Mechanical stiffness of thermally sprayed coatings and elastic constants for stress evaluation by X-ray diffraction

Thermally sprayed coatings of several materials, ceramic, metallic and cerm ets, were used to study the mechanical stiffness and the X-ray elastic cons tants (XEC) needed for an evaluation of internal stresses by X-ray diffract ion. Two spraying techniques were used: atmospheric plasma spraying, and hi gh velocity oxygen fuel. The stiffness was studied by the four-point bendin g method, applied to the coating-substrate composite beam, and by cantileve red flexure, applied to coatings removed from the substrates. The values fo und are lower than those for similar material produced by other industrial processes. This behaviour is explained by microstructural features which, i n turn, are related to the spraying parameters. It was tried to determine t he XEC by the four-point bending method. The results often show a non-linea r relationship between the strains determined by X-ray diffraction and the applied mechanical strain controlled by strain gages. This phenomenon is ex plained by the different volume scale of the material sampled by each techn ique. The results suggest that an XEC calculation based on the mechanical l oading of the samples, is either unreliable, or even impossible. However, c onsidering the characteristics of dense material to which X-ray diffraction is sensitive, the use of XEC calculated by methods of the elasticity theor y, for the same phase of the bulk material, is suggested as a procedure for stress determination in thermally sprayed coatings. (C) 1999 Published by Elsevier Science S.A. All rights reserved.