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T. Glechner:
"Phase stability and thermomechanical properties of sputter deposited Ta-C-N coatings";
Supervisor: H. Riedl, P.H. Mayrhofer; E308, 2018; final examination: 2018-04-19.

Exploring novel thin film materials with outstanding mechanical properties - especially, high hardness but simultaneously high damage tolerance - as well as superior temperature stability is for a major goal in the field of protective coating-applications. Therefore, thin film materials such as transition metal (TM) carbides, nitrides, and borides are highly popular in this field. Among all these ceramic thin films, the binary Ta-C system exhibits the highest melting temperature combined with further outstanding properties such as chemical inertness, or high thermal conductivity. However, the extreme brittle behavior of this compound drastically limits the field of use. Therefore, new concepts such as tailoring thermomechanical properties by substituting carbon with nitrogen atoms is a highly interesting approach, changing the bonding state towards a more metallic like character through non-metals - hence increasing ductility. Based on ab initio calculations we could experimentally proof, that up to a nitrogen content of about 65 % on the non-metallic sublattice, Ta-C-N crystals prevail a face centered cubic structure in sputter deposited thin films. The cubic structures are partly stabilized by both non-metal and Ta vacancies, whereas metal defects are decisive for nitrogen rich compositions. With increasing nitrogen content super-hard fcc-TaC0.71 thin films weaken from around 40 GPa to 25 GPa for TaC0.32N0.68, respectively, accompanied by a linear decrease in indentation modulus. In accordance with a slightly reduced thermal stability, meaning phase degradation detected around 1300 °C in DSC/TG analysis, this indicates a reduced bonding strength hence a reduced covalent and more metallic like bonding character - also confirmed by PDOS. Pugh´s semi-empirical criterion for ductility (B/G > 1.75) even suggest ductile behavior with the addition of nitrogen. This finding is confirmed by various micromechanical testing methods on a sputter deposited 110-oriented Ta0.47C0.34N0.19 film. Although this film exhibits super-hardness (43.3±1.4 GPa) plastic deformation is observed in micro-pillar compression tests over a yield stress, σY, of 16.89±1.39 GPa with {111} <011 ̅> as the most active slip system. In addition, single cantilever bending tests result in a stress intensity factor, KIC, of 2.9±0.25 MPa·m1/2 whereas pillar splitting tests suggest KIC=7.12±0.8 MPa m1/2. Furthermore, superior fracture toughness of Ta0.47C0.34N0.19 compared to TaC0.81 is proofed by cantilever testing. This study emphasizes an alternative alloying concept for ceramic like thin film materials, by forming solid solutions on the non-metallic sublattice. The investigated Ta-C-N system gives a promising prospective for further ultra-high temperature ceramics.

Ta-C-N; Thermomechanical Properties