ICMCTF2002 Session A3-3: Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are now internationally accepted as a technology capable of improving the performance of gas turbine engines. They allow either an increase in turbine entry temperatures, and therefore an increase in power output, or a decrease in the metal surface temperature, with the associated increase in component life, or a combination of both.

Reducing the thermal conductive, must lead to further performance improvements. This paper aims to review alternative strategies aimed at lowering the thermal conductivity of TBC systems.

TBC thermal conductivity not only reflects the thermal properties of the ceramic layer, but also its method of manufacture. The latter influences the coating structure, its porosity, the porosity distribution and thus the thermal conductivity gradients within the coating.

Starting with zirconia-8wt% yttria, this paper will review thermal transport mechanisms within TBCs and how the thermal conductivity can be changed by dopant additions. The selection of alternative ceramics based on pyrochlore and garnet phases will also be examined.

The role of coating microstructure will also be discussed with reference to the microstructures developed within plasma sprayed and EB-PVD thermal barrier coatings. Suggestions for refining the coating microstructures, aimed at designing lower thermal conductivity coatings will be proposed.

Ceramic thermal barrier coatings are widely used in modern gasturbines and aircraft engines for temperature reduction of the base material. In this way the efficiency can be increased or the life time of the engine can be improved.

For application on turbine blades columnar EB-PVD coatings are used. By manipulating the columnar structure during the process a so called "herringbone" structure can be obtained. This structure, with a zig zag type of columns, can result in a more than 25% reduction in thermal conductivity. Furnace cycle tests and burner rig testing showed that this reduction in thermal conductivity is possible without harming the mechanical, oxidation and / or corrosion properties of the coating.

Since the introduction of electron beam (EB) physical vapour deposition (PVD) thermal barrier coatings (TBCs) and their application to moving components in the hot gas stream, erosion has become a prime concern. EB PVD TBCs, due to their unique columnar microstructure are far more strain tolerant than their plasma sprayed (PS) counter parts and can thus be used under more exacting operating conditions. It is under these operating conditions that erosion of the coated components is of primary importance.

The main aim of this project was the development of a computer model capable of predicting the erosion rate of EB PVD TBCs under various different conditions. In order to do this it was first necessary to determine the erosion mechanisms of EB PVD TBCs as well as their mechanical properties. Steady state erosion and single impact studies together with SEM were used to determine the erosion mechanisms, which have been discussed in a previous paper. While nano indentation techniques were used to obtain the hardness and the Young’s Modulus of the EB PVD TBCs.

All these findings were then used in the development of a Monte-Carlo based computational erosion model capable of predicting the erosive wear rate of EB PVD TBCs. The model is capable of predicting the erosion rate of EB PVD TBCs to ±2% to ±15%, depending on the test conditions, so long as the erosion falls within defined mechanisms, which can easily be checked against an erosion map, which has been drawn.