H. Riedl

Thermal expansion of rock-salt cubic AlN

We combine continuum mechanics modeling and wafer curvature experiments to characterize the thermal expansion coefficient of AlN in its metastable cubic rock-salt (B1) structure. The latter was stabilized as nm thin layers by coherency strains in CrN/AlN epitaxial multilayers deposited on Si (100) substrates using reactive magnetron sputtering. The extraction of the B1-AlN thermal expansion coefficient, from experimentally recorded temperature dependent wafer curvature data, is formulated as an inverse problem using continuum mechanics modeling. The results are cross-validated by density functional theory calculations.

Thermal conductivity and mechanical properties of AlN-based thin films

While many research activities concentrate on mechanical properties and thermal stabilities of protective thin films, only little is known about their thermal properties being essential for the thermal management in various industrial applications. Based on the 3ω-method, we show the influence of Al and Cr on the temperature dependent thermal conductivity of single-phase cubic structured TiN and single-phase wurtzite structured AlN thin films, respectively, and compare them with the results obtained for CrN thin films. The dc sputtered AlN thin films revealed a highly c-axis oriented growth for deposition temperatures of 250 to 700 °C. Their thermal conductivity was found to increase strongly with the film thickness, indicating progressing crystallization of the interface near amorphous regions during the sputtering process. For the 940 nm AlN film, we found a lower boundary for the thermal conductivity of 55.3 W m-1 K-1. By the substitution of only 10 at. % Al with Cr, κ significantly reduces to ∼5.0 W m...

Composition driven phase evolution and mechanical properties of Mo–Cr–N hard coatings

Although many research activities concentrate on transition metal nitrides, due to their excellent properties, only little is known about Mo-N based materials. We investigate in detail the influenc ...

Oxidation behavior and tribological properties of multilayered Ti-Al-N/Mo-Si-B thin films

The impact of highly oxidation resistant Mo1−x−ySixBy on the oxidation resistance and tribological properties of arc evaporated face-centered-cubic Ti0.57Al0.43N is investigated in detail through Mo1−x−ySixBy/Ti0.57Al0.43N and Mo1−x−ySixByN/Ti0.57Al0.43N compound coatings as well as Ti0.57Al0.43N/Mo1−x−ySixBy multilayer coatings. The Ti0.57Al0.43N/Mo0.54Si0.30B0.16 and Ti0.57Al0.43N/Mo0.68Si0.12B0.20 multilayers with bilayer periods of 35 and 70 nm (consisting of 6 and 15 nm thin sputtered Mo0.54Si0.30B0.16 and Mo0.68Si0.12B0.20 layers), respectively, significantly outperform all coatings investigated during 900 °C ambient-air-oxidation treatments. Consequently, the oxidation resistance of the multilayer coatings is higher as compared with their individual layers. Even after 1000 min exposure, only ∼1.0 μm of the Ti0.57Al0.43N/Mo0.68Si0.12B0.20 multilayer is consumed to form a dense adherent and protective oxide scale. The latter is composed of Si-, Al-, Ti-, and Al-rich layered outer oxides preventing th...

Ab initio studies on the adsorption and implantation of Al and Fe to nitride materials

The formation of transfer material products on coated cutting and forming tools is a major failure mechanism leading to various sorts of wear. To describe the atomistic processes behind the formation of transfer materials, we use ab initio to study the adsorption energy as well as the implantation barrier of Al and Fe atoms for (001)-oriented surfaces of TiN, Ti0.50Al0.50N, Ti0.90Si0.10N, CrN, and Cr0.90Si0.10N. The interactions between additional atoms and nitride-surfaces are described for pure adhesion, considering no additional stresses, and for the implantation barrier. The latter, we simplified to the stress required to implant Al and Fe into sub-surface regions of the nitride material. The adsorption energies exhibit pronounced extrema at high-symmetry positions and are generally highest at nitrogen sites. Here, the binary nitrides are comparable to their ternary counterparts and the average adhesive energy is higher (more negative) on CrN than TiN based systems. Contrary, the implantation barrier ...

Ab initio inspired design of ternary boride thin films

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.

Effect of Mo on the thermal stability, oxidation resistance, and tribo-mechanical properties of arc evaporated Ti-Al-N coatings

The general tendency in industrial machining and forming operations toward further reduced operation time and increased efficiency requires continuous improvements of protective coatings regarding their tribo-mechanical properties. Alloying arc evaporated Ti1-xAlxN with small amounts of molybdenum (Mo) has shown very promising results to further improve the tribological properties of the industrially widely used Ti-Al-N. Therefore, the authors study in detail the effect of Mo on the thermo-mechanical properties of Ti1-xAlxN coatings, as well as their wear behavior at elevated temperatures. First principle density functional theory (DFT) calculations are conducted for face-centered cubic c-Al1-yMoyN, c-Ti1-yMoyN, and c-Ti1-x-yAlxMoyN showing increasing energy of formation with increasing Mo incorporation for all structures investigated. However, the thereby (DFT) obtained/calculated lattice parameters for comparable compositions perfectly matches with those of the single-phase face-centered cubic structure...