Keywords: residual stress, thermal spray coatings, modelling.

Thermal spray technology has become an important part of modern industry for the provision of coatings that offer customised surface properties for a variety of industrial applications. The technology, which has proved useful and cost effective, basically involves coating of a component referred to as the substrate with a molten or semi-molten material possessing good physical properties. The coating material, upon solidification enhances the properties of the base metals of the substrate and increases its service life and operational efficiency. The mechanisms at play, at the coating-substrate interface, as well as between the particles making up the coating is an area that in many cases is still subject to speculation [1]. Also, the deterioration in bond strength, particularly with thick coatings, is of concern specifically with regard to premature debonding, which is a unique feature with such coatings.

This paper is concerned with the residual stresses that arise when a steel-alloy coating is sprayed onto a copper-alloy substrate. This material combination was used recently to enhance the thermal and mechanical efficiency of the pressure die casting process. A difficulty with the spraying of steel on copper is the attainment of appreciable thickness of the coating due to debonding during the thermal spraying process. Residual stress is recognised to be a particularly influential cause of debonding and is the focus of the research presented in the paper. An investigation into the thermal spray process is performed using experimentation, simplified numerical modelling and finite element modelling. The development of residual stress for a range of process parameters, i.e. deposited layer thickness, interval of layer deposition and the number of layers in a coating (i.e. block deposition versus multilayer deposition for a desired coating thickness) is recorded. The results from the three investigation methods agreeably indicate a progressive change in average interfacial residual stress from compressive towards tensile with increase in thickness of deposited layer; and a tensile interfacial stress in a two-layer coating, which increases with increase in interval of deposition between the two layers. On the whole, the observations from the results suggest an increase in potential for coating debonding with increase in both deposited layer thickness and layer deposition interval. The results further suggest higher potential for coating debonding with block deposition compared to multilayer deposition for a desired coating thickness.

1

Araujo, P., Chicot, D., Staia, M. and Lesage, J., "Residual Stresses and Adhesion of Thermal Spray Coatings", Surface Engineering, Vol. 21, No. 1, 35-40, 2005. doi:10.1179/174329405X30020

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