Effects of WC phase contents on the microstructure, mechanical properties and tribological behaviors of WC/a-C superlattice coatings

Nanocomposite WC/a-C coatings with variable contents of tungsten carbide (WC1-x) and amorphous carbon (a-C) were successfully fabricated using a magnetron sputtering process. The microstructure, mechanical properties and tribological behaviors of the as-fabricated coatings were investigated and compared. The results showed that the "superlattice coating" feature of an alternating multilayer structure with a-C and WC1-x nanocrystallites layers on the nanoscale was formed. These multilayer superlattice structures led to diminished residual stress and improved the strength of the adhesion to the substrate. The WC/a-C coating with W 5.43 at.% exhibited low friction coefficients of 0.05 at 25 degrees C and 0.28 at 200 degrees C. This significant improvement in the tribological performances of the WC/a-C coating was mainly attributed to the superior "superlattice" microstructure and the formation of a continuously compacted tribofilms, which was rich in graphitized carbon at 25 degrees C and dominated by the friction triggered WO3 at 200 degrees C. Moreover, the WC/a-C coating with W 5.43 at.% achieved optimal anti-wear properties at 25 degrees C due to the synergistic combination of the enhancement effects of the WC1-x, nanoparticles and the partition effect from the transfer film that restricted direct contact of the steel ball with the coating and thus prevented further intense wear. The accelerated wear of the WC/a-C coating with the increase of the WC phase content at 200 degrees C might be due to the combination of oxidation wear and abrasive wear that originated from the WC1-x phase. (C) 2015 Elsevier B.V. All rights reserved.