A complete set of TiC/a-C samples have been presented along with pure a-C references. These coatings have been prepared by controlling the power ratio applied to both Ti and C magnetrons. This ratio is found to control the chemical composition and microstructure of the coatings. The samples can be classified depending on the amount of the a-C phase, from polycrystalline TiC columnar coatings to dense nanocomposite TiC/a-C films. The microstructure evolution is found to be in agreement with the Barna model. The mechanical and tribological properties can be classified into two domains depending on the a-C amount. If the TiC content dominates, high hardness, medium friction and wear resistances are obtained. The hardness maximum is achieved at 50% a-C. If a-C dominates, the coatings exhibit lower hardness but the tribological performance notably improves (low friction and wear) due to the influence of the lubricant properties of this second phase. In the particular case of pure carbon coatings, they possess certain sp3 C fraction, especially for the biased sample, which improves the hardness and the wear resistance due to the higher film cross-linking and compactness but at the expense of a friction increment due to a reduction of the supply of sp2-C “graphite-like” phases. No influence of stresses in these results can be inferred, except for the biased pure a-C sample.
Our results are compared with much reported literature data that appear to follow similar trends with respect to hardness and friction coefficient dependence on chemical composition. With respect to the friction coefficient, three different categories of samples can be distinguished. For amounts of C above 60–65% the friction remains low in the range of 0.1–0.2. Reducing the C content, a second group of samples are identified, with f between 0.2 and 0.3. In the third group are included samples that show higher friction coefficients (fN0.4) and are composed by high amounts of Ti (N40%). In summary, the wear resistance of the nanocomposite TiC/a-C coatings is found to be controlled by the friction of the tribological system rather than mechanical properties of the coating such as hardness, toughness or resilience. This influence is determined by the predominant phase (a-C, TiC or Ti-rich compounds). An optimal value of 60–65% a-C can be identified since low values of friction coefficient (fb0.2) and wear rate (~10− 7 mm3 N−1 m−1) are achieved, whereas the hardness is moderate (10–15 GPa). The low levels of macro and microstresses in combination with adequate toughness properties qualify these nanocomposites coatings as excellent candidates for tribological applications.
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