24th International Colloquium Tribology - January 2024 107 Early Stages of Tribo-Oxidation in Single Crystalline Copper Ines L. Kisch 1, 2 , Julia S. Rau 3 , Vahid Tavakkoli 4 , Lisa T. Belkacemi 5, 6 , Baptiste Gault 6, 7 , Christian Greiner 1, 2, * 1 Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 2 IAM-ZM MicroTribology Center µTC, Karlsruhe, Germany 3 Department of Physics, Chalmers University of Technology, Gothenburg, Sweden 4 Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 5 Leibniz-Institut für Werkstofforientierte Technologien (IWT), Department of Physical Analysis, Bremen, Germany 6 Max-Planck-Institut für Eisenforschung GmbH, Department of Microstructure Physics and Alloy Design, Düsseldorf, Germany 7 Department of Materials, Royal School of Mines, Imperial College London, London, UK * Corresponding author: christian.greiner@kit.edu 1. Introduction Tribological loading on metallic surfaces often leads to accelerated oxidation, which can significantly influence the resulting friction and wear behavior as well as component lifetime. It is therefore important to better understand the underlying chemical and mechanical mechanisms in order to engineer materials with better friction and wear performance. By performing experiments with varying cycle numbers with a copper-sapphire model system, we were able to monitor the sequence of stages in the microstructural evolution and oxide formation, and investigate especially the very early stages of tribo-oxide formation. 2. Materials and Methods Copper single crystals were used in order to eliminate the influence of pre-existing grain boundaries, while sapphire counter bodies were chosen due to their hardness and chemical inertness. The samples underwent heat treatment and were then ground and polished. Details of the procedure are given in [1], [2]. Immediately before the tribological experiment, the samples were electropolished to provide a surface with significantly reduced pre-existing deformation that also exhibits only minimal amounts of native oxidation. Experiments were conducted at room temperature with a low normal load of 1.5 N, in a reciprocating motion with a sliding speed of 0.5 mm/ s. The relative humidity of the surrounding atmosphere was kept at 50 % (± 3 %), while the number of sliding cycles was varied (10 and 100 cycles respectively). In order to assess the oxidative features developed under the tribological loading, two protective platinum layers (first with the electron, then with the ion beam) were deposited onto the region of interest to preserve the surface. Subsequently, scanning (transmission) electron microscopy (S(T)EM) as well as focused-ion beam milling (FIB) were employed to create cross-sections and foils, revealing the oxide evolution into the depth of the material. Additionally, the foils were characterized by energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS) and high resolution transmission electron microscopy (HR-TEM). These techniques allowed for the analysis of the chemical composition as well as the crystallographic structure of the oxides. 3. Results Figure 1 shows TEM images from a single crystalline copper sample after ten sliding cycles. The foil was cut parallel to the sliding direction and perpendicular to the sample surface. Figure 1a is using an high-angle annular dark field (HAADF) contrast that gives an elemental contrast. Dark, randomly distributed clusters can be seen below the surface (exemplary ones marked with arrows), these were confirmed by EDS to be rich in oxygen. In order to analyze the clusters further, HR-TEM was performed, the results for one exemplary cluster are depicted in Figure 1b. Fast Fourier transformation (FFT) was performed to reveal the crystallographic structure and showed that the oxide clusters are a mix of amorphous and nanocrystalline areas. Figure 1: Copper sample after 10 sliding cycles. a) HAADF TEM image from a foil cut perpendicular to the surface. The dark contrast shows oxide clusters just below the surface. b) HR-TEM image of an exemplary oxide cluster. FFT shows that there are crystalline as well as amorphous areas within the cluster. We speculate that oxidation initiates at surface defects, where oxygen diffusion into the material leads to the initial formation of supersaturated, amorphous clusters. With the formation of these clusters, a new phase boundary is formed, that subsequently acts as a preferred pathway for oxygen diffusion. This eventually leads to higher levels of oxygen in the outer areas of the clusters, which eventually reach the Cu 2 O stoichiometry and therefore, the formation of crystal- 108 24th International Colloquium Tribology - January 2024 Early Stages of Tribo-Oxidation in Single Crystalline Copper line Cu 2 O can be observed starting from the outer particle borders. In a second experiment, a sample was loaded with 100 sliding cycles, results from the respective TEM analysis are depicted in Figure 2. At this stage, the copper exhibits a fully formed oxide layer with a mean thickness of 0.3-µm. The HAADF image in Figure 2a shows a distinct, line-like feature in a depth of approximately 100-nm below the sureface, which is reminiscent of the dislocation trace line (DTL) observed e.g. in [1]. The darker contrast suggests a higher oxygen content in that area. To confirm this assumption, EELS measurements were performed on the indicated region of interest (ROI), the results are shown in Figure 2b. A line profile measured across the region indicated by the black box shows the atomic percentage of copper and oxygen respectively, plotted over the depth into the material. At the height of the presumed DTL, an elevated oxygen concentration of up to 50 at % is observed. We speculate that a DTL is formed in the early stages of sliding, and that the locally increased dislocation density leads to a locally higher oxygen concentration. Figure 2: Copper sample after 100 sliding cycles. a) HAADF image from foil perpendicular to the surface. At the sample surface, a fully formed oxide layer is visible (dark contrast). On the right hand side, there is a magnified image of the ROI. b) EELS measurement in the ROI. Green indicates copper, while red indicates oxygen. The graph on the right shows a depth profile measured across the indicated area and plots the atomic percentage of Cu and O respectively over the depth into the material. At the height of the DTL, a near 1: 1 composition is observed. The observed Cu and O composition potentially allows for the formation of CuO. The area was subsequently analysed with HR-TEM, and FFT analysis of the images showed a crystalline area with a monoclinic structure, further supporting the hypothesis that CuO has formed locally. Additionally, the EELS measurements show copper rich areas depleted in oxygen just below the surface. These are likely caused by an increased upward diffusion of copper through the oxide. This assumption is supported by the observation of potential Kirkendall pores at the copper-oxide interface. 4. Conclusion The results of this study suggest that there is a distinct correlation between microstructural features that evolved during sliding and the diffusion and local distribution of oxygen within the material. This results in the local formation of different copper oxides. Investigating and understanding the fundamental mechanisms at play will, in the long term, enable the targeted development of frictionand wear-optimized surfaces. References [1] C. Greiner, Z. Liu, L. Strassberger, P. Gumbsch, „Sequences of Stages in the Microstructure Evolution in Copper under Mild Reciprocationg Tribological Loading”, ACS Appl. Mater. Interfaces, Vol. 8, no. 24, pp. 15809 - 15819, 2016. [2] Z. Liu, T. Höche, P. Gumbsch, C. Greiner, „Stages in the tribologically induced oxidation of high-purity copper”, Scr. Mater., Vol. 153, pp. 114 - 117, 2018.