An approach to elucidate the different response of PVD coatings in different tribological tests

In this paper, the different performances of duplex (i.e. with a prior plasma nitriding treatment) and non-duplex PVD coatings in micro-abrasive wear tests and impact tests are discussed, taking into account some key properties, such as nanohardness (H), elastic modulus (E) and the H/E ratio. The nanohardness and elastic modulus of the coatings were determined by nanoindentation and were characterised by means of several analytical methods, such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, Knoop hardness measurements, scratch tests and scanning electron microscopy (SEM). Similar H/E ratios were found for all coatings under investigation. Duplex (Ti,Al)N coatings exhibited the best wear resistance in micro-abrasive wear tests. The results obtained in such tests suggested the occurrence of abrasive wear by plastic deformation, with the hardest coatings displaying the lowest wear rates. The highest wear rates were recorded for duplex and non-duplex Cr-N coatings. Conversely, a duplex Cr-N coating exhibited the best performance in impact tests using a tungsten carbide ball. The duplex Cr-N coating displayed the lowest elastic modulus among all coatings tested, which can be attributed to the presence of a small amount of α-Cr metallic phase. This coated system also showed the smallest difference between the substrate and coating elastic moduli, indicating that low elasticity mismatch between the coating and substrate is desirable for achieving good performance in impact tests. The differing behaviours exhibited by duplex (Ti,Al)N and Cr-N coatings in different tribological tests reveal the importance of selecting the best coating for a given tribological application.