Effects of B content on microstructure and mechanical properties of nanocomposite Ti–Bx–Ny thin films

Abstract

Thin films of Ti–Bx–Ny were deposited on Si(100) at room temperature by reactive unbalanced dc magnetron sputtering in an Ar–N2 gas mixture. The effects of B content on microstructure and mechanical properties of these films have been analyzed using x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, x-ray diffraction, transmission electron microscopy, atomic force microscopy, micro-indentation measurements, and an optical interference method. Microstructure studies revealed that depending upon the amount of B addition, the films showed two- or three-phase nanocomposite structure. At B contents below about 10at.%, the films consisted of mainly TiN bondings with a small amount of TiB and BN bondings. As the B content increased, TiB gradually transformed to TiB2 and the films consisted of nanocrystalline (nc-) TiN embedded in an amorphous (a-) TiB2 matrix. A maximum hardness of ∼44GPa was observed in a film with B content of 19at.%. The improved mechanical properties of Ti–Bx–Ny films with the addition of B into TiN were attributed to their densified microstructure with development of fine grain size and different phase combination. The reduction in grain size has also been supported by means of a Monte Carlo simulation. When B contents reached ∼42at.% or above, an amorphous-like nanocomposite of nc-TiN∕a-TiB2∕a-BN was formed. The effect of thin a-TiB2 layer in stabilizing nc-TiN structure is also elucidated and explained on the basis of structural and thermodynamic stability.