Influence of the bias voltage on the structure and the tribological performance of nanoscale multilayer C/Cr PVD coatings

Nanoscale multilayer C/Cr coatings have been deposited by utilising the combined steered cathodic arc/unbalanced magnetron sputtering technique. The coating microstructure and tribological performance have been investigated as a function of the bias voltage, ranging from Ub=−65 to −350 V. The XRD results revealed that C/Cr coatings are amorphous at low Ub, but became more crystalline when the Ub increased to −350 V. High-resolution XTEM analysis indicated coating densification and smoothening as well as formation of novel amorphous nanostructure, in which carbon-rich clusters are surrounded by a Cr-rich matrix, leading to the formation of self-organised multilayer structure as the bias voltage was increased from −65 to −350 V. An increase of the bias voltage from −65 to −350 V resulted in an increase in the hardness from 8 to 25 GPa and Young's modulus, E from 186 to 319 GPa. A pin-on-disc test showed that the friction coefficient was reduced from 0.22 to 0.16 when the bias voltage was increased from −65 to −95 V. However, a further increase in the bias voltage to −350 V led to an increase in the friction coefficient to 0.31. The lowest wear coefficient Kc∼6.25×10−17 m3 N−1 m−1 was achieved at Ub=−120 V. Standard HSS drills, 8 mm in diameter, coated with C/Cr have been tested using solution annealed AISI 304 stainless steel as the work piece material. An improvement of the lifetime by a factor of ∼9 has been achieved as compared to the uncoated tools. In this test, the C/Cr coating outperformed a number of commercially available PVD coatings, such as TiCN, TiAlCrN and showed similar performance to TiAlCrYN.