Tantalum nitride seed layers for Bcc tantalum coatings deposited on steel by magnetron sputtering

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.