In this article, a thorough investigation of a-C:H films deposited by rf glow discharge from methane-argon mixtures, containing different Ar fractions, onto silicon substrate is presented. The structural, mechanical, and morphological properties of these films were investigated by complementary techniques, such as x-ray reflectivity, Raman spectroscopy, Brillouin light scattering, pin on disk test, and atomic force microscopy. Experimental results are examined to develop a coherent picture of the relationships among deposition parameters, microstructural features, and macroscopic properties. The consistency of the experimental results are checked with a new theoretical physical model that predicts the reaction probability for methyl radicals interacting with the surface of amorphous hydrogenated carbon films.