Effect of high temperature annealing on ultrahard polycrystalline diamond in air and vacuum conditions

The effect of high temperature annealing on ultrahard polycrystalline diamond (UHPCD) has been investigated in air and vacuum conditions up to 1500 degrees C. The thermal stability, carbon bonds, morphologies and wear resistance of UHPCD were evaluated by thermal gravimetric-differential scanning calorimetry (TG-DSC), Raman spectroscopy, scanning electron microscopy (SEM) and wear tester. The thermal analysis results indicated that the thermal stability of chemical vapor deposited (CVD) diamond was better than that of polycrystalline diamond (PCD) even though it was weakened by high pressure high temperature treatment, while no graphitization was observed on UHPCD in flowing argon up to 1500 degrees C. When the UHPCD annealed in air, the oxidation damage with the extension of cracks and spalling holes was observed on CVD diamond as the evolution of temperature. The result confirmed by the changes of diamond peak position and full width at half maximum (FWHM) in Raman spectra curves. The PCD had shown the damage with cracks induced exfoliation of binder regions and cracks ruined diamond grains. However, the diamond peak position and FWHM of CVD diamond and PCD showed slight reduction as a function of vacuum annealing temperature with no detectable change of morphologies. The high temperature annealing has strong impact on the wear resistance of UHPCD in air while slightly in vacuum.