The oxidization behavior and mechanical properties of ultrananocrystalline diamond films at high temperature annealing

•The selective thermal oxide behavior happened at the early stage of annealing.•The nanoindentation depth of UNCD films kept ∼90nm with annealing time increasing.•The mechanical properties of films tend to be stable with annealing time increasing.•A steady 30-nm-thick oxidized layer is formed on the top-surface of oxidized film. Ultrananocrystalline diamond (UNCD) films prepared by hot filament chemical vapor deposition (HFCVD) were annealed at 1000°C in low degree vacuum under a pressure of 4000Pa. The correlation between the mechanical and structural properties was investigated to understand the oxidization behavior of UNCD films after high temperature annealing. At the early stage of annealing (∼30min), the amorphous carbon and graphite in grain boundaries are selectively oxidized firstly along the clusters’ gaps, with the Young's modulus and hardness decreasing rapidly revealed by nanoindentation results. A special annealing time of ∼30–60min is found to exist as a turning point that the mechanical properties changing trend has a transition, because of the diamond grains starting to be oxidized. With the annealing time increasing to 180min, the nanoindentation depth increases from ∼70 to ∼90nm and the Young's modulus and hardness decrease more slowly with almost keeping constant of ∼383 and ∼35GPa, respectively. X-ray photoelectron spectroscopy (XPS) results show that a steady 30-nm-thick oxidized layer has been formed on the top-surface and keeps a balance of the speed between films being oxidized and the carbon oxidation being broken down.