Residual stresses and adhesion of thermal spray coatings

Residual stresses generated in coatings during thermal spraying could have very different inside intensity and distribution, depending on the materials and processing conditions. As it is recognised that residual stresses play a major role in the adhesion of coatings, it is necessaryto evaluate precisely their influence. It is not possible to conduct these measurements directly, and no indication on how the stresses should be taken into account has been reported in the literature. Moreover, depending on the test used to evaluate adhesion, different volumes of the coatingcan participate in the delamination process. In order to take into account these observations, it is proposed to define two stress parameters related either to the stresses in the coating or to the stresses at the interfacial zone. In these conditions, it is possible to explain the variationin adhesion as a function of the coating thickness, i.e. to explain the maximum value obtained for the bonding strength deduced from tensile tests and the monotonic increase in adhesion toughness deduced from interfacial indentation tests.