||Determination of adhesive properties of coatings is one of the most important problems for the extension of the use of coated materials. Numerous studies are devoted to the research of new tests which have to face practical as well as scientific problems. In this work, we propose to evaluate adhesion of coatings by means of an apparent interfacial toughness. The interface indentation test provides a relation between the applied load (P) and the length of the crack (a) created at the interface between the coating and the substrate. Representing this relation in bilogarithmic coordinates shows that the relation is linear and may be expressed by the equation a = αPn where the exponent n is a function of the coating thickness. When the residual stresses due to solidification of the coating are removed by an appropriate annealing treatment, the straight lines corresponding to different thicknesses intersect at the same point. We have shown earlier that this point corresponds to the half diagonal of the indent and then to a limit (ac) for which no crack is formed by indentation. Therefore this point (critical point PC) may be considered as a criterion representative of the adhesive properties of the coating on its substrate. In order to give a usable value of these adhesive properties, we suggest to represent adhesion by an apparent interfacial toughness based on the critical load Pc. As the interfacial test is used to create and propagate a crack, some authors have proposed in a recent past to transpose the principle of indentation tests used to determine toughness of brittle materials. In this study, we expressed an apparent interfacial toughness in function of the critical point (Pc, ac), apparent elastic modulus E1 and apparent hardness H1 of the interface:
As the indentation is performed at the interface between the coating and the substrate, hardnesses and elastic properties of both the substrate and the coating are concerned by the indentation process. The (E/H)1 ratio had necessarily to be dependent on these properties. Considering the plastic deformations as well as the indentation diagonals into the substrate and into the coating, we may define the following relation:
The critical load is representative of adhesive properties of the coating, then the apparent interfacial toughness should vary in the same way. We have verified this point on a chromium carbide coating thermally sprayed on various metallic substrates and on a stellite coating on an austenitic steel. The proposed model gives toughness values in good accordance with the critical indentation loads necessary to initiate a crack at the interface between the coating and its substrate. As a consequence, this apparent interfacial toughness may be considered as a criterion to represent adhesion of a coating on a substrate.