Thermal fracture mechanisms in ceramic thermal barrier coatings

Ceramic thermal barrier coatings represent an attractive method of increasing the high temperature limits for systems such as diesel engines, gas turbines and aircraft engines. However, the dissimilarities between ceramics and metal, as well as the severe temperature gradients applied in such systems, cause thermal stresses which can lead to cracking and ultimately spalling of the coating. This paper reviews the research which considers initiation of surface cracks, interfacial edge cracks and the effect of a transient thermal load on interface cracks. The results of controlled experiments together with analytical models are presented. The implications of these findings to the differences between diesel engines and gas turbines are discussed. The importance of such work for determining the proper design criteria for thermal barrier coatings is underlined

Thermal Stresses Under Engine Heat Flux-Part 1: Ceramic Coating on Metal Substrate

The thermal stresses in a ceramic coating bonded to a metal substrate generated by heat flux conditions in an engine were studied. Edge delamination of the coating was related to the displacements of an interface crack between the ceramic and the metal. The effects of varying the thermal expansion coefficients of the ceramic, the bond coat and the metal, thin moduli of elasticity, their thicknesses and the initial stress-free temperature were determined. Introduction Ceramic coatings are being developed for internal combustion engines in order to improve their efficiency, reduce cooling requirements, and improve reliability. In order to quantify the reduction in heat flux through the use of ceramic thermal barrier coatings, a heat flux gage was designed and built by Tree [1] which consisted of a cylindrical 304 stainless steel body with a partially stabilized, plasmasprayed zirconia surface as shown in The motivation for this work resulted from failures that were observed as a result of placing the heat flux gages in the engine. The failures were in the form of ceramic-metal interface cracking, as well as delamination and cracking of the thin film thermocouple. Therefore, the heat flux gage was modeled as an axisymmetric body with the thin film also assumed as having an axisymmetric configuration. The effect of the engine heat flux at the time when the thermal stresses are maximum was calculated on the ceramic and the thin film. The first paper addresses the effects of changing material properties and geometry on the thermal stresses in the ceramic coating. A second paper describes the thermoelastic-plastic stresses on the thin film determined using the local-global approach to the finite element method Ceramic coatings used for purposes of providing a thermal barrier usually fail as a result of the transient thermal stresses which are generated by the temperature gradients and material property mismatches. Previous studie