This book assesses the state of the art of coatings materials and processes for gas-turbine blades and vanes, determines potential applications of coatings in high-temperature environments, identifies needs for improved coatings in terms of performance enhancements, design considerations, and fabrication processes, assesses durability of advanced coating systems in expected service environments, and discusses the required inspection, repair, and maintenance methods. The promising areas for research and development of materials and processes for improved coating systems and the approaches to increased coating standardization are identified, with an emphasis on materials and processes with the potential for improved performance, quality, reproducibility, or manufacturing cost reduction.
In this program, the corrosion of ceramics is studied from a mechanistic point of view. In an integrated fundamental approach the gaseous corrosion investigation is followed by that of deposit-modified gaseous corrosion and eventually by deep melt corrosion. The study of the gaseous corrosion of Al H CO/sub 2/ and SO/sub 2/-SO/sub 3/ is almost completed. Particular emphasis has been placed on the interaction between SO/sub 3/ and Al/sub 2/O/sub 3/ at the lower temperatures. A few additional experiments remain to characterize better the products and check some interpretations but the following conclusions have been obtained: (1) SiO/sub 2/ and Al/sub 2/O/sub 3/ are very resistant to gaseous corrosion, (2) pure oxides had no measureable weight changes during exposure to all gases at 700/sup 0/C and 1000/sup 0/C, and (3) at 1400/sup 0/C, silica devitrified and was reduced in wet hydrogen. The reaction of the aluminas at this temperature is related to their impurities and no reaction occurred with higher purity alumina single crystal. The reaction of Al/sub 2/O/sub 3/ with SO/sub 3/ follows qualitatively the general trends predicted by thermodynamics: (1) decrease in sulfate formation with decreasing SO/sub 3/ pressure, (2) decrease in sulfate formation with increasing temperature, and (3) however the sulfate product formed at slightly lower SO/sub 3/ pressures than expected from thermodynamics. The corrosion products on the higher purity polycrystalline Al/sub 2/O/sub 3/ are not uniform. Some regions of th samples formed little or no product. This appears associated with different grains in polished specimen and different faces of grains in the as-received alumina suggesting impurity and orientation effects. The distribution and morphology of the products will be studied in more detail