Helmut Riedl

Bio: Helmut Riedl is an academic researcher from Vienna University of Technology. The author has contributed to research in topics: Thin film & Materials science. The author has an hindex of 17, co-authored 57 publications receiving 840 citations. Previous affiliations of Helmut Riedl include University of Leoben.

Ab initio inspired design of ternary boride thin films.

Abstract: The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB2, VB2, etc.) tend to be chemically more stable in the AlB2 structure type, whereas late transition metal diborides (WB2, ReB2, etc.) are preferably stabilized in the W2B5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB2 or W2B5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB2, TcB2, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.

Origin of high temperature oxidation resistance of Ti–Al–Ta–N coatings

Abstract: Alloying Ti–Al–N coatings with Ta has proven to enhance their hardness, thermal stability, and oxidation resistance. However, especially for arc-evaporated Ti–Al–Ta–N coatings only limited information on the detailed influence of the elements on various properties is available. Therefore, we have developed arc-evaporated Ti 1−x−y Al x Ta y N coatings with various Al (x = 0.50–0.65) and Ta (y = 0.00–0.15) contents. While the thermal stability of our coatings during annealing in inert He atmosphere increases with increasing Ta content, best results are obtained for specific Ta–Al ratios during oxidation. Single phase cubic Ti 0.32 Al 0.60 Ta 0.08 N yields a mass-gain of only ~ 5% after 5 h at 950 °C in synthetic air, whereas Ti 0.35 Al 0.65 N is completely oxidized after 15 min. This is in part based on the suppressed anatase and direct rutile TiO 2 formation at a defined Ta–Al content. Consequently, the anatase-to-rutile transformation, generally observed for Ti 1−x Al x N, is absent. This reduces the generation of pores and cracks within the oxide scale and especially at the nitride–oxide interface, leading to the formation of a protective rutile and corundum based oxide scale. This is also reflected in the pronounced decrease in activation energy for the protective scale formation from 232 kJ/mol for Ti 0.35 Al 0.65 N down to 14.5 kJ/mol for Ti 0.32 Al 0.60 Ta 0.08 N. Based on our results we can conclude that especially phase transformations within the oxide scale need to be suppressed, as the connected volume changes lead to the formation of cracks and pores.

Thermal stability and oxidation resistance of sputtered TiAlCrN hard coatings

Abstract: In recent years, Ti Al Cr N coatings attract broad research interests owing to their excellent mechanical and tribological properties. Here, we study the thermal stability and oxidation resistance of two Ti1 − x − yAlxCryN coating series, having up to 47 at.% Cr (of the metal fraction, hence, y ≤ 0.47) for Al/(Ti + Al) ratios of ~ 0.55 and ~ 0.65. All as-deposited Cr-containing coatings are single-phase face-centered cubic (c) structured with hardness values between 32.0 and 33.9 GPa. For Cr contents below y N bonds) leading to a significant hardness reduction above 700 °C. Nevertheless, the oxidation resistance constantly increases with increasing Cr and Al content of our single-phased c-Ti1 − x − yAlxCryN coatings, because both favor the formation of a dense well-adherent oxide scale and retard the growth of Ti-rich scales. Therefore, our Ti0.18Al0.35Cr0.47N coating – exhibiting the best oxidation resistance among all coatings studied − only exhibits a 1.3-μm-thin oxide scale after 20 h exposure to ambient air at 900 °C.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

AlScN: A III-V semiconductor based ferroelectric

Abstract: Ferroelectric switching is unambiguously demonstrated for the first time in a III-V semiconductor based material: Al1-xScxN—A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator, and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in Al1-xScxN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 μC/cm2—which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from −0.3% to + 0.4% and a paraelectric transition temperature in excess of 600 °C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors—that of their spontaneous polarization.

High-entropy ceramics: Review of principles, production and applications

Abstract: High-entropy ceramics with five or more cations have recently attracted significant attention due to their superior properties for various structural and functional applications. Although the multi-component ceramics have been of interest for several decades, the concept of high-entropy ceramics was defined in 2004 by producing the first high-entropy nitride films. Following the introduction of the entropy stabilization concept, significant efforts were started to increase the entropy, minimize the Gibbs free energy and achieve stable single-phase high-entropy ceramics. High-entropy oxides, nitrides, carbides, borides and hydrides are currently the most popular high-entropy ceramics due to their potential for various applications, while the study of other ceramics, such as silicides, sulfides, fluorides, phosphides, phosphates, oxynitrides, carbonitrides and borocarbonitrides, is also growing fast. In this paper, the progress regarding high-entropy ceramics is reviewed from both experimental and theoretical points of view. Different aspects including the history, principles, compositions, crystal structure, theoretical/empirical design (via density functional theory, molecular dynamics simulation, machine learning, CALPHAD and descriptors), production methods and properties are thoroughly reviewed. The paper specifically attempts to answer how these materials with remarkable structures and properties can be used in future applications.