In Ar-rich Ar–H2–CH4 gas mixture the presence of H2 is found to be beneficial to the plasma stability. On the other hand, too high H2 percentages lead to materials showing a high surface roughness. In the present work, diamond films were grown on p-type Si (100) substrates screening different quantities of H2. The plasma phase and plasma–substrate interface were investigated by in-situ optical emission spectroscopy and pyrometric interferometry to determine the behavior of emitting species and the deposition rates, respectively. The obtained films were characterized by Raman micro-spectroscopy, AFM and SEM techniques. For H2 percentages between 6.3 and 10%, the structure and morphology are characteristic of nanocrystalline films, affording low roughness values when a buffer layer was grown between the diamond coating and the treated silicon surface. ► We explore the limits of the hydrogen percentage in Ar–H2–CH4 gas mixtures. ► We develop a new kind of buffer layer between the diamond films and treated silicon substrate. ► We obtain a low roughness of the coatings without modifying the Astex-type reactor.