1,058 research outputs found
Data for: Hard Ti-Al-N endowed with high heat-resistance through alloying with Ta and Ce
No full textThese are the eps file formats of the figures presented in the paper "Hard Ti–Al–N endowed with high heat-resistance through alloying with Ta and Ce" by H. Asanuma, F.F. Klimashin, P. Polcik, S. Kolozsvári, H. Riedl, and P.H. Mayrhofe
Epitaxial growth of Al-Cr-N thin films on MgO(111)
Get PDFCubic rock salt structure Al0.60Cr0.40N and Al0.68Cr0.32N films of different thicknesses were grown epitaxially onto MgO(111) substrates by reactive unbalanced magnetron sputtering at 500°C. Rutherford backscattering spectroscopy reveals stoichiometric nitrides with Al/Cr ratios close to the ones of the used compound targets of 60/40 and 70/30. High resolution x-ray diffraction proves epitaxial growth over the whole film thickness up to thicknesses of ~1.8 µm. Reciprocal space maps and selected area electron diffraction show that the AlxCr1-xN films grow fully relaxed. Scanning and transmission electron microscopy imaging reveals columnar microstructures with column widths between 12–16 nm and {001} surface faceting on individual columns. The fully relaxed growth and the columnar structure can be attributed to limited ad‑atom mobility on the initial AlxCr1-xN(111) growth surface.Original publication: H. Willmann, M. Beckers, J. Birch, P.H. Mayrhofer, C. Mitterer, and L. Hultman, Epitaxial growth of Al-Cr-N thin films on MgO(111), 2008, Thin Solid Films, (517), 2, 598-602.http://dx.doi.org/10.1016/j.tsf.2008.07.003. Copyright: Elsevier B.V., http://www.elsevier.com
Influence of Yttrium on the Thermal Stability of Ti-Al-N Thin Films
Get PDFTi(1-x)Al(x)N coated tools are commonly used in high-speed machining, where the cutting edge of an end-mill or insert is exposed to temperatures up to 1100 degrees C. Here, we investigate the effect of Yttrium addition on the thermal stability of Ti(1-x)Al(x)N coatings. Reactive DC magnetron sputtering of powder metallurgically prepared Ti(0.50)Al(0.50), Ti(0.49)Al(0.49)Y(0.02), and Ti(0.46)Al(0.46)Y(0.08) targets result in the formation of single-phase cubic (c) Ti(0.45)Al(0.55)N, binary cubic/wurtzite c/w-Ti(0.41)Al(0.57)Y(0.02)N and singe-phase w-Ti(0.38)Al(0.54)Y(0.08)N coatings. Using pulsed DC reactive magnetron sputtering for the Ti(0.49)Al(0.49)Y(0.02) target allows preparing single-phase c-Ti(0.46)Al(0.52)Y(0.02)N coatings. By employing thermal analyses in combination with X-ray diffraction and transmission electron microscopy investigations of as deposited and annealed (in He atmosphere) samples, we revealed that Y effectively retards the decomposition of the Ti(1-x-y)Al(x)Y(y)N solid-solution to higher temperatures and promotes the precipitation of c-TiN, c-YN, and w-AlN. Due to their different microstructure and morphology already in the as deposited state, the hardness of the coatings decreases from similar to 35 to 22 GPa with increasing Y-content and increasing wurtzite phase fraction. Highest peak hardness of similar to 38 GPa is obtained for the Y-free c-Ti(0.45)Al(0.55)N coating after annealing at T(a) = 950 degrees C, due to spinodal decomposition. After annealing above 1000 degrees C the highest hardness is obtained for the 2 mol % YN containing c-Ti(0.46)Al(0.52)Y(0.02)N coating with similar to 29 and 28 GPa for T(a) = 1150 and 1200 degrees C, respectively
Adjusting the Oxidation Behaviour of arc evaporated Al1-xCrx Intermetallics and Substoichiometric Oxides
No full textInitiated by our recent works on intermetallic Al-Cr-Fe droplets [1-3], which indicate the ability to trigger the nucleation of corundum-structured (Al,Cr,Fe)2O3 crystallites, the oxidation behaviour of intermetallic Fe-doped Al0.70Cr0.30 coatings and corresponding substoichiometric oxides was investigated. The structure and composition of the outermost oxide scale and the oxidation temperature at which it forms is determined by the initial microstructure. Columnar intermetallic coatings, for instance, oxidise via an outermost metastable Al2O3 scale, which transforms into the thermodynamically stable corundum-phase starting at 1000-1050 °C. However, nano-composite-like structures, present in substoichiometric coatings, show a more diverse oxidation at the surface and complex elemental separation within the unoxidised material
In the present work, we extent our studies on the oxidation and oxide phase formation to intermetallic and substoichiometric coatings prepared from powder-metallurgically produced Al1-xCrx cathodes with nominal compositions of x=0.10, 0.25, 0.30, 0.50, and 0.75. The structural evolution of the outermost scale and coating underneath-studied by X-ray diffraction and cross-sectional electron microscopy-is related to the Cr content, oxygen flow rate during the synthesis process and the oxidation conditions.
[1] C.M. Koller, J. Ramm, S. Kolozsvári, J. Paulitsch, P.H. Mayrhofer, Role of droplets and iron on the phase formation of arc evaporated Al-Cr-oxide coatings, Surf. Coatings Technol. 276 (2015) 735-742. doi:10.1016/j.surfcoat.2015.05.012.
[2] C.M. Koller, R. Hahn, J. Ramm, S. Kolozsvári, P.H. Mayrhofer, Microstructural modifications in powder-metallurgically produced Al0.675Cr0.275Fe0.05 targets during cathodic arc evaporation, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 34 (2016) 21603. doi:10.1116/1.4938407.
[3] C.M. Koller, A. Kirnbauer, R. Hahn, B. Widrig, S. Kolozsvári, J. Ramm, et al., Oxidation behavior of intermetallic Al-Cr and Al-Cr-Fe macroparticles, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 35 (2017) 61601. doi:10.1116/1.4986928
Stereoscopic PIV measurement in laminar rotating plane Couette flow
No full textPlane Couette flow with spanwise system rotation shows structures of streamwise-oriented roll cells that arise due to the instability by the Coriolis force when the system rotation is in the opposite direction to the mean flow vorticity. The momentum transport caused by such roll cell structures makes the flow tend to exhibit zero absolute vorticity. In the present study, stereoscopic PIV measurements in the rotating plane Couette flow were carried out in order to further illuminate the vortex structures and transport phenomena in this flow. The Reynolds stresses and some terms of its transport equation were evaluated to discuss the transport phenomena caused by the coherent structure. Furthermore, the wall shear stress was evaluated based on the measurement results of the Reynolds and viscous shear stresses and its variation with the system rotation rate is also presented
Influence of Al-content on structure, mechanical properties and thermal stability of (Al,Ta,Ti,V,Zr)-nitride coatings
No full textIn the field of materials research, a novel alloying concept, so-called high-entropy metal-sublattice ceramics have gained a lot of interest in the last years. These consist of a solid solution of 5 or more binary ceramics leading to a high-entropy metal-sublattice. Previous investigated material systems based on this concept have shown to be promising candidates for industrial applications due to their high thermal stability and good mechanical properties [1,2] Within this work, we investigate the structure, mechanical properties, and thermal stability of thin films based on (Al,Ta,Ti,V,Zr) with different Al contents.
For coating synthesis, we used a single powder-metallurgically produced composite target and additional Al cubes placed along the racetrack to increase the Al content within the coatings. The coatings were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), as well as nanoindentation. Furthermore, vacuum annealing treatments were carried out to gain information about the thermal stability and the change of mechanical properties upon heat treatment.
The coatings in as-deposited state exhibit a fine-columnar growth morphology. X-ray diffraction (XRD) analysis shows that the coatings independent of the Al-content crystallise in a single-phase face-centred cubic (fcc) structure. The hardness and the indentation modulus of the investigated coatings in as-deposited state is ~31 GPa and 400 GPa, respectively, and are as well independent on the Al-content. We studied the influence of the Al content on the thermal stability by investigating the structural evolution of our coatings by XRD, as well as nanoindentation. Furthermore, also powdered coating material was investigated by XRD after heat treatment up to 1500 °C. All the coatings keep their hardness up to an annealing temperature of 1000°C. After annealing at higher temperatures, the precipitation w-AlN causes a decrease of the hardness. Furthermore, the XRDs of powdered coating material reveal a distinct influence of the Al-content on the decomposition of the solid solution into an fcc-matrix, a nitrogen depleted hexagonal phase, and Al-rich domains.
[1] A. Kirnbauer, A. Kretschmer, C.M. Koller, T. Wojcik, V. Paneta, M. Hans, J.M. Schneider, P. Polcik, P.H. Mayrhofer, Mechanical properties and thermal stability of reactively sputtered multi-principal-metal Hf-Ta-Ti-V-Zr nitrides, Surf. Coatings Technol. 389 (2020) 125674. https://doi.org/10.1016/j.surfcoat.2020.125674.
[2] A. Kirnbauer, A. Wagner, V. Moraes, D. Primetzhofer, M. Hans, J.M. Schneider, P. Polcik, P.H. Mayrhofer, Thermal stability and mechanical properties of sputtered (Hf,Ta,V,W,Zr)-diborides, Acta Mater. 200 (2020) 559–569. https://doi.org/10.1016/j.actamat.2020.09.018
Superlattice effect for enhanced Fracture Toughness of Hard Coatings and its Intrinsic Origins
No full textCeramic hard coatings, for instance transition metal nitrides like TiN, protect tools from severe loads and harsh environments and thus enhance their lifetime. In this contribution, we show that TiN/CrN superlattice structures exhibit a peak in fracture toughness, similar to the well-known effect in indentation hardness [1].
We found -using micromechanical cantilever bending tests- that the fracture toughness increases with decreasing bilayer period, reaching a maximum at Λ ~ 5 nm (depending on the substrate material and the specific growth conditions). Thinner layers lead to intermixing and a subsequent loss of its unique superlattice properties.
The hardness enhancement in superlattice systems is a plasticity driven phenomenon, and thus determined by inhibition of dislocation movement [2]. In contrast, the fracture toughness follows a strictly linear elastic behaviour until fracture with no signs of plasticity. Hence, we assume underlying bilayer period dependent intrinsic effects to be responsible for the fracture toughness enhancement, such as coherency stresses and oscillating young's moduli across the individual layers.
[1] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, and P.H. Mayrhofer, (2016) Scripta Materialia, 124, 67 - 70.
[2] X. Chu, S.A. Barnett, (1995) Journal of Applied Physics, 77, 4403
Generalized diagnostic scaling for high-order moments in turbulent boundary layers
No full textThe present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20'000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region
PV architectures for DC microgrids using buck or boost exclusive microconverters
No full textDC microgrids can connect directly dc renewable energy sources with increasing amount of dc loads. In this paper it is looked for possible architectures for integrating PV panels into dc microgrids, by means of microconverter strings. Three topologies are considered, featuring only buck microconverters and only boost microconverters, since they promise higher efficiency due to fewer semiconductors in the current path. The topologies under exam are tested with a perturb and observe MPPT, in the cases of abrupt local shading and uneven shading over a solar panels' array. Among the three, the best topology by response time and control ease is found.Accepted Author ManuscriptOld - EWI-ESE-DC&S DC systems & StorageElectrical Power Processin
Fracture toughness enhancement in superlattice hard coatings
No full textPhysical vapour deposited (PVD) ceramic hard coatings are widely used in industrial applications as protective, wear reducing coatings. Their combination of good mechanical properties such as high hardness, a low friction coefficient, and their chemical resistance enable the application in harsh environments. However, a strong limitation is the relatively low fracture tolerance (brittle behaviour), depicting especially in cutting applications a major challenge.
In this contribution, we show experimental results of in-situ microcantilever bending tests on nanolayered TiN-CrN coatings, referred to as superlattices, overcoming this unfavourable behaviour. We found a maximum in fracture toughness (KIC) at bilayer periods of ~6 nm [1], similar to the well-known peak for the indentation hardness reported by Helmersson et al. [2]. For both, KIC and the hardness, we observe an increase by ~50 % compared to the rule of mixture of the constituents. The beneficial effect of a careful structural design on the fracture toughness will be shown for reactive magnetron sputtered as well as arc evaporated superlattice coatings. Importantly, the coatings synthesized in the industrial scale arc evaporation plant (Oerlikon Balzers Innova) show an even more pronounced superlattice effect and thus unite high hardness with reasonable toughness.
While mechanisms based on dislocation activity explain the increase in hardness, the linear elastic behaviour during our micromechanical tests suggests a different mechanism responsible. To describe this, we conducted density functional theory (DFT) calculations as well as finite element studies.
Complementary, we studied the microstructure of our coatings by X-ray diffraction experiments, scanning electron microscopy and high-resolution transmission electron microscopy. The thermal stability of our films was investigated by annealing in vacuum and ensuing experiments (XRD, hardness and fracture toughness) [3] along with differential scanning calorimetry (DSC) investigations.
[1] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, P.H. Mayrhofer, Superlattice effect for enhanced fracture toughness of hard coatings, Scripta Mat. 124 (2016) 67.
[2] U. Helmersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, Growth of single‐crystal TiN/VN strained‐layer superlattices with extremely high mechanical hardness, J. Appl. Phys. 62 (1987) 481.
[3] R. Hahn, M. Bartosik, M. Arndt, P. Polcik, P.H. Mayrhofer, Annealing effect on the fracture toughness of CrN/TiN superlattices, Int. J. Refract. Met. H. 71 (2018) 352-356
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