ICMCTF2015 Session TSP: Symposium TS Poster Session

The natural brittleness of oxide ceramics heavily inhibits their more extensive applications. By employing plasma electrolysis oxidation to in-situ grow alumina layers on Al foil, a highly flexible Al₂O₃/Al/Al₂O₃ hybrid composite was fabricated with a ductile Al substrate and an outside layer of nanostructured polycrystalline oxide ceramic of which the ceramic part is composed of nanosized grains with the size less than 20 nm. Due to shear band formation, nanosized circle bubbles prolonging the crack path, grain rotation and deformation, the fabricated Al₂O₃/Al/Al₂O₃ hybrid composite contains no observable cracks even after being bent on a cylindrical bar with a curvature of 1.5 mm. The composite exhibits alumina stiffness at the elastic stage and aluminum ductility during plastic deformation, which provides high flexibility with the well-integrated properties of the components. In a synergistic interaction, the alumina on the outside exhibited a strain of 0.33% at room temperature, which was higher than optimum value of 0.25% presented by reported most flexible oxide ceramic. With the unique characteristics and properties, the Al₂O₃/Al/Al₂O₃ composite demonstrates a great potential for engineering applications in various industries.

Biofilms are referred as microbial populations in a matrix of extracellular polymeric substances (EPSs) adhering to substrates. Many industries (e.g. maritime, water systems, dentistry and hospitals) suffer from the biofilm growth which can result in significant costs in cleaning and maintenance. On the other hand, biofilms can be useful in removing contaminants (e.g. metals, oil spills and nitrogen compounds) and for the purification of industrial waste water.

The mechanical properties of biofilms determine their resistance to stress and eventual dispersal mechanism. Similar to many other biological materials, biofilms also exhibit viscoelastic behaviour. To assess the mechanical properties of the intact soft biofilms, nanoindentation is one of the best techniques to use. For the motile bacteria growing at hydrodynamic environment, the typical biofilms are truncated ellipsoidal and mushroom-like shapes. Due to the complex morphology, it is not easy to obtain the reliable mechanical properties of the biofilms. Therefore, finite element modelling was adopted to study how the morphology of the biofilms would affect the determination of their viscoelastic properties.

A comprehensive characterization of the Nb_(1-x)Ti_xN_y films is performed in order to shed light on the interrelated physicochemical and structural properties of these materials. The ternary Nb_(1-x)Ti_xN_y (with 0≤x≤1 and 0.9<y<1.1) have been deposited onto Si substrates at temperatures around 400 °C by using reactive magnetron sputtering. The analysis includes XRD, AFM, TEM, SEM, EPMA, XPS, nanohardness and optical and electrical measurements. In this paper we report on the influence of chemical and structural properties on the electronic properties of Nb_(1-x)Ti_xN_y films. The optical properties have been investigated by specular reflectivity and spectroscopic ellipsometry, and the electrical resistivity was measured by the van der Pauw method from 7 K to 300K. In the ternary nitrides the fcc rocksalt phase is the only phase observed. The dielectric function of these nitrides exhibits high sensitivity to the phase composition and in particular to the nitrogen content in the films allowing a complete and consistent description of the evolution of the Nb_(1-x)Ti_xN_y films as a function of the chemical composition.

Since 2010 the Institute for Electrical Engineering and Plasma Technology at the Ruhr-University Bochum has been investigating a large-area, capacitively coupled multi frequency plasma source (MFCCP) for deposition of ceramic layers.

The advantages of CCPs over the established magnetrons consists of a nearly 100% target utilization, a weak target poisoning, application of isolating (e.g. ceramic) and magnetizable (ferromagnetic) targets, a good homogeneity of the deposited layers on large areas and the capability for a separate control of the ion energy and the ion flux.

The main research objective is to understand the coherences between the plasma process parameters respectively plasma parameters and the properties of the deposited layers.

The MFCCP is powered by three frequencies 13.56 MHz, 27.12 MHz and 60 MHz. The two lower frequencies are driven in a phase locked mode with an adjustable phase shift. By varying the phase shift the dc self-bias respectively the ion energy onto the target can be changed very accurate without effect on the ion flux. The applied voltage at 60 MHz generates a high plasma density.

In recent investigations process parameters (e.g. dc selfbias) respectively plasma parameters (e.g. electron density, temperature, ion energy distribution) and its influence to the properties of aluminium nitride layers (e.g. thickness, stress) are examined by using various plasma diagnostics and coating analytics.

Particularly the impact of the ion energy on the growing Al and AlN-layers is studied by measuring the ion energy distribution (IED) with a retarded field energy analyser at different positions on the substrate electrode. The measurement is divided in three steps: First of all there are ion energy measurements in the MFCCP only with the classical frequency to get a first idea of the system. In the second step the plasma is additional driven with the second harmonic of the classical frequency to investigate the IED by using the electrical asymmetry effect. Finally the measurements are done by using all three frequencies of the MFCCP to obtain the plasma at higher electron density and ion flux. After all the results will be correlated with the Al and AlN-layer properties (e.g. thickness, stress).

The MFCCP is a sub-project of the collaborative research centre SFB-TR 87 funded by the German Research Foundation (DFG). We thank Dr. Carles Corbella and Prof. Dr. Julian Schulze for supporting the IED measurements and the inspiring discussions. We also thank the Institute of Materials (coating analytics) at the Ruhr-University Bochum for providing success for presenting the corporate results.

Dye-sensitized solar cells (DSSCs) have been intensively studied in many laboratories due to their relatively low cost, high light-to-electricity conversion efficiency, and easy fabrication compared to silicon based solar cell. Generally, DSSCs are composed of as parts titanium dioxide (TiO₂) which carries an anchored organic dye on working electrode layer, counter electrode layer made of Pt and an electrolyte containing a redox couple (I^-/I₃^-) between them.

However, a number of problems remain to be solved in order to improve their efficiency. In particular, one of the main limiting factors is the electron recombination that occurs due to contact between the transparent conductive oxide and the redox electrolyte.

For high efficiency solar energy conversion, the photoelectrode must facilitate effective light harvesting, electron injection, and reducing recombination of the photoinjected electrons. To avoid recombination, one of the strategies is to introduce a ZnO passivating layer to the FTO conducting glass/TiO₂ interface. Several researches have introduced ZnO to reduce the recombination at electrode/electrolyte interface as having a more negative conduction band edge than TiO₂. In addition, 1-dimensional TiO₂ nanostructures have attracted need to avoid the limited electron transport in the nanocrystal boundaries of TiO₂ nanoparticles and the electron recombination with the electrolyte during the electron migration process.

And, one method of increasing the efficiencies of DSSCs is to enhance their ability to harvest light. Commercially available dyes (N-719) only absorb in the wavelength range of 400–800 nm, and most of the ultraviolet region is not used. If ultraviolet radiation could be converted to visible light by a suitable conversion-luminescence process and reabsorbed by the dye of a DSSCs, a larger part of the solar irradiation could be used and the photocurrent of the DSSCs will be effectively enhanced.

Thus, we fabricated hybrid photoelectrode of TiO₂ nanotube/phosphor mixed TiO₂ nanoparticles on ZnO passivating layer in terms of dye adsorption, charge transport and reducing the electron recombination.

In this paper, we described the improvement in the photovoltaic characteristics of dye-sensitized solar cells by using a ZnO passivation layer, TiO₂ nanotube/phosphor mixed TiO₂ photoelectrode. The crystalline structure and morphology were characterized by X-ray diffraction and using a field emission scanning electron microscope. The absorption spectra were measured using an UV-Vis spectrometer. The incident photocurrent conversion efficiency was measured using a solar simulator (100 mW/cm²).

In this research, the behavior of iron boride growth and adhesion of the coating formed on the surface of AISI 4140 steel by thermochemical treatment boriding, boride with paste, boxed dehydrated is studied temperature of 1273 °K and 1223 °K, with an exposure time of 2 to 0.5 h. Adhesion was evaluated by the Rockwell C technique prescribed by the German standard VDI 3198 adhesive. By light microscopy the morphology and growth rate of layer FeB / Fe₂B on the material surface, by exposure to boriding paste is observed. The presence of phases of boride FeB / Fe₂B were determined by X-ray diffraction (XRD). The technique of scanning electron microscopy (EDS) shows the distribution of the alloying elements on the FeB / Fe₂B / Subtrato layer formed on the surface of the material to study. The elastic modulus and hardness were obtained with the temperatures of 1273 °K and 1223 °K, with a time of 2 to 0.5 h for each temperature, nanoindentation technique on FeB / Fe₂B layers formed on the steel surface. Finally, this study determined the type of delamination layer / substrate, the distribution of the alloying elements on the FeB / Fe₂B / phases and mechanical properties FeB / Fe₂B AISI 4140 steel boriding, paste dehydrated boron. This study aims to contribute to the growth, morphology and behavior of boride formed in the boriding steel with dried paste.

Metal silicide thin films are of technological relevance in silicon-based microelectronics devices, as they are largely used as ohmic contacts, Schottky barrier contacts, gate electrodes, or diffusion barriers. The mechanisms of silicide formation at the metal-silicon interface are relatively complex and depend on the nature of the metal as well as substrate orientation. Pd-Si is a model system because of the formation of a single silicide phase, Pd₂Si, below 700°C. Extensive studies have been carried out to identify the dominant diffusing species and study the growth kinetics as well as stress development/relaxation processes during solid state reaction; however, the growth dynamics and stress build-up during thin film deposition of Pd on Si remains still largely unexplored.

In the present work, we have used in situ and real-time multiple beam optical stress sensor (MOSS) and four-probe resistivity measurements, combined with ex-situ (XRD, AFM, TEM) structural characterization, to investigate the early growth stages of sputter-deposited Pd thin films. The nucleation conditions were varied by depositing the Pd films on Si(001) wafers covered either by native SiO₂ (~2 nm), amorphous a-Si (2 to 20 nm) or chemically etched with dilute HF. We also deposited Pd/Si and Pd/Ge multilayers to highlight the role of the interface. The influence of substrate temperature, up to 200°C, will be also discussed.

From the non-monotonous stress evolution recorded at room temperature (RT), different growth regimes have been evidenced: a 3D growth mode (islands nucleation and coalescence) for films grown on SiO₂, and the formation of interfacial layer prior to the steady-state growth of pure Pd for films grown on a-Si. In all cases, the Pd films are strongly (111)-textured, the lowest mosaicity (2.2°) being surprisingly obtained on a-Si. XRD investigations on (a-Si/Pd)_n [resp. (a-Ge/Pd)_n] multilayers reveals that the interfacial layer corresponds to crystalline Pd₂Si [resp. Pd₂Ge] phase of ~2-3 nm thickness when deposition occurs at RT. At higher substrate temperature, competition between interfacial silicide formation and growth of fcc Pd takes place, the former process being limited by the initial thickness of a-Si layer.

Plasma electrolytic oxidation (PEO) is a modern knowledge-based technology for applying protective coatings on surfaces of light metals and their alloys. The PEO process is held at high voltages (400-1000V) which results in the coatings with high wear and corrosion resistance. To design PEO process control system, it is necessary to solve the diagnostics problem for the coating thickness; this is a challenge because a direct measurement of the surface layer properties during the treatment using conventional techniques is infeasible.

In this paper a solution for this problem is proposed based on the analysis of microdischarge video image processing and the process modelling. On the first stage using multifactorial experimental design a set of experimental data regarding the coating thickness, surface roughness and microdischarge images was acquired for DC PEO in the voltage range from 450 to 600V. With a help of a general regression neural network F₁ a connection between the output characteristics and the input variables (U, t) was established. An algorithm for microdischarge image processing and assessing the statistical characteristics of their population was developed and implemented into a software. The software allows automatic identification of the microdischarge location and size, as well as calculation of statistical characteristics of their population. With the help of neural network F₂ a connection between the microdischarge characteristics, inputs (U, t) and image exposure was formalized. It was shown that for the surface state diagnostics it is enough to use histogram characteristics: microdischarge population density and average size of micro-discharges.

According to the analysis of change in the characteristics of microdischarge populations, two ways to diagnose the coating thickness during PEO process were developed. The first method is based on the application of an integral information parameter, which does not depend explicitly on the voltage and time, but uses the properties of the microdischarge population. This parameter has a physical meaning of the surface covered by microdischarges, integrated over time. The coating thickness is further calculated from piecewise linear calibration curves. The second method is based on the application of neural network F₃ transforming the set of informative parameters of microdischarges into the set of the coating thickness and surface roughness.

It was shown that the developed methods can be used for indirect measurements of the coating thickness during plasma electrolytic oxidation, and applied in the PEO process control system with a new feedback loop considering the surface state parameters.

We present our recent progress in utilizing calorimetric probes for plasma diagnostic purposes in thin film deposition. The developed technique - called the transient method – allows for a faster calibration of the probe and energy flux measurements as well as for simultaneous and more precise determination of plasma parameters like plasma and floating potential and plasma density [1].

By combining the conventional method of passive calorimetric probes [2] with the well-established electrostatic probe, it is now possible to build a model of the energy influx and its contributions by electrons, ions, recombination and other surface processes.

The presentation includes the basic principle of calorimetric measurements as it was already used by J.A. Thornton in the 1970’s [3]. By addressing the drawbacks of the conventional method, we developed a method to shorten the time needed for the calibration process, which is necessary for a quantitative determination of the total energy influx from plasmas towards a surface. By using this procedure in a capacitively coupled RF plasma using Argon as process gas we demonstrate the applicability in a plasma environment. Argon was chosen in order to keep the number of different contribution to the integral energy influx small. However, the method can also be used in more complex situations, e.g. magnetron sputtering. In this case contributions from the neutral sputtered particles as well as the film formation itself have to be included.

Plasma parameters are determined from the electrical currents as it is typical for electrostatic probes. By using the same probe for current end energy flux measurements we avoid discrepancies caused by different geometries and conditions. Using the currents and the plasma parameters, one can calculate the various contributions caused by the different species and surface processes easily. Comparing the sum of all calculated contributions with the energy influx determined from the temperature curve obtained during the probe voltage sweep allows for verification of the model.

[1] S. Bornholdt and H. Kersten, Eur. Phys. J. D. 67(8):167 (2013)

[2] S. Bornholdt et al, Surf. Coat. Tech. 205:388-392 (2011)

In dental prostheses, metallic alloy and all ceramic copings, e.g. of Yttria Partially Stabilised Zirconia (YPSZ) are coated with feldspathic porcelain veneers to deliver an aesthetically pleasing finish and to produce a surface of similar hardness to human teeth. A prominent failure mode of such coatings is associated with the chipping of the near-interface porcelain. Studies into failure at the porcelain-YPSZ interface have revealed that there is a complex interplay between tempering, phase transformation and coefficient of thermal expansion mismatch at this location. Recent Transmission Electron Microscopy has also provided evidence that the interaction between tensile stresses and high temperatures at the interface region are sufficient to induce creep in the porcelain coating at sub-micron length scales. Several previous studies have focused on the bending and compression creep rate behaviour in silica-alumina ceramics. These fail to account directly for the importance of pure tensile loading that is likely to induce micro-cavitation and promote fracture. We report the development of a new experimental technique capable of loading a specially designed tensile creep specimen. Tensile creep rate analysis was performed on porcelain samples made from Cerec Mark II Vitablocs over a range of temperatures (between 650°C and 800°C) and stresses (50MPa to 125MPa) that are similar to the conditions that arise during veneer sintering. The steady state power law creep rate equation was found to be representative of the strain behaviour observed. The activation energy was evaluated to be equal to 243.4 ± 3.3 kJmol^-1 and the stress exponent ranged from 1.24 to 1.52 across the range of temperatures. This variation may be an indicative of varying stress sensitivity at different thermal energy levels. The results provide an important quantified basis for numerical modelling of the temperature dependent stress evolution in porcelain veneers due to creep.

Hard and wear-resistant coatings based on the system aluminium and chromium represent the state of the art in the protection of tools used in metal cutting operations. Using physical vapour deposition techniques, nitrogen and/or oxygen can be added to the plasma to synthesise nitrides, oxides or oxynitrides. A frequently employed method is cathodic arc deposition which is characterised by a high degree of ionisation, enabling an optimisation of the growth condition. In addition high deposition rates can be achieved which reduces the process time and, hence, the production costs. Multi-element cathodes containing both elements are employed to facilitate an easier process control and reproducibility. However, the plasma conditions in the cathodic arc plasma using multi-element cathodes and the erosion behaviour of theses cathodes in the different gaseous atmospheres are scarcely studied.

In the present investigation, AlCr composite cathodes with composition of 75/25, 50/50 and 25/75 at.% were exposed to a cathodic arc plasma in Ar, N2 and O2 atmosphere. Due to periodic heating and cooling of the cathode’s near-surface region in the cathode spots, an intermixing of the elements Al and Cr occurred and was analysed by cross-sectional analysis using focused ion beam and scanning electron microscopy. Recording the spatial distribution of the elements revealed regions of virgin Cr grains embedded in the Al matrix as well as regions of partial or complete intermixing of both elements. Due to exposure of the cathode surface to nitrogen or oxygen atoms or molecules, nitride and oxide phases were noticed which is typically referred to as cathode poisoning. The results regarding the cathode erosion are put in context with recently reported ion charge state distributions and ion energy distribution functions obtained with the same AlCr cathodes and gas atmospheres [1, 2].

References:

[1] R. Franz, P. Polcik, A. Anders; IEEE Trans. Plasma Sci. 41 (2013) 1929–1937

[2] R. Franz, P. Polcik, A. Anders; Surf. Coat. Technol. (2015) under review, arXiv:1412.5772