Oxidation and spallation of FeCrAl alloys and thermal barrier coatings:in situinvestigation under controlled temperature gradient

Abstract

Alumina forming alloys and multi-material systems such as thermal barrier coatings (TBC) used for high temperature applications in aeronautical gas turbines are subject to severe thermo-mechanical loading. During service cycles, complex high stress is generated, resulting from the thermal variations that establish through the thickness of the multi-materials system. The occurrence of such thermal gradients may specifically provoke spallation and enhance the damage of the material surface. The cooling rate value is also an important factor influencing the material’s life. In order to reproduce as close as possible those conditions of materials utilization, a dedicated, cyclic oxidation equipment is designed and implemented. It is able to impose a controlled and measurable thermal gradient through the material thickness and allows to monitor in situ the oxidation cycles using various optical means. Furthermore, the heating and cooling conditions are easily adjustable. This versatile real-time approach allows the identification and analysis of the spallation mechanisms for different microstructural and time scales.Preliminary results showing the impact of a thermal gradient as well as its magnitude on the propensity of an iron –chromium–aluminum ODS alloy and electron bean-physical vapour deposition TBC systems to spall upon thermal shock or thermal cycling are presented.