Thermal barrier coatings (TBCs) are used to protect underlying base materials from thermal degradation from high temperature of operation in high temperature environments. Such environments include hot sections of turbomachinery. Over time, refinements have been made to thermal barrier coatings to impart additional properties to thermal barrier systems so that they are more survivable in turbine environments.
Yttria stabilized zirconia (YSZ) is one of the most widely used coating systems for thermal barrier applications. In a gas turbine engine, hot gases of combustion moving over the TBCs cause erosion of the TBC material, while ingestion of particles into the engine can cause foreign object damage (FOD) when the particles impact the TBCs. To improve the performance of a TBC such as YSZ, Dense Vertically Cracked (DVC) TBCs were developed that have improved erosion resistance and strain compliance without impacting the thermal performance of the TBC. Where abradability such as due to blade rubs has been identified as a problem, other improvements have relied on a patterned controlled porosity to impart sufficient abradability, while maintaining both erosion resistance and low thermal conductivity.
In high temperature operation, YSZ may destabilize, degrading the erosion resistance and abrasion resistance of the thermal barrier coating significantly. To overcome problems with high temperature operation, low thermal conductivity materials are required for use at elevated temperatures, while offering further reductions in thermal conductivity and improved abradability. Ideally, such thermal barrier coating materials overcome problems with brittleness that may pose a problem with regard to FOD.
What is desired is a thermal barrier coating suitable for use at elevated temperatures having strain tolerance and resistance to crack initiation and crack growth/propagation, particularly when deposited as an abradable coating. Various problems with such a structure have retarded its development.