Structural, hardness and toughness evolution in Si-incorporated TaC films

Ta-Si-C films were deposited by DC magnetron co-sputtering using TaC and Si targets in an Ar-discharge atmosphere. Increasing the current of Si target from 0.0 to 0.5 A led to a continuous increase of Si content from 0.0 to 30.8 at%. The effects of Si content on microstructure were systematically investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). At low Si content (<= 5.6 at%), Si occupied C vacancies to form a solid solution Ta(C, Si). Further increasing the Si content, some Si atoms bonded with C atoms resulting in the formation of amorphous phase (a-C:Si), and the films presented a nanocomposite structure consisting of solid solution Ta(C, Si) surrounded by a-C:Si matrix. At the highest Si content (30.8 at%), the film exhibited finally X-ray amorphous structures. The hardness (H) and fracture toughness (K-f) were observed to initially increase and then decrease as the Si content was increased. At 5.6 at% Si, the film exhibited a maximum in H (44.9 +/- 2.7 GPa) and K-f (3.61 +/- 0.16 MPa m(1/2)), which can be ascribed to the formation of a solid solution. On further increasing the Si content, an amorphous phase gradually appeared, leading to a decrease in H and K-f.