Interface science of thermal barrier coatings

Excerpt

The drive for greater efficiency in propulsion and industrial/power production machinery has pushed metallurgists to develop ever better alloys and taken existing metallic components to their reliability threshold. Nowhere is that better illustrated than in gas turbine engine materials. The nickel-based superalloys currently in use for the most demanding areas of the engines melt at 1,230–1,315 °C and yet see combustion environments ~1,600 °C. The result is that these components require thermal protection to avoid failure from phenomena such as melting, creep, oxidation, thermal fatigue, and so on [1]. The stakes are high as the equipment must remain reliable for thousands of take-offs and landings for aircraft turbine engines, and at least 40,000 h of operation in power generating land-based gas turbines [2, 3]. The most critical items that see both the greatest temperatures and experience the highest stresses are the hot-section components, particularly the high pressure turbine blades. Two strategies have been adopted to help the superalloy turbine blades survive the demanding environment: active air cooling and ceramic thermal protection coatings, which together can reduce metal surface temperatures by >300 °C [2]. The combination of turbine blade external film cooling and internal air cooling requires an exceptionally complex structure with flow passages and sets of small holes in the blades where air bled from a matching stage of the compressor is directed through the blade and over the surface. Stecura [4] was among the first to describe a successful coating system, and today’s ceramic insulating layer alone is credited with reducing metal temperatures as much as 165 °C [1, 5]. …