Microstructures of titanium oxide thin films grown continuously on stainless steel wires by PVD in an inverted cylindrical magnetron: Towards an industrial process

Abstract

Producing colored coatings by Physical Vapor Deposition (PVD) in an in-line way is an interesting industrializing process. Moreover, the durability of vacuum coatings is an advantage for architectural applications such as outdoor decoration. In this project, titanium dioxide coatings were produced over moving stainless steel wires via direct current (DC) reactive magnetron sputtering PVD in an inverted cylindrical magnetron. The samples were characterized by transmission electron microscopy (TEM) coupled with ASTAR (Automated crystal orientation mapping on TEM -ACOM- with precession tool -DigiSTAR-) orientation-phase mapping and EDS (Energy Dispersive X-ray Spectroscopy) chemical mapping. The microstructure of TiOx thin films is analyzed as a function of the O2 flux in the chamber. In the metallic mode, a 300 nm polycrystalline hexagonal Ti film is obtained in 2 min 30 s at 1000 W. For the same power and same time but in the poisoned mode, a 40 nm titanium dioxide film composed of both rutile and anatase columnar grains is obtained. A peculiar mode is the transition one, where in a single pass through the chamber a multilayer of ≈230 nm hexagonal Ti and ≈70 nm pure rutile titanium dioxide is grown. This multilayer is a result of a heterogeneous magnetic field, hence of the chamber poisoning. All these thin films present chemical, structure and grain size gradients because of an original process (in-line and cylindrical inverted magnetron) with plasma density heterogeneity.