Phase formation of powders sputtered from X2BC targets and XC+XB powder mixtures {X = Nb, Ta, W}

Abstract

In this work, the phase formation and thermal stability of X-B-C powders are investigated, where X = {Nb, Ta, and W}. X-B-C powders are synthesized by pulsed-DC magnetron sputtering and compared to annealed mixtures of commercially available monocarbide and monoboride powders. The phase formation of all powders is investigated before and after annealing to 1500°C in a protective atmosphere. Before annealing, powders prepared from coatings sputtered from X2BC targets exhibited a nanocomposite structure with small XC grains in the case of the Nb-B-C and Ta-B-C powders. No crystalline phases could be identified in the W-B-C powder. Annealing resulted in a significant increase in crystallite size and the formation of additional phases in all sputtered coating powders. Commercial powder mixtures showed XC and XB phases before and after annealing with no evidence of forming additional phases and minor exothermic reactions were observed by DSC measurements, which were linked to grain growth. No formation of the X2BC phases was observed. This can be explained by the energetically favorable binary XC and XB phases over the ternary X2BC phase in the case of Ta and W. The ternary Nb2BC phase was not observed despite being energetically favorable over binary phases, likely due to kinetic limitations.