Growth morphology and corrosion resistance of magnetron sputtered Cr films

Chromium (Cr) films are commonly used as corrosion-resistant coatings because they form a passive protective scale. The film morphology and microstructure are important aspects of corrosion resistance. This paper presents growth mechanisms and morphologies of these films to provide insight into correlation between microstructure and corrosion resistance. Thin films of Cr were deposited on silicon wafers, and AISI 1018 carbon steel substrates using a plasma-enhanced magnetron sputtering technique. A tungsten filament was used to produce a plasma that enhances ion bombardment on the workpiece during the magnetron sputter deposition process. The deposited films were characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The film growth morphology and microstructure are correlated with sample orientation (with respect to the magnetron) and the deposition parameters. One important deposition parameter affecting Cr film properties is the level of plasma ion bombardment. This investigation demonstrated that at a low level of ion bombardment, a columnar structure is formed with large column widths resulting in a film that is discontinuous. As the level of ion bombardment increases, the films become denser and more continuous. Using the filament induced plasma enhancement method, a fully dense chromium film without indication of columnar structure was achieved. The corrosion behavior of the deposited films was studied using potentiodynamic polarization techniques. This study demonstrated that adequate ion bombardment is necessary to achieve good corrosion resistance.