Two-layer nanocomposite WC/a-C coatings produced by a combination of pulsed arc evaporation and electro-spark deposition in vacuum

Abstract

New cost-efficient technology combining pulsed arc evaporation (PAE) and electro-spark deposition (ESD) in vacuum was developed and implemented to coat Ti with two-layer WC/a-C coating in a single technological run using the same WC-6%Co electrode. A thick bottom ESD layer (70–90 μm) was composed of β-Ti (~70%), β-(W,Ti)C1-x and a small amount of α-Ti. A thin upper PAE layer had two sublayers (3.2 and 1.6 μm thick) sequentially deposited in an Ar and C2H4 atmosphere. Both sublayers had a nanocomposite, nearly amorphous structure, in which small disordered β-WC1-x crystallites were dispersed in a-C matrix. A superior tribological performance of the PAE/ESD coatings was attributed to gradual changes in phase composition and grain size, as well as a combination of tailored mechanical properties (hardness gradually increased from 3.6 GPa (Ti substrate) to 20 GPa (PAE layer)), enhanced toughness and high thickness of the ESD sublayer, which prevented the substrate from plastic deformation, and low friction coefficient of the top PAE layer due to its nearly amorphous WC/a-C structure with a large amount of a-C phase acting as a solid lubricant. In-situ mechanical TEM tests demonstrated superior adhesion strength of the PAE/ESD interface able to withstand high stresses of 560 MPa.