Effect of sputtering parameters on Ta coatings for gun bore applications

Tantalum offers a number of attractive properties for gun bore coating appl ications, including a high melting temperature, high ductility, and an envi ronmentally friendly deposition method. However, vapor-deposited tantalum c an appear in both the characteristic bcc phase found in the bulk material, and in a very brittle and less desirable 'beta' phase. Presence of the beta phase in bore coatings is considered undesirable because of its brittlenes s and resulting failure as the coating is stressed. A high-rate triode sput tering system with a cylindrical coating geometry was used to produce thick tantalum coatings on 4340 steel smooth bore cylindrical substrates. A syst ematic series of tests were performed to evaluate the effects of sputtering gas species (Ar, Kr, Xe) and substrate temperature (100-300 degreesC) duri ng deposition on the phase and microstructure of the coatings. Heavier sput tering gases and higher substrate temperatures were found to promote the fo rmation of bcc-phase tantalum coatings. Use of a movable target assembly wa s shown to promote the production of dense, single-phase tantalum coatings. (C) 2000 Elsevier Science B.V. All rights reserved.