The influence of surrounding atmosphere on tribological properties of hard protective coatings

• Hard coatings were tribologicaly tested in different atmospheres: ambient air, nitrogen and oxygen.
• In the nitrogen atmosphere the oxidation processes were suppressed resulting in reduced wear rate.
• In oxygen atmosphere all the hard coatings exhibited higher friction coefficient than in ambient air atmosphere.
• Formation of different wear debris depends on the atmosphere type.

In this research paper we compare the tribological properties of conventional hard coatings (TiN, CrN, TiAlN) and more advanced protective coatings (CrxA11 − xN and CrxV1 − xN). Numerous factors can influence the coating tribological properties; thus we limit our research to evaluate the tribological behaviour in different surrounding atmospheres. We preformed tribological tests in the air (high, medium and low humidity), nitrogen and oxygen environment. Experiments indicate that oxygen-rich environment causes the formation of oxides at lower temperatures while the use of the nitrogen environment can effectively stop the oxidation process which results in a reduction of wear. We encountered different friction and wear behaviour in dependence of the atmosphere type. To fully understand the processes during tribological testing, the wear mechanisms were investigated for each hard coating separately. The wear debris and the wear tracks were analysed by SEM, focused ion beam (FIB), 3D-stylus profilometry and confocal optical microscopy.