| Summary: | Humid air is a very important service environment, in which metal friction parts should be enhanced to offer excellent corrosion resistance and wear resistance. The B<sub>4</sub>C coating is an excellent candidate material to enhance the corrosion resistance and tribological behaviors. The purpose is to investigate the effect of negative bias voltages on the tribological properties of B<sub>4</sub>C coatings under a high relative humidity environment. Amorphous B<sub>4</sub>C coatings were successfully prepared by closed field unbalanced magnetron sputtering technology and its microstructure, hardness, elastic modulus, adhesive force and tribological properties were systematically studied. Results demonstrate that the B<sub>4</sub>C coatings deposited at each negative bias voltage have a columnar structure and the surface roughness remained unchanged (about 1.0 nm), while the thickness, hardness, elastic modulus and adhesion force increase first and then decrease with the negative bias voltage increasing. Among them, the B<sub>4</sub>C (−50 V) coating showed the best mechanical properties. It should be noted that the B<sub>4</sub>C (−50 V) coating with an excellent corrosion resistance also exhibits the lowest friction coefficient (~0.15) and wear resistance (7.2 × 10<sup>−7</sup> mm<sup>3</sup>·N<sup>−1</sup>·m<sup>−1</sup>) under humid air (85% RH). This is mainly due to the tribochemical reaction of B<sub>4</sub>C during a sliding process to produce boric acid at the sliding interface. B<sub>4</sub>C coatings can provide an excellent corrosion resistance and high wear resistance due to their high chemical stability and high hardness.
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