Effect of nitrogen flow rate on the mechanical properties of (V,Mo)N thin films

Abstract

The objective of this research was to study the effect of nitrogen flow rate on the structure and mechanical properties of (V,Mo)N thin films. Five sets of (V,Mo)N thin films were deposited with different nitrogen flow rates ranging from 1.2 to 6.0 sccm on Si (100) substrate using unbalanced magnetron sputtering. The results showed that the N/metal ratio increased from 0.47 to 0.85 with increasing nitrogen flow rate. The microstructure of the specimen deposited at the lowest nitrogen flow rate (D12) was characterized by transmission electron microscopy, where the microstructure contained three phases, including Mo, VN and (V,Mo)N. The X-ray diffraction patterns also indicated the presence of minor Mo metal phase for the specimens deposited at nitrogen flow rate lower than 2.5 sccm. The texture of the thin films was mostly (200) and switched to random texture at nitrogen flow rate from 4.0 to 6.0 sccm. The lattice constant of the specimens was linearly related to the N/metal ratio. The specimen D12 had the highest hardness 29.3 GPa, which could be due to the existence of Mo metal phase in between (V,Mo)N phase, thereby retarding the crack propagation. By contrast, the hardness of all the other specimens showed slight variation ranging from 23.6 to 26.1 GPa, which may be attributed to the nanograin size (<15 nm) where deformation was controlled by grain boundary-mediated mechanisms. Fracture toughness (Gc) of the thin films was evaluated using internal energy-induced cracking method. The resultant Gc of the specimens, ranging from 16.4 to 30.1 J/m2, linearly increased with increasing N/metal content, suggesting that the MoN bonding may be crucial on increasing fracture toughness of (V,Mo)N thin films. For specimen D12, the higher Gc may be caused by the presence of Mo metal phase that could arrest the crack propagation. The results showed that Gc of the (V,Mo)N thin films was higher than that of VN and TiN thin films, which was consistent with the theoretical predictions.