Characterization and corrosion of BCC-tantalum coating deposited on aluminum and steel substrate by DC magnetron sputtering

Tantalum is one of the most versatile highly refractory corrosion-resistant metals. Tantalum coating can be used as an effective corrosion barrier if it is continuous, free from defects and it adheres well to the substrate. A DC magnetron sputtering technique was used to deposit tantalum on aluminum and steel. X-ray diffraction measurements revealed that the tantalum films produced by sputtering had two crystallographic structures: a body centered cubic α-phase and a metastable tetragonal β-phase. Tough and ductile alpha phase tantalum is preferred over the hard and brittle beta phase tantalum, as a protective coating. Alpha phase coatings were successfully deposited on aluminum and steel at room temperature by depositing interlayer of tantalum nitride prior to tantalum and also on heated substrates of aluminum and steel at 400°C with argon sputtering gas. Alpha phase was also obtained on sputter-etched aluminum at room temperature, while beta phase was obtained at room temperature if aluminum was not sputter-etched. The tantalum nitride films were investigated by X-ray diffraction and by four-point probe resistivity measurements. The resistivity was found to increase, as concentration of nitrogen in sputtering gas was increased. Corrosion resistance of tantalum coating was evaluated using potentiodynamic and potentiostatic methods. Potentiodynamic test carried out on bcc tantalum coated aluminum and steel revealed a very low corrosion current density, indicating good corrosion resistance. Tetragonal tantalum coated on aluminum was evaluated by potentiostatic test, which indicated some corrosive effects after four days of continuous testing. This study confirms that high quality coatings of tantalum can be deposited on steel and aluminum by DC magnetron sputtering. The coatings of alpha phase tantalum on metal such as steel and aluminum have potential applications as corrosion barriers