In this work, a-C:H:Si (DLC-Si) films were produced onto crystalline silicon and polycarbonate substrates by the r.f.-PACVD technique from gaseous mixtures of CH4 + SiH4 and C2H2 + SiH4. The effects of self-bias and gas composition upon mechanical and optical properties of the films were investigated. Micro-hardness, residual stress, surface roughness and refractive index measurements were employed for characterization. Films deposited at high self-bias showed very high deposition rates, low stress and high hardness. However, high self-bias values lead to surface damage of the samples. Smooth films could be obtained keeping self-bias voltages \V-B\ below 600 V to avoid surface heating and damage. C2H2-based films seemed to be more attractive as carbon source because they showed higher deposition rates, with similar hardness and surface roughness values than CH4-based films in the same conditions. The viability of producing DLC-Si films deposited by r.f.-PACVD as protective coatings for polycarbonate substrates has been demonstrated as well-adherent films were produced with low surface damage from the plasma.