Mutual promotion on the mechanical and tribological properties of the nacre-like self-lubricant film designed for demanding green tribological applications

The inverse relationship between tribological and mechanical properties of environmental friend self-lubricant films, induced by adding soft lubricant agents which can diffuse fast at elevated temperatures, has hindered the widespread use of these materials in industrial applications. This paper took this challenge to break through the above established relationship, by developing a novel nacre-like multilayered Mo₂N-SiN_x/Ag-SiN_x self-lubricant films, using a RF magnetron sputtering system, for real applications where harsh conditions at elevated temperatures exist. The multilayered films, deposited by alternating deposition of Mo₂N-SiN_x and Ag-SiN_x modulation layers, exhibited tri-phases of fcc-Mo₂N, fcc-Ag and SiN_x, where SiN_x encapsulated the nano-crystalline Mo₂N and Ag phases in each layer, to successfully induce a ‘brick and mortar’ nacre-like microstructure (in the area without the coherent structure). The epitaxy growth of Ag-SiN_x layers, below 6 nm, on the Mo₂N template resulted in an extraordinary increase in both hardness and elastic modulus, able to block the serious degradation of the mechanical properties by the addition of Ag. The room temperature anti-friction property could be enhanced by the increase of the Ag-SiN_x layer thickness due to the excellent lubricant nature of Ag acting in synergy with Mo₂N, while the wear-resistance was due to the high mechanical strength giving rise to values below 4×10^-8 mm³/(N.mm). The tribological properties at 600 °C also benefited from the interlocked multilayered architecture which allowed an extreme low friction coefficient of ~0.12 and a negligible wear rate. This behavior was justified by the synergism between the lubricant action of Ag and Mo₂N, and the tribo-phase transformation from Ag₂Mo₄O₁₃ to Ag₂MoO₄.