Phase transformation in hot pressed cobalt and cobalt–diamond materials

Abstract

The present paper describes the effect of oxygen and hard phase (diamond) content on the allotropic phase transformation in hot pressed cobalt. Investigations were carried out both on samples made of cobalt and cobalt–diamond materials. All specimens were obtained by hot pressing in graphite moulds. X-ray diffraction (XRD) was used in order to estimate the regular and hexagonal phase content of the cobalt. The XRD measurements were supplemented with microstructural observations by means of scanning electron microscopy and transmission electron microscopy. The obtained results provided evidence that occurrence of phase transformation in hot pressed cobalt upon cooling to room temperature strongly depends on the oxygen content, and it is hindered by a very fine grain structure and the cobalt oxides. The effect of residual stress produced in cobalt–diamond materials, which is the effect of the difference between linear expansion coefficients of metal matrix and diamond on the phase composition of the cobalt, proved to be of secondary importance.

Keywords:

• COBALT
• DIAMOND
• HOT PRESSING
• MICROSTRUCTURE