PECVD coatings for magnesium casting protection

In this study SiO2-like films have been deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) on Mg alloys employed in the automotive industry in different experimental conditions in order to optimize the protective effectiveness of the deposited layers. The experimental apparatus used was a capacitive coupled (CC) parallel-plate-reactor with an asymmetric electrode configuration, the electromagnetic radiation was applied to a gas mixture containing tetraetoxysilane (TEOS), oxygen and argon in different ratios. The deposition was performed at 100 mTorr of pressure, by varying the input power, on as cast and polished samples submitted to a pre-treatment step in H2-containing plasma. A set of Mg alloy specimens has been treated with a standard anodising process (AP). From the overall experimental results, PECVD appears an attractive surface modification technique which allows, through the deposition of SiO2-like films, to increase the corrosion resistance of Mg alloys, usually protected with anodising treatment or with chromium-based coatings. The check of the transferability of deposition process to a medium scale range is now running. Magnesium frames representative of real automotive parts coated in a PECVD industrial chamber, assembled and painted are submitted to accelerated corrosion test in order to verify the corrosion resistance of these painted elements manufactured following the production steps of a real component.