Deformation behavior of plasma-sprayed thick thermal barrier coatings

A fundamental deformation study of several candidate diesel engine coating materials, independent of a substrate, has been conducted. Both plasma-sprayed 8%Y2O3-ZrO2 and CaTiO3 material specimens were subjected to various isothermal mechanical tests from room temperature to 800 degrees C to understand their basic constitutive behavior. In this work, it was found that all of the coating materials showed significant irreversible deformation behavior during cyclic loading. In addition, both time- and temperature-dependent behavior were observed through stress relaxation and strain-rate sensitivity tests. This inelastic behavior was seen to occur under all conditions, including tests at room temperature. In a diesel engine, this irreversible deformation behavior will give rise to detrimental tensile stresses when subjected to in-service compressive strain cycles. SEM observations suggest that the combined closing of pre-existing microcracks and sliding along microcrack surfaces results in the deformation behavior observed. In addition, a glassy phase found in the microstructure is thought to aid these microstructural deformation processes at high temperature. (C) 1999 Elsevier Science S.A. All rights reserved.