Effect of argon addition on CH₄-H₂ microwave plasma: Self-consistent simulation and nanodiamond coating deposition

High-quality nanodiamond coatings have immense potential in cutting tools, electronic devices, and biomedicine. Introducing Ar is a crucial method of enhancing the quality of these films. This study examines the influence of Ar on the spatial distribution of CH₄/H₂ microwave plasma parameters, group density, and the subsequent deposition of nanodiamond coatings. The electron number density, discharge volume, gas temperature and molecular mean free path increase with the increase of Ar volume fraction. In addition, the dissociation rate of H₂ increases with the increase of Ar volume fraction, which leads to more H. For the Ar-rich deposition atmosphere, the hydrocracking rate of CH₄ reduces and the hydrocracking rate of C₂H increases, resulting in a decline in the number density of CH₃ and an increase in the amount of C₂ at the core of the discharge region. C₂H₂ is the primary neutral hydrocarbon in the discharge, while CH₃ is the leading neutral single‑carbon hydrocarbon. The total flux of atomic hydrogen and C₂ increases with Ar, which is a key factor in improving the quality of nanocrystalline diamond crystals and enhancing renucleation. The optical emission spectroscopy of CH₄/H₂/Ar plasma analysis indicates an accelerated generation rate of H and C₂ with an increase in Ar volume fraction. The deposition experiments demonstrate that an increase in Ar volume fraction results in a reduction in the size of diamond grains, an augmentation in the coating coverage on the substrate surface, and an acceleration in the coating deposition rate. Meanwhile, the quality of diamond crystal is almost unaffected.