Microstructure and phase composition dependent tribological properties of TiC/a-C nanocomposite thin films

TiC/a-C nanocomposite films were deposited by rf magnetron sputtering on silicon substrates. A thin Cr inter-layer was interposed to enhance the interfacial adhesive strength. At a low deposition power of 25 W, the formation of TiC phase with (200) orientation was observed. Cross sectional scanning electron microscopy (SEM) image of this film showed growth with columnar microstructure. However, at higher deposition powers, 75 W and 125 W, several XRD peaks related to TiC were observed including prominent TiC (111) orientation. These films were found to be composed of fine spherical grains embedded in an amorphous carbon (a-C) matrix. Weak tribological performance and poor film/substrate adhesive resistance were observed in the 25 W deposited columnar structured film. However, films deposited at higher power exhibited superior tribological and scratch resistant properties such as low friction coefficient and high wear resistance. Such a large discrepancy in friction and wear behavior of these films could be explained in terms of microstructure and phase composition. Films with columnar microstructure possess residual compressive stress, low hardness and reduced fracture toughness. In contrast, spherical shaped TiC nanograins embedded in a-C matrix exhibited superior hardness and high fracture toughness.