| Coating or plating on the surface of the substrate are effective methods to improve surface properties of materials, which meets practical application needs. The periodic structure of multilayers attributes to its fantastic mechanical, electrical, magnetic and optical properties. Because of the interaction between each layers, the physical properties of multilayers are different from the ones of the monolithic materials which compose it. And it is also shown that the system of surface multilayers has more advantages than the monolithic one in practical applications. Therefore the research of thin film-substrate composite material becomes increasingly important. In the experimental researches of multilayers films, nanohardness test, in which there are still some problems such as bigger errors and more dispersal results, is one of the most important methods. In this paper, numerical simulation is used to analyze the stress and strain response of multilayers through simulating nanohardness experiments under the condition that indenter has indented on it. A three-dimensional Vickers indenter is simplified to a two-dimensional wedge indenter with an angle 136°, therefore the problem becomes a plane strain problem. It is assumed that the indenter is made of rigid material, the film and substrate are made of isotropic elastic-perfectly plastic material, and Mises yield condition is obeyed. The friction between indenter and film is considered. Because this is a physical and boundary condition nonlinear problem, an incremental iterative method is used. And the needed computing programs are developed on the platform FEPG (Finite element program generating system) software. The research work based on the model and programs above are listed below:1. The influence of different soft layers material on the stress in the film is studied when the material of soft layers is Al and Ti, respectively. When the soft layers are on the top, Al film, which is softer, could better disperse the force transferred from above. When the soft film is underneath, the stress of the x-direction on the interface increases because the Al film cannot support the hard layers above well. This increase may cause the expansion of longitudinal cracks.2. With a certain film thickness, the influence of the ratio of soft and hard layer thickness on stress in the film is studied. The ratios of soft and hard layers are 1:2, 1:1 and 2:1, respectively. In the lower layers, increasing the thickness of soft layers can average the distribution of press and reduce the stress in the films and sheer stress on the interface. In the layers near the surface, increasing the soft film will reduce the hardness of the film surface and increase the tensile stress on the surface, which may cause the expansion of longitudinal cracks on the surface.3. The influence of the number of layers on the stress field of multilayers films is studied. It is indicated that increasing the number of layers can protect the substrate,and reduce the stress on the interface of the films and substrates and reduce tensile stress in the films. However, the thickness of the hard layers on the film surface decreases with the increase of the number of layers, which reduces the hardness of the surface. This aggravates the deformation of the film surface when the indenter indents, therefore increases the tensile stress in it, which may cause the expansion of longitudinal cracks; and also increases the plastic strain of the soft film near the surface, which may cause cracks and flaking.4. Nanohardness tests are simulated by the program provided in this paper. The results show that in the nanohardness tests of TiN/Ti six-layer films, in order to obtain the hardness of the films, the proportion of indenter depth to the thickness of sub-layer should be more than 23%; and in the nanohardness tests of TiN/Ti mono-layer films, in order to avoid the plastification of substrate, the proportion of indenter depth to the thickness of film should be less than 20%.In the last part of the paper , accor...
|
PDF Full Text Request