Toward low friction in water for Mo2N/Ag coatings by tailoring the wettability

Abstract Increasing demands for robust surfaces in harsh conditions, such as erosion, abrasion and sea-water, has stimulated the development of self-lubricated protective coatings. Meanwhile, due to the oil crisis, research in water lubrication again attracts much attention from both academics and practical engineers. Here, a higher hydrophilicity accompanying with a remarkable drop of friction coefficient in water environment was achieved successfully in Mo 2 N/Ag coatings by increasing Ag content. To do these, the Mo 2 N/Ag coatings with different Ag content were deposited by co-sputtering, which exhibit a nanocomposite structure consisting of precipitate Ag embedded in the Mo 2 N matrix. The high hydrophilicity can be ascribed to the combined contributions of the partial oxidation of Mo 2 N and high polarity of Ag precipitates. The decrease of friction coefficient is illustrated by the colloidal friction products and a mode with electric double layer. In which, enhanced hydrophilicity will result in forming a thin “water film” layer between the interface of counterpart and the coatings. And the MoO x /Ag 2 Mo 4 O 13 derived from the hydrolysis action of Mo 2 N/Ag sliding in water could function as lubricant phase. Meanwhile, these negative charged MoO x /Ag 2 Mo 4 O 13 colloidal particles induce the rearrangement of positive ions in the “water film” and form an electric double layer, which also contributes to the decrease of friction coefficient.