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–SiNx /Ag–SiNx 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–SiNx and Ag–SiNx modulation layers, exhibited three phases of face-centered cubic (fcc) Mo2N, fcc Ag and SiNx , where SiNx 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–SiNx 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–SiNx 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). This behavior was attributed to the synergism between the lubricant action of Ag and Mo2N and the tribo-phase transformation from Ag2Mo4O13 to Ag2MoO4 .