- _ ----- ORNL/CP-99990 Surfnce-Roughness Induced Hesidual Stresses in Thermal Barrier Coatings: Computer Simulations C. H. Hsueii I, P. F. Becher I, Edwin R. Fuiler, Ir. ’, Stephen A. Langer’, and W. Craig Carter ’ Oak Ridge Nation4 Laboratory, Oak Ridge, Tennessee 3783 1-6068, U.S.A. ’National Institute of Standards and Technology, Gaithersburg, Maryland 20899, U.S.A. presently at Massachusetts Institute of Technology, Cambridge, MA 02139-4307, U.S.A. lUQV 2 4 1998 3OSTi Keywords: asperities; computer simulations; finite element analysis; microstructure; OOF; plasma spray; residual stress; spallation; surface roughness; thermal barricr coatings Abstract Adherence of plasma-sprayed thermal barrier coatings (TBC’s) is strongly dependent on mechanical jntedocking at tho interhce between the ccramjc coating and the underlying metallic bond coa~ Typically, a rough bond-coat surface topcdogy is required to achieve adequate mechanicaf bonding However, the resultant interfacial asperities modify the residual stresses that develop in the coating system due to thermal expansion differences, and other misfit strains, and generate stresses that can induce progressive fracture and eventual spallation of the ceramic coating. Far a flat it~terface, the principal residual stress is pjrallel to the interface, as the stress normal to the interface is zero. However, the residual stress normal to the intcrface becomes non-zero, when the interface has the requkcd interlocking morphology. In the present study, an actuat microstnrcture of a plasma-sprayed TBC sygtcm was numeridy simulated and anayzed with a recently developed, chject-oriented finite element analysis program, OOb: to give an estimate of the localized residual stresses in a TBC system. Additionaiiy, mod4 TBC microstructures were examined to evaluate the manner in which the topology of interfacial asperities infiuences residual strcsses. Results arc present for several scenafios of modifying intcrfhcial roughness. lntroduction Ceramic thermal barrier coatings (TBC’s) are commonjy used to protect air-coolcd superalloy hardware in hot-sections of gas turbine engines by reducing the surfacetempetatures of metallic componerits [ 1-41. A typical plasma-sprayed TBC system consists of an oxidation-resistant rnetaliic bond coat overlaid by a porous, thermally-insulating ceramic coating [I-31, Since adherence of the piasma-sprayed ceramic coating to the metallic surface is enhanced by mechanical interlocking, a rough bond-coat surface topoiogy is required to achieve the level of mechanical adtresion needed for the severe thermal cycles of a turbine engine [3,5,6]. Plasma-spray deposition of the bond coat provides an inherently rough and irregular rnetaUic surface, with the resultant asperities providing good bonding along the ceramidmetal interhe. Funhermore, the nature of the bond-coat surfaceroughness can be tailored to a limited extent by control of plasma-spray parameters andor powder size distributions. Currently, the magnitude and morphology of bond-coat roughness that provide optimum TBC durability have not been established. Substantiai residual stresses exist in TBC systems due to the large mismatch in metal-ceramic thermal n during cooling and lo high-temperature oxidation of the bond coat 12-71 During high- thin oxide scale (predominantly, a-Al,O,) forms at the irregular bond-mad g in a constrainedvolumetric increase dong the interface [2-7). The interfacial ic coating have significantly lower coeEcients of thermal expansion (CTE) than mponents, and thus, are subjected to significant compressive residual strcsses ntcrface during cooling [4,S,9]. The residuai stress normal to the interface (Le., the out-