Effect of some thermal treatments on interface adhesion toughness of various thick thermal spray coatings

Abstract

Interfacial indentation is used to create and propagate a crack in the interface plane between a coating and its substrate. It has been shown earlier that this methodology allows the measurement of an apparent interface toughness, which can represent the adhesion of the coating to its substrate. In service thermal spray coatings are subjected to various external influences, which could be detrimental to their adhesion, e.g. thermal shock, fatigue, wear, corrosion and combinations of these. The objective of the present work is to study some of these effects and their influence on the interface adhesion toughness of various substrate/coating systems based on new phenomenological approaches to the residual stress state existing in the coating: (a) annealing treatment – for as sprayed specimen, it was first shown that the interface toughness is directly proportional to the reciprocal of the squared coating thickness. Extrapolated to an infinite thickness, the toughness could be assumed then to represent the adhesive properties of the coating. The same tests, performed on annealed samples, for which, it was expected to see significant modification of the residual stresses state, led to an apparent interface toughness independent of the coating thickness. Moreover, this value corresponds to the value extrapolated for the as sprayed specimen. The difference between the calculated values of K_ca on both as sprayed samples and on annealed ones is discussed in terms of a stress amplitude factor defined as the difference between the residual stresses in the coating and in the substrate. The obtained result is very important since it confirms that there exists a value which, being independent on the thickness, can represent the adhesion of the coating; (b) hydrogen contamination – using this methodology, it was found that, besides the embrittlement of the coating, adhesion was also affected because the critical load necessary to initiate a crack at the interface was reduced in the presence of hydrogen. Using apparent interface toughness, it was also possible to compare the effect of hydrogen with that of the residual stresses (thickness) effect, according to the stress amplitude factor at the interface and to the capability of the substrate and/or the coating to trap atoms of hydrogen in the neighbourhood of the interface; (c) thermal treatments – in addition, it is demonstrated here that the methodology allows quantification of the effects of the thermal treatments such as thermal shock or thermal cycling on the adhesion properties of the coatings. It was shown that thermal treatments for particular conditions of time or temperature lead to a reinforcement of the metallurgical bond and, consequently, to an increase in the adhesion of the coating on its substrate.