Nano-scratch hardness and the Lateral Size Effect (LSE)

Abstract

Nano-scratch testing has been used throughout this thesis in order to deepen the understanding of the processes occurring during the nano-scratch test, and to develop a method for calculating the nano-scratch hardness, from the quantifiable variables that are obtainable from the technique. Scratch testing on the macro scale is a well-established technique, however when reducing the scale of a mechanical test, whilst interest is focussed on the yield strength or hardness of the material, plasticity size effects must be explored. A literature survey concludes that size effects in nano-scratch testing have not been investigated in the past, thus this is the main subject of this thesis. In order to carry out this investigation a number of methods were trialled to calculate the nano-scratch hardness of pure, polycrystalline, oxygen free copper using both the edge forward and the face forward tip orientations of a diamond Berkovich indenter. The results obtained were compared to the indentation hardness and the most theoretically suitable method was adopted for later experiments carried out in this work; the technique for obtaining measurements was optimised, such that a genuine lateral size effect (LSE), whereby the nano-scratch hardness increases with decreasing scratch size, was observed. To further investigate the lateral size effect, nano-scratches were performed on a sample of single crystal copper in different work hardened states. It was observed that the nano-scratch hardness not only increases with decreasing scratch size, but also increases when the spacing between the dislocations in the material is reduced; when the level of work hardening in the sample increases, the density of dislocations increases, thus the spacing between these obstacles is reduced. In addition to this, the anisotropy of the nano-scratch hardness was investigated by altering the scratch direction in the (100) plane of the single crystal copper. It was found that the nano-scratch hardness is anisotropic and that the scratch hardness is largest when the scratch direction is parallel to the slip plane. It is known that the yield strength of a material increases with decreasing average grain size and therefore the effect of grain size on the nano-scratch hardness was considered. By reducing the grain size of pure, annealed, oxygen free copper, the nano-scratch hardness was observed to increase. In all experiments, the nano-scratch hardness values of scratches performed in the face forward tip orientation were larger than that of scratches performed in the edge forward tip orientation, when scratching the same sample condition. This suggests that scratch hardness is tip geometry dependent and in order to develop a method of calculating a tip orientation-independent scratch hardness, the shape of the indenter and the plastic flow of material around the indenter in that orientation, must be known and incorporated into the calculation, possibly as a drag coefficient. In addition to the geometry of the plastic flow, and therefore the plastic zone size, it was found that the nano-scratch hardness is also governed by the interaction between the geometry of the indenter and the grain boundaries in the material. Finally a number of experimental issues from the nano-scratch test are highlighted and researchers are encouraged to consider these precautions when using the nano-scratch test