Properties of thick sputtered Ta used for protective gun tube coatings

Thick tantalum coatings were deposited on the bore surfaces of 25 mm I.D. cylindrical 4340 steel substrates using a high-rate triode sputtering apparatus. Sputtering parameters affecting the tantalum phase and microstructure were investigated. Prior work has indicated that the sputtering gas species and substrate temperature during deposition affect the characteristics of the tantalum coatings. In the work presented here, we report on experimental studies aimed at evaluating additional phase and microstructural effects resulting from changes in sputtering gas species and substrate bias during the deposition. Tantalum deposits of 75–140 μm thicknesses were evaluated using X-ray diffraction, optical microscopy, and microindentation hardness measurements. Coatings deposited using krypton gas, a 200°C substrate temperature, and 0.45 Pa gas pressure also showed little variation when deposited at substrate biases ranging from −50 to −150 V. However, the tantalum coatings produced under similar conditions with an unbiased (floating) substrate were found to be the beta phase of the material and had a columnar microstructure. Beta-phase tantalum was produced at low substrate biases (from 0 to −50 V) when using Xe as the sputtering gas. Both phase and microstructure of the tantalum coating could be altered in mid-run by adjusting the sputtering parameters and appeared independent of the phase and microstructure of the underlying coating.