Dagmar Dietrich

Influence of composition and structure on the mechanical properties of BCN coatings deposited by thermal CVD

BCN films were deposited by isothermal chemical vapour deposition from gaseous mixtures of trimethylborazine, toluene and ammonia. The films were analysed with respect to chemical state, composition, morphology and microstructure on the one side oxidation behaviour and hardness on the other side. X-ray spectroscopy (WDX), Raman and infrared spectroscopy, differential thermal analysis, X-ray diffraction and transmission electron spectroscopy were employed for film characterization. A microhardness of maximum 20 GPa was achieved, affected by carbon content, by the way of its incorporation into the hexagonal turbostratic lattice as well as by the crystallite size and its texture.

CVD-coated boron nitride on continuous silicon carbide fibres: structure and nanocomposition

Abstract Boron nitride is isoelectronic with graphite, has a similar bonding structure, but a stronger localization of the π-states, and a better oxidation resistance. Therefore, it is a promising alternative to pyrolytic carbon as a tool for tayloring special fibre/matrix interlayers in composites for gas turbine and aircraft applications. To understand the CVD processing via a boron-organic precursor and to improve the thermomechanical and hydrolytic stability of the BN fibre coatings, we focussed our investigations on the question: “In what way do carbon and oxygen become incorporated into the BN layer?” HREM studies showed the turbostratic structure of the BN, forming cells of 5–10 nm in diameter. Energy filtered electron microscopy (EFTEM) and the nanometre resolved analysis of the electron energy-loss near edge structure (ELNES) of the individual elements revealed a correlation between microstructure and chemical composition. There is a general deficiency of 10–15% of nitrogen with respect to boron, which is partly compensated for by contents of up to 10 and 15% of oxygen and carbon, respectively, in the layer. It could be concluded that most of the carbon is precipitated in between the BN cells, and a smaller amount is incorporated within the hexagonal BN structure.

Preparation of boron nitride thin films by microwave PECVD and their analytical characterisation

Hexagonal boron nitride films on Si(100) substrates are formed from the system N2/B(OCH3)3 (TMOB) by microwave plasma enhanced chemical vapour deposition (PECVD). The parameters substrate temperature, deposition time and composition of the process gas mixture were varied. The stability of the deposited layers mainly depends on the substrate temperature during the deposition process. Layers formed below 650°C showed strong decomposition features by contact with air humidity. Above this temperature very stable layers could be produced. This was confirmed by hydrolysis tests. The structural growth of the boron nitride layers was investigated by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Three different growth zones were observed within the layers. The composition was determined by electron probe microanalysis (EPMA) and elastic recoil detection analysis (ERDA). Infrared and Raman spectroscopy were used for qualitative investigation of the BN layers.

Raman spectroscopy on Psaronius sp.: a contribution to the understanding of the permineralization process

Samples of the permineralized tree fern Psaronius sp. from the Lower Permian Rotliegend of Chemnitz were examined to find out their chemical composition and structural behavior. The inhomogenic distribution of elementary constituents of oxides, sulfates and carbonates within the silica-matrix consisting of α-quartz and chalcedony were formerly detected by analytical X-ray microscopy. Most of the compounds could be revealed by Raman spectroscopy. Calcite and barite developed in small fissures. Iron of different oxidation level is the prevailing pigment in the agate preserved adventitious roots. Coalified cellulose whose structure mainly corresponds to anthracite reflects the anatomical detail of the former organic tissues.

Electroplated Nickel Composites with Micron- to Nano-Sized Particles

Electroplated nickel coatings provide ductility, excellent corrosion resistance and good wear resistance, which qualifies them to meet complex demands of engineering, microtechnology and microelectronics. The co-deposition of particles is a promising alternative to deposit layers with adequate microstructure and properties avoiding the rise of residual stress. The incorporation of the sufficient quantity of particles, monodisperse distribution and downsizing to nanometre scale affect the amount of strengthening by dispersion hardening. To avoid agglomeration in the electroplating bath as well as in the layer is a challenge which has been met by simple Watts nickel electrolyte with a minimum of organic additives and adequate bath agitation comprising sonication, i.e. the exposure of the bath to high-frequency sound waves. Well-dispersed hard particles (titanium oxide and silicon carbide) were incorporated in nickel films. The focus was set on the correlation between the gained microstructure of the composites with particles from micron to nanometre scale and the electrochemical and mechanical properties. Corrosion was quantified from polarisation curves and volumetric erosion measurements. Wear resistance was evaluated by scratch energy density studies, oscillating sliding wear testing and cavitation wear testing and compared to indentation hardness results. Sonication and particle downsizing result in matrix grain refinement and dispersion hardening. Incorporation of different particles with respect to different material and size proved to meet different demands. Submicron TiO2 is best for high corrosion resistance, sonicated nickel without particle incorporation is best for high abrasion resistance, nano TiO2 is best for oscillating sliding wear resistance and submicron SiC is best for cavitation wear resistance.

A microstructure study on silicified wood from the Permian Petrified Forest of Chemnitz

Three typical plant taxa from the fossil assemblage of the 290-million-year-old Chemnitz Petrified Forest (Zeisigwald Tuff Horizon, Leukersdorf Formation) were studied with regard to the microstructure of the petrifactions: samples of the tree fern Psaronius sp., the seed fern Medullosa stellata, and the gymnosperm Dadoxylon sp. The plant’s tissues are anatomically preserved by silica exhibiting different crystalline order and by other mineralisations. Specimens were studied by means of electron backscatter imaging and electron backscatter diffraction in a scanning electron microscope. The cell walls were largely preserved by quartz crystals, the cell lumina by cryptocrystalline silica. The former organisation and chemical composition of the vascular tissue are mirrored by varying grain formation and grain size. Results are discussed in terms of extant xylem cell wall organisation showing highly hydrophilic cellulose and hemicellulose cross-linked by hydrophobic lignin. The effect of polar and non-polar wood components on the precipitation of silica from aqueous solution and on the formation of crystals is convincing, and the reported results provide a better understanding of how silica replaced organic matter during the petrifaction process.KurzfassungDrei typische Kieselhölzer des 290 Millionen Jahre alten Versteinerten Waldes von Chemnitz (Zeisigwald Tuff-Horizont, Leukersdorf-Formation) wurden hinsichtlich ihrer Mikrostruktur untersucht: Proben des Baumfarns Psaronius sp., des Farnsamers Medullosa stellata, und des Nacktsamers Dadoxylon sp. Mittels Rückstreuelektronen-Abbildungen und Beugungsuntersuchungen im Rasterelektronenmikroskop wird die vorzügliche anatomische Erhaltung des Pflanzengewebes durch Silifizierung und andere Mineralisationen gezeigt. Die Zellwände weisen zumeist Quarz in gut ausgebildeten Kristallen auf, während die eingeschlossenen Zelllumina durch kryptokristalline Siliziumdioxid-Varietäten konserviert wurden. Die unterschiedliche Kristallinität sowie die Anordnung und Größe der Kristallite spiegeln den ursprünglich vorhandenen Aufbau der Zellwand und deren chemische Zusammensetzung im Holzteil wider. Die Ergebnisse werden mit Bezug auf die Zellwandorganisation des Xylems rezenter Pflanzen diskutiert, das aus stark hydrophiler Zellulose und Hemizellulose besteht, die durch hydrophobes Lignin vernetzt sind. Der Einfluss der polaren und unpolaren Holzbestandteile auf die Ausfällung von Silica aus einem wässrigen Sol und auf die Kristallisation ist auffallend. Im Umkehrschluss ermöglichen die vorgestellten Ergebnisse ein vertieftes Verständnis, wie während des Versteinerungsprozesses die organischen Bestandteile silifiziert wurden.

PETRIFACTIONS AND WOOD-TEMPLATED CERAMICS: COMPARISONS BETWEEN NATURAL AND ARTIFICIAL SILICIFICATION

Fascination with petrified wood has stimulated interest in understanding the process of natural petrifaction. Early attempts of modeling natural petrifaction in the laboratory have been limited to mimicking incipient permineralization resulting in the creation of silica casts of pore spaces and inner cell walls. Silica lithomorphs produced through artificial silicification provided a possible avenue for studying microstructure of wood. More recently artificial petrifaction is motivated by the goal of creating advanced ceramic materials for engineering applications. The concept of using wood as a biotemplate has led to the creation of porous ceramics by cell wall replacement. To some extent artificial and natural petrifaction processes are comparable; although, some of the materials and procedures used in the laboratory are not found in nature. Research focused on the composition and structure of fossil wood from different-aged deposits is compared with research focused on the development of wood-templated porous ceramics. Differences and similarities in the pathways of natural silicification and creation of biomorphous ceramics are discussed. The comparison between artificial and natural silicification highlights the particular significance of the degree to which (de)lignification is needed for silica permeation.

EBSD und STEM an hochgradig plastisch verformten Aluminiumlegierungen

Abstract After aluminum alloys undergo severe plastic deformation there is an interest in what grain refining is obtained and is determined from the orientation data detected from backscatter electron diffraction in lateral resolution. The quality of backscatter diffraction depends on the fraction of successful indexings, which presupposes an appropriate preparation of the surface. Vibration polishing results in a matrix nearly free of deformation and excellent levelling of the precipitates whereas relief formation (often occurring in oxide polishing) and dissolution (often occurring in electro-polishing) of those phases will reduce the band contrast and indexing quality. The results thus obtained from backscatter electron diffraction are backed by scanning electron microscopy images in transmission.

Transmission electron microscopic investigations on SiC- and BN-coated carbon fibres

Carbon fibres were coated with layers of silicon carbide (SiC) and boron nitride (BN) by conventional chemical vapour deposition. The SiC films were deposited by thermal decomposition of methyltrichlorosilane, whereas the BN films were deposited using the stepwise disproportion reaction of boron chloride with ammonia. Samples for electron microscopic investigations were prepared by separating film from fibre or by conventional mechanical thinning and subsequent ion milling of cross sections of coated fibres. Bright- and dark-field images of both planar and cross-sectional electron microscopic investigations on the fibre coatings gave detailed information on film thickness and morphology. High-resolution images improved the structural information of electron diffraction patterns. Crystal dimensions in the SiC film vary between 10 and 40 nm. Electron diffraction revealed the crystal structure to be a mixture of disordered hexagonal 2H-SiC and cubic β-SiC. High-resolution images showed the (1 1 1)-planes to be preferred for deposition. In BN films, a hexagonal turbostratic structure similar to turbostratic carbon was observed. Apart from amorphous regions, nanocrystalline parts were detected, which have a higher structural perfection in the stacking sequence of their (0 0 2)-planes compared to the (0 0 2)-planes of the turbostratic carbon fibre. High-resolution images located the film-fibre interface that was confirmed by electron energy loss spectroscopy.

Microstructural evolution in the bonding zones of co-extruded aluminium/titanium

In the present study, the interfacial microstructure of dissimilar Al/Ti joints formed by a co-extrusion process has been investigated. The material combinations used for the experiments were commercially pure aluminium and titanium, respectively, in the alloys EN AW-6082 and TiAl6V4. X-ray diffraction, energy-dispersive X-ray spectroscopy, electron microscopy and electron backscatter diffraction revealed titanium aluminide formation in the interface, the development of deformation bands in the sleeve material, and the occurrence of grain size refinement and twinning in the core material. The results are discussed with respect to the concept of a hard core–soft sleeve co-extrusion comprising plastic deformation to promote solid-state diffusion. The study shows the phase distribution and the microstructural evolution in the bonding zone with the aim to improve the quality of the bonding by adequately adjusting the process parameters.