Densification and Surface Carbon Transformation of Diamond Powders under High Pressure and High Temperature

A new type of poly-diamond plate without a catalyst was produced via the high-pressure high-temperature (HPHT) compression of diamond powders. The densification of diamond powders and sp³ to sp² carbon on the surface under HPHT compression was investigated through the characterization of the microstructure, Raman spectroscopy analysis and electrical resistance measurement. The densification and sp³-sp² transformation on the surface are mainly affected by the pressure, temperature and particle size. The quantitative analysis of the diamond sp³ and sp² carbon amount was performed through the peak fitting of Raman spectra. It was found that finer diamond particles under a higher temperature and a lower pressure tend to produce more sp² carbon; otherwise, they produce less. In addition, it is interesting to note that the local residual stresses measured using Raman spectra increase with the diamond particle size. The suspected reason is that the increased particle size reduces the number of contact points, resulting in a higher localized pressure at each contact point. The hypothesis was supported by finite element calculation. This study provides detailed and quantitative data about the densification of diamond powders and sp³ to sp² transformation on the surface under HPHT treatment, which is valuable for the sintering of polycrystalline diamonds (PCDs) and the HPHT treatment of diamonds.