Simultaneously achieving superior mechanical and tribological properties in WC/a-C nanomultilayers via structural design and interfacial optimization

Nanomultilayered WC/a-C coatings with modulation period raning from 1.3 to 11.5 nm were successfully fabricated using unbalanced magnetron sputtering process and the evolution of their microstructure, mechanical and tribological properties with the modulation period were systemically investigated. It has been demonstrated that the columnar structure in WC/a-C nanomultilayers originates from the surface irregularities of intermediate Cr/WC/C layer and the column diameter is correlated with the initial Cr layer thickness in this intermediate stage. Hardening and toughening through the nanomultilayer structure design have been achieved and particularly pronounced when the modulation period ranges from 5.8 to 10 nm. The enhanced hardness and fracture toughness are mainly attributed to dislocation/crack pinning effect from the thin individual layer and the heterointerfaces betweenWC and a-C layers. It is also found that the more heterointerfaces, the lower internal stress. What's more, the WC/a-C nano-multilayers with small modulation period (<= 5.8 nm) exhibit low COF about 0.05 and good wear resistance due to the interfacial sliding between a WO3-rich tribofilm and a carbon-rich wear surface; once the modulation period above 5.8 nm, it fails to build up such a low friction interface between the tribopairs, which combining with the high intrinsic friction caused by its own structure characteristics result in high COF about 0.18-0.2 and poor wear resistance. (C) 2016 Elsevier B.V. All rights reserved