Preparation of ultramicro molybdenum carbide powders and study on wear properties of their coating

Abstract

Using specially designed mechanochemical ball-mill equipment, ultramicro molybdenum carbide (MoC) powders were prepared by high-energy ball milling from pure molybdenum powders in civil coal gas atmosphere at room temperature. The structure and the particle size of the powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Transmission Electron microscopy (TEM). The results showed that after milling for 30 h, the MoC powders of hexagonal structure were obtained, and their average particle size was around 100 nm. It was found that chemisorption of CO in coal gas onto the fresh molybdenum surfaces created by milling was the predominant processes during the solid-gas reaction, and the energy input due to the introduction of highly dense grain boundaries and lattice defects provided the activation energy for the transition from Mo-C chemisorption to MoC. A coating was formed on the 40Cr steel base using plasma spray by mixing Ni60 alloy powders and ultramicro MoC powders of 5 wt.%, 10 wt.%, and 15 wt.%, respectively. Coating abrasion tests under the condition of dry-grinding, 2 h wear time, and 300 N load showed that the wear resistance property of the coating added with ultramicro MoC powders could be improved greatly, and the wear resistance property of the coating increased with the increase of ultramicro MoC content. The wear mechanisms of ultramicro MoC coating is mainly plough wear and flaking wear assisted. In the abrasion process, the evenly distributed ultramicro MoC particles play a dispersion strengthening and self-lubricating role in the coating.