Nanoscratching of polycrystalline copper examined through strain gradient crystal plasticity

Abstract

Nanoscratch testing is a method pivotal for evaluating the mechanical and tribological characteristics of materials which involves the controlled scratching of specimens with a nano-scale indenter. This research delves into the analysis of effect of grain size on the deformation mechanisms and material responses during nanoscratch tests on polycrystalline copper. By utilizing finite element method and adopting a lower-order strain gradient crystal plasticity framework, this study investigates results such as reaction forces on the indenter, apparent friction coefficients, and alterations in pile-up topography. These factors are examined with the aforementioned strain gradient theory, employing calculations of the density of geometrically necessary dislocations to obtain size-dependent material response. The crystal plasticity framework is implemented into ABAQUS as a user material subroutine (UMAT), and the model's accuracy is affirmed through comparisons with experimental data from single crystal copper studies available in the literature. 3D geometries are generated to model a single crystal and three polycrystal materials with average grain diameters of 5 µm, 15 µm, and 50 µm. These specimens are subjected to deformation by a rigid Berkovich indenter to simulate nanoscratching tests in the numerical examples, where the key point is to examine the grain size effects while keeping fixed any other variables that may influence the results.