Vacancy-driven extended stability of cubic metastable Ta-Al-N and Nb-Al-N phases

Abstract

Quantum mechanical calculations had been previously applied to predict phase stability in many ternary and multinary nitride systems. While the predictions were very accurate for the Ti-Al-N system, some discrepancies between theory and experiment were obtained in the case of other systems. Namely, in the case of Ta-Al-N, the calculations tend to overestimate the minimum Al content necessary to obtain a metastable solid solution with a cubic structure. In this work, we present a comprehensive study of the impact of vacancies on the phase fields in quasi-binary TaN-AlN and NbN-AlN systems. Our calculations clearly show that presence of point defects strongly enlarges the cubic phase field in the TaN-AlN system, while the effect is less pronounced in the NbN-AlN case. The present phase stability predictions agree better with experimental observations of physical vapour deposited thin films reported in the literature than that based on perfect, non-defected structures. This study shows that a representative structural model is crucial for a meaningful comparison with experimental data.

Biomimetic hard and tough nanoceramic Ti–Al–N film with self-assembled six-level hierarchy

“…29,31,49 Recent reports suggested that the nanolamellae formation could be a result of a phase separation at the deposition temperature via surface diffusion or kinetically-controlled oscillating reactions at the film surface. 30,50,51 The regular morphology of the herringbone crystallites with well-developed {100} interfaces between platelets ( Fig. 2) as well as the geometrical and compositional matching at nanolamellar boundaries reported by Zalesak et al 30 in monolithic epitaxial TiAlN films suggest, however, that the nanolamellae's self-assembly is most likely a consequence of kinetically controlled oscillatory reactions at the growing film's surface described already by Bartsch et al 34 In other words, the nanolamellae are formed as a result of a sequential epitaxial overgrowth of {100} facets by individual Aland Ti-rich sub-layers.…”

Tuning structure and mechanical properties of Ta-C coatings by N-alloying and vacancy population

“…by introducing vacancies, as high temperature phases are typically of higher symmetry but also increased disorder). This has recently been verified through detailed ab initio calculations, showing that depending on the vacancy type (metallic or non-metallic sublattice) and content, δ-TaN can be stabilized over π-TaN even at 0 K 21 . For Ta-C, non-metallic vacancies are more decisive than metal vacancies for promoting a stabilization of the cubic structure, whereas for Ta-N, metallic vacancies are of crucial importance 1 , 22 .…”

Phase Identification and Ordered Vacancy Imaging in Epitaxial Metallic Ta2N Thin Films

“…There is as yet no consensus on the range of structural defects or exact phases present in epitaxially grown TMNs. The crystal structure that TMNs adopt is often highly dependent on stoichiometry and growth parameters 8,9 and is a subject of continued research interest. 6,10−18 An important question is whether epitaxial TMNs adopt the parent polar crystal structure of their hexagonal III-N or SiC growth substrate, P6 3 mc, or retain the desired P6 3 /mmc of the hexagonal phase, γ-Ta 2 N. The film polarity may have wide-ranging effects on the properties of devices incorporating epitaxial TMNs on polar substrates.…”