Effect of mechanical damage on thermal conduction of plasma-sprayed coatings
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Effect of mechanical damage on thermal conduction of plasma-sprayed coatings Lanhua Wei,a) Antonia Pajares,b) and Brian R. Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 20 September 1995; accepted 29 February 1996)
A thermal wave methodology for monitoring the thermal conduction of ceramic coatings with accumulating mechanical damage is described. Tests are conducted on a model alumina coating containing laminar defect intralayers. Controlled subsurface damage introduced with a spherical indenter is observed using a presectioned specimen. Microcrack damage accumulates progressively with increasing contact load and number of cycles. Associated changes in thermal diffusivity, specifically in the through-thickness direction, are imaged and quantified point-by-point using laser-generated thermal waves. The effective thermal resistance of the coating increases with crack density, up to the point of failure.
Plasma-sprayed ceramics are used to provide protective coatings on metal substrates.1–3 They are particularly useful as thermal barrier coatings, where thermal insulation is paramount to the survival of underlayer metal components.1–4 However, such coatings tend to have a high defect content, and are susceptible to damage accumulation, particularly in cycling, and therefore have finite mechanical lifetimes. Accordingly, it can be important to understand how thermal properties change with damage evolution. In the present study we use a thermal wave technique5,6 to investigate the effect of mechanical damage accumulation on the thermal properties of ceramic coating structures. The technique provides point-by-point information on the distribution of damage within the structures. Hertzian testing with a spherical indenter is used to introduce the damage in a simple but controlled way, and to follow the evolution of this damage as a function of contact load and number of cycles. The Hertzian test, with its highly concentrated stress field, is especially useful in probing weak points within the coatingysubstrate structure. Such testing has already been used in earlier studies to investigate damage accumulation in alumina-based ceramic coatings, as well as in other coating system.7,8 Although alumina is not a common component of practical thermal barrier coatings, we retain this material from those earlier studies as an ideal model system for demonstrating the thermal wave procedure.
a)
Guest Scientist, on leave from Department of Physics and Astronomy, Wayne State University, Detroit, Michigan. b) Guest Scientist, from Departamento de F´ısica, Universidad de Extremadura, 06071 Badajoz, Spain.
Nominally pure alumina coatings were air plasma sprayed directly onto grit-blasted soft steel substrates ø3 mm thick (i.e., without bond coat) to produce coatingysubstrate coupons.7,8 The coatings were deposited to a thickness ø500 mm in four interrupted torch passes, leaving defective intralaye
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