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
.