Comparative performance of nanocomposite coatings of TiC or TiN dispersed in a-C matrixes

Abstract

Titanium carbide (TiC) and nitride (TiN) are two of the most used materials in the field of protective coatings, due to their optimal mechanical and tribological properties. The addition of the second phase can provide extra benefits to the coating, like improved hardness, reduced friction and/or oxidation resistance. In this work, we present two series of coatings in which hard crystalline TiC and TiN phases are mixed at the nanometric level with a soft lubricant phase like amorphous carbon (a-C). Both series of TiC/a-C and TiN/a-C nanocomposite coatings were prepared by double magnetron sputtering of C and Ti(N) targets in a Ar atmosphere ( P = 5 × 10 − 3 Torr) by changing the power ratio applied to each magnetron. The chemical composition has been measured by electron energy loss spectroscopy, and the phase composition changes gradually from pure C to pure TiC or TiN through nanocomposite structures with variable phase contents. These structures are confirmed by transmission electron microscopy and diffraction techniques, like X-ray diffraction and electron diffraction. The mechanical and tribological properties are found to be mainly controlled by the hard/soft phase ratio present in the coating. The changes in hardness values follow similar trends in both types of nanocomposite samples. Introducing a small amount of TiN or TiC into a-C matrix causes a hardness reduction, but further addition of crystalline phase makes increase the hardness. The best tribological properties are found for nanocomposite coatings (both TiN/a-C and TiC/a-C) with high amount of a-C (> 65%), showing low friction values ( f ∼ 0.1) and high wear resistance ( k about 10 − 7 mm 3 N − 1 m − 1 ). However, coatings with 50–60% a-C show a good compromise between tribological and mechanical properties.