Coating Standards
Coating Standards : Note: Organizations such as the Society of Automotive Engineers and ASTM address feedstock composition and powder size for thermal spray processes with limited coating property measurements. Foreign standards, notably British and German, primarily address coating thickness with limited attention to physical or mechanical properties. Testing standards are particularly important for thermal barrier coatings (TBCs) in that they include a ceramic layer(s) and the inherent scatter in the mechanical properties of ceramics is accentuated by the complex microstructure produced by thermal spraying or electron-beam physical vapor deposition (EB-PVD). The lack of standard test methods and data analysis and interpretation techniques for relatively fundamental properties (e. g. , strength, adhesion and cohesion, strain-to-failure, and ductility) is accompanied, not unexpectedly, by a lack of standards for more complex properties (e. g. , thermal shock, fatigue, wear and erosion, corrosion, and toughness). Although basic and applied research has been conducted to understand coating behavior and to relate processing and microstructure and microchemistry to properties and performance, little of this effort has resulted in standards. The necessity to determine accurately appropriate properties for thermal-spray-deposited coatings has been recognized (Berndt et al. , 1992; Dapkunas, 1993), but a similar perspective for coatings applied by other processes has not been developed. For TBCs, the lack of understanding of failure mechanisms hinders the identification of required standards. This situation that exists for current superalloy components is also present for future materials such as monolithic ceramics and ceramic-matrix composites, which may require coatings for oxidation protection. The measurement of properties for use in coating micromechanical design is a significant area that has been neglected. Measurements of properties (e. g. , modulus of elasticity, coefficient of thermal expansion (CTE), inter-and intragranular strength, and toughness) would be particularly valuable for design of TBCs and the functionally graded materials that are similar in concept. The difficulty of measurement on the micrometer scale required for these materials is largely responsible for this situation. Test techniques currently in development (e. g. , nanoindentation) may alleviate this situation. High-temperature coatings have not been the specific subject of standards development, and many of the standards developed for other applications have been used where appropriate. The status of standard testing and analysis procedures varies with the specific aspect of coating technology considered and is summarized in the following section
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