Graphitic regulation of C/Cr composite film for 304 stainless steels bipolar plates

Abstract

This work aims to deposit conductive and corrosion-resistant protective coatings for bipolar plates in proton exchange membrane fuel cells. The films' graphitization degree is regulated to obtain excellent conductivity and corrosion-resistance. A series of Cr-doped graphite-like carbon films with different substrate bias voltages and deposition times (600 V 2 h, 900 V, 2 h, 900 V, 3 h; and 1200 V, 2 h) were prepared on 304 stainless steels by plasma enhanced chemical vapour deposition method combined with magnetron sputtering technique. The results showed that an increase of substrate bias voltage increased substrate current density. The substrate current density was the highest when the substrate bias voltage was 1200 V at 400 °C. Subsequently high substrate bias voltage caused ID/IG to increase, indicating that the increase of energy contributes to graphitisation of the films. The particle size of the films decreased with increasing bias voltage, and the cross section showed that the prepared films were homogeneous and distinctive. The interface contact resistance (ICR) of the films at 1.4 MPa decreased with an increase of substrate bias voltage, showing that deeper graphitisation improves the conductivity of the films. The coated 304 stainless steels showed a considerable decrease in self-corrosion current density in comparison to the 304 stainless steel substrate. According to the VDI-3198 indentation test, the adhesion of the films and substrate belongs to the HF1 level. After long-term service, chromium oxides, iron oxides, and defects were formed on the surface, leading to an increase in ICR. The study revealed that the optimum technical parameters were 1200 V and 2 h.