Plasma polymerized hexamethyldisiloxane (PPHMDSO) thin films have been obtained in a DC glow discharge from the monomer vapor. Such films are widely used as protective coatings, gas barriers, biocompatible layers and many other applications. An increased wear resistance is often achieved by modifying the surface structure and composition of the deposited polymer films. We have studied two different approaches, both of them leading to a reduction in the carbon content of the film, which is commonly referred to as a silica-like (SiOxCy:H) structure: first, the deposition of a thin overlayer by plasma polymerization of a HMDSO/O-2 gas mixture; second, the modification of the existing polymer films by immersion in oxygen plasmas. The structure and composition of the films have been characterized by X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), and Transmission Electron Microscopy (TEM); their tribological properties have been studied by means of the depth-sensing nanoindentation technique. Detailed studies have been conducted on the hardness of films with different surface treatments. The protective performance of the films and their wettability have been studied.