Structure and tribological behaviour of nanoscale multilayer C/Cr coatings deposited by the combined steered cathodic arc/unbalanced magnetron sputtering technique

Nanoscale multilayer C/Cr coatings (bilayer thickness similar to2 nm) were produced by the combined steered cathodic arc/unbalanced magnetron deposition technique. The mirror-polished M2 substrates were treated by Cr+ ion etching/implantation followed by the deposition of a 0.25 mum thick CrN base layer. A 1.6 mum thick C/Cr multilayer coating was then deposited by non-reactive unbalanced magnetron sputtering while rotating the substrates in front of three graphite and one Cr targets. During the multilayer deposition, a bias potential between -65 and -95 V was applied to the substrates. The ion flux measured by a flat electrostatic probe was 1.2 mA cm(-2) and the deposition rate was 0.4 mum h(-1), which resulted in ion-to-neutral ratio of J(i)/J(n)=5.2. Cross-sectional transmission electron microscopy diffraction and Z-contrast imaging investigations revealed a novel nanostructure in which the basic nano-lamellae obtained as a result of substrate rotation in front of the C and Cr targets were modified by ion-irradiation-induced nanocolumnar structure. The intense ion-irradiation of the immiscible film components caused local enrichment of Cr and C that propagated in the growth direction resulting in Cr-rich nanocolumns separated by C-rich boundaries in an overall amorphous structure. Tribological studies of the composite C/Cr coatings were conducted using a pin-on-disk apparatus under a load of 5 N, at Velocities of 0.13-0.15 m s(-1) and for distances of similar to500 m in dry nitrogen (similar to 0% humidity) and open air (30% relative humidity). Results indicated that the C/Cr composite coatings provided friction coefficients of 0.7-0.8 in dry nitrogen, while the values were significantly lower in open air, 0.21-0.24, during sliding against both the coated and uncoated balls for coatings deposited at bias voltage of -75 V The friction coefficient decreases to 0.16 when the bias voltage of -95 V was used. (C) 2003 Elsevier B.V. All rights reserved.