Fracture initiation in FCC and BCC metals during tribology

• Applicant: Privatdozent Dr. Steffen Brinckmann
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324591046

• Final Report Year: 2021

Final Report Abstract

Improving the metals resistance against wear requires an fundamental understanding of the mechanisms during the initial stages of wear. Initially, the counter surfaces get into contact and the harder material’s asperities indent the softer materials surface. This first stage is mimicked by the indentation of a spherical diamond nanoindenter tip into the counter metal. We observed that: Dislocation motion is activated first on positively inclined slip planes. This slip activity results in slip-steps on this slip-planes. The Hertz solution is a suitable approximation of the stress distribution during that stage of deformation. As a result of this deformation, surface pile-ups form. - Due to the pile-ups, the surface is non-planar and the contact is not captured by the Hertz solution. This topography change results in a change of the stress state and the activated slip planes. This change in stress state results in slip on negatively inclined slip planes. - While slip on positively inclined planes occurred in the contact zone of the indenter, the surrounding of the imprint is dominated by slip on negatively inclined slip planes. - Plastic flow in the material surrounding the imprint is determined by the inclination angle of the slip-plane with respect to the surface. At a transition angle of 55 o, 58o, the flow changes its direction from the forward (away from imprint) to sideways. - Large strain FEM simulations verified an inversion of the resolved shear stress at this transition angle. The second part of the initial wear deformation is the transition from static indentation to dynamic wear. This evolution from stationary to sliding contact was again mimicked by using single micrometer-sized roughness peaks. In this study we focused on a wider range of materials: copper, cementite and austenitic steel. We observe that the conservation of contact area, elastic recovery, and the front pile-up development are the dominant mechanisms that determine the plasticity and wear volume. The elastic recovery leads to an additional contact area at the backside of the asperity. The influence of the crystallographic orientation was found to be negligible during the initial increase in wear depth but significant during the later stages. Moreover, the partial wear depth recovery is observed only in ductile materials.

Publications

• Deformation of pearlitic steels during nanotribology. 46th Leeds-Lyon Symposium on Tribology 2019, Lyon, France. 2019-09-02 – 2019-09-04
  Tsybenko, H.; Brinckmann, S.; Dehm, G.
  
• Unraveling indentation-induced slip steps in austenitic stainless steel, Mater. Des., 183, 108169, (2019)
  Xia, W., Dehm, G., Brinckmann, S.
  
• (2020). Plasticity during micro-wall wear test: single dislocation wall formation. Nanobrücken 2020: A Nanomechanical Testing Conference. 2020, Düsseldorf, Germany. 2020-02-04 – 2020-02-06
  Xia, W., Brinckmann, S., & Dehm, G.
  
• Insight into indentation-induced plastic flow in austenitic stainless steel. J. Mater. Sci., 55, 9095–9108 (2020)
  Xia, W., Dehm, G. & Brinckmann, S.
  
• Investigation of single asperity wear at the microscale in an austenitic steel, Wear 452–453, 203289 (2020)
  Xia, W., Dehm, G., Brinckmann, S.
  
• On the commensuration of plastic plowing at the microscale, Tribol. Int., 151, 106477 (2020)
  Tsybenko, H., Xia, W., Dehm, G., Brinckmann, S.