Self-lubricating mechanisms in magnetron-sputtered Cr–Mo–N/Ag composite films: Role of Mo content and tribo-oxide evolution

Green lubricant development remains a key challenge for zero-friction, zero-emission technologies. In this work, Cr–Mo–N/Ag composite films were engineered by integrating a CrN hard matrix with the self-lubricating effects of Ag and Mo, and deposited via RF magnetron sputtering with varying Mo content. All films exhibited a dual-phase structure of fcc-CrN and fcc Ag. Mo incorporation below 17.7 at.% enhanced hardness and elastic modulus to around 22 GPa and 280 GPa, respectively. At room temperature, Mo addition up to 17.7 at.% reduced the coefficient of friction (COF) and wear rate (WR) to about 0.45 and 3.5 × 10 −6 mm 3 /N·mm, attributed to matrix hardening, Ag-induced lubricity, and tribo-oxide formation. At 500 °C, the COF further dropped to 0.21 for the 22.6 at.% Mo film due to the formation of low-shear tribo-oxides (MoO₃, Ag₂MoO₄, Ag₂Mo₄O₁₃). However, excessive debris adhesion hindered accurate WR measurement. These results highlight an effective strategy for designing robust, self-lubricating coatings for harsh thermal environments. • The Cr–Mo–N/Ag film was deposited using RF magnetron sputtering system. • The incorporation of Mo enhanced the hardness from 17 GPa for the CrN/Ag reference to 22 GPa. • At 500 °C, the COF continued to decrease to reach as low as 0.21 for the 22.6 at.% Mo film. • The optimized composition of 17.7 at.% Mo achieved a balance between hardness and tribological performance.