Failure Mechanisms of a Ceramic-Metal Interface Under the Application of High Temperature Loading

Abstract

Controlled experiments with a thermal spray coating of aluminum oxide (alumina) on high purity nickel metal base were carried out to examine the mechanisms of cracking and delamination. Control of these phenomena will enhance the life of thermally sprayed components subjected to thermal shock. These experiments along with computer models will advance the fundamental understanding of this ceramic-metal interface. A plasma spray torch was used to prepare the test specimens, and a high power radiative furnace was used to apply a thermal shock load to the specimens during testing. Geometric effects studied include thickness ratio, thickness to diameter ratio, absolute coating thickness, and the effect of a peak temperature. Experimental results show that using the same critical thermal cycle, specimens with 1mm thick coating failed by cracking, specimens with coating thickness below 1mm did not show any type of failures, and specimens with coating thickness higher than 1mm failed by spallation inside the coating layer. A finite element model incorporating the rate-dependent inelastic behavior of nickel at high temperature was developed to provide a realistic estimate of the residual stresses that develop during the thermal cycle. From transient heating to room temperature cooling, residual stress gradient from compressive to tensile was observed in the coating layer. With decreased coating thickness, this phenomenon was less pronounced.