Thermochemistry of contacts between silicon nitride ceramics and steels

The effects of C and Cr content of iron alloys on the kinetics of reaction of silicon nitride ceramics with pure iron and steels are investigated. Kinetics were evaluated from diffusion couples reacted at 1150 degreesC under Ar, for periods of 5-80 h. The reaction is diffusion controlled, with an induction time at the onset for some of the couples. Reaction constants increase with increasing Cr content and with decreasing C content of the steels. Assessed by thermodynamic data (SGTE), the computer software package Chemsage was used to predict equilibrium concentrations at the advancing interface between the alpha/(alpha + gamma)-Fe phases on the steel side of the diffusion couples. A linear relationship was found between the calculated values of silicon concentration at this interface and the resistance of the ceramic to iron attack, this dependence being tentatively linked to the properties of the asymptotic solution of the appropriate diffusion model. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AII rights reserved.