CERAMIC-METAL JOINING FOR ELEVATED TEMPERATURE APPLICATION

Abstract

The recent years have seen extreme progress in the use of ceramic'as a construction material. Due to its covalent bonding characteristics, ceramic have advantageous high temperature properties in comparison to metals. But the ceramics are brittle in nature. To obtain optimum properties of ceramic and metal, it is necessary to obtain good ceramic - metal joints for use in ceramic rotors in the gas turbine engines. Main problem in joining ceramic-metal joining is wetting and residual stresses in the joint. Many conventional methods such as direct bonding and mechanical attachment are used for joining, but active metal brazing is the most widely used technique for ceramic-metal joining. The aim of this work is to study the preparation of strong joint at hot pressed silicon nitride (HPSN) with ferritic stainless steel using high temperature brazing material (PdCo) for elevated temperature applications beyond 900°C. Studies have been carried out to produce the ceramic-metal joint by the methods as (1) Metallising of stainless steel part to provide flat coverage, followed by its brazing with HPSN and (2) Preactivation of HPSN by D.C. Magnetron sputtering of active material (Ti) followed by brazing with stainless steel. The physical and chemical characteristics of various stages of the process for joint preparation have been studied. The strength of the ceramic-metal. brazed joints has been characterized by the 4-Point bending test at ambient temperature. While metallising with the metallising fixture, increase in load on the steel block lead to decrease in wetting angle of filler metal with steel block. This increases the wettability of steel block by metallising filler alloy. First and second methods lead to bending strength of 161 and 75 MPa respectively. The decrease in sputtering thickness leads to increase in 4-point bending strength. The strength of joint was improved because the sputtering film improves the wettability of HPSN.