Publications

Cathodic arc evaporated Ti1−xAlxN coatings were deposited on a Nimonic c-263 superalloy and tested under low temperature hot corrosion conditions. Treated with a MgSO4/Na2SO4 salt-mixture, all samples were annealed at 700 °C in a 2800 ppm(volume) SOx-rich atmosphere for 1, 5, 15 and 30 h. A significantly reduced corrosion severity was exhibited by all Ti1−xAlxN variants over the uncoated NiCrCo alloy. A synergistic fluxing mechanism was found to be the dominant factor for the coating breakdown. Depending on the relative Al-to-Ti content on the Ti1−xAlxN sublattice, differences in the coatings’ corrosion and scaling behavior were observed.

The request to reduce carbon emissions as well as fuel consumptions of modern aerospace and aviation mobility, heavily motivated the development of lighter and more durable high temperature materials. γ-TiAl bulk materials meet many of these requirements due to their unique properties, such as low density, high strength or excellent creep resistance. However, improving their oxidation resistance above 750 °C is still challenging, especially without deteriorating other material properties. Recently, we showed that magnetron sputtered Al-rich γ-TiAl coatings are ideal candidates for well-established TNM bulk alloys (Ti-43.5Al-4Nb-1Mo-0.1B, in at%) to increase their oxidation resistance and to block oxygen inward diffusion. Within this study, we present detailed microstructural investigations of the appearing phase transformations and morphological changes in the coating due to ambient-air-exposure at 850 °C for up to 1000 h. These show that only a 4-µm-thin, well-adhering α-Al2O3-based thermally grown oxide (TGO) forms on top of an initial 16.5-µm-thick coating. Cross-sectional nanobeam diffraction in conjunction with high resolution chemical as well as structural analysis during transmission electron microscopy after these exposures highlight that the Al-rich γ-TiAl coating is perfectly intermixed with the TNM substrate material. Already after 100 h oxidation at 850 °C, no interface between the Al-rich γ-TiAl coating and the TNM alloy can be identified chemically or structurally. The structural homogenization is governed by the transformation of all Al-rich phases (i.e., TiAl3 or Ti2Al5) – also present in the as-deposited state – towards γ-TiAl. After 1000 h at 850 °C, the predominant phase within the original coating region is γ-TiAl, next to the highly dense and well-adherent α-Al2O3-based scale.

The corrosion resistance of cathodic arc evaporated Al0.7Cr0.3-xVxN coatings with a vanadium content up to 22.3 at.% has been electrochemically tested in a 0.1 M NaCl-solution. Significant improvement in the open porosity and corrosion rate was observed for coatings with higher V-contents, due to a denser and more refined coating morphology. Further reduction in the open porosity rate was achieved through an annealing step in air at 700 °C. Here, the formation of an AlVO4 top-oxide and underlying oxygen-rich V-depletion zone provides additional sealing of the coating surface, whilst reducing the corrosion current density to a final 1.59×10-9 A/cm2.

The Si-based alloying of transition metal diborides is a promising strategy to improve their limited oxidation resistance in high-temperature environments. In this study, we investigate the oxidation resistance of ternary and quaternary Cr-(Mo)-Si-B2-z coatings sputter-deposited from alloyed CrB2/TMSi2 targets (TM = Cr or Mo). The as-deposited Cr-(Mo)-Si-B2-z coatings are stabilized in the single-phased hexagonal AlB2-structure, except the high-Si containing Cr0.26Mo0.11Si0.24B0.39 presenting amorphous character. The Mo-containing Cr-Mo-Si-B2-z films exhibit relatively high hardness compared to their ternary Cr-Si-B2-z counterparts, obtaining up to 26 GPa due to the formation of (Cr,Mo)B2 solid solutions. The Si-alloying in ternary and quaternary coatings provides oxidation resistance up to 1200 °C, owing to the formation of highly protective double-layered scales consisting of SiO2 with a Cr2O3 layer on top, inhibiting oxygen inward diffusion. The quaternary Cr0.31Mo0.07Si0.15B0.47 coating is distinguished by superior oxidation resistance with lower porosity and void formation compared to the ternary Cr0.37Si0.16B0.47. Mo proved to be the key element for the higher stability and enhanced oxidation resistance due to the evolution of the MoSi2 phase at ~600 °C. This phase formation controls the Si diffusion and mobility within the microstructure, thus reducing the porosity and governing the Si supply to form SiO2 scale. The quaternary Cr0.31Mo0.07Si0.15B0.47 coating maintained an oxidation resistance up to 30 h at 1200 °C by forming a 2.5 μm dense amorphous Si-based oxide scale with a thin Cr2O3 on top.

Superlattice structures enable the simultaneous enhancement in hardness (H) and fracture toughness (KIC) of ceramic-like coatings. While a deeper understanding of this effect has been gained for fcc-structured transition metal nitrides (TMN), hardly any knowledge is available for hexagonal diborides (TMB2). Here we show that superlattices can—similarly to nitrides—increase the hardness and toughness of diboride films. For this purpose, we deposited TiB2/WB2 and TiB2/ZrB2 superlattices with different bilayer periods (Λ) by non-reactive sputtering. Nanoindentation and in-situ microcantilever bending tests yield a distinct H peak for the TiB2/WB2 system (45.5 ± 1.3 GPa for Λ = 6 nm) but no increase in KIC related to a difference in shear moduli (112 GPa). Contrary, the TiB2/ZrB2 system shows no peak in H, but for KIC with 3.70 ± 0.26 MPa∙m1/2 at Λ = 4 nm originating from differences in lattice spacing (0.14 Å), hence causing coherent stresses retarding crack growth.

To overcome the limited oxidation resistance of the emerging class of transition metal borides, we suggest within this study novel quaternary diborides, Ti-TM-Si-B 2 ± z (TM = Ta, Mo), achieving the compromise between excellent oxidation resistance and requirements of hard coatings. Single-phase AlB 2-type structured Ti-TM-Si-B 2 ± z films (3-5 µm) are sputter-deposited from TiB 2 /TMSi 2 targets. The Ti-Ta-Si-B 2 ± z coatings exhibit 36 GPa in hardness, while maintaining strongly retarded oxidation kinetics till 1000°C. Ti-Mo-Si-B 2 ± z coatings preserve a hardness up to 27 GPa, although out-performing all their counterparts by featuring outstanding oxidation resistance with 440 nm oxide scale thickness after 1 h at 1200°C.

The impact of Si-segregates and varying deposition conditions on the structural and mechanical properties of sputter deposited, high-temperature oxidation-resistant Cr-Si-B2±z coatings is studied from ambient, to elevated temperatures. Overstoichiometric, AlB2-structured Cr-Si-B2±z thin films with Si-content up to 15 at.% were synthesized on Ti-6Al-4V by magnetron-sputtering using a substrate bias of −120 V. The enhanced surface diffusion promotes mechanically superior, (001)-oriented coatings with hardness of H∼30 GPa up to a Si-content of 3 at.%. Higher Si-concentrations result in significant hardness loss to H∼20 GPa, related to a bias-independent solubility-limit in the CrB2-structure and the formation of mechanically-weak Si grain-boundary segregates. The as-deposited hardness of all Cr-Si-B2±z compositions is maintained after annealing to 800 °C, despite the initiation of material recovery. A B/Cr-ratio-independent oxidation resistance up to 1400 °C is demonstrated, underlining a minimum Si-content of 8 at.% to form a stable SiO2-based scale. In line with the room-temperature hardness, increasing Si-contents are accompanied by decreasing fracture toughness, reducing from KIC∼2.9 (Cr0.28B0.72) to ∼1.7 MPa√m (Cr0.24Si0.10B0.66). High-temperature cantilever bending up to 800 °C revealed a brittle-to-ductile-like transition for Cr0.28B0.72, resulting in an increased fracture toughness of KIC∼3.3 MPa√m. Si-alloyed coatings show decreasing fracture resistance up to 400 °C, whereas beyond, Si-segregates enable high-temperature plasticity and thus a significantly increased damage tolerance.

Structural imperfections such as nano-columnar tissue phases constitute a morphological design feature in protective thin films. Especially in the rising group of PVD transition metal diborides, excess Boron is known to form nm-sized B-rich precipitations in-between nano-crystalline domains. Here, we focus on super-stoichiometric TiB2+z thin films, varying the stoichiometry from TiB2.04 to TiB4.42 (B: 67–82 at.%). The tissue phase fraction and thickness is mainly governed by the B content and corresponding deposition conditions. A decreasing tissue phase width from ≈ 3–1 nm leads to an increasing critical stress intensity factor KIC of ≈ 40%.

Si doped transition metal (TM) diborides are a promising class of thin films materials due to their outstanding oxidation resistance at high temperatures. In detail, the excellent oxidation resistance was proven i.e. for Hf-Si-B_2±z being stable up to 1500 °C, whereby the low growth kinetics are attributed to a Si rich oxide formed on top. This scale formation is accompanied with a decomposition/clustering of Si within the TMB_2±z structure, which can be understood as a transition from the metastable as deposited state to the thermodynamically stable phases as suggested by the ternary phase diagrams. Furthermore, the oxidation of Si was shown to be thermodynamically preferred, and hence if the transport of Si to the coating/oxide interface is ensured, a protective Si rich oxide can be expected. This requires on the one hand a certain Si content within the coatings as well as a distinct activation temperature. A generalized criterion for the required Si content is proposed, which empirically holds at least for Ti-, Cr- and Hf-Si-B_2+z coatings. This criterion basically relates the minimum Si content to the amount of access boron in the diboride based coatings.

A complex and untraceable mechanical and thermal loading situation in rolling-sliding contacts can lead to the formation of white etching layers (WELs), depicting critical crack initiation sites. For our detailed studies, to obtain a holistic view of the microstructural characteristics and micro-mechanical properties, we prepared a WEL on a decommissioned (after 200,000 km service life) rail wheel by laser surface treatments with a defined energy input. This WEL is predominantly martensitic down to a depth of 30–40 μm, after which a transition to the deformed ferritic-pearlitic microstructure of the hypoeutectoid rail wheel steel is present. The martensitic region is with 6.98 ± 0.68 GPa significantly harder than the transition zone (5.17 ± 0.39 GPa) and the deformed ferritic-pearlitic base material (3.30 ± 0.33 GPa). In-situ V-notched micro-cantilever bending experiments of the martensitic (and thus most brittle) region show crack initiation and propagation – mostly along the boundaries of the martensitic grains – besides a plastic behavior. Applying the elastoplastic fracture mechanics allows to derive the local fracture toughness KIQ, which is 16.4 ± 1.2 MPam^1/2 for this martensitic region of the WEL. The results outpoint the application of micro-cantilever bending tests in addition to hardness testing as a promising tool to discuss the relationship of microstructural characteristics with its micro-mechanical properties.

This work investigates the influence of Si-alloying up to 17 at.% on the structural, mechanical, and oxidation properties of magnetron sputtered CrB2±z-based thin films. Density-functional theory calculations combined with atom probe tomography reveal the preferred Si occupation of Cr-lattice sites and an effective solubility limit between 3 to 4 at.% in AlB2-structured solid solutions. The addition of Si results in refinement of the columnar morphology, accompanied by enhanced segregation of excess Si along grain boundaries. The microstructural separation leads to a decrease in both film hardness and Young’s modulus from H ~ 24 to 17 GPa and E ~ 300 to 240 GPa, respectively, dominated by the inferior mechanical properties of the intergranular Si-rich regions. Dynamic thermogravimetry up to 1400 °C reveals a significant increase in oxidation onset temperature from 600 to 1100 °C above a Si content of 8 at.%. In-situ X-ray diffraction correlates the protective mechanism with thermally activated precipitation of Si from the Cr-Si-B2±z solid solution at 600 °C, enabling the formation of a stable, nanometer-sized SiO2¬¬-based scale. Moreover, high-resolution TEM analysis exposes the scale architecture after dynamic oxidation to 1200 °C (10 K/min heating rate) – consisting only of ~20 nm amorphous SiO2 beneath ~200 nm of nanocrystalline Cr2O3. In summary, the study provides detailed guidelines connecting the chemical composition with the respective thin film properties of high-temperature oxidation resistant Cr-Si-B2±z coatings.

Pitting corrosion of sputtered and arc evaportaed fcc-AlCrN coated low-alloy steel substrates was studied in a 0.1 M NaCl solution, using a three-electrode-cell. Depending on the deposition technique, several diffusion mechanisms were identified by high-resolution techniques (i.e. APT, TOF-SIMS). For arc evaporated AlCrN, incoherently embedded macro-particles provided the majority of fast-track diffusion pathways and pit-initiation sites, while their pristine coating matrix proved protective against chloride inward diffusion. Contrarily, the more coarse-grained sputtered AlCrN morphology with a highly orientated crystal growth featured diffusion paths along column boundaries, where chloride permeated the coating structure and initiated pit formations at the coating-substrate interface. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request.

Transition-metal disilicides (TMSi2) based thin films are suggested as promising, novel protective coating materials used in various high-temperature applications. In this study, we investigate the phase formation, microstructure, and mechanical properties (i.e. H, E, and KIC) of sputter-deposited TMSix films (TM = Mo, Ta, Nb) in correlation with the varied bias potential. The as-deposited TaSix and MoSix coatings show Si sub-stoichiometries with Si/Me (x)< 2, while all the NbSix coatings are overstoichiometric in Si. All TaSix and NbSix coatings are stabilized in their preferred hexagonal structure, whereas the MoSix coatings exhibit small fractions of T1-Mo5Si3 next to the dominant metastable hexagonal β-phase. The oxidation behaviour of the coatings was examined up to 1400 °C. MoSix based films are distinguished by an outstanding oxidation resistance, forming dense and protective silica scales of only 650 nm after 100 h at 1200 °C – also obtaining an extremely high interfacial stability. In contrast, TaSix suffers accelerated oxidation at 1200 °C due to the formation of mixed, non-protective scales consisting of Ta2O5 and SiO2. Moreover, NbSix coatings show retarded oxidation kinetics up to 60 h at 1200 °C, forming a dense and uniform SiO2 scale of only 533 ± 131 nm. Micro-cantilever bending experiments reveal that TaSi1.7 coating exhibit the highest fracture toughness, KIC, of 2.7 ± 0.2 MPa∙m1/2 compared to 2.3 ± 0.1 and 1.7 ± 0.1 MPa∙m1/2 for NbSi2.4 and MoSi1.9, respectively.

Hexagonal transition metal diborides embody promising material systems for the purpose of protective thin films. Here, we focus on DC magnetron sputtered TiB2+z coating materials, comprehensively revisiting the impact of the stoichiometry on the structure-mechanical properties, from nearly stoichiometric TiB2.07 (B: 67 at. %) up to super-stoichiometric TiB4.42 (B: 82 at. %). The structural analysis confirmed the apparent correlation between the deposition pressure and the preferred {0001} orientation, which is essential to gain super-hardness (>40 GPa). In contrast, the hardness decreases for >10 GPa for 101¯1 and 1000 oriented thin films, underlining the pronounced anisotropy of TiB2+z. The broad stoichiometry variation revealed no predominant hardness effect based on a B-rich tissue phase. The excess B contributes to a decreasing column size correlating with a decreasing hardness of ≈ 7 GPa (B/Ti ratios >2.5) due to column boundary sliding events. Micro-cantilever bending experiments proved a declining fracture toughness from 3.02 ± 0.13 MPa√m for TiB2.43 to 2.51 ± 0.14 MPa√m for TiB4.42 to be column size dependent.

Fatigue failure through sustained loading of ductile materials manifests in irreversible motion of dislocations, followed by crack initiation and growth. This contrasts with the mechanisms associated with brittle ceramics, such as nanostructured physical vapor deposited thin films, where inhibited dislocation mobility typically leads to interface-controlled damage. Hence, understanding the fatigue response of thin films from a fundamental viewpoint – including altered atomic bonds, crystal structures, and deformation mechanisms – holds the key to improved durability of coated engineering components. Here, a novel method utilizing quasi-static and cyclic-bending of pre-notched, unstrained microcantilever beams coupled with in situ synchrotron X-ray diffraction is presented to study the fracture toughness and fatigue properties of thin films under various loading conditions. Investigating a model system of sputter-deposited Cr and Cr-based ceramic compounds (CrN, CrB2, and Cr2O3) demonstrates that the fatigue resistance of such thin films is limited by the inherent fracture toughness. In fact, cantilever cycling close to the critical stress intensity is sustained up to 107 load cycles on all materials, without inducing noticeable material damage, structural or stress-state changes. The observed variation in fracture toughness is put into context with linear-elastic fracture theory and complementary micro-pillar compression, thereby elucidating the wide range of values from as low as 1.6±0.2 MPa√m for Cr1.79O3 up to 4.3±0.3 MPa√m for Cr1.03B2, respectively. Moreover, possible mechanisms governing the elastic-plastic deformation response of all coatings, both in quasi-static and cyclic-loading conditions, are discussed. Our findings contribute key-insights into the underlying mechanisms dictating the damage tolerance of PVD coated components by relating fatigue strength limits to fundamental material properties.

The exceptional mechanical properties of transition metal carbide coatings are known to be governed by the carbon content and its morphological distribution. Here, we verify the influence of the target peak power density on the chemical composition, microstructure, and mechanical properties of NbCx coatings grown by non-reactive high-power impulse magnetron sputtering (HiPIMS). By tuning the pulse parameters, the power density can be increased from 0.11 to 1.48 kW/cm2 leading to a decrease in the C/Nb ratio from 1.52 to 0.99 within the films – proven by combined elastic backscattering and time-of-flight elastic recoil detection analysis. This decrease in the C/Nb ratio is accompanied by microstructural changes from nanocomposite morphologies with an average grain size of 6.6 ± 2.5 nm at 0.13 kW/cm2 into more columnar structures with an average column width of 65.2 ± 18.7 nm at 1.48 kW/cm2. Independent from the C/Nb ratio, all films exhibit a single face-centered cubic structure. The mechanical properties correlate with the enhanced growth behavior dominated by ions at higher peak power densities and the varied C/Nb ratios. A maximum in hardness and fracture toughness of H = 38.7 ± 3.6 GPa and KIc = 2.78 ± 0.13 MPa∙m1/2 (at 3.2 GPa residual compressive stress), is obtained for the nearly stoichiometric NbC coating exhibiting C/Nb ratio of 1.06.

Within physical vapor deposited Hf-Si-B2±z thin films, selective diffusion-driven oxidation of Si is identified to cause outstanding oxidation resistance at temperatures up to 1500 °C. After 60 h at 1200 °C, the initially 2.47 µm thin Hf0.20Si0.23B0.57 thin film exhibits a dense oxide scale of only 1.56 µm. The thermally induced decomposition of metastable Hf-Si-B2±z leads not only to the formation of Si precipitates within the remaining thin film (related to a non-homogenous Si distribution after the deposition) but also to pure Si layers on top and bottom of the Hf-Si-B2±z coatings next to the excellent adherend SiO2 based scales.

The increased demand for sustainability requires, among others, the development of new materials with enhanced corrosion resistance. Transition metal diborides are exceptional candidates, as they exhibit fascinating mechanical and thermal properties. However, at elevated temperatures and oxidizing atmospheres, their use is limited due to the fact of their inadequate oxidation resistance. Recently, it was found that chromium diboride doped with silicon can overcome this limitation. Further improvement of this protective coating requires detailed knowledge regarding the composition of the forming oxide layer and the change in the composition of the remaining thin film. In this work, an analytical method for the quantitative measurement of depth profiles without using matrix-matched reference materials was developed. Using this approach, based on the recently introduced online-LASIL technique, it was possible to achieve a depth resolution of 240 nm. A further decrease in the ablation rate is possible but demands a more sensitive detection of silicon. Two chromium diboride samples with different Si contents suffering an oxidation treatment were used to demonstrate the capabilities of this technique. The concentration profiles resembled the pathway of the formed oxidation layers as monitored with transmission electron microscopy. The stoichiometry of the oxidation layers differed strongly between the samples, suggesting different processes were taking place. The validity of the LASIL results was cross-checked with several other analytical techniques.

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.