Friction and electrical contact resistance in reciprocating nano-scale wear testing of metallic materials

Reciprocating contacts occur in a wide variety of practical wear situations including hip joints and electrical contacts. In developing tribological tests for candidate materials with improved durability in these contacts it is beneficial that the contact conditions (e.g. sliding speed) can be reproduced. Hence, a fully instrumented capability for rapid high-cycle linear reciprocating nano-scale wear tests has been developed. It is multi-sensing with high data acquisition measurements of probe displacement data, friction, cumulative frictional energy dissipation and electrical contact resistance. In comparison with other nanoindenters the design has the high level of lateral rigidity which provides sufficient stability to perform nano- or micro-scale wear tests for extended duration (e.g. several hours, up to 300 m sliding). In this study, reciprocating nano-wear tests with diamond probes have been performed on the biomedical alloys Ti6Al4V and 316L stainless steel, and with electrically conductive metallic probes on gold and silver alloys. The stainless steel exhibited a ductile response with low friction throughout the load range. At higher loads on Ti6Al4V, there was an abrupt transition to higher friction and fracture-dominated wear after ~20 cycles. Improved detection of the onset of wear and the subsequent failure mechanisms sliding against conductive probes was possible by a multi-sensing approach simultaneously monitoring friction and electrical contact resistance (ECR). Changes in ECR exhibited a complex correlation with changes to the measured friction. The reciprocating tests of noble metal-noble metal contacts (Au–Au and Ag–Ag) showed much longer endurance than gold vs. steel contacts although occasional isolated failures were observed. A new approach for the analysis of repetitive nano-scratch test data was also developed enabling improved data mining.