A SIMPLE INDENTATION STRESS-STRAIN RELATION FOR CONTACTS WITH SPHERESON BILAYER STRUCTURES

A simple formulation for the indentation stress-strain behavior of bil ayers in contacts with hard spheres is proposed. The formulation is ba sed on empirical power-law stress-strain relations for each of the ind ividual constituent bulk materials, acid thence for the bilayer coatin g/substrate composite. For the constituent materials, two regions of r esponse are considered: at low loads, an elastic region, with linear s tress-strain response (exponent = 1); at high loads, an elastic-plasti c region, with nonlinear stress-strain response (exponent < 1). The ma terial responses in each of these two regions are characterized by eff ective moduli. For the composite bilayer, the transition in load-beari ng capacity from coating to substrate with increasing load is represen ted by a three-parameter Weibull asymptotic function of ratio contact radius to coating thickness. This function conveniently defines the ge ometrical aspects of the contact response separately from the material properties. The power-law formulation is tested, and the Weibull para meters calibrated, against FEM-generated and experimental indentation stress-strain data for selected coating/substrate systems with widely different interlayer elastic-plastic mismatch. The formulation allows a priori predictions of the composite bilayer indentation stress-strai n curves from control data on the constituent materials. Conversely, m easurements of indentation stress-strain responses on given bilayers m ay be used to evaluate otherwise undeterminable elastic-plastic proper ties of coatings on well-characterized substrates. (C) 1998 Elsevier S cience S.A. All rights reserved.