Joining of chromized carbon-steel and Si₃N₄

A 0.96 mass%C steel coated with chromium carbides was brazed to Si₃N₄ by using Ag-Cu/Ti alloy. Chromium carbides were coated on the steel at 1373 K for 2.5 to 36 h in an [Ar+HCl] mixture gas using the Cr powder pack process. The obtained coating layer consisted of M₇C₃ and M₂₃C₆(M = Fe, Cr). The bonding treatment was performed at 1173 K for 1 h in vacuum. The shear strength of the joint increased with increasing coating layer thickness up to 20 μm. The average and maximum shear strengths of the joint with a coating layer of 20 μm in thickness were 209 and 258 MPa, respectively. Those of the joint without the coating layer were 149 and 218 MPa, respectively. The Weibull slopes for the shear strength were 9.4 in the former and 3.8 in the latter. The coating layer was considered to play an important role as an active layer of interfacial reaction and as a relaxation layer of thermal stress in the joint. The difference in interfacial structures between these joints was also discussed on the basis of TEM observation.