Oxidation and graphitization of micro‐grained diamond in vacuum and atmosphere

Abstract Thermal stability of diamond is crucial for its applications. However, conflicting reports exist regarding the sequence of oxidation and transformation and the temperature range at which non‐diamond phases form. In addition, systematic and quantitative studies are insufficient. This study systematically investigates the oxidation and graphitization behavior of micro‐grained diamond powder after heat treatment in air and vacuum environments. The temperature ranges and volume fraction for non‐diamond phase formation were estimated, and the oxidation and non‐diamond phase transition temperatures were directly measured using synchronous thermal analysis. The results indicate that: (1) when annealed in air, the diamond powder oxidizes and then graphitizes as the temperature increases, while the graphitized diamond is then oxidized; (2) diamond powder remains stable in air at temperatures below 596°C, with significant oxidation occurring about 669°C; (3) the conversion of diamond to graphite occurs at 728°C in air and approximately 1300°C in vacuum, with oxygen significantly promoting the graphitization of diamond; and (4) increased specific surface area (smaller particle size) and longer heating times exacerbate thermal instability.