimental results.