Substrate-affected indentation contact parameters of elastoplastic coating/substrate composites
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In coating/substrate bilayer systems, the indentation contact behavior transitionally varies from coatinglike to substratelike behaviors. Spatial confinement effects of the substrate induce very complicated plastic flows in the coating beneath the indenter, leading to a crucial difficulty that is not accounted for by any of the present quantitative analytical/ theoretical predictions for the substrate-affected contact hardness. In this work, the author presents finite-element-based studies on the elastoplastic indentation contact mechanics of coating/substrate systems. The effect of the substrate is taken into account by introducing the spatially variable elastic modulus and the yield stress; this approach quantitatively describes the substrate-affected stress/strain field in the spatially localized area beneath the indenter. The elastoplastic constitutive relationship of the contact hardness for semi-infinite homogeneous bulks combined with these spatially variable material characteristics are successfully applied to analytically as well as quantitatively predict the substrate-affected contact hardness of bilayer composite systems having wide ranges of elastoplastic coating/substrate characteristics. The experimental procedures for determining the elastic/plastic parameters both of the coating and the substrate are also discussed, in which the importance of the experimental determination of substrate-affected indentation contact radius/area is emphasized. I. INTRODUCTION
Extensive as well as intensive studies have examined the effect of the substrate on the indentation contact behaviors of coating/substrate systems.1–24 The most practical interest in applying indentation contact mechanics to thin films is the quantitative characterization of “film-only” properties in terms of the contact parameters of the layered composite that are, in general, significantly affected by the substrate. Experimental determination of these film-only properties requires a quantitative value of the contact area, Ac, of the layered composite for subsequent characterization of the contact parameters, including the Meyer hardness H and the elastic modulus E0 [E0 = E/(1 n2); E is the Young’s modulus, and n is the Poisson’s ratio]. The elastic approximations of Oliver and Pharr for cone/pyramid indentation and Field and Swain for spherical indentation have been applied to a homogeneous half-space to estimate Ac. These elastic approximations, however, cannot a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/jmr-policy DOI: 10.1557/JMR.2009.0102 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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be extended to layered composites because of the significant substrate effect. Three major issues remain as obstacles to the experimental application of indentation contact mech
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