Low stress thermal spray TBCs

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Thermal barrier coatings applied by thermal spray coating processes find a tremendous level of applications in the aerospace, ground based power generation and other industrial markets as engines and components are required to run hotter and hotter for better efficiency. Plasma sprayed 8% yttria stabilized zirconia and 12% yttria stabilized zirconia are popular choices. Such thermal sprayed TBCs usually have an underlying bond coat of NiCrAly or CoCrAly or CoNiCrAly. However, delamination and spalling of thermal barrier coatings applied by thermal spray processes caused by residual stresses in the coating leads to failure of the thermal barrier property and the coating is rendered useless. Hundreds of studies are conducted, especially by thermal spray research scientists in trying to understand the stress generation mechanism of thermal spray TBCs. Analytical and computational models are used and x-ray diffraction techniques for determining residual stresses in thermal spray coatings are employed to come up with numerical values for the stresses generated. Both operational mechanical stresses developed in a component thermal spray coated with TBCs as well as oxidation stresses developed contribute to potential coating failure. Fatigue cracks in the coating and their propagation rates will also determine the life cycle of the coated component. A major problem that one has to deal with is the fact that unlike testing a virgin machined component, testing a component that has a thermal spray coating, YSZ TBC or not, is that the level of exact duplication of thermal barrier coatings from batch to batch is extremely difficult in thermal spray; I am referring to the exact duplication of porosity levels, porosity distribution, etc., the latter being of more interest to me. Yet the question that arises in the mind of the practical thermal spray coatings operator is that while research in the subject of reducing stresses in thermal barrier coatings is not complete by any means, what can he or she do NOW based upon what is known until today, to at least keep the thermal barrier coating at its lowest stress levels and optimum functionality.

I would say that the first thing to do is to apply the bond coat with the optimum parameters so that it is as clean as possible with as low an oxide content as possible. Obviously, if the only process that you have at your disposal is air plasma spray, then your bond coat cannot meet the cleanliness of LPPS standards. However, there are NiCrAly bond coat parameters for air plasma spray that produce lousy bond coats and there are those that produce much better quality bond coats. So having a clean bond coat is a major step forward in reducing the failure rate of bond coat - thermal barrier systems. Secondly, the time lapse between bond coat application and top coat TBC application should be minimal. I have come across cases where a million parts are all bond coated in one set up and then days later the same million parts get all top coated.( may be days later the same million parts all failed in service prematurely! ) Contamination after bond coat application is a recipe for failure. Thirdly, I would keep the NiCrAly or CoNiCrAly bond coat thickness to the high limit allowable by the specification. If I am a specification writer, then instead of specifying a bond coat thickness of .002 inches to .004 inches or .004 inches to .008 inches, I would specify a bond coat thickness of .006 inches to .012 inches. Finally, top coat thermal barrier coating needs to have the prescribed porosity as allowable by the specification. Making TBCs with minimal porosity is not a good idea. More importantly, porosity distribution needs to be uniform. Non-uniform porosity distribution in TBC top coats contributes to pre-mature coating failure. Every coating batch must be properly metallographically evaluated for uniform porosity distribution with standards set in place. Now, metallographic evaluation of thermal barrier coatings is a topic in itself for another time and we will not deal with it here; it is suffice to say for now that if the metallurgical polishing technique is flawed and pull outs during the polishing process are construed as inherent porosity in the coating, then the evaluation technique will take you nowhere.