<em>In situ</em> study of mechanical properties and structural transformations during heating of WC–TaC–Co cemented carbides in a transmission electron microscope column

Abstract

This study investigated the hardness of lamella with varying thickness, obtained from a massive, fine-grained cemented carbide comprising WC–6 %Co–0.2 %TaC, characterized by an average grain size of approximately 5 μm. The picoindentation method was employed for this analysis. Picoindentation was carried out using a Berkovich diamond indenter with a radius of curvature around 50 nm, and the experimental data were analyzed using the Oliver–Pharr model. The results revealed a significant correlation between hardness and lamella thickness. The hardness of the electron transparent section (thickness less than 100 nm) of the lamella measured 11.3±2.8 GPa, while the electron nontransparent section (thickness more than 200 nm) exhibited a hardness of 20.8±1.2 GPa. The lower hardness in electron transparent objects (thickness ~100 nm) is likely attributed to a combination of factors, including the potential bending of thin cobalt layers, the presence of edge effect, and closely spaced structural defect dislocations on the lamella surface. In situ TEM studies were conducted to examine structural transformations during the heating of WC–6 %Co–0.2 %TaC lamella, including in the presence of oxide phases (WOx ). Oxide phases on the lamella’s surface were generated by oxidizing the lamella at 200 °C in an air atmosphere. The results indicated that heating up to 500 °C did not bring about significant changes in the structure. However, at 600 °C, there was a notable thinning of cobalt layers due to intense surface diffusion of cobalt. Simultaneously, the formation of nanosized particles of the Co3W3C phase, ranging in size from 5 to 20 nm, was observed in the binder. These particles resulted from a shift in the equilibrium phase composition of the carbide, changing from a two phase region (WC + γ) to a three phase region (WC + γ + Co3W3C) as a consequence of the lamella’s oxidation.