Thomas Lampke

Interface behaviour in nickel composite coatings with nano-particles of oxidic ceramic

Advances in micro-technology demand that new functional materials be developed so that the technical properties of micro-devices can be improved at reasonable cost. The co-deposition of nanoscaled particles during an electroplating process has been shown to bring such an improvement. This work focuses on particles of oxidic ceramics, in this case those of Al2O3 and TiO2. The diameters of the primary particles ranges from 10 to 30 nm, electrodeposited by means of a conventional Watts nickel electrolyte. A series of nickel nano-ceramic composites were produced, with co-deposition of particles as a single primary particle in the nanometre range at one end of the scale and as agglomerates up to a size of a micrometer at the other. The influence of the presence of particles on crystallisation behaviour, residual stress and texture of the deposited nickel coatings was examined by X-ray diffraction (XRD). There is a report on the interfaces between the nickel grains and the oxidic ceramic particles, investigated using transmission electron microscopy (TEM). A decreasing corrosion stability indicates an attack along the interface nickel/particles.

Methods to determine surface energies of natural fibres: a review

To tailor the interaction across composite interfaces especially for the development of green composites, i.e. composites made completely from renewable materials, information about the fibre surfaces is required. We review the current state of the art of methods to determine the surface tension of natural fibres and discuss the advantages and disadvantages of techniques used. Although numerous techniques have been employed to characterise surface tension of natural fibres, it seems that commonly used wetting techniques are very much more affected by the non-ideal character of natural fibres. Inverse Gas Chromatography (IGC) is a much better suited technique to determine the surface energetic properties of natural fibres than wetting techniques. The surface tension of natural reinforcements, determined using IGC, was reported for nanosized bacterial cellulose as well as bamboo, cornhusk, flax, hemp and sisal, covering a wide range of cellulose content. The effect of methods to separate/extract fibres from the plants as well as of a few surface modification procedures on the fibre surface properties is also reviewed. The dispersive part of the natural fibre surface tension γ d S varies from 32 to 61 mJ/m2. The fibre surface tension increases with increasing cellulose content of natural fibres. We also found that a higher basicity (Donor Number, K B to Acceptor Number, K Aratio) was observed for fibres containing more cellulose. This may be reflective of higher crystalline cellulose content in the surfaces of the fibres, as only the ether linkage of the cellulose is labile for hydrogen bonding.

Microstructural Features and Mechanical Properties after Industrial Scale ECAP of an Al 6060 Alloy

Future applications of ultrafine-grained, high performance materials produced by equal-channel angular pressing (ECAP) will most likely require processing on an industrial scale. There is a need for detailed microstructural and mechanical characterisation of large-scale, ECAP-processed billets. In the present study, we examine the microstructure and mechanical properties as a function of location and orientation within large (50 x 50 x 300 mm³) billets of an Al 6060 alloy produced by ECAP (90° channel angle) with different magnitudes of backpressure. The internal deformation is analysed using a grid-line method on split billets. Hardness is recorded in longitudinal and cross-sectional planes. In order to further characterise the local, post-ECAP mechanical properties, tensile tests in different layers are performed. Moreover, low voltage scanning transmission electron microscopy observations highlight relevant microstructural features. We find that the homogeneity and anisotropy of mechanical properties within the billets depend significantly on the geometry of the shear zone. We demonstrate that deformation gradients can be reduced considerably by increasing the backpressure: The opening-angle of the fan-shaped shear zone is reduced from ψ ≈ 20 ° to ψ ≈ 7 ° when the backpressure is increased from 0 to 150 MPa. Backpressures of 150 MPa result in excellent homogeneity, with a relative variation of tensile mechanical properties of less than 7 %. Our investigation demonstrates that ECAP is suitable for processing homogenous, high performance materials on a large scale, paving the way for advanced industrial applications.

Splat Formation and Adhesion Mechanisms of Cold Gas-Sprayed Al Coatings on Al2O3 Substrates

The metallization of ceramics by means of cold gas spraying (CGS) has been in the focus of numerous publications in the recent past. However, the bonding mechanism of metallic coatings on non-ductile substrates is still not fully understood. Former investigations of titanium coatings deposited on corundum revealed that a combination of recrystallization induced by adiabatic shear processes and hetero-epitaxial growth might be responsible for the high adhesion strengths of coatings applied on smooth ceramic surfaces. In the present work, the interface formation between CGS aluminum and alumina substrates is examined for different particle sizes and substrate temperatures. Furthermore, the influence of subsequent heat treatment on tensile strength and hardness is investigated. The splat formation of single particles is examined by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to study the Al/Al2O3 interface. First results suggest that mechanical interlocking is the primary adhesion mechanism on polycrystalline substrates having the roughness in sub-micrometer range, while the heteroepitaxy between Al and Al2O3 can be considered as the main bonding mechanism for single-crystalline sapphire (α-Al2O3) substrates with the surface roughness in nanometer range. The heteroepitaxial growth is facilitated by deformation-induced recrystallisation of CGS aluminum.

Effect of additive and current mode on surface morphology of palladium films from a non-aqueous deep eutectic solution (DES)

Electrodeposition of palladium from a non-aqueous electrolyte solution [choline chloride/urea/palladium(II)chloride] has been carried out by direct and pulse current electroplating. In this study, the influence of an organic additive (nicotinic acid amide), current mode (direct current or pulse current deposition) and hydrodynamic on the surface morphology of electroplated palladium films was investigated. In order to determine the surface morphology and thickness of the electrodeposited palladium layers, a scanning electron microscope and an energy dispersive X-ray fluorescence spectroscope were used. In addition, the cell voltage during the different electrodeposition experiments was recorded and analysed. The experimental results showed that the surface morphology of the palladium deposits could be remarkably affected either by addition of the additive or by applying pulse current. Pulse plating and the selected inhibitor mutually interfere with each other, causing changes in the microstructure of the palladium deposits (e.g. smoothening or forming of micro-cracks). It was possible to optimise the palladium deposit quality by applying pulse current or by addition of the appropriate inhibitor.

Electrodeposition of palladium films from ionic liquid (IL) and deep eutectic solutions (DES): physical–chemical characterisation of non-aqueous electrolytes and surface morphology of palladium deposits

Abstract The physical and electrochemical characteristics such as density, viscosity, electrical conductivity and cyclic voltammetry of three different non-aqueous palladium electrolytes were analysed. The cyclic voltammetry behaviour showed typical reduction and oxidation peaks corresponding to the deposition and stripping of palladium in the electrolytes employed. The electrodeposition of palladium films from choline chloride/ethylene glycol (ChCl-EG), choline chloride/urea (ChCl-urea) and 1-butyl-3-methylimidazolium chloride-tetrafluoroborate (BMIM-Cl-BF4) solutions was demonstrated. Compact deposits were obtained with galvanostatic electrolysis. The scanning electron micrographs of the deposits revealed predominantly nodular Pd particles.

Anodizing – A Key for Surface Treatment of Aluminium

This article describes the flexibility and the potentials of the most important finish for aluminium-based materials. After a suggestion for the classification of anodizing processes, the historical development of the electrolytic anodic oxidation (EAO) and the plasma-electrolytic anodic oxidation (PAO) are presented. In the following section the focus is given on selected topics concerning the process parameters, the microstructure including the layer composition and properties. The variability of the anodizing process parameters and the used materials provide this variety of application. Furthermore, both differences and similarities of the EAO and PAO processes are shown. The conclusion of this review emphasizes that there is still some need for further research especially in the interaction of the process, the formed microstructure and the resulted properties determining the final application. Hence new possibilities for this surface treatment will be open.

Corrosion characteristics of an ultrafine-grained Al-Mg-Si alloy (AA6082)

The corrosion behaviour of the aluminium alloy, AA6082, processed by equal-channel angular pressing (ECAP) after different passes (route E, room temperature) was studied in comparison to the coarse-grained counterpart. The results of the electrochemical investigations (cyclovoltammetry; electrochemical impedance spectroscopy, EIS) are presented in correlation with the microstructure before and after the corrosion examinations. Both, chemical (precipitations, phases) and physical (dislocations, high-angle grain boundaries, grain size, low-angle grain boundaries) inhomogeneities characterize the microstructure of this commercially used Al-Mg-Si alloy. Results indicate an improved resistance against pitting of the ECAP material expressed by a reduced pitting density of up to 50 % and lower pit depths. EIS measurements and microstructural examinations (scanning electron microscopy, transmission electron microscopy, 3D topography measurement) confirm that ECAP modifies the number, size and distribution of these inhomogeneities, which leads to a more favourable corrosion behaviour.

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.