MODELING OF RESIDUAL-STRESS IN PLASMA-SPRAYED COATINGS - EFFECT OF SUBSTRATE-TEMPERATURE

Post-fabrication cool down (secondary cooling) is said to be the major cause of the residual stress in thermally sprayed coatings. Primary cooling, the cooling of spray particles to substrate temperature after solidification, is seldom considered. The purpose of the present work is to demonstrate the importance of primary cooling. A model including both primary and secondary cooling was developed to predict the effect of the substrate temperature on residual stress. The following three sequential mechanisms were considered: (1) shrinkage of the spray particles after solidification; (2) deformation of the deposit during spraying; (3) deformation of the deposit after spraying. When the expansion coefficient of the coating is less than that of the substrate, the model predicts that a high substrate temperature induces compressive stress and that a low substrate temperature generates tensile stress. Therefore an optimum substrate temperature exists to minimize residual stress. The modelling results were confirmed by experiments. When the relation between expansion coefficients is reversed, the stress is always tensile throughout the spraying procedure. The present model explains the complicated behaviour of residual stress in sprayed coatings. The importance of primary cooling is confirmed.