Coating 19
Coating 19 : Testing and Standards: Broadly accepted test methods, standards, and specifications are of great value to both vendors and purchasers of coating services and coated products. They are essential communication mechanisms for purchasers to describe the critical aspects of required coatings and for coating suppliers to unambiguously understand the requirements. Coatings are only used in gas turbines after a substantial testing and evaluation program by the engine manufacturer and, where appropriate, by the coating vendor. Testing and analysis procedures are usually developed by the engine manufacturer to: (1) Develop procedures, specifications, and controls for the deposition process; (2) Serve as a quality control measure to ensure that coated products meet specified properties; and (3) Provide data for performance and lifetime prediction by evaluation under conditions simulating engine conditions. These procedures are developed by engine manufacturers to address specific conditions expected in each manufacturer's engine. Notable examples of this approach are found in the several types of high-temperature tests developed by engine manufacturers to simulate the corrosive behavior found in specific engines operating under specific conditions. Burner rigs of various designs have evolved at each manufacturer that, through experience, can generate data for corrosion and thermal-shock resistance. Many different methods of testing and analysis have evolved in the research community to develop new coating compositions, microstructures, and processes as well as to understand coating behavior on a fundamental level. The measurement of coating properties must be viewed in the context of a coating/substrate system. Coating compositions and microstructures are complex and become more so during service at high temperature in corrosive environments. Thus the use of material properties for design or lifetime prediction is usually based on the measurement of the coating systems rather than the bulk materials, the properties of which may differ significantly from the same nominal materials present as a coating. While substantial efforts have been made by individual companies and research organizations to develop satisfactory evaluation procedures, relatively few broadly accepted test methods are available for the evaluation of high-temperature coatings. Consequently, much of the data publicly reported consist of measurements that are directly compared with the behavior of widely used materials. This is a conservative approach suitable for the expensive turbines that are expected to have high reliability. However, the need for increased productivity in the materials and gas-turbine fields argues for the use of commonly accepted test methodologies that allow more cost-effective data generation and increased commonality of property specification. Definitions for the several terms used for standards have been developed by the American Society for Testing and Materials (ASTM) and are followed in this discussion. A standard is defined as a rule for an orderly approach to a specific activity, formulated and applied for the benefit and with the cooperation of all concerned. Six types of full consensus standards are identified by the ASTM: (1) Classification. A systematic arrangement or division of materials, products, systems, or services into groups based on similar characteristics (e. g. , origin, composition, properties, or use). (2) Guide. A series of options or instructions that does not recommend a specific course of action. (3) Practice. A definitive procedure for performing one or more specific operations or functions that does not produce a test result. (4) Specification. A precise statement of a set of requirements to be satisfied by a material, product, system, or service that also indicates the procedures for determining whether each of the requirements is satisfied. (5) Terminology. A definition or description of terms or an explanation of symbols, abbreviations, or acronyms. (6) Test method. A definitive procedure for the identification, measurement, and evaluation of one or more qualities, characteristics, or properties of a material, product, system, or service that produces a test result. In general, company specifications dominate the commercial market and address characteristics such as composition, microstructure, thickness, and strain-to-failure. The U. S. military has published specifications that address limited aspects of high-temperature coatings, primarily for the thermal spray (plasma and detonation gun) processes. Broadly available standards developed by consensus through 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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