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

Abstract

Tailoring mechanical properties of transition metal carbides by substituting carbon with nitrogen atoms is a highly interesting approach, as thereby the bonding state changes towards a more metallic like character and thus ductility can be increased. Based on ab initio calculations we could prove experimentally, that up to a nitrogen content of about 68% on the non-metallic sublattice, Ta-C-N crystals prevail a face centered cubic structure for sputter deposited thin films. The cubic structure is partly stabilized by non-metallic as well as Ta vacancies – the latter are decisive for nitrogen rich compositions. With increasing nitrogen content, the originally super-hard fcc-TaC0.71 thin films soften from 40 GPa to 26 GPa for TaC0.33N0.67, accompanied by a decrease of the indentation modulus. With increasing nitrogen on the non-metallic sublattice (hence, decreasing C) the damage tolerance of Ta-C based coatings increases, when characterized after the Pugh and Pettifor criteria. Consequently, varying the non-metallic sublattice population allows for an effective tuning and designing of intrinsic coating properties.

A review of the intrinsic ductility and toughness of hard transition-metal nitride alloy thin films

“…However, refractory ceramics typically possess only one or the other. In investigations designed to address the physics behind the long-standing challenge of realizing simultaneously high strength and high ductility in ceramics films, electronic structure results have shown that VEC is an important parameter for predicting mechanical properties in TM nitride alloys [54,60,98,101,105,107,108,117]. The overall results demonstrate that hardness decreases (maximum hardness is expected at VEC ~ 8.4), while ductility increases with increasing VEC; and optimum toughness is predicted for alloys with VEC ~ 10.…”

“…An alternative approach to increasing toughness, but also based upon VEC, was proposed by Glechner et al [117]. In this DFT-based work, instead of tuning the number of valence electrons by alloying the TM sublattice, they used TaC1-xNx as a model system to investigate alloying on the non-metal anion sublattice and found that both the hardness and elastic modulus of cubic TaC1-xNx decreases with increasing N content (i.e., with increasing VEC), which they attributed primarily to changes in bonding leading toward a more metallic character.…”

Point-defect engineering of MoN/TaN superlattice films: A first-principles and experimental study

“…Additionally, an enormous versatility in structural and mechanical properties can be accomplished by intentionally using the typically unwanted products of Physical Vapour Deposition (PVD) processes: vacancies and point defects in general [16][17][18][19][20][21] . Recent work by Buchinger et al 22 particularly underlined the important role of theory-guided defect design, showing an impressive fracture toughness enhancement in TiN/WN x superlattices, 4.6 MPa √ m for Λ = 10 nm, which presents one of the highest records among transition metal nitrides.…”

Exploring Physical Properties of Tantalum Carbide at High Pressure and Temperature

“…With the development of the aerospace and marine industries, ultrahigh temperature ceramics (UHTCs) have been and continue to be the most widely used multifunctional materials due to their high strength, good fracture toughness, and excellent resistance to oxidation. , As a subset of UHTCs, transition metal carbides possess a high melting point; for example, the melting points of TiC, ZrC, NbC, VC, and HfC are 3373 K, 3805 K, 3883 K, 3083 K, and 4232 K, respectively. − The TaC ceramics, as a vital member of the transition metal carbides (TMCs), have widely been used in refractories, abrasives, cutting tools, and coating materials . However, the hardness and fracture toughness of TaC ceramics show tremendous variations at different preparation conditions and diverse processing methods.…”