Characterization and properties of low-friction, multilayered Cr-doped diamond-like carbon coatings prepared by pulse biased filtered cathodic arc deposition

The aim of the article was to investigate the effect of pulsed high-voltage bias on phase composition, surface morphology, mechanical properties, and the tribological performance of the multilayered Cr/CrN/DLC coatings prepared by the filtered cathodic arc deposition (FCAD) process. The coatings were deposited using a laboratory arc source device equipped with two macroparticle filtering systems. All coatings were deposited at substrate temperatures below 200 degrees C. Cr/CrN/DLC multilayered coatings were analysed using X-ray photoelectron spectroscopy, a glow discharge optical emission spectrometry technique, scratch testing, nano-indentation measurements (nano-hardness and Young's modulus), and optical microscopy. Tribological tests employing a ball-on-disc tribosystem were performed in dry friction conditions in an air-conditioned room (RH 50 per cent, 23 degrees C). The results proved that the substrate bias voltage exerts a significant influence on the mechanical properties of the Cr/CrN/DLC multilayered coatings by the changing chemical composition of the DLC top layers. With an increase of bias voltage from 2 to 6 kV, the Cr content increases from 2 to 4 per cent and the sp(2)/sp(3) ratio decreases from 1.06 to 0.44. The Cr-doped DLC multilayered coatings were characterized by a very smooth surface, a low friction coefficient (0.13-0.15), moderate hardness (8-11 GPa), and a Young's modulus of 95-130 GPa. Due to the moderate adhesion strength of DLC top layers, their wear resistance was not satisfactory for real applications. In each tribological test, the delaminating process of the DLC layers was observed after approximately 200-300 load cycles. To improve the mechanical properties and adhesion of a DLC layer, the optimization of the transition and DLC layers is achieved by changing the parameters of the physical vapour deposition process.