Molecular Simulation And Experimental Study Of Nanoindentation Of Polymer

Nanoindentation has become one of the most commonly applied methods for determining of Modulus and Hardness of materials at nano to micrometer length scales, such as polymer film and coating. Performing nanoindentation experiments, the mechanical behavior of various materials has been found to be dependent on the indentation depth------ hardness and elastic modulus will be increased with the decrease of the maximum indentation depth. Size effect is a kind of inherent attribute of material, all of our studies aim to find a hardness model which can accurately indicate the inherent properties of material. However, the hardness models existing now are on the basis of experiment, which can also cause the size effect, so they can’t reflect the inherent size effect attributes of materials. The main content of this paper is to use molecular simulation to eliminate the effect from experiment, trying to provide a method to set up the nano indentation hardness model. We chose to simulate PE and PVAC. In the final section, we performed nanoindentation experiments for epoxy resin and a kind of PVAC composite blended with a small amount of Au nanoparticles.Firstly, we introduced the basic principle of molecular mechanics and molecular dynamics simulation, and given the details of PCFF force field functions and parameters applied in the numerical simulations of PE and PVAC, which has a good reference value for the detailed understanding of the PCFF force field. We calculated the elastic constants of PE and PVAC by MD simulations. and provided the polymer modeling method and simulation technology, which was of instructional significance.Molecular mechanics method was used to simulate the nanoindentation of PE and PVAC. Their deformation mechanism was analyzed in detail. During the process of loading, the matrix firstly gets elastic deformation. When the load reaches the first peak on the load-displacement curve, the molecular chains in the matrix start to slip and get irreversible rotation(plastic deformation), then the load drops immediately and sharply. Before the load reaches the second peak on the load-displacement curve, the matrix gets elastic deformation again, and it would drop immediately at the second peak. Such repeated behavior is performed in the matrix until the end of loading. This process clearly explains the serrated load-displacement curve.We provided a way to calculate the hardness of materials which were loaded by a truncated cone-shaped indenter in molecular simulation, and a hardness model was developed to describe the size effect by this way. Since the molecular simulation of nanoindentation completely eliminates various error factors from experiments, we believe the given hardness model reflects the real inherent size effect attributes of materials. We performed nanoindentation experiments for epoxy resin and a kind of PVAC composite. Using the modeling method of hardness model given in our simulation, we set up a model which can describe the hardness data of the experimental very well.The molecular modeling method provided in this paper can be used as a reference for other molecular modeling, and the hardness calculation method is of great instructional significance for calculating the hardness of other materials in molecular simulation. The hardness model established is of theoretical and practical value to understand and describe the inherent size effect attributes of materials.