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 the tribological and mechanical properties of environmentally friendly self-lubricant films, induced by the addition of soft lubricant agents that can diffuse quickly 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 novel nacre-like multilayered Mo₂N–SiN_x/Ag–SiN_x self-lubricant films via an radio frequency (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 three phases of face-centered cubic (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 the Ag–SiN_x layers with thickness below 6 nm on the Mo₂N template resulted in an extraordinary increase in both the hardness and elastic modulus, which was able to prevent severe degradation of the mechanical properties caused by the addition of Ag. The room-temperature anti-friction property could be enhanced by increasing the Ag–SiN_x layer thickness due to the excellent lubricant nature of Ag, which acts in synergy with Mo₂N, while the wear rate below 4×10⁻⁸ mm³/(N·mm) was due to the high mechanical strength. 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 (WR).