FGM (Functionally Graded Material) Thermal Barrier Coatings for Hypersonic Structures - Design and Thermal Structural Analysis

Abstract : A feasibility study of a relatively simple and economical method for producing functionally graded thermal barrier coatings using the Wet Powder (Slurry) and Sintering method showed promising results. This method is relatively undeveloped and under utilised compared to more expensive techniques such as chemical vapour deposition, physical vapour deposition, plasma spraying and powder metallurgy. It utilizes a pressurized spray gun to spray a slurry mixture of the powdered coating material suspended in a liquid solution directly onto a substrate surface followed by sintering using an oxyacetylene torch. The effects of slurry composition, type of ceramic powder, compatibility with substrates, spraying and sintering conditions were studied. Several TBC specimens have been fabricated under various conditions to develop a procedure which produce good quality coatings (of up to 3 layers) with little or no spallation. The optimum time, heat flux and applied pressure level for sintering were deduced. TBCs produced from a 40-45% ceramic, 4% binder and 0.4% dispersant composition and sintered for 30 minutes with an applied pressure of 30 MPa produced good quality coatings with a uniform and very smooth surface. Scanning electron micrographs of the fabricated TBC coatings showed good contact between the grain boundaries of the ceramic powder. In phase 2 of this project, the Wet Powder and Sintering method will be further developed. Improvements to this method include automating the sintering procedure and optimizing the fabrication conditions. A much more extensive micromechanical testing program will be conducted to obtain a more qualitative measure of the quality of the FGM thermal barrier coatings and to develop high temperature constitutive models for input into the FE modeling.