Effects of Mo–N–Cu doping on microstructural, mechanical, and tribological properties of thick ta-C coatings for cryogenic applications

• The properties of a Mo–N–Cu–ta-C coating are explored. • Appropriate doping metals for cryogenic ta-C coatings are identified. • Mo doping reduces the wear of the counterpart material. • Cu doping increases the fracture toughness and reduces the disk wear. • Properties suitable for application in a liquid nitrogen environment are obtained. This study investigates the microstructural, mechanical, and tribological properties of Mo–N–Cu tetrahedral amorphous carbon (Mo–N–Cu–ta-C) coatings fabricated using simultaneous filtered cathodic vacuum arc and unbalanced magnetron sputtering deposition in air (296 K) and liquid nitrogen (LN 2 , 77 K) environments. The 1 μm-thick Mo–N–Cu–ta-C coating consisted of nanocomposite Mo carbide and a nanolayered Cu structure. Compared with un-doped ta-C, Mo doping reduced counterpart wear by 82 %, while Cu doping enhanced the fracture toughness by 22 % and decreased disk wear by 86 %. The addition of N 2 gas further promoted phase separation, thereby strengthening the combined effect of Mo and Cu to achieve a balanced wear response. Furthermore, Cu and N 2 minimized the thermal expansion and strain mismatches between the substrate and coating with decreasing temperature, thereby facilitating cryogenic applications and alleviating thermal mismatch stress. At 296 K, the Mo–N–Cu–ta-C coatings exhibited superior adhesion, controlled fracture toughness, and stable wear behavior, while maintaining reliable performance in the LN 2 environment. These findings highlight that ternary metallic doping combined with nitrogen-assisted deposition provides significant advantages over conventional ta-C coatings, which closes a critical research gap for protective coatings in cryogenic conditions.