The deposition parameters in the synthesis of CVD microcrystalline diamond powders optimized by the orthogonal experiment

•Microcrystalline diamonds are grown on the scratched substrates by HFCVD.•The pressure has the strongest influence on the nucleation of microcrystals.•The morphology of nuclei plays a decisive role on that of final crystals.•As-grown diamonds directly meet size requirements of commercial powders.

In the present work, microcrystalline diamond powders are deposited by using a bias-enhanced hot filament chemical vapor deposition (HFCVD) apparatus. Mirror-polished silicon wafers are served as substrates, pretreated by the scratching process for 10–15 s. A systematic investigation is under taken into the combined effects of deposition parameters on nucleation and growth characteristics of microcrystalline diamonds, based on the orthogonal collocation method. The results show that the morphology of final microcrystals depend mainly on that of nuclei rather than the deposition parameters, while the quality and grain size of crystals largely depend upon the deposition parameters. A high reactor pressure (3–4.5 kPa) in the nucleation process is a necessary condition for depositing the ideal nuclei with the single-crystal structure and euhedral diamond faces. Then under a set of optimized growth parameters, the final single crystals exhibit the regular-shaped morphology and smooth surfaces. The CVD microcrystals with various grain sizes in the range of 0.3–2 μm can be obtained by regulating the deposition time; moreover, they have a dramatically narrow particle size distribution, meeting the requirements on certain types of commercial powders without the process of sieving grain.