Characterisation of duplex and non-duplex (Ti, Al)N and Cr-N PVD coatings

In this paper, non-duplex (i.e. without plasma nitriding) and duplex (Ti, Al)N and Cr-N coatings produced by ion plating are 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) on fractured and polished cross-sections. After the duplex treatment, an increase in surface roughness of hard PVD coatings could be detected, probably due to an effect of sputter etching by argon/nitrogen ions on the substrate surface during the plasma nitriding step. XRD results indicated that the coatings deposited with and without the plasma nitrided substrate exhibited different dominant phases, which can be attributed to the lower deposition temperatures used for producing the duplex (Ti, Al)N and Cr-N coatings. GDOES results indicated that both non-duplex and duplex (Ti, Al)N coatings had compositions close to Ti_0.50Al_0.50N. The non-duplex and duplex Cr-N coatings, however, showed different compositions, with the non-duplex Cr-N coatings displaying a two-layer structure. Finally, scratch tests indicated that the duplex coatings exhibited higher critical loads than their non-duplex counterparts, confirming that a plasma nitriding treatment prior to coating deposition increased the load-bearing capacity of steel substrates. The highest critical loads were recorded for the duplex (Ti, Al)N coating.