AN EXAMINATION OF THE VALIDITY OF THE INTERFACE INDENTATION TEST - APPLICATION TO THERMAL SPRAYED COATINGS

One of the most important limitation to the use of coated materials is the adhesion of the coating on its substrate. Among the various metho ds of measuring or estimating adhesion, indentation tests seem to offe r interesting prospects. Indentation fracture has gained widespread ac ceptance as a means to determine the toughness of brittle materials [1 ]. More recently, attempts have been made to transpose this test to co aling mid substrate couples in order to produce and measure a mechanic ally stable crack into the coating ard substrate interface. For thin c oatings, indentation is generally performed on the surface of the samp le [2] whereas for thick coatings, indentation into a cross section ei ther nearby the interface [3] or precisely at the interface [4] has be en proposed. From indentation tests performed at the interface (Fig. 1 ) between, an hypersonic thermal sprayed Cr3C2 - 25%NiCr coating and s everal metallic substrates for which Table I gives the hardness, we sh ow that it is possible to obtain a critical load which may represent a dhesion. This critical indentation load is independent of the coating thickness if an appropriate annealing treatment is realized, order to remove the residual stresses appearing during the cooling of the spray ed material. From tests performed on as received specimens, the effect of residual stresses is also discussed.