Microstructural evloution mechanisms and mechanical properties of vacuum kinetic sprayed TiN films

Abstract

Vacuum Kinetic Spray (VKS) is a relative new ceramic film fabrication method. Pristine single crystal titanium nitride powder was used to fabricate titanium nitride film using vacuum kinetic spray process. Titanium nitride coatings having thicknesses from 500 nm to 5 µm were successfully made, and the process was optimized based on study of various spray conditions. Spray gas flow rate, nozzle scanning speed were considered as most important parameters that affect particle deposition behavior and film microstructure. Film grains translated from elongated particles to anisotropy nanocrystalline coexist with amorphous phase with increase gas flow rate, while the deposition efficiency come to a peak value than going down with increasing gas flow rate. Tamping effect is significantly contributed to fragments accumulation; therefore, lower nozzle scanning speed facilitates the formation of dense structure.

In addition, transition microstructures in atomic scale were studied to investigate the ceramic particle deposition mechanisms. Dislocation activation was universally observed. Distorted lattice, edge dislocation, twin, stacking fault, amorphization, internal damages, grain boundaries and some other complex transition structures were characterized and discussed, as well as their role in nanograin formation and film growth. Plastic deformation features (i.e., subgrain formation, rotated grain and planar defects) reveal elastic to inelastic transition of titanium nitride particles at high strain rate and high pressure induced by hypervelocity impact. Plastic wave duration is vital for mechanical bonds formation between particles. Microplasticity and internal damage are two significant approaches to drainage particle kinetic energy during the onset of collision. Apparent yielding of ceramic particle is considered as a critical condition for successful deposition.

Nanoindentation and scratch test were used to examine the microhardness and the adhesion strength, respectively. Hardness and adhesion strength increased with the increase of gas flow rate, in which maximum hardness of ~23 GPa and adhesion strength of ~40 N were achieved.