Optimal bias voltage of substrate for formable nanocrystalline Niobium precoatings of metallic bipolar plates

Metallic bipolar plates (BPPs) are key components of the proton exchange membrane fuel cells. To further improve the manufacturing efficiency of metallic BPPs, precoatings have attracted much attention recently. However, precoatings tend to crack and loss the integrity during the forming process. Therefore, formable precoatings are in great demand, and the relationship between deposition and formability of precoatings needs to be studied. In this research, Niobium (Nb) precoatings with different bias voltages of substrate are deposited on SS316L with unbalanced magnetron sputtering process. To evaluate the formability, uniaxial tensile tests are conducted. As the bias voltage increases, the formability of the precoatings firstly increases and reaches the maximum when the bias voltage is −60 V. With further increase of bias voltage, the formability is deteriorated. To understand the effect of bias voltage, the microstructure and surface morphology of the samples are characterized. Nb precoatings with great compactness and nanocrystalline state can sustain 30 % engineering strain when the bias voltage is −60 V, which is the optimal bias voltage for formable Nb precoatings.