Texture, structure and phase transformation in sputter beta tantalum coating

Structural properties of tantalum are of interest because of its potential application in high temperature wear and erosion. In this paper, we report on beta tantalum coatings, which were sputter-deposited onto inner surface of steel cylinders, and flat steel and glass plates. Two forms of beta tantalum coatings were generally observed: high (002) fiber-texture at low sputter gas pressure, and more random oriented beta tantalum at higher sputter gas pressure. Two-dimensional XRD and pole figure analyses showed both belong to the same tetragonal structure. Structure simulation was made using a tetragonal cell, a=1.0194 nm, c=0.5313 nm, space group P42/mnm and a very similar cell, a=1.0211 nm, c=0.53064 nm, space group P-421m by Frank–Kasper (1958, 1959) and Arakcheeva (2002). Diffraction pattern generated using the former space group allows (00l) reflections for even l, while the latter allows both even and odd (00l) reflections. The latter model provides better interpretation of our data. Upon annealing, the (002) grains in random oriented tantalum became unstable at 300 °C, and complete beta to alpha tantalum phase transformation occurred at ∼750 °C, resulting in alpha tantalum with (110) preferred orientation. In highly textured (002) beta tantalum, hot hardness measurements showed hardness decreased drastically between 250 and 350 °C to hardness values of alpha tantalum, suggesting a phase transformation approximately 300 °C. XRD data showed partial beta to alpha phase transformation and re-orientation of the (002)-grains at 100 °C, and was more intense at 300 °C.