The invention relates to an improved diamond-like nanocomposite composition comprising networks of a-C:H and a-Si:O wherein the H-concentration is between 40% and 80% of the C-concentration and having a coefficient of friction against steel which is smaller than 0.1 in air with a relative humidity up to 90%, or in water. The invention relates also to a process for depositing the composition on a substrate in a vacuum chamber. The composition comprises preferably 30 to 70 at % of C, 20 to 40 at % of H, 5 to 15 at % of Si and 5 to 15 at % of O and can be doped with transition metals.