Tantalum, a tough refractory metal with excellent corrosion resistance, is an attractive protective coating material for steel subjected to mechanical wear in harsh chemical environments. The coatings deposited by sputtering usually contain a hard and brittle tetragonal phase mixed with the desirable tough and ductile bcc phase of Ta. One of the methods of promoting the growth of the bcc phase at a moderate substrate temperature is deposition of an interfacial or seed layer of tantalum nitride. Ta coatings were deposited by dc magnetron sputtering in argon, and the interfacial nitride layers by reactive sputtering performed in the same chamber with nitrogen gas added to argon. The critical thickness, stoichiometry, and structure of the seed layer required for bcc Ta growth were studied by scanning electron microscopy and X-ray diffraction. The results of adhesion testing of the Ta coatings deposited on the interfacial layers are also reported.
Tantalum has mainly two phases: alpha phase (bcc structure) and beta phase (tetragonal structure). The meta-stable beta phase is usually obtained in sputtered films. Alpha phase is preferred over the beta for some applications as beta phase is very brittle. One of such application is to protect steel from the erosive and the corrosive wear. It was found that with the intermediate layer of tantalum nitride the preferred alpha phase was grown on steel by DC magnetron sputtering technique. Electrical and structural properties of these films were studied by four-point probe measurement and x-ray diffraction (XRD). Stoichiometry of the interfacial tantalum nitride layer was investigated by nuclear reaction analysis (NRA). Influence of the interfacial film thickness and the ratio of argon and nitrogen gas during reactive deposition of tantalum nitride on the tantalum phase were investigated. This work also reports on the dependence of tantalum phase on the substrate temperature (100-400°C) during sputtering in Ar and Kr gases.
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