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B Schwartz:
"Influence of Ta on the oxidation resistance of WB2 based thin films";
Supervisor: H. Riedl, P.H. Mayrhofer; Institute of Materials Science and Technology/E308-01, 2019; final examination: 2018-07-18.

High-performance components are used in various industrial fields such as aviation, transportation, as well as energy production applied for aero engines (e.g. blades, bearings, or gears), industrial gas turbines (IGTs), steam turbines, or drive train systems. To withstand the mostly extreme environments involving highest temperatures and aggressive gases, the development of protective coatings is of great interest - also considering an emission reduction of greenhouse gases and a sustainable usage of resources. A very promising material class for novel protective thin films - revealing outstanding characteristics - are transition metal diborides (TMB2). Especially, tungsten diboride (WB2−z), obtaining an outstanding fracture toughness at simultaneously high hardness, emphasizes to be a very suitable base material for the application as a protective coating. Latest studies on WB2−z indicate, that the metastable α-phase (space group 191 - P6/mmm) is stabilized through vacancies introduced during sputter deposition. Without the addition of these structural defects, the energetically preferred but more brittle ω-phase (space group 194 - P63/mmc) would form. Furthermore, the alloying of tantalum significantly enhances hardness and thermal stability of α-WB2−z by only minor influencing the ductile character of α-W1−xTaxB2−z. However, to overcome the insufficient resistance against oxidation, the aim of this diploma thesis is to observe how Ta influences the oxidation resistance of α-WB2−z. In a first step, various W1−xTaxB2−z thin films have been sputter deposited obtaining Ta contents, x, of 0, 0.15, 0.34, 0.58, 0.81, and 1.0, respectively. These chemical compositions were realized by varying the power densities of the co-sputtered WB2−z and TaB2−z targets. Subsequently, coating thicknesses of around 3.75 µm, after 60 min deposition time at around 300 ± 15°C substrate temperature utilizing a bias potential of -50 V have been obtained. Structural analysis by X-ray diffraction revealed mainly α structured coatings up to contents of x ≈ 0.58 obtaining a very fine grained, micro- to nano-crystalline morphology. At higher Ta contents a clear phase identification was not possible due to extremely broad XRD peaks - confirming the nano-crystalline character - but obtaining slight indications for dual phased coatings. This behavior was also confirmed during mechanical analysis showing a hardness increase from 31 ± 1.9 GPa to 36 ± 1.6 GPa with increasing Ta content, based on solid solution hardening and the decreasing grain size with increasing x. The oxidation resistance of all deposited coatings was evaluated after annealing in ambient air at 500, 600, and 700°C for 1, 10, 100, and 1000 min, respectively. After 100 min annealing time at 600°C, the formed oxide layer thickness decreases with increasing Ta content from ∼ 2.2 µm for WB2−z to 0.1 µm for TaB2−z. The enhancement in oxidation resistance gets even more pronounced after 1000 h at 600°C, where WB2 is nearly fully oxidized (oxide layer thickness of ∼ 8.1 µm) while the coating with a Ta concentration of 0.58 exhibit a solely ∼ 1.5 µm thick oxide layer. Structural analysis by TEM revealed a tetragonal WO3 crystal structure of the formed oxides, on top of WB2−z coatings, which is also confirmed by EDS line scans, indicating a W to O ratio of around 1 to 3. All Ta alloyed coatings formed well adherent scales highlighting smallest scale thickness for x = 0.58 next to pure TaB2−z thin films. Analyzing the oxidation kinetics by linear or parabolic growth laws (e.g. kl, kp), a more linear scale growth for WB2−z-rich compared to a parabolic dominated one for TaB2−z-rich thin films could be observed. W0.42Ta0.58B2−z obtained an oxide growth rate, kp, of 1.3 · 10−3 µm²/s at 700°C after 100 min oxidation. In summary, the investigations clearly depict the advanced oxidation resistance of Ta alloyed α-WB2−z thin films, whereas a minimum in scale thickness and kinetics is suggested to be around x ≈ 0.55.

Project Head Helmut Riedl:
CDL-SEC