Crystallographic structure and mechanical properties of tantalum coatings on steel deposited by DC magnetron sputtering

Tantalum metal exhibits excellent erosion and corrosion resistance property. Hence, tantalum coatings are investigated for protection of steel surface exposed to mechanical stresses, high temperature, and corrosive environment. Such conditions exist inside large army gun barrels. The goal of this work was to contribute to the development of technology for tantalum coating of steel, which would replace the presently used electrochemical chromium deposition. A DC magnetron sputtering process was used to deposit tantalum coatings on steel substrates. In sputtering, tantalum is deposited in two phases: ductile, body-centered cubic (bcc) phase (a-phase) and a metastable, brittle tetragonal ß-phase. Only a-phase of tantalum is considered suitable for protective coatings. Such coatings were successfully deposited on steel substrates heated to 400°C (with Ar sputtering gas) and 350°C (with Kr), and also at room temperature if tantalum nitride interlayers were first deposited on steel substrates. Tantalum phase composition was determined by x-ray diffraction (XRD). The crystallographic phase of the nitride interlayer, studied by XRD, and its composition measured by the nuclear reaction analysis, showed that the stoichiometric TaN is required for a-phase deposition. Adhesion of sputter deposited tantalum coatings was evaluated using a scratch test method. It was found that strongly adhering a-phase tantalum coatings can be deposited on sputter etched steel substrate with tantalum nitride interlayer, and also on substrates heated to elevated temperatures. Adhesion failure of coatings was different for the ductile a-phase or predominantly a-phase coatings, and the brittle ß-phase coatings, which under an optical microscope revealed numerous small cracks, before delamination. This study confirms the superiority of the a-phase tantalum over ß-phase tantalum in protective coating applications. Parameters of DC magnetron sputtering deposition process for obtaining a-phase tantalum coatings were also determined