Influence of temperature and pressure on the static contact resistance of vacuum heat-treated polyacrylonitrile films

The protection against corrosion of low-level electrical contacts is usually insured by metallic terminal coatings. However, they can be degraded by wear and corrosion, which finally causes the failure of the contacts. The field we have been investigating, consists of replacing the terminal metallic layer by an organic conducting film, derived from polyacrylonitrile (PAN) by heat-treatment. In this paper, we present results concerning the static contact resistance (Rc) measured in a ball/plane configuration on metallic coupons over-layered with PAN films and heated under high vacuum at several temperatures: Rc at room temperature under a constant normal load; Rc at room temperature varying the normal load; Rc at a constant normal load varying the temperature of the contact. These experiments reveal a transitional heat-treating temperature. Above this temperature, the film is very conducting and the behavior of the contact is ruled by the metallic electrodes. Below this temperature, the film is moderately conducting and then responsible for the contact resistance. In that case, pressure and temperature dependencies of the contact show that variable-range hopping is the most probable transport mechanism. At the transitional annealing temperature, the values of the contact resistance are scattered. Electrical investigations with a modified atomic force microscope (AFM) show that the film is covered by a very thin layer, non-adherent and poorly conducting.