Formation of Solid Lubricants during High Temperature Tribology of Silver-Doped Molybdenum Nitride Coatings Deposited by dcMS and HIPIMS

The coating system MoN-Ag is an interesting candidate for industrial applications as a low friction coating at elevated temperatures, due to the formation of lubricous molybdenum oxides and silver molybdates. Film deposition was performed by high-power impulse magnetron sputtering and direct current magnetron sputtering. To facilitate a future transfer to industry Mo-Ag composite targets have been sputtered in Ar/N2 atmosphere. The chemical composition of the deposited MoN-Ag films has been investigated by wavelength dispersive X-ray spectroscopy. Morphology and crystallographic phases of the films were studied by scanning electron microscopy and X-ray diffraction. To obtain film hardness in relation to Ag content and bias voltage, the instrumented indentation test was applied. Pin-on-disc tribological tests have been performed at room temperature and at high temperature (HT, 450 °C). Samples from HT tests have been analyzed by Raman measurements to identify possible molybdenum oxide and/or silver molybdate phases. At low Ag contents (≤7 at.%), coatings with a hardness of 18–31 GPa could be deposited. Friction coefficients at HT decreased with increasing Ag content. After these tests, Raman measurements revealed the MoO3 phase on all samples and the Ag2Mo4O13 phase for the highest Ag contents (~23–26 at.%).