MECHANISM OF REACTION BETWEEN SI3N4 AND FE-CR ALLOY UNDER AR ATMOSPHERE

In relation to the joining of silicon nitride ceramics to metal, the r eaction products and reaction mechanism between Si3N4 and Fe-Cr alloy have been investigated under Ar atmosphere at temperatures from 1223 t o 1573 Ii. Using Si3N4/Fe-62.5 mass%Cr alloy powder mixture, the react ion rates were thermogravimetrically determined, and the reaction prod ucts were identified by X-ray diffraction. Cr2N was produced below 127 3 K. Above 1273 Ii, the solid solution and silicides were produced, re sulting in the mass loss. At higher temperatures and on prolonged heat ing, the silicides changed in the following order: Cr3Si, Cr5Si3 and C rSi. Two types of Cr5Si3 were found; hexagonal crystal above 1323 Ii a nd tetragonal crystal above 1423 Ii. Iron is considered to form the so lid solution with chromium silicides. The initial rate was described b y the linear rate law. The activation energy was 264 kJ/mol. An interf acial reaction may be the rate-determining step: When the alloy partic les were covered with the reaction product layer, the kinetics obeyed the parabolic rate law. The reaction rate is probably controlled by th e solid-state diffusion through the reaction layer.