Ceramic matrix composites (CMCs) are exciting new materials for use in gas turbine engines, in which an environmental barrier coating (EBC) is needed to protect the CMCs from water vapour attack. The coating also serves as a thermal barrier coating (TBC). The coating acts to increase the operating lifetime and can also offer higher operating temperature with improved fuel efficiency. Significantly, the environmental barrier coating is a prime reliant coating (unlike a TBC), that must not fail in service, since without it the CMC components would have very limited life in the harsh gas turbine engine operating environments.

It is known that one major factor governing the life of an environmental barrier coating is its residual stress state, which results from the quenching process of its manufacture via the thermal spray process and the cooling after spraying. The coating stress state also changes with thermal excursions such as heat-treatment and engine operations. This is because of factors that include the mismatch of coefficients of thermal expansion (CTE), thermal gradients, coating crystallization, phase change and sintering, among others. However, there is little knowledge of how these combine to change the stress state, without which it is not currently possible to predict their evolution, nor the effect on the EBC lifetime.

The aim of this project is to develop new understanding and models for the evolution of coating stress with time and temperature. The objectives include new observations of the effects of EBC crystallization, phase change, sintering, and thermal mechanical loading on the stresses within the coating and its integrity. The project will apply novel experimental methodologies to investigate the state of stress and strain at high resolution, including synchrotron X-ray diffraction, synchrotron and laboratory X-ray computed tomography, and micro-scale hole-drilling using focused ion beam milling, analysed by 2D and 3D digital image correlation. The study will mostly use ex situ observations of thermally treated coatings, but in situ studies may be achieved using synchrotron X-rays at UK National Facilities. The results will be interpreted using modelling by the finite element method in simulations that will incorporate thermophysical processes such as creep of the bond coat between the substrate and the EBC.

This project falls within the EPSRC Materials engineering - ceramics research area' in the Engineering theme listed on this website https://epsrc.ukri.org/research/ourportfolio/themes/

This is an iCASE project, with the support of Rolls-Royce Plc.