Papers by Author: S. Sampath

Abstract: Thermal and cold spraying encompasses a great variety of techniques for deposition of fully or partially molten or cold particles of material to produce coatings with a specific microstructure and properties for the purpose of surface enhancement. Among the many factors influencing sprayed coating integrity, residual stress is very important since it will be the driving force for possible crack propagation and consequent coating failure. The very complex physical processes occurring during coating deposition make first principles treatment difficult. A number of empirical models have been proposed to predict and describe quantitatively the stress distribution in the coating/substrate system. However, there are a limited number of experimental measurements on through-thickness stress distribution and for only a few materials and few spraying techniques, to validate any model and to define its area of applicability. Several metal and ceramic coatings produced by different spraying techniques were measured by means of neutron diffraction. Through-thickness stress profiles were obtained and treated in the frame of the empirical progressive coating deposition model. The comparison between experimental and simulated results is discussed.

Abstract: Thermal spray is a well-established, versatile method of producing protective and functional coatings. As with most thin- or thick-film structures, residual stresses developed during processing play an important role in determining the performance and life of sprayed coatings. Diffraction methods (X-ray or neutron) and in situ curvature measurement have been widely used to measure stresses nondestructively, yet results in coating stress measurements seen in the literatures are sometimes ambiguous or conflicting. This is due not only to the experimental error associated with the measurement and simplifying assumptions, but also the complexity and heterogeneity of the coating structure. During deposition, molten, semi-molten or solid particles successively impinge onto a substrate surface, thus forming a layered structure comprised of ‘splats,’ separated by interfaces, cracks and pores. In this study, X-ray micro-diffraction with a 2-D detector has been used to determine the stress magnitude of both splats and coatings on substrates. Neutron diffraction stress measurements have been made through the entire coating thickness. The process of depositing and cooling has been monitored by in situ curvature measurement. Micro- and macro stresses have been examined. The relation between process and splat and coating residual stresses has been evaluated and interpreted by recourse to microstructural and morphological observations under SEM. This study bridges the behaviors of microscopic single splats and macroscopic coatings, hence helps to fundamentally understand the stress generation during thermal spray process.