Deposition and some properties of nanocrystalline, nanocomposite and amorphous carbon-based coatings for tribological applications

Ti–C:H, W–C:H and Si:C:H (generally X–C:H) coatings were deposited by pulsed reactive magnetron sputtering in an Ar/C2H2 atmosphere. The flow rate of acetylene was changed from zero, where coatings of pure titanium, tungsten or silicon were obtained, to the value where sputtering was carried out from the totally poisoned target. It was observed that Ti–C:H and Si:C:H coatings deposition rates slightly fluctuated within the whole range of an acetylene flow applied. Whereas, a W–C:H coating deposition rate reaches, for certain values of acetylene flow, over a twice higher value in relation to the deposition rate of tungsten coating. It was revealed that mechanical and tribological properties of X–C:H coatings may control within a broad range through a change in mutual concentration of a crystalline phase of a carbide (XC) and an amorphous hydrogenated carbon (a-C:H) matrix. The structural, mechanical and tribological properties of the deposited coatings were examined as a function of X component concentration and a-C:H content. X-ray, TEM and Raman spectroscopy investigations have revealed that Ti–C:H and W–C:H coatings with titanium content over 49.2 at.% and tungsten over 58.7 at.%, respectively, have the nanocrystalline structure. Ti–C:H coatings with titanium content over 41.3 at.%, and W–C:H coatings with tungsten content over 51.9 at.% have the nanocomposite structure of a TiC/a-C:H or WC/a-C:H type, i.e. composed of a carbide phase (TiC or WC) and an a-C:H amorphous matrix. Si–C:H coatings within the whole range of silicon content are amorphous in character. High hardness (Hp > 41 GPa) probably connected with the nanocomposite effect of hardening was observed, especially for TiC/a-C:H coating with titanium content 41.3 at.%. The friction coefficient and wear rate of coatings decreased as titanium, tungsten or silicon content was decreasing, in effect with an increase of an amorphous a-C:H phase. If the X-component content in coatings drops below 20 at.%, the friction coefficient approaches the value of 0.1, and the wear rate kvc of coatings is smaller than 10−6 mm3 N−1 m−1. If an X-component concentration is 4–6 at.%, the coatings show a very low friction coefficient (f < 0.06 for Ti–C:H and Si–C:H, f < 0.1 for W–C:H) and high abrasive wear resistance (kvc < 10−7 mm3 N−1 m−1). Coatings with properly selected architecture are marked by good adhesion. The critical load LC1, which causes the cohesive damage (mainly cracks) exceeds 20 N. The critical load LC2 which causes the damage of coatings inside the cracks, in case of W–C:H coatings, exceeds 60 N.