ELASTIC FIELD OF A THIN-FILM SUBSTRATE SYSTEM UNDER AN AXISYMMETRICALLOADING/

This paper presents the elastic solution of a layered half space with perfect interfacial bonding under an axisymmetrical compressive loadin g on the plane surface. The analysis is intended to model the nano-ind entation of thin-film coating/substrate systems. Unlike most of the ex isting work of which the substrate is assumed as rigid and the numeric al results are obtained by finite element analysis, the present paper presents theoretical solutions for the elastic coating/substrate syste ms. The surface displacement profiles and the stress fields are shown to be sensitive to the thickness of the coating layer and the ratio of the elastic modulus of the coating material to that of the substrate. When the film thickness is comparable to the loading contact radius, the film elastic property cannot be accurately determined by using Sne ddon's half-space indentation solution. Furthermore, there are pronoun ced differences in the stress fields of the hard-coating and the soft- coating systems. When an indentation load is applied to a soft-thin-fi lm/hard-substrate system, most the stress components are compressive. But for a hard-thin-film/soft-substrate system, the radial and hoop st resses in the film near the film/substrate interface change from tensi on to compression as the film thickness decreases. The normal and shea r stress results are compared with those obtained from finite element analysis (Jayachandran, R., Boyce, M. C. and Argon, A. S. (1995) Mecha nics of the indentation test and its use to assess the adhesion of pol ymeric coatings. In Adhesion Measurement of Films and Coatings. ed. K. L. Mittal. VSP, pp. 189-215) for the rigid substrate system. Also, th e load-indentation depth results are compared with the experimental da ta of Oliver and Pharr (Oliver, W.C. and Pharr, G.M. (1992) An improve d technique for determining hardness and elastic modulus using load an d displacement sensing indentation experiments. Journal of Material Re search 7(6), 1564-1583) for tungsten subjected to elastic indentation. The agreement is quite satisfactory. Implications of the elastic fiel d on the failure mechanisms of coating/substrate systems are also disc ussed. (C) 1997 Elsevier Science Ltd.