In-depth investigation of a third body formed by selective transfer in a NiCr / AgPd electrical contact

Abstract

The aim of this work is to investigate the physical and chemical mechanisms involved in the formation process of a third body in an electrical contact. The device studied is an electric rheostat position sensor consisting of a low pressure sliding contact between a resistive body (NiCr) and a contactor (AgPd). The contact is lubricated with a silicone oil/PTFE grease and operating under boundary lubrication condition. It was previously shown that the degradation of the electrical behavior of the contact is mainly due to a severe abrasive wear of the NiCr surfaces, enhanced by the formation on the AgPd contactor of a hard and adhesive transfer enriched in Nickel (Ni). In the current work, In-depth cross-section analyses combining focused ion beam (FIB) with analytical transmission electron microscopy (TEM) techniques are used to identify the structural and chemical changes that undergo by the interfacial contact involving NiCr and AgPd surfaces. The results highlight a selective character (Ni alone transferred from NiCr alloy) and a nanocrystalline structure of the Ni third body adhering to the noble counterpart. Several hypotheses involving severe plastic deformation (SPD) of the NiCr surface, catalytic effect of Pd element and/or Cr salts volatility are discussed. Furthermore, it was found that the deposition of alumina powder on the NiCr surface using a tumble finishing treatment inhibits the formation of the detrimental third body, which preserves the contact from severe wear and leads to a significant improvement of the electrical behavior of the position sensor. The role of alumina in preventing the contact from wear is discussed and a scenario explaining the evolution of the interfacial contact at operating conditions is proposed.