Investigation of magnetron sputtered nitrogen-containing chromium coatings with additions of copper and silver

In the past two decades, research in the field of plasma-assisted Physical Vapour Deposition (PVD) has been intensively concentrated on hard and superhard coatings, particularly ceramic-ceramic and ceramic-metal nanocomposite film, in order to achieve improved tribological properties, such as wear resistance and low friction. However, sufficient (rather than extreme) high hardness, combined with low elastic modulus (high H/E ratio), often proves to be more effective again wear than (extreme) high hardness by itself, especially so for coatings on soft or compliant substrates. In addition, self-lubricating PVD tribological thin films have been intensively investigated in recent years. Among these, PVD nanocomposite coatings containing soft metals (e.g. Ag or Cu), as a solid lubrication phase, embedded in a hard wear-resistant matrix, such as a transition metal nitride, carbide, oxide or supersaturated solid solution, and mixtures of them (in ternary/quaternary/nanocomposite coating systems), have all been extensively studied, with the promise of improved tribological performance during transient and/or cyclic temperature changes. Therefore, a PVD coating with similar elastic modulus as soft or compliant metallic substrates (e.g. around 200 GPa or less), and promising solid-lubricating properties over a (relatively) large range of temperatures, would be of great application interest. This work concentrates on nanocomposite metallic (or partially metallic) PVD CrCuAgN coatings. The main aims of the study are to preserve the metallic state of Cr and, instead of creating stoichiometric ceramic nitride phases (CrN or Cr2N), to supersaturate it with interstitial nitrogen atoms, in order to obtain coatings with high H/E ratios. Moreover, the transport behaviour of solid lubricants (i.e. Cu and Ag) to coating surfaces under medium high temperature would also be important for the tribological and antimicrobial properties of these coatings. Guidelines of coating design for specific applications can be obtained by comprehensive study of the abovementioned studies. Coatings were produced by asymmetric pulsed-DC unbalanced magnetron sputtering, on AISI 316 stainless steel coupons. Then they were divided into three groups for post-coat annealing at different temperatures, which were as-deposited (no annealing), 300 °C and 500 °C, respectively, with a fixed annealing duration of 2 hours. The as-deposited and annealed coatings were then analysed and/or evaluated using X-ray diffraction (XRD) analysis, scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM), high-resolution TEM and scanning TEM (STEM) with EDX, nanoindentation, scratch test, high-temperature reciprocating sliding wear test, in order to investigate the phase composition, chemical composition, surface and fracture morphologies, phase- and elemental distribution, mechanical properties and tribological properties, etc. The result show that, in coatings with a nitrogen concentrations of up to 16 at.% (i.e. N/(Cr+N) atomic ratio up to 0.18), a metallic Cr solid solution with supersaturated interstitial nitrogen was preserved, even after post-coat annealing at 300 °C and 500 °C. At relative higher N/(Cr+N) atomic ratios, the coexistent state of α-Cr solid solution with embedded Cr2N nitride phase is relatively stable over a large range of nitrogen concentrations (i.e. N/(Cr+N) ratios from 0.18 to 0.55 in this investigation), which can also survive at relatively high temperature (e.g. 500 °C). Nitrogen-free CrCuAg coatings exhibited relatively low hardness, ranging from 5 GPa to less than 10 GPa. With the introduction of nitrogen, the hardness of CrCuAgN coatings increased significantly, to ~15 GPa with a preserved metallic Cr solid solution matrix. And the tribological test results showed that, a significant decrease in the coefficients of friction, over 50% compared to that of the substrate (from 0.31 to 0.14 with diamond indenter, and from 0.83 to 0.40 with alumina ball, respectively) was obtained for the CrCuAgN coatings, showing effective solid lubricating behaviour. The co-existence of Cu in a PVD coating containing Ag was found to be beneficial to Ag transport onto the coating surface, resulting not from the preconceived ‘intergranular network channels’ of Cu, but from the much higher detachment (from small precipitates to larger ones) rate of Cu than Ag, which would ‘leave a path’ for Ag to transport after Cu. Therefore by proper design and processing, CrCuAgN coatings with appropriate elemental compositions and solid-solution matrix can be obtained, in which the precipitation rate of Ag can be (partially) controlled by a moderate concentration of Cu (4.5 at. % appears to be sufficient), resulting in a controlled supply of solid lubricants, hence being highly promising in the application fields of solid lubricating coatings.