Publications

The concept of Si alloyed transition metal (TM) diborides – well explored for bulk ceramics – is studied for five different physical vapor deposited TM-Si-B2±z (TM = Ti, Cr, Hf, Ta, W) coatings, focusing on the oxidation behavior up to 1200 °C. In their as deposited state, all coatings exhibit single phased AlB2 prototype structures, whereby the addition of Si results in dense, refined morphologies with no additional phases visible in the X-ray diffractograms. With already low amounts of Si, the slope of the mass increase during dynamic oxidation flattens, especially for Ti-Si-B2±z, Cr-Si-B2±z, and Hf-Si-B2±z. Above distinct Si contents, the formation of a steady state region exhibiting no further mass increase is promoted (starting at around 1000 to 1100 °C). Best results are obtained for Hf0.21Si0.18B0.61 and Cr0.26Si0.16B0.58 (both around 2.4 μm thick in the as deposited sate), revealing drastically retarded oxidation kinetics forming 400 nm thin oxide scales after 3 h at 1200 °C in ambient air (significantly lower compared to bulk ceramics). This highly protective oxidation mechanism is attributed to the formation of an amorphous Si rich oxide scale. The Si content needed to form these oxide scales largely differs between the TM-Si-B2±z coatings investigated, also diversifying the prevalent oxidation mechanism, especially for Cr-Si-B2±z.

Transition metal diboride-based thin films are promising candidates to replace state-of-the-art protective and functional coating materials due to their unique properties. Here, we focus on hexagonal WB2−z , showing that the AlB2 structure is stabilized by B vacancies exhibiting its energetic minima at sub-stoichiometric WB1.5. Nanoindentation reveals super-hardness of 0001 oriented α-WB2−z coatings , linearly decreasing by more than 15 GPa with predominant 1011 orientation. This anisotropy is attributed to differences in the generalized stacking fault energy of basal and pyramidal slip systems, highlighting the feasibility of tuning mechanical properties by crystallographic orientation relations.

γ-TiAl alloys have gained much attention due to their low density, excellent fatigue resistance, and high specific strength at elevated temperatures. Throughout this study, we showcase physical vapor deposited Al-rich γ-TiAl coatings as a considerable solution to extend the durability and longevity of γ-TiAl bulk materials in high temperature regimes (> 750 °C). In detail, the oxidation resistance is enhanced up to at least 850 °C for 1000 h, whilst exhibiting a maximum oxygen affected area of 8.4 µm (16 µm coating). Based on various oxidation treatments (800, 850, and 900 °C), we estimated a mean parabolic rate constant of about 1.03∙10−10 cm²/h and a logarithmic oxygen inward diffusion rate of approximately 1.20∙10−4 cm/h. The corresponding activation energies validate the considerably retarded oxide scale kinetics – particularly, for the thermally grown oxides. Furthermore, the thermal exposure introduced excellent intermixing between the Al-rich γ-TiAl coating and γ-TiAl bulk material, resulting in a coalescence of the coating-substrate interface.

The influence of the non-metal species on the oxidation resistance of transition metal ceramic based thin films is still unclear. For this purpose, we thoroughly investigated the oxide scale formation of a metal (Hf), carbide (HfC0.96), nitride (HfN1.5), and boride (HfB2.3) coating grown by physical vapor deposition. The non-metal species decisively affect the onset temperature of oxidation, ranging between 550 °C for HfC0.96 to 840 °C for HfN1.5. HfB2.3 and HfN1.5 obtain the slowest oxide scale kinetic following a parabolic law with kp values of 4.97∙10⁻¹⁰ and 5.66∙10⁻¹¹ kg² m⁻⁴ s⁻¹ at 840 °C, respectively. A characteristic feature for the oxide scale on Hf coatings, is a columnar morphology and a substantial oxygen inward diffusion. HfC0.96 reveals an ineffective oxycarbide based scale, whereas HfN1.5 features a scale with globular HfO2 grains. HfB2.3 exhibits a layered scale with a porous boron rich region on top, followed by a highly dense and crystalline HfO2 beneath. Furthermore, HfB2.3 presents a hardness of 47.7 ± 2.7 GPa next to an exceptional low inward diffusion of oxygen during oxidation. This study showcases the strong influence of the non-metallic bonding partner despite the same metallic basis, as well as the huge potential for HfB2 based coatings also for oxidative environments.

Time-averaged and time-resolved ion fluxes during reactive HiPIMS deposition of Ti1-xAlxN thin films are thoroughly investigated for the usage of Ti1-xAlx composite targets – Al/(Ti+Al) ratio x = 0.4 and 0.6. Ion mass spectroscopy analysis revealed, that increasing x in the target material or reducing the N2 flow-rate ratio leads to a proportional increase of the Al⁺-ion count fraction, whereas that of Tiⁿ⁺-ions (n = 1, 2) remains unaffected despite of comparable primary ionisation energies between Al and Ti. In fact, energetic Ti²⁺-ions account for the lowest flux fraction incident on the substrate surface, allowing for a high Al-solubility limit in cubic-structured Ti1-xAlxN thin films (xmax ~ 0.63) at low residual stresses. In addition, time-resolved plasma analysis highlights the simultaneous arrival of metal- and process-gas-ions throughout the entire HiPIMS pulse duration. These ion-bombardment conditions, which were dominated by gas-ion irradiation with a significant contribution of Al⁺-ions (up to ~ 20 %) and negligible energetic Ti²⁺-ions, allowed for the growth of cubic Ti0.37Al0.63N coatings exhibiting high indentation hardness of up to ~36 GPa at a low compressive stress level (σ = -1.3 GPa).

The outstanding oxidation resistance, thermo-mechanical stability, and chemical inertness of alumina, but also the synthesis of phase pure polymorphs attract particular attention in academia and industry. Especially, the difficulties regarding the synthesis of α- or γ-structured Al2O3 by physical vapor deposition techniques are still strong limitations. Within this study, we investigated in detail the influence of 2 at.% tungsten in the Al-target on the process stability and phase formation during reactive DC magnetron sputtering as well as high power impulse magnetron sputtering (HiPIMS) of Al2O3-based coatings. The small addition of W to the Al target allows to increase the oxygen partial pressure by more than 200 % while maintaining a stable deposition process. Ion mass spectroscopy measurements yield a promising high fraction of ¹⁶O⁺ and ³²O2⁺, when operating the W-containing target in the metal-to-poisoned transition mode. A significant increase of ¹⁶O⁺ is further provided by the target surface oxide in poisoned mode. Detailed time-of-flight ion mass spectroscopy investigations during one HiPIMS pulse show a clear temporal separation of the individual ions arriving at the substrate plane during the pulse on-time, allowing for controlled ion attraction by synchronizing the bias pulse to the discharge impulse. Equal amounts of ²⁷Al⁺ and ³²O2⁺ can be attracted using a bias on-time between 400 μs and 900 μs in the “off-time” (after glow) leading to a dense and nano-crystalline coating. Detailed electron microscopy investigations show the presence of metallic phase fractions for higher duty cycles (7.5 %). Decreasing the duty cycle to 3.75 % leads to amorphous coatings when operating the Al-target at the highest oxygen partial pressure in metallic mode.

Diffusion driven high-temperature oxidation is one of the most important failure mechanisms of protective thin films in industrial applications. Within this study, we investigated the diffusion of oxygen at 800 to 1100 °C through nano-laminated crystalline Ti-Al-N and amorphous Mo-Si-B based multilayer coatings. The most prominent oxygen diffusion pathways, and hence the weakest points for oxidation, were identified by combining ¹⁸O tracer diffusion and atom probe tomography. An oxygen inward diffusion along column boundaries within Ti-Al-N layers in front of a visually prevalent oxidation front could be proven, highlighting the importance of these fast diffusion pathways. Furthermore, the amorphous Mo-Si-B layers act as barriers and therefore mitigate the migration of oxygen by accumulating reactive O species at a nanoscale range. Preventing oxygen diffusion along column boundaries – through the implementation of amorphous interlayers – lead to paralinear oxidation behavior and stable scales even after 7 h at 1100 °C. Our results provide a detailed insight on the importance of morphological features such as grain and column boundaries during high-temperature oxidation of protective thin films, in addition to their chemistry.

Ternary W 1−x Ta x B 2−z is a promising protective coating material possessing enhanced ductile character and phase stability compared to closely related binaries. Here, the oxidation resistance of W 1−x Ta x B 2−z thin films was experimentally investigated at temperatures up to 700 °C. Ta alloying in sputter deposited WB 2−z coatings led to decelerated oxide scale growth and a changed growth mode from paralinear to a more linear (but retarded) behavior with increasing Ta content. The corresponding rate constants decrease from k* p = 6.3 ⋅ 10 −4 µm 2 /s for WB 2−z , to k* p = 1.1 ⋅ 10 −4 µm 2 /s for W 0.66 Ta 0.34 B 2−z as well as k l = 2.6 ⋅ 10 −5 µm/s for TaB 2−z , underlined by decreasing scale thicknesses ranging from 1170 nm (WB 2−z), over 610 nm (W 0.66 Ta 0.34 B 2−z) to 320 nm (TaB 2−z) after 10 min at 700 °C. Dense and adherent scales exhibit an increased tantalum content (columnar oxides), which suppresses the volatile character of tungsten-rich as well as boron oxides, hence being a key-factor for enhanced oxidation resistance. Thus, adding Ta (in the range of x = 0.2-0.3) to α-structured WB 2−z does not only positively influence the ductile character and thermal stability but also drastically increases the oxidation resistance.

Hard protective coating materials based on transition metal nitrides and carbides typically suffer from limited fracture tolerance. To further tune these properties non-metal alloying – substituting C with N – has been proven favorable for magnetron sputtered Hf-C-N based thin films. A theoretically predicted increase in valence electron concentration (from 8.0 to 9.0 e/f.u. from HfC to HfN) through nitrogen alloying lead to an increase in fracture toughness (KIC obtained during in-situ SEM cantilever bending) from 1.89 ± 0.15 to 2.33 ± 0.18 MPa·m1/2 for Hf0.43C0.57 to Hf0.35C0.30N0.35, respectively. The hardness remains close to the super-hard regime with values of 37.8 ± 2.1 to 39.9 ± 2.7 GPa for these specific compositions. Already the addition of small amounts of nitrogen, while sputtering a ceramic HfC target, leads to a drastic increase of nitrogen on the non-metallic sublattice for fcc single phased structured HfC1-xNx films, where x = N/(C + N). The here obtained results also provide experimental proof for the correlation between fracture characteristics and valence electron concentration.

Reactive high-power impulse magnetron sputtering (R-HiPIMS) is seen as a key-technology for the deposition of future hard and multifunctional coatings. Increased ionisation rates allow for additional possibilities in tuning specific coating characteristics based on growth mechanisms varied by surface-diffusion. Especially within the well-established Ti-Al-N system, the Al solubility limit (xmax) of metastable face-centred-cubic (fcc) Ti1−xAlxN is an everlasting scientific topic. Here, we investigate in detail the dependence of xmax on various deposition parameters (i.e. pulse frequency and duration, N2-to-Ar flow rate ratio, and substrate bias potential) during R-HiPIMS of Ti-Al-N coatings using Ti0.6Al0.4, Ti0.5Al0.5 and Ti0.4Al0.6 composite targets. The systematic studies showed that the highest solubility limit of xmax ∼0.55 could be obtained for duty cycles around 3.75% (peak power densities of ∼1.0 kW/cm2) and a N2-to-Ar flow rate ratio of 0.3. A further decisive fact for the deposition of high Al containing fcc-structured Ti1−xAlxN coatings is surface diffusion controlled by bias potentials (DC as well as modulated pulses) ensuring sufficient intermixing of the arriving film species. Despite the presence of very small amounts of wurtzite-typed phases, excellent hardness values of ∼36 GPa for Ti0.40Al0.60N – which further increased to ∼ 40 GPa upon annealing for 1 h at 700 °C – could be achieved for a DC bias potential of -50 V, irrespective of all variations conducted. Based on our results we can further conclude, that the ratio and energy of Tin+- and Aln+-ions, simultaneously arriving at the substrate surface, are decisive for stabilising the highly preferred cubic modification. A distinct promotion of specific discharge regimes – selected by synchronised bias pulses – can thus positively influence the cubic phase formation through altered gas-to-metal ion ratios arriving at the film surface.

In the field of hard protective coatings, nano-crystalline Ti-B-N films are of great importance due to the adjustable microstructure and mechanical properties through their B content. Here, we systematically study this influence of B on Ti-B-N during reactive as well as non-reactive DC magnetron sputtering. The different deposition routes allow for an additional, very effective key parameter to modify bond characteristics and microstructure. Plasma analysis by mass spectroscopy reveals that for comparable amounts of Ti+, Ti2+, Ar+, and Ar2+ ions, the count of N+ ions is about 2 orders of magnitude lower during non-reactive sputtering. But for the latter, the N+/N2+ ratio is close to 1, whereas during reactive sputtering this ratio is only 0.1. This may explain why during reactive deposition of Ti-B-N, the Bsingle bondN bonds dominate (as suggested by X-ray photoelectron spectroscopy), whereas the Bsingle bondB and Tisingle bondB bonds dominate for non-reactively prepared Ti-B-N. Chemically, reactively versus non-reactively sputtered Ti-B-N coatings follow the TiN-BN versus TiN-TiB2 tie line, respectively. Detailed X-ray diffraction and transmission electron microscopy studies reveal, that up to 10 at.% B can be dissolved in the fcc-TiN lattice when prepared by non-reactive sputtering, whereas already for a B content of 4 at.% a BN-rich boundary phase forms when reactively sputtered. Thus, we could not only observe a higher hardness (35 GPa instead of 25 GPa) as well as a higher indentation modulus (480 GPa instead of 260 GPa), but also a higher fracture energy (0.016 instead of 0.009 J/m during cube-corner indentations) for Ti-B-N coatings with 10 at.% B, when prepared non-reactively.