layer along the substrate interface, many experimental researches based on the scratch methods have been carried out in past decade [1-5]. However, theoretical analyses connected with the scratch experiments are very few [1]. This is because that any theoretical study must deal with on the complicated failure geometry of the scratch test. It is obvious that a three-dimensional elastic-plastic deformation problem must be solved, and a robust theoretical model for describing the scratch failure behavior is needed. Must theoretical researches have been based on the simple geometry of the scratch failure strap and the simple mechanics equilibrium to simulate the scratch failure behavior [1-5]. However, it is difficult to use a simple model to describe the strong influence of plastic deformation on the micro-scratch behavior. It is well known that plastic deformation has a strong shielding effect on the interface cracking [6-8]. So that in an elastic-plastic failure process more energy is dissipated than that in a pure elastic failure process. Therefore, it is important to develop a reasonable mechanics model for scratch test simulation. The failure characteristics of the scratch test for ductile thin film materials[2-4] are somewhat similar with the thin film peeling problems. Therefore, in micro-scratch test research, the analytical method for the thin film peeling problem [8] is relevant. It is important to obtain a reasonable relation between the critical driving force and the parameters of the materials and scratch strap geometry. In the present research, based on the three-dimensional character of failure strap, a new mechanics model describing the interface separation and the thin film shear failure, i.e., a double cohesive zone model will be presented. Using the new model, a relation between the scratch horizontal driving force and the parameters of the materials will be set up and used to predict the scratch work. Finally, the simulation results will be applied to an experimental result for Pt/NiO from [4]. FUNDAMENTAL DESCRIPTION AND SIMPLIFICATION From failure characteristics for ductile film scratching, the scratch test process can be described by figure 1. This process can consist of two stages. One stage is a normal scratch before thin film delamination occurs along interface. With the indenter moving forward and downward with scratch depth increase, especially when indenter tip is near the interface, a region of thin film or coating layer near the indenter tip will be delaminated from interface. Thereby, the scratch process is transferred to another stage. The failure character changes from the indenter tunnel growth to the delaminated film strap formation and growth (or post-scratch process). For simplifying the analysis, the problem is divided into two sub-problems. One problem is "plate bend" under elastic-plastic large deformation for the delaminated thin film part BCD, see figure 1. This sub-problem has been solved successfully in [8]. Another problem is a three-dimensional delaminating problem for a part of thin film BA and jointed substrate. In the present research, our attention will focus on the latter problem. The solution of the former problem [8] will be taken as the boundary condition and exerted on the section B directly. For simplification, we present and adopt a new double cohesive zone model to simulate the film failure process and the scratch work for the post-scratch process. The new model is shown in figure 2. In this model, there are three cohesive zones, one is the separation-dominated cohesive zone and other two are shear-dominated cohesive zones. DOUBLE COHESIVE ZONE MODELS AND MECHANICS DESCRIPTIONS In figure 2, the thin film delaminates from interface of thin film/substrate (plane x2=0), and this failure process can be simulated by a separation-dominated cohesive zone surface. Simultaneously, the curved film layer is cut off from two sides of the delaminated region (planes x3=-W and 0). The cutting process for each plane can be described by the shear-dominated cohesive zone deformation. In figure 2, and cn δ ct δ are critic relative displacements for the separation and shear cohesive zone surfaces, respectively. The separation cohesive zone model under plane strain case has been widely adopted and completely formulated in [6-8]. In the following, we shall discuss and give a brief description and generalization for the two kinds of cohesive zone models for the three-dimensional case.
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