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 cermets, were used to study the mechanical stiffness and the X-ray elastic constants (XEC) needed for an evaluation of internal stresses by X-ray diffraction. Two spraying techniques were used: atmospheric plasma spraying, and high velocity oxygen fuel. The stiffness was studied by the four-point bending method, applied to the coating–substrate composite beam, and by cantilevered flexure, applied to coatings removed from the substrates. The values found are lower than those for similar material produced by other industrial processes. This behaviour is explained by microstructural features which, in turn, are related to the spraying parameters. It was tried to determine the XEC by the four-point bending method. The results often show a non-linear relationship between the strains determined by X-ray diffraction and the applied mechanical strain controlled by strain gages. This phenomenon is explained by the different volume scale of the material sampled by each technique. The results suggest that an XEC calculation based on the mechanical loading of the samples, is either unreliable, or even impossible. However, considering the characteristics of dense material to which X-ray diffraction is sensitive, the use of XEC calculated by methods of the elasticity theory, for the same phase of the bulk material, is suggested as a procedure for stress determination in thermally sprayed coatings.