Coating 21
Coating 21 : Thermal Properties: Thermal properties are a particular concern for ceramic TBCs. Knowledge of thermal conductivity, preferably at the temperature of intended use, is important for design and life prediction, while knowledge of thermal expansion coefficients are critical to understanding the adherence to the substrate and stresses in the coating. Thermal conductivity of bulk materials and coatings is routinely measured by the use of a laser flash apparatus that provides a controlled heat input to the front of a sample and measures surface temperature change at the back. Direct measurements are obtained through the use of the guarded hot plate technique that uses a well-insulated apparatus to heat one side of a specimen, while the temperature is measured on the other side. Standards for the guarded hot plate technique (ASTM C-177) and the laser flash (ASTM E-1461) are available for uncoated specimens and may be extended to coated specimens. The ease of the laser flash technique makes it attractive for commercial use and argues for the development of a standard reference material that can be related to results from the guarded hot plate method. Heat transfer through coatings is influenced by the thermal emissivity as well as the thermal conductivity of the coating. Techniques for measuring emissivity are available and will become more important as operating temperatures increase. These techniques are not available as standards for coatings. The thermal comparator method has been used to determine the conductivity of films thinner than 1 micron. This work has shown that these materials can exhibit conductivities as much as two orders of magnitude lower than bulk materials of the same composition and that significant interfacial thermal resistance can develop (Lambropoulos et al. , 1993). This observed behavior has implications for the use of thin multilayer TBCs and warrants the evaluation of this class of materials by techniques for which standard methods are available. CTEs can be measured on coatings removed from a substrate using conventional dilatometry, but the graded composition and microstructure of TBCs adds a degree of complexity that can result in specimen bowing. Although bowing may complicate conventional measurements, this phenomena could, in principle, be used as an alternative method to determine coating expansion. Standards for determining the CTE for coatings are unavailable
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