Coatings are used throughout the gas turbine to improve and enhance surface properties of bulk materials that do not have the combination of all the required characteristics demanded by the operating conditions. For example, applications range from anti-seizure coatings of nut threads to prevent seizure to turbine blade coatings which enable the turbine materials to withstand the very high combustion chamber exit temperatures. Other applications include hard coatings to enhance wear resistance and the use of tailored soft coatings to provide an abradable seal.
Thermal Barrier coatings
The thermal barrier coating on combustor cans, nozzle guide vanes (NGV) and HP turbine blades provides a good example of a complex multi-step coating process. A “bond coat” is applied by electroplating or thermal spray, followed by heat treatment and then finally, the application of a zirconia-based ceramic coating by EB-PVD or thermal spray.
(Micrograph of abradable structure)
To increase efficiency throughout the gas turbine it is desirable to minimise gas leakage over the blade tips. To achieve this, in the compressor case, each stage is lined with a thermally sprayed abradable coating. The coating is soft enough to allow the blade tips to cut a path into the abradable layer whilst resilient enough to maintain structural integrity and resist erosion from the gas stream. Other abradable coating materials include felt metal and filled honeycomb structures.
Gas turbine labyrinth seals use a continuous raised fin, for example, around a compressor drum. These are designed to prevent air leakage past the stator vanes. As with the blade tip sealing problem, an abradable liner surrounds the labyrinth seals. The seal fins are coated with an electroplated layer that contains co-deposited abrasive ceramic particles. These allow the seal fins to cut a groove in the abradable liner, maintain clearances and prevent gas leakage.
While the turbine blade aerofoils are exposed to the highest temperatures from the gas stream, other parts of the blade may need to be protected from the effects of combustion products and debris ingested into the engine. Platinum aluminising and platinum chromising are two diffusion coating systems that provide such protection and are commonly applied to turbine blade shanks to improve sulphidation resistance. The effect of this surface modification is to increase the life of the blade system from 100s of hours to 1000s of hours.