ELSEVIER Thin Solid Films 293 ( 1997) 25 1- 260 Effect of substrate and bond coat on contact damage in zirconia-based plasma-sprayed coatings Sataporn Wuttiphan Antonia Pajares a.2, Brian R. Lawn '.*, Christopher C. Berndt '' Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg MD 20899, USA Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11 794, USA Received 1 1 April 1996; accepted 5 June 1996 Abstract This paper reports a Hertzian indentation study of damage modes in zirconia-based plasma- sprayed coatings on metal substrates, with and without bond coats. The structure of the study is as follows: (i) measurement of Hertzian indentation stress- strain curves, first on individual bulk material components (controls) and then on the composite layer structures, to quantify the degree of plasticity; (ii) micrographicanalysis of the corresponding subsurface damage modes, particularly of the yield zones, in both coatings and underlayers; (iii) finite element modelling of the elastic- plastic stress fields in the adjacent layers, again with a focus on the yield zones. It is demonstrated that the substrate can have a profound influence on the damage distribution, depending on the degree of elastic- plastic mismatch relative to the coating. The bond coat, by virtue of its relative thinness, plays a lesser role in the damage intensity, notwithstanding an apparent improvement in substrate adhesion. Indentation variables followed are applied load, to examine the evolution of damage, and number of cycles, to examine fatigue. The results indicate the power and simplicity of the Hertzian technique as a route to mechanical characterization of coating structures: for identifying damage modes, especially yield (but also fracture, in the present case delamination fracture); for evaluating damage parameters, such as Young's modulus and the yield stress, from FEM analysis of stress- strain curves and yield zone microscopy; and for quantifying design concepts, e.g. maximum sustainable bearing stress and damage tolerance. Keywords: Bond coat; Coating; Contact damage; Delamination ; Indentation stress- strain curve; Plasticity; Substrates;Thermal spray coatings; Zirconia 1. Introduction Plasma-spray ceramic coatings on metal alloy substrates have many applications, for thermal barrier protection, wear and corrosion resistance, enhanced biocompatibility in pros- thetic devices, and so on [ 1 - 31. Ultimate performance of the coating depends sensitively on the choice of substrate as well as the coating material itself, in addition to a myriad of ther- mal spray variables [4]. Bond coats add to the effectiveness of the ensuing structures, and afford extra adhesion of the overlying coating to the underlying substrate. Despite (or perhaps because of) their high defect contents, plasma- sprayed coating systems show uncommon damage tolerance, with impressive lifetime performance under most severe operating conditions. At the same time modes of failure, when * Corresponding author. ' This work representspart of a PhD dissertation, Department of Materials and Nuclear Engineering,University of Maryland, College Park, MD 20742, USA. Guest Scientist, from Departamento de Fisica, Universidad de Extre- madura, 0607 1 Badajoz, Spain. they do occur, have not been well characterized, especially in relation to material microstructures. Recently, we have developed a Hertzian contact technique for investigating damage accumulation in plasma-sprayed structures [5-71. Experimentally, the Hertzian test procedure is the essence of simplicity, requiring little more than a hard spherical indenter, a conventional mechanical testing machine, and routine microscopy facilities. Theoretical inter- pretation of the contact damage, on the other hand, is rela- tively complex, involving analysis of a highly nonlinear, inhomogeneous elastic-plastic field, usually by finite element modelling (FEM) [8]. Qualitatively, the method enables identification of intralayer and interlayer deformation and fracture damage modes, and tracks the evolution of these damage processes with increasing load or number of cycles. It is especially effective in mapping out subsurface "yield" zones beneath the contact. Such yield zones tend to be exten- sive in the metal substrate, where conventional dislocation- driven processes operate (plasticity); but they also occur to a limited extent in the ceramic coating, from the cumulative effect of microstructurally discrete shear faults ( "quasi-plas- 0040-6090/97/$17.00 O 1997 Elsevier Science S.A. All rights reserved PIISOO40-6090(96)08992-4