Tantalum films for protective coatings of steel

Deposition of tantalum films for protective coatings on steel by DC magnetron sputtering have been investigated. Conditions resulting in the deposition of bcc and tetragonal phases of tantalum and the evolution of the phase composition and crystallinity during the film growth from nanometers to tens of micrometers are described. Heating the substrates to only 400 °C resulted in the growth of pure bcc phase tantalum, which is tough and ductile and thus preferred for protective coatings, while lower temperatures promote nucleation and growth of the hard and brittle tetragonal phase. Adhesion tests by scratching with a diamond tip showed that while well adhering coatings of both phases can be obtained, the tetragonal phase films develop cracks under load, well below the critical value that results in delamination. This has important implications for application of tantalum films for protection against corrosion of steel elements subjected to mechanical stress and wear, as the cracks would locally expose the substrate to the surrounding environment. With proper choice of deposition conditions and moderate heating that does not compromise the steel properties, coatings of the preferred bcc phase of tantalum with excellent adhesion can be deposited by DC magnetron sputtering.