Effect of the TiN-to-CrN Layer Thickness Ratio on the Mechanical, Tribological and Corrosion Properties of TiN/CrN Multilayer Coatings

TiN/CrN multilayer coatings with bilayer periods of 20 and 30 nm and TiN:CrN layer thickness ratios of 1:1, 1:2 and 2:1 were deposited on 40Kh steel substrates by pulsed direct current magnetron sputtering (pDCMS). The phase composition (XRD), microstructure (SEM), mechanical (nanoindentation, scratch testing), tribological (dry sliding and boundary lubrication), and corrosion (potentiodynamic polarization in 3.5 wt.% NaCl) characteristics of the coatings were examined. Increasing the TiN volume fraction increased nanohardness and elastic modulus reaching a maximum H3/E2 = 0.186 GPa. The Λ30-TiN(20)/CrN(10) coating showed the best adhesion (Lc1 = 14.4 N, Lc2 = 17.7 N, CPRS = 48 N2) and the lowest wear rate both under dry sliding and under boundary lubrication (1.2 × 10−5 mm3/(N·m)). In contrast, CrN dominance (Λ30-TiN(10)/CrN(20)) sharply degraded the tribological behaviour (the wear rate increased by almost an order of magnitude) and caused catastrophic localized pitting in the corrosion tests. At the same time, the best corrosion resistance (Ecorr ≈ −0.47 V, the most extended quasi-passive region) was achieved at the smallest bilayer period. A balanced composition, Λ20-TiN(10)/CrN(10), owing to the highest density of interlayer boundaries, effectively blocks through-thickness growth defects. The architectures that are optimal in terms of mechanical/tribological and of corrosion criteria do not coincide, which indicates that the TiN:CrN ratio has to be chosen according to the dominant type of service loading. The results obtained show that varying the thickness ratio of the TiN and CrN layers at bilayer periods of 20 and 30 nm makes it possible to tune the balance of hardness, wear resistance, adhesion and corrosion resistance of multilayer coatings on structural steels.