Residual stresses, interfacial adhesion and tribological properties of MoN/Cu composite coatings

Ceramic thin film coatings, such as MoN/Cu, are attractive for engine applications due to their low friction, high hardness and high wear resistance. However, there is a need to establish a reliable connection between the deposition parameters of the thin film and its tribological performance. In this study, two coating compositions, MoN and MoN with Cu were deposited on H-13 tool steel substrates in order to correlate compositional variables that result from processing to their respective tribological properties. The efficacy of the coating in protecting surfaces is highly dependent on its adhesion to the substrate and its tribological properties. Residual stresses resulting from the coating deposition were evaluated using an X-ray microprobe. Scratch testing was performed to measure the coating adhesion energy, and the wear rate was determined using a ball-on-flat contact configuration on a high frequency reciprocating test rig. It was observed that coatings with lower copper content performed better in the wear test and exhibited higher coating adhesion energy. A primary wear damage mechanism was coating removal by delamination and spallation, which is related to the adhesion energy. Since coating processing variables determine the structure and properties, and hence affect the tribological properties of these MoN based coatings, these parameters can be used to optimize coating composition for enhanced tribological performance.

► Tribological properties of MoN/Cu coatings assessed.
► Residual stresses directly affect coating adhesion and wear resistance.
► Copper addition weakens adhesion and increases wear.