Coating 14
Coating 14 : Mechanical Properties: Mechanical properties, particularly as a function of temperature, are critical to the performance of a coating and consequently are the subject of measurement in the coatings field. The properties of greatest concern for metallic overlay or conversion coatings are ductile-to-brittle transition, fatigue, thermal-shock resistance, adhesion, and strain-to-failure. Test procedures for these properties are generally of the elevated- temperature uniaxial tensile, creep, stress-rupture, or fatigue type developed for metals and alloys. The evaluation of ceramic overlay coatings used for thermal barriers focuses on adhesion to the substrate and cohesion within the coating. Traditional methods for the qualitative evaluation of adhesion (e. g. , bending, scratching, or impacting) that were developed for less complex materials (e. g. , zinc coatings) are of limited value but are nonetheless included in ASTM, British Standards Institute, and International Standards Organization (ISO) standards. The most commonly accepted adhesion test is ASTM C 633-79, the tensile tab test, which is comparable to DIN 50 160-A, AFNOR NF A91-202-79, and JIS H866680 (Berndt, 1990). This technique is limited by the use of an epoxy adhesive grip attachment at test temperatures significantly lower than service temperatures. Brown et al. (1988) have provided a review of methods used to measure the adherence of coatings applied by thermal spray, including flexure and fracture mechanics techniques, and conclude that widely used tests do not provide the information required and that simulation of service conditions is vital. Thermal-shock tests can provide a qualitative measure of adhesion and have been developed for the porcelain-coated steel industry. The importance of determining the mechanical properties of coatings has encouraged the development of compressive, tensile fatigue techniques for coatings removed from substrates (Beardsley, 1992). These methods have not been codified as standards. Recognition of the importance of more subtle properties such as fracture toughness, thermal-shock response, and thermomechanical fatigue has been manifested in research on the modeling of coating behavior and was the subject of a recent conference (Kokini, 1993). However, standards for the measurement of these properties are not available. Hardness is commonly used as a process control measure, and its application to coatings has been recognized in the development of BS 5411-part 6, Vickers and Knoop microhardness, for metallic coatings. Fracture mechanics analysis has been combined with microindentation to measure the fracture toughness of ceramics and has been applied to ceramic coatings (Besich et al. , 1993). More recently, instrumented microindenters and nanoindenters that provide data on deformation as a function of load have been developed that can provide a measure of elastic properties. Nanoindentation offers the potential to measure hardness and elastic modulus of specific portions of microstructures as small as several micrometers that can be applied to modeling. None of these latter techniques have been developed into standards
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