Title

Microstructure of consolidated nanocomposite tungsten carbide-cobalt

Date of Completion

January 1996

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Composites of tungsten carbide/cobalt (WC/Co) are used as cutting tools, drilling bits, and in hard-facing applications. Nanostructured WC/Co composites having high hardness and greater wear life synthesized by a spray conversion process, and consolidated by processing (i) below and (ii) above the eutectic temperature, have been examined by transmission electron microscopy. An unusual phenomenon of precipitation of nanometer sized cobalt inside sub-micron tungsten carbide has been observed on consolidated samples of the material. Results from high resolution electron microscopy (HREM), diffractogram analysis of HREM images, and micro-diffraction revealed that there were two crystal forms of the precipitates--smaller precipitates are hexagonal close packed and larger ones are face centered cubic cobalt. A size dependent phase transformation occurs during growth of the cobalt crystals inside WC under the constraint of the WC matrix. Based on examination of starting powder, the consolidation process, and the final microstructure, mechanisms for formation of the precipitates and their transformation are proposed. Role of VC in retarding the microstructural coarsening of WC phase is discussed. ^