Mechanism of graphitization and optical degradation of CVD diamond films by rapid heating treatment

Polycrystalline CVD free-standing diamond films have been heat treated through a rapid heating process using a direct current (DC) arc plasma jet system within the temperature range of 1500–2000°C. The optical transmission of the diamond film decreases when the treatment temperature exceeds 1500°C. When the diamond film was annealed to 1800°C for just 1min, the infrared (IR) transmittance at λ=10.6μm of the film was significantly decreased from 60.5% to 4.0%, due to the transformation from diamond to graphite. Further increase of the temperature to 2000°C leads to complete loss of optical transmittance in only 30s, due to the detrimental occurrence of graphitization. XRD patterns indicate that graphite ultimately grows preferentially along the (0002) plane by forming disordered or turbostratic graphite as intermediate product. Raman spectra and SEM images verify that at 1500–1850°C graphitic carbon transforms from the diamond surface in the form of protruding submicron crystallites, which were still containing a number of sp3 CC bonds. In addition to graphitization on the external surface, the diamond-graphite phase transition also occurs along the grain boundaries and the calculated activation energy of the grain boundary graphitization process is 276kJ·mol−1 for the CVD diamond. [Display omitted] •Polycrystalline CVD free-standing diamond films have been heat treated through a rapid heating process by H2/Ar arc plasma method within the temperature range of 1500–2000°C.•The relationship between IR transmittance and graphitization were investigated after the polycrystalline CVD diamonds were heat treated.•Amorphous or turbostratic graphite forms as an intermediate product during the graphitization process.•The activation energy of graphitization process along grain boundaries is calculated.