THE QUENCHING STRESS IN THERMALLY SPRAYED COATINGS

Various material properties which influence residual stresses in plasma-sprayed deposits are examined. A simple expression to evaluate the residual stress in a plasma-sprayed coating is given for the case where the coating thickness is much smaller than the substrate thickness. The equation contains two terms, the stress owing to quenching of sprayed splats and the macroscopic thermal stress arising from differential thermal contraction. The variables in the equation have been studied experimentally. Methods of measuring the quenching stress are described and values are reported for a wide range of materials and deposition conditions. For metallic powders, correlation can be established between the magnitude of the quenching stress and the propensity of the material to undergo creep deformation and/or plastic yielding. For a ceramic powder such as Al2O3 this is not possible and the quenching stress is very low. Mechanisms are suggested for the reduction in the quenching stress, microcracking being a prominent suggestion. Poor bonding between splats and the incorporation of small disintegrated particles are probably also of significance. Mechanical and thermal properties of sprayed materials, such as Young's modulus and the coefficient of thermal expansion, have been measured. Effects of porosity are examined using the Eshelby equivalent homogeneous inclusion method. The importance of using measured in situ properties in quantitative modelling work is emphasized.