Maik Fröhlich

Rotating dust ring in an RF discharge coupled with a dc-magnetron sputter source. Experiment and simulation

During an experiment involving coating of dust grains trapped in an RF discharge using a sputtering dc-magnetron source, a rotating dust ring was observed and investigated. After the magnetron was switched on, the dust cloud levitating above the RF electrode formed a ring rotating as a rigid body. Langmuir probe diagnostics were used for the measurement of plasma density and potential. It was discovered that the coupling of the dc-magnetron source to the RF discharge causes steep radial gradients in electron density and plasma potential. The rotation of the dust ring is attributed to the azimuthal component of the ion drag force, which appears due to the azimuthal drift of the ions caused by crossed radial electric and axial magnetic fields. In order to get more insight into the mechanism of dust ring rotation, a Particle-in-Cell simulation of a rotating dust cloud was performed. The results of the experiment and simulation are presented and discussed.

Oxidation behaviour of TiAl-based intermetallic coatings on γ-TiAl alloys

Abstract TiAl-based intermetallic layers with additions of Ag, Cr, Y and Hf were deposited on γ-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated Ti-45Al-8Nb (at.%) alloy was investigated at 850 °C and 900 °C under cyclic oxidation conditions in air. The ternary Ti-50Al-2Ag (at.%) coating did not provide oxidation protection to the substrate material at 850 °C. Two-phase γ-TiAl + Ti(Cr, Al)2 Laves coatings exhibited high oxidation resistance at 900 °C for up to 1000 cycles of 1 h dwell time at high temperature. During thermal exposure phase dissolution and transformation occurred. The slow oxide growth rates observed for the Ti – Al – Cr intermetallic layers were associated with the formation of a thin continuous alumina top scale established by the Laves, B2 and Z-phases. Small additions of hafnium and yttrium improved the oxidation resistance. In contrast, Ti – Al – Cr – Ag coatings exhibited poor oxidation behaviour at 900 °C.

Thermally grown oxide scales on γ-TiAl coated with thermal protection systems

Abstract Thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on gamma TiAl samples using electron-beam physical vapour deposition. The specimens were coated with intermetallic Ti –Al – Cr layers and CrAlYN/CrN nanoscale multilayer coatings. The lifetime of the TBC systems was determined performing cyclic oxidation tests in air at temperatures between 850 and 950–C. The TBC systems with Ti –Al – Cr and CrAlYN/CrN layers did not fail at 850 and 900–C during the maximum exposure time period of 1000 cycles of 1 h dwell time at high temperature. No spallation of the thermal barrier coatings was observed. As revealed by post-oxidation microstructural analysis, the protective coatings were severely degraded when exposed at 900–C, resulting in growth of mixed oxides on the substrate. Underneath the thermal barrier coating an outer oxide scale with a columnar structure was observed, consisting of rutile and α-Al2O3. Energy-dispersive X-ray spectroscopy analysis revealed zirconia and chromia being dissolved in the outer oxide scale. The columnar structure and the presence of zirconia indicated an effect of the TBC on the morphology of the outer oxide scale. The zirconia top coat exhibited an excellent adherence to this oxide scale formed on the protective layers when degraded, and at defects like cracks. When thermally cycled at 950–C, the TBC system on specimens coated with Ti –Al – Cr failed by spallation of the thermally grown mixed oxides, whereas the thermal barrier coating was well adherent to the outer oxide scale at this temperature, too.

Investigation of a Commercial Atmospheric Pressure Plasma Jet by a Newly Designed Calorimetric Probe

A modified calorimetric probe for the investigation of a commercial atmospheric pressure plasma jet is presented. The design of the probe is adapted to the conditions provided by the plasma jet, in particular the high energy density and gas flow rates. The results of a parametric study are shown to prove the functionality of the newly designed probe and to characterize the plasma jet. Typical values of the energy influx during operation have been determined to 200-350 W/cm2 at a distance of 4-6 mm from the plasma jet. Moreover, the radial profile exhibits a high symmetry.

Calorimetric Probes for Energy Flux Measurements in Process Plasmas

This chapter gives an overview of the method of calorimetric probes which are used for characterizing the interaction between low-temperature plasmas and substrates in materials processing. Although the focus is on low-temperature non-equilibrium plasmas most of the concepts can also be transferred to thermal plasmas or are in fact adopted from fusion research. An introductory section showing the importance and complexity of plasma wall interactions is followed by a section providing an overview and comparison of various probe concepts, which have been developed in the last decades. Special focus is on the type of probes which are similar to the probes used by J.A. Thornton (In fact, Thornton was not the first, to use this type of probe. From his work from 1978 [1], one can follow the citations back to the work of Jackson in 1969, who used equation (6.7) for the determination of the power dissipated by a copper block located at substrate position in a sputtering discharge [2]) who was one of the pioneers connecting plasma characteristics with resulting surface properties. Thereafter, a short section gives a basic overview of the different contributions to the total energy influx and which are of importance for the plasma wall interaction. The last section shows different examples of applications of calorimetric probes. It demonstrates the applicability and flexibility of these types of probes for characterization of different low-temperature plasmas. The examples mainly focus on complex plasmas, where the variety of the involved processes also causes a specific plasma wall interaction. Such complex situations can be found in systems where reactive gases or gas mixtures, nano or micro particles (dust), magnetic fields or large surface to volume ratios are involved.

Influence of different annealing processes under various atmospheres on the oxidation behaviour of γ-TiAl

Abstract A detailed study of the oxidation behaviour of bare γ-TiAl based alloy Ti–45Al–8Nb under various conditions, such as different atmospheres, pressures, temperatures (900°C, 1000°C) and times (100–200 h) is presented. Under high vacuum conditions (10–6 mbar) a continuous zone of α-2Ti3Al was formed at the surface with an oxygen-enriched phase on top. No oxide scale formation was obvious. During thermal treatment under Ar-atmosphere at low vacuum pressure (approximately 50 mbar) mainly nitrides (TiN, Ti2AlN) and Al2O3 particles were formed at the surface with an α-2layer below. Annealing γ-TiAl in hydrogen atmosphere (about 1040 mbar) led to the formation of a thick reaction zone. A TiO2 layer was formed on top, followed by a mixed oxide scale. Beneath that scale a thick region with alumina, σ–Nb2Al and α-2Ti3Al was observed. Moreover, the oxidation behaviour of several thermally pre-treated samples was tested by cyclic oxidation at 900°C in air. The microstructure of the oxide scale formed after testing can be compared with that of non pre-treated material. γ-TiAl annealed under high vacuum conditions exhibits the lowest oxidation rate, while the mass gain of specimens pre-treated under Ar-atmosphere increased rapidly in the first cycles. All pre-treated specimens exceeded a lifetime of 600 cycles at minimum. The reference material failed after 520 cycles.

Oxidation and Decomposition of Ti2AlN MAX Phase Coating Deposited on Nickel-Based Super Alloy IN718

In this work, the oxidation behavior of Ti2AlN coating deposited on nickel super alloy IN718 was investigated. The coating was obtained by DC-magnetron sputtering at 540°C and subsequent vacuum annealing at 800°C for 1h. The coating morphology as well as the chemical composition were analyzed using SEM, EDS and XRD, respectively. The XRD results revealed that the coating mainly composed of Ti2AlN MAX phase. Cycling oxidation was performed at 700 °C and 800 °C in air. The XRD and SEM results proved the interaction between substrate and coating and the formation of the quaternary Ti3NiAl2N phase during oxidation at the interface. Due to the Ni diffusion towards the surface, the Ti3NiAl2N phase grew continuously and the Ti2AlN phase decomposed gradually resulting in a coating failure. The results indicate that the oxidation behavior of the coating is essentially controlled by the interdiffusion of Ni from substrate into the coating.