Structure, mechanical properties and corrosion resistance of amorphous Ti-Cr-O coatings

In this work, titanium-chromium oxide (TCO) coatings were investigated aiming to produce a functional hard corrosion resistance coating based on the well-known properties of the individual TiO2 and Cr2O3 oxides. The TCO coatings were deposited on low carbon steel and Si substrates using reactive magnetron sputtering from a crystalline TiCr target. Amorphous TCO coatings with increasing oxygen content up to 65 at.% were obtained by rising the oxygen flow rate during the reactive deposition. Coatings deposited using constant O-2 flow rate of 1.5 (6.2% of oxygen) were selected to evaluate the corrosion resistance, hardness and to determine the effect of annealing treatments. Electrochemical evaluation was done using impedance spectroscopy and potentiodynamic polarization in 3.5 wt% NaCl solutions showing a significant protection efficiency. The coatings were heat treated at different temperatures (200, 400, 500, 600 and 800 degrees C) for 2 h in air. The coatings were analyzed by X-ray photoelectron spectroscopy to obtain the composition, X-ray diffraction (XRD) and scanning electron microscopy for microstructural and morphological characterization, respectively. The XRD results show that the heat-treated coatings maintained the X-ray amorphous structure up to 500 degrees C. Above this temperature, the coatings crystallize forming large Cr2O3 crystals segregated to the surface leaving an amorphous TiCr bottom layer.