APPARENT INTERFACE TOUGHNESS OF SUBSTRATE AND COATING COUPLES FROM INDENTATION TESTS

Determination of adhesive properties of coatings is one of the most im portant problems for the extension of the use of coated materials. Num erous studies are devoted to the research of new tests which have to f ace 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 in terface between the coating and the substrate. Representing this relat ion in bilogarithmic coordinates shows that the relation is linear and may be expressed by the equation a = alpha P-n where the exponent n i s a function of the coating thickness. When the residual stresses due to solidification of the coating are removed by an appropriate anneali ng treatment, the straight lines corresponding to different thicknesse s intersect at the same point. We have shown earlier that this point c orresponds to the half diagonal of the indent and then to a limit (a(C )) for which no crack is formed by indentation. Therefore this point ( critical point P-C) 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 repres ent adhesion by an apparent interfacial toughness based on the critica l load P-C. As the interfacial test is used to create and propagate a crack, some authors have proposed in a recent past to transpose the pr inciple of indentation tests used to determine toughness of brittle ma terials. In this study, we expressed an apparent interfacial toughness in function of the critical point (P-C a(C)), apparent elastic modulu s E(I) and apparent hardness H-I of the interface: [GRAPHICS] As the i ndentation is performed at the interface between the coating and the s ubstrate, hardnesses and elastic properties of both the substrate and the coating are concerned by the indentation process. The (E/H)(I) rat io had necessarily to be dependent on these properties. Considering th e plastic deformations as well as the indentation diagonals into the s ubstrate and into the coating, we may define the following relation: [ GRAPHICS] The critical load is representative of adhesive properties o f the coating, then the apparent interfacial toughness should vary in the same way. We have verified this point on a chromium carbide coatin g thermally sprayed on various metallic substrates and on a stellite c oating on an austenitic steel. The proposed model gives toughness valu es in good accordance with the critical indentation loads necessary to initiate a crack at the interface between the coating and its substra te. As a consequence, this apparent interfacial toughness may be consi dered as a criterion to represent adhesion of a coating on a substrate .