STEM analysis of WC-Co coatings modified by nano-sized TiC and nano-sized WC addition

Abstract

Thermally sprayed hardmetal WC-Co-based coatings are widely applied in order to provide good wear resistance to mechanical components working in demanding environment. The WC-Co coatings are obtained by various spraying methods such as High Velocity Oxygen Fuel (HVOF) or High Velocity Air Fuel (HVAF) techniques. Additional improvement of coatings' properties can be achieved by adding very fine carbides to the WC-Co powder feedstock. The usual approach is the agglomeration and sintering of submicron-sized carbides without or with micro-sized carbides (bimodal distribution). In this work, WC-Co powders were modified by blending them with 5wt% nano-sized TiC (<100nm) or 5wt% nano-sized WC (<100nm). The coatings were deposited by High Velocity Air Fuel (HVAF) on a steel substrate. As reference, pure WC-Co coating was also sprayed. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were employed for the microstructural characterization of the deposited coatings. The formation of solid solution of tungsten in cobalt with the composition Co0.9W0.1 by selected area electronic diffraction, was detected. W dissolution effect was observed to be lower in the case of nano-sized TiC-added coatings. Microstructural investigation by STEM indirectly indicates that nano-sized WC completely dissolved in the cobalt matrix, inducing solid solution of tungsten in cobalt formation in the 5wt% WC-added coating. The formation of new phases containing Ti, except TiC, was not found in the coating. Obtained results suggest that nano-sized TiC may increase the stability of metallic matrix microstructure in a WC-Co-based coating.