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 degreesC) 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 x 10(-17) m(2) s(-1) for the least reactive system and 486 x 10(-17) m(2) 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. (C) 2002 Elsevier Science Ltd. All rights reserved.