Depth Dependence of Nanoindentation Pile-Up Patterns in Copper Single Crystals

STANISAW KUCHARSKI 0 DARIUSZ JARZA BEK 0 0 STANISAW KUCHARSKI, Head, and DARIUSZ JARZ ABEK, Assistant, are with the Surface Layer Laboratory, Institute of Funda- mental Technological Research , Pawin skiego 5b, 02-106 Warsaw, Poland . Contact A study of the dependence of nanoindentation pile-up patterns on the indentation load and crystallographic orientation is presented. Three different orientations(001), (011), and (111)of single crystal copper were investigated. Experiments were conducted on a CSM ultrananoindentation tester using a Berkovich tip. The topographic images were obtained using an atomic force microscope. The evolution of pile-up patterns with different applied loads was observed. The results show that for applied loads equal to 0.45 mN and smaller the pile-up patterns do not depend on the crystallographic orientation of the indented surface; instead, they depend on the tip's geometry. On the other hand, in the case of indentation loads bigger than 2 mN, pile-up patterns on the surfaces of (001)-, (011)-, and (111)-oriented single crystals have fourfold, twofold, and sixfold (or threefold) symmetry, respectively. An intermediate state was also reported. Furthermore, a detailed analysis of residual impressions with maximal applied loads equal to 2 mN and bigger reveals that both pile-up and sink-in patterns are present. - Nevertheless, in the case of anisotropic material, the response around the impression is more complex. Hence, much research in recent years has focused on the anisotropy associated with nanoindentation tests of a single crystal in different crystallographic orientations. Some initial attempts focusing on this topic can be found in the work of Vangroenou and Kadijk.[2] More recently, Wang et al.[3] presented a study of the dependence of nanoindentation pile-up patterns and microtextures on the crystallographic orientation using high purity copper single crystals. Their results showed that the pile-up patterns on the surfaces of (001)-, (011)-, and (111)-oriented single crystals have four-, two-, and six-fold symmetry, respectively. They evaluated the crystallographic directions in which the pile-ups should appear for specific oriented surfaces and they observed that at ambient temperature the active glide systems of copper consist of (111) glide planes and h110i slip directions. A similar effect was also observed by Liu et al.[4] and was compared with finite element simulations. However, in this work, threefold instead of sixfold symmetry was observed in the case of (111)-oriented surfaces. A detailed 3D study of the microstructure and texture below a conical nanoindent in a (111) Cu single crystal at nanometer-scale resolution was carried out by Zaafarani et al.[5,6] These experiments revealed pronounced deformation-induced 3D patterning of the lattice rotations below and around the indent. On the other hand, the crystal plasticity finite element method (CPFEM) has been used to describe the phenomenon of nanoindentation pile-up patterns. This approach was presented by Casals et al.[7,8] In their first mentioned paper, CPFEM simulations of pyramidal indentation (both Vickers and Berkovich indenters) in copper single crystals were compared with experimental results. In the second one, CPFEM was used to investigate the anisotropy in the contact response of fcc and hcp single crystals. To this aim, spherical indentation experiments were simulated at mesoscale. Furthermore, Gan et al.[9] performed numerical simulations of wedge indentation into fcc cubic single crystals. In this work the stresses, shear strains, and crystal lattice rotation map within the material were obtained and the numerical predictions were validated with the experimental results (nanoindentation tests and EBSD investigation of the indented area). Moreover, both spherical and pyramidal indentation tests in fcc single crystals were simulated by Alcala et al.[10] and Narayanan et al.[11] Zahedi et al.[12] developed their own crystal plasticity models to investigate spherical indentation in copper single crystals. Not only Cu but also single crystals made of other materials have been investigated recently. The nanoscale anisotropic elasticplastic behavior of single crystal aragonite using nanoindentation with a Berkovich probe and scanning force microscope imaging was studied by Kearney et al.[13] Ju et al. used molecular dynamic (MD) simulations to elucidate the anisotropic characteristics (pile-up patterns and atomic stress distribution) in the material responses for nickel substrates with different surface orientations.[14] Yoshida et al.[15] performed nanoindentation tests with a Berkovich probe on aluminum single crystals in order to investigate the effect of crystallographic planes on the yield load and the hardness of the metals in nearly perfect crystals. Britton et al. combined nanoindentation (Berkovich probe), electron backscatter diffraction (EBSD), and CPFEM simulation to examine the anisotropy in the indentation behavior of individual grains within alpha Ti-polycrystals.[16] Xie et al.[17] reported that in an Ni-based single crystal superalloy indented on the (001) and (011) planes, the deformation also depends on the crystallographic orientation. The residual imprints in anisotropic (multilayer) coatings were investigated using AFM, and simple model of the elasticplastic penetration of a Berkovich pyramid has been developed by Kravchuk et al.[18] Unfortunately, few researchers have addressed the problem of nanoindentation experiments with small loads and pyramidal tips. Therefore, the pile-up pattern observed in these experiments depends mainly on the crystal structure of the indented surface. Conversely, Chiu and Ngan,[19,20] who investigated single crystals of Ni3Al, conducted nanoindentation experiments in the (111) plane using a Berkovich indenter with forces ranging from 0.8 to 8 mN. Next they investigated the impressions using transmission electron microscopy (TEM). They reported that indentation plasticity zones consist of two regions: a core with a very high dislocation density and a surrounding region where the dislocation density is much lower. They observed that the core and a surrounding low dislocation density zone did not evolve in a self-similar manner as the load increased. It was suggested that this is a new aspect of the indentation size effect (ISE) whose implications should be further investigated. Therefore, in this paper, we present the results of nanoindentation using a Berkovich tip and very small indentation depths (down to tens of nanometres). For the experiments, we indented single crystalline copper specimens with different orientations. To imagine the sample surface, scanning force microscopy was used. We have observed that at very small indentation loads, the pile-up patterns have a random character and are closer to the tip symmetry rather than the crystal symmetry. This effect may strongly influence the results of th (...truncated)