Structure characterization and mechanical properties of industrial PVD-TiA1N coatings
工業用 PVD-TiA1N 塗層的結構分析及其力學性能
Student thesis: Doctoral Thesis
Titanium-aluminium-nitride (Ti1-xAlxN) films were deposited onto unheated silicon (100) substrates and hardened M42 tool steels by reactive close-field unbalanced magnetron sputtering. The effects of aluminium content (x) on chemical, structural, mechanical and tribological properties of these films have been studied. By XPS the stoichiometric composition of TiN and AlN was found as forming a ternary phase of Ti-Al-N, while no unbound Al and Ti atomic species were detected in films. The XRD θ-2θ scans exhibited the structural changes in Ti1-xAlxN films with different Al contents. The films were essentially cubic B1-NaCl TiN with (111) oriented grains in the range of x = 0 to 0.48. The grains size decreased with the Al content. A two-phase structure consisting of B1-NaCl (TiN) and B4-wurtzite (AlN) was observed at x = 0.57, while at higher Al contents, a single-phase structure of B4-wurtzite (AlN) grains was formed. HRTEM and Raman investigations revealed an increase of disordered phase surrounding the nanocrystalline TiN grains when x increased. The films at x=0.41 had the highest hardness of about 31 GPa. Nanoindentation measurements combined with AFM and cross-sectional SEM showed that the improved mechanical properties of Ti1-xAlxN films with the addition of Al into TiN compound were attributed to their densified microstructure with development of fine grains and reduced surface roughness. By applying the height-height correction functions to the measured AFM images, a steady growth roughness exponent α=0.94±0.03 was determined for all the Ti1-xAlxN films. The value of α is consistent with growth model predictions incorporating surface diffusion. Calculations based on a semi-empirical method for determining the hardening origin of the Ti1-xAlxN films showed that the improved hardness in Ti1-xAlxN films primarily originates from the effect of solid solution hardening, with both intrinsic hardening and grain boundary hardening having a very weak contribution for films with relatively low Al amount. By scratch and dynamic impact tests, it was also found that the films at x=0.41 exhibited the best adhesion and cohesive strength. In the wear tests, tribo-oxidation wear was the main wear mechanism for the coatings sliding against WC-6wt%Co balls. The better performance of TiN could be explained by the formation of friction-induced TiO2 oxide layer during sliding process, which was evidenced by the XPS and Raman spectroscopy. On the other hand, the formation of Al2O3 on the Ti-Al-N coatings could be the reason of higher wear rate and friction coefficient recorded. In the practical machining performance, the hardest Ti-Al-N coatings at x=0.5 showed an excellent performance in the turning tests with an increase in tool life of 80% compared to TiN or commercial tool grades. It was also found that the best results in drilling were obtained for Ti1-xAlxN coatings at x=0.7. In that case, a decrease in cutting force and torque rather than high hardness was the key factor since it prevented from coating delamination. The pre-treatment of WC-6wt%Co tooling materials by Ti ion-implantation prior to deposition provided a better performance on the turning test. With the use of ion implantation energy of 120 keV, there was 200% enhancement of tool life in the dry and high speed cutting test. The greatly enhanced turning performance can be explained by the formation of the gradient layer, which can substantially reduce the stress gradient at the interface and the possibility of crack formation during cutting, and the enhanced hardness of the tooling materials, which can retard the plastic deformation when loading.
- Physical vapor deposition, Titanium nitride, Protective coatings