Tribological behaviour of microindented 100Cr6 steel surfaces in dry contact conditions

In the present work, we studied the dry tribological behaviour of a 100Cr6 steel, the spherical surface of which was texturized with microindentation. The purpose of adopting a mechanical indentation technique on a non-planar surface was to simultaneously evaluate the effectiveness of adopting a fast, deformation-based technique for improving the contact tribological properties. Specifically, dimples were created using an automatic microhardness tester equipped with a Vickers indenter, setting a load of 0.5 N. Friction tests were performed at different speeds considering textured surfaces with two different void ratios (VRs). Textured and untextured surfaces were tested using a ball-on-disc tribometer. In addition, the effect of dimple size was evaluated by producing Vickers indented surfaces at a load of 5 N per each indentation, while keeping the VR values unchanged and testing the frictional properties of such surfaces at a fixed speed of 4.18 mm/s. Textured surfaces were deeply investigated to motivate the improvement of tribological properties. Notably, compared to the untextured samples, the microindented samples exhibited a much lower coefficient of friction (COF), with a friction reduction compared to the untextured case ranging from 45 to 65%, depending on the VR values. The adoption of large dimples allowed the reduction of the COF, already at smaller VR value but, in such a case, the presence of bulges at the edge of the dimple worsens the wear resistance of the counter surface. In addition to reducing the contact area and the capability to trap any debris in the dimples, the local measurement of strength allowed to clarify that the friction reduction is also determined by the work hardening effect produced by the microindentation texturing. Considering the significant improvements recorded in terms of COF and the high ability to indent even non-planar surfaces, the proposed approach can be considered very promising and, therefore, industrially applicable (e.g. using a specifically designed multi-indenter tool) to affect the friction behaviour of components, even locally, during both their use and their production.