A new criterion to quantify the DC and RF electrical response of an electrical contact subject to fretting wear

Microwave connectors are extensively used in numerous industrial applications, including modern means of transport such as aviation and car industries. Connectors are subjected to vibrations and micro-displacements in the contact interface which lead to fretting wear damage. The present study investigated the influence of fretting wear on the transmitted high-frequency signal. A new structure suitable for radiofrequency (RF) measurements, comprising a cross-cylinders specimen adapted on a transmission line, was designed for fretting tests. An electrical contact transmitting a radiofrequency signal was modeled by an RLC (resistor, inductor and capacitor) equivalent electrical circuit. Various coating materials were tested: tin (Sn), white bronze (WB), gold (Au) and silver (Ag). The radiofrequency signal attenuation (insertion loss) and DC electrical contact resistance (ECR) evolutions using a four wire method were measured simultaneously as a function of fretting cycles. The threshold criterion for contact damage in the radiofrequency band was defined as a 0.1 dB increase in signal attenuation. The corresponding DC ECR failure (i.e., NcRF=NcDC,RF) was established for a ECR threshold about ΔRRF = 40mΩ whatever the studied material. Hence the RF failure can be established from a simple DC analysis. SEM and EDX analyses of debris layer confirm that as for Micro Cut (MC) threshold (ΔRMC = 4 mΩ), the fretting scar at the RF failure is characterized by a quasi-elimination of noble elements (Au or Ag) and the formation of an insulating oxide debris layer. However, the transition from MC to RF failure appears to be induced by a drastic increase of the proportion of low conducting copper oxides.