Investigation of Transient Oxygen Loss From Thermal Barrier Coatings With Engine Qualification

Modern gas turbines increasingly rely on zirconia-based thermal barrier coatings (TBC) to achieve their performance goals.  Being both porous and white in colour, these ceramic coatings are extremely sensitive to contamination ingress and discolouration.  Occasionally, newly produced coatings will exhibit diffuse grey spots and streaks that raise the suspicion of contamination and the question whether the coatings can be reliably used in operation.  Stripping and re-coating is expensive, time-consuming, and is not guaranteed to produce visually acceptable coatings.  Laboratory examination of discoloured areas has failed to find any specific foreign material(s) imbedded or dissolved into the coating material, however it has shown that the depth of spots can range to be the full thickness of the coatings.  Purposeful contamination experiments of foreign material mixed into the coating powder produced defects that appeared different from the spots in question.  Experience from high temperature exposure of porous zirconia in vacuum has shown that the material can turn grey when oxygen-transparent zirconia becomes oxygen deficient.  This study explores the phenomena of coating spots caused by hydrocarbon-driven oxygen removal during heat treatment processes.  The sources of hydrocarbon exposure for TBC are common manufacturing environmental fluids.  In the absence of any detrimental contamination it was concluded that the discoloured TBC, while unsightly, was suitable for service.  To demonstrate this conclusion, production coatings with spots were run for 32,000 hours with natural gas fuel in an industrial gas turbine with a 1065 ֯C turbine inlet temperature (surface coating temperature estimated at 990 ֯C).  Post service, the discoloured areas of the coating exhibited no difference from the white areas.  Both areas exhibited no cracking, peeling, blistering or thinning.  The study conclusion was that the discoloured coatings are fully acceptable for service.