High temperature degradation of plasma sprayed thermal barrier coatings in oxidizing environment

The thrust and efficiency of turbine engines are directly related to the operating temperature. In order to increase temperature without damaging the metallic parts of the engine, thermal barrier coatings (TBC) made of thick ceramic layers are currently deposited on the surface of high pressure turbine blades and vanes of aero engines. However, in the hostile engine environment, the TBC is subjected to degradation phenomena that limit its reliability and time of life. Thus, the comprehension of the TBC degradation process in this aggressive environment entails the greatest importance. The currently adopted TBCs consist of two parts: an external ceramic top coat (TC) facing the gaseous atmosphere, a bond coat layer (BC) put between the TC and the metallic substrate. Due to the interconnected porosity of the TC, oxidation of BC occurs, resulting in the formation in exercise of a thermally grown oxide (TGO) layer. The TGO formation involves significant volume increase and causes elastic stresses to arise inside the TBC, while other residual stresses are generated by the different thermal expansion coefficients of the TBC components. These two effects are both responsible for the deterioration and the final failure of the TBC system. The life-time of the TBC as well as its failure are strictly related with the growth of the TGO intermediate layer. In this chapter, CMSX4 nickel superalloy specimens with Air Plasma Spray (APS) deposited TBC have been submitted to different oxidation treatments at temperatures typical of aircraft gas turbine working conditions. TBC deterioration during high temperature oxidation was monitored by compositional and microstructural observations, thermal diffusivity and residual stresses measurements. In particular, residual stresses were measured by a technique based on the analysis of the deformation of the Debye rings, by means of a 2D detector in a X-Ray microdiffraction system. The effect of Thermally Grown Oxide (TGO) layer growth was studied in order to compare kinetic data, microstructure, residual stresses and thermal diffusivity. TGO grew according to diffusion in a first stage of the oxidation, followed by a transition to a faster rate when cracking occurred. This was detected by the stabilization of residual stresses values and the reduction of thermal diffusivity. The results of residual stress measurements were consistent with FEM simulations that kept into account the typical undulations of the TGO interface.