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A study on the influence of the silicate composition on the self-healing ability of a promising environmental barrier coating (EBC) material is reported. The EBC material consists of silicon carbide particles dispersed in a ytterbium silicate matrix. The composition of the silicate was varied from pure monosilicate to pure disilicate including a mixture of 50 vol-% of both phases. Pre-cracked specimens were mechanically tested before and after thermal annealing in air at 1400 °C accompanied by microstructural investigations. It is shown, that the self-healing mechanism is based on the application of compressive surface stresses due to graded oxidation of the SiC particles. With increasing amount of disilicate strength and self-healing ability increase. [ABSTRACT FROM AUTHOR]

Hardness, high mechanical strength, wear and corrosion resistance, for instance, are the most important properties of silicon nitride. Next to technical application, the material becomes more and more important for medical applications, especially for implants and the focus in research for this increase steadily. Up to now, the demands on the geometry of silicon nitride-based components could largely be met by using conventional shaping methods such as pressing and green machining or injection moulding. However, requirements in terms of complexity and function are constantly increasing. By using additive manufacturing (AM) technologies, the complexity of components can be significantly increased and additional functions such as transport channels or features like triple periodic minimal surfaces (TPMS), which seem to be advantageous for osseointegration can be integrated, for instances. High-resolution AM processes such as stereolithography enable the production of very accurate and complex ceramic components. In this work a silicon nitride suspension suitable for biomedical applications was developed and the influence of the formulation on the properties (especially flow and curing behavior) were analyzed. Afterwards, various test samples were generated by ceramic additive manufacturing via vat photopolymerization (CerAM VPP), thermally processed and the sintered parts were characterized with common ceramic methods, like mechanical tests and microstructure analyses, showing satisfying results for the described development state. [ABSTRACT FROM AUTHOR]

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible. [ABSTRACT FROM AUTHOR]

Abstract Additive manufacturing processes become more and more important in almost all areas, also for medical implant application. Medical implants made of ceramics are known for their excellent biological compatibility. For increasing the ingrowth of cells into an implant, a function-optimized structure with a defined porosity seems to be advantageous. Additive manufacturing is suited to realize defined and filigree structures. Lithography-based ceramic manufacturing (LCM), for example, has a high resolution, but still a limited material portfolio. Alumina toughened zirconia is a very interesting material for medical applications, but not commercially available for LCM. In this study, the development of an ATZ suspension usable in the LCM process with a bulk material density higher than 99% after sintering is presented. Furthermore, printing of lattice test structures for the evaluation of accuracy in dependence of printing dimension is demonstrated. [ABSTRACT FROM AUTHOR]

The so-called Freeze Foaming method aims at manufacturing ceramic cellular scaffolds for diverse applications. One application is dedicated to potential bone replacement material featuring open, micro and interconnected porosity. However, the main challenges of this foaming method is to achieve a homogeneous pore morphology. In a current project, the authors throw light on the bubble/pore and strut formation of this process by in situ computed tomography. This allows for evaluating varying process parameter’s effects on the growth of the ceramic foam during the foaming process. As first result and basis for CT analysis, a stable and reproducible model suspension was developed which resulted in reproducible foam structures. In dependence of selected process parameters like pressure reduction rate or air content in the ceramic suspension resulting Freeze Foams became adjustable with regard to their pore morphology. Pore size and distribution data as well as the porosity were characterized and evaluated accordingly. [ABSTRACT FROM AUTHOR]

Superhard silicon carbide-bonded diamond materials were synthesized by liquid silicon infiltration of diamond-containing preforms. The properties of the materials were strongly influenced by the strength of the interfaces between the diamond and the silicon carbide. Interface formation was investigated through local analysis of the microstructure in the interface regions using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) as well as X-ray diffraction (XRD). The results of these experiments revealed a pronounced orientation relationship between SiC and diamond at their interfaces and, as a result, strong bonding of the diamond particles to the ceramic matrix. There was also an orientation relationship between the nano-sized SiC grains, which were embedded in residual silicon near the diamond interfaces, and diamond. Additionally, the different morphologies and phenomena occurring in the microstructures of the diamond-SiC composites and their dependence on the infiltration temperature were studied. [ABSTRACT FROM AUTHOR]

Liquid phase-sintered B 6 O materials with Al 2 O 3 /Y 2 O 3 additives were recently developed to yield higher fracture toughness and strength levels than those of pure B 6 O-materials. However, the high additive contents of hot-pressed and spark plasma-sintered (FAST/SPS) materials led to the formation of strength-limiting boride precipitates. Systematic variation of the B 2 O 3 content of the grain boundary phase and thermodynamic calculations carried out in the present work showed that it is possible to avoid boride formation by controlling the decomposition of B 2 O 3 during densification and adjusting the B 2 O 3 content of the composition through direct addition of B 2 O 3 or through reaction of added transition metal oxides such as TiO 2 with the starting composition to form B 2 O 3 . [ABSTRACT FROM AUTHOR]

Abstract: SiC-fiber/SiCN ceramic matrix composites were manufactured by means of polymer infiltration and pyrolysis. The fiber preform was made by slurry infiltration and winding using a computer-controlled winding module. Multiple infiltration steps using a Si–C–N precursor were included to increase the density. The influence of the sintering conditions on the microstructure of the CMC was demonstrated. Pipe sections made of the CMC materials were joined using a laser-supported heating technology with an Y–Al–Si–O glass–ceramic filler. The thermal response of the CMC components was controlled by the anisotropic thermal conductivity. Fast heating by laser beam was achieved for elements rotating in the direction of the fiber winding. SEM micrographs of the joints showed the good wettability of the CMC by the glass–ceramic filler. Nearly defect-free joints were obtained using a nitrogen process atmosphere. The laser-supported technology was shown to be promising for the joining of CMC components. [Copyright &y& Elsevier]

Abstract: Dense boron rich boron carbides were reactive sintered by hot pressing at 2050°C using elementary boron carbon compositions with carbon contents of 9.1, 11.1, 13.3 and 18.8at.%. The following material characteristics are presented: relative density, SEM images, EDX, X-ray diffraction and corresponding lattice parameters, Seebeck coefficient, electrical conductivity and thermoelectric power factor. Significant grain growth has been obtained with increasing boron content. A deeper understanding of the boron and carbon reaction and the overall sintering process is gained by thermal and chemical analysis in combination with X-ray diffraction. Additionally a thermal experiment with boron and carbon layers illustrates the solid state diffusion behaviour. The found results of boron carbide properties of this paper correspond with results by other authors. The aim is to correlate technological aspects of sintering procedure with material properties. This should help to improve the thermoelectric efficiency of boron carbide based materials. [Copyright &y& Elsevier]

Abstract: Sintered silicon carbide materials have found widespread use due to their high corrosion stability. This corrosion stability can be affected by electrochemical processes. Electrochemical corrosion experiments conducted on a SSiC material in H2SO4 at different voltages and subsequent detailed investigation of the formed surfaces was carried out. The first time a systematic local measurement of the thickness of the oxide layers was carried out. The measurements revealed the formation of SiO2 surface layers with thickness up to 125μm. The measured values also showed a strong deviation from grain to grain. The thickness of the layers does not correlate with the crystallographic orientation of the grains or the SiC-polytypes. The data indicate that the behaviour is caused by the variation of the resistivity of the grain boundaries. The measured thicknesses as a function of the electrical charge transferred indicate that the electrochemical oxidation results in the SiO2 and carbon dioxide. [Copyright &y& Elsevier]

Abstract: Materials with enhanced wear resistance are of great interest for modern industry. The different ways of producing superhard ceramic materials are shortly reviewed. In detail, the preparation of diamond–silicon carbide composites by reactive infiltration of diamond preforms with liquid silicon at ambient pressure is investigated. In addition to homogeneous SiC/diamond materials with nearly 50vol.% of diamond layered composites were also prepared. The microstructures of the materials prepared under different infiltration conditions were analyzed using X-ray diffraction, electron microscopy, electron back-scatter diffraction (EBSD) and Raman spectroscopy. The properties of the materials depend strongly on the formation of graphite interlayers at the boundary between SiC and diamond. Hardness of 47GPa was reached for materials where the formation of the interlayer was prevented. [Copyright &y& Elsevier]

Abstract: B6O/TiB2 composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900°C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B6O material showed that the addition of TiB2 in a non-reactive sintering route promotes the B6O densification. Further improvement was obtained by sintering reactive B–TiO2 mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0MPam1/2 (SEVNB) and 2.6–5.0MPam1/2 (IF method) while simultaneously a high hardness of up to 36GPa (HV0.4) and 28GPa (HV5) could be preserved. The high temperature properties at 1000°C of hardness, thermal conductivity and CTE were up to 20GPa, 18W/mK and 6.63×10−6/K, respectively. [Copyright &y& Elsevier]

Abstract: The influence of initial pH and energy input during suspension homogenization on the stabilizing performance and coordination type of commercial available polyacrylate dispersant were studied. Additionally to widely used rheology and electroacoustic measurement techniques the alumina suspensions were analysed with centrifugal separation and in situ ATR-FTIR to study the impact of varied powder processing in detail. In contrast to zeta potential analysis and viscosity measurements only the determination of sedimentation properties by centrifugal separation shows the effect of macroscopic changes in powder processing. A combination of positively charged alumina surface and a high shear homogenization leads to the most stable suspension. Accordingly ATR-FTIR results show a correlation between improved suspension stability and inner-sphere coordination of polyacrylate. Moreover it was possible to determine an optimal pH range for inner-sphere adsorption. It can be shown that macroscopic changes in powder processing influence the coordination of dispersant and thus the suspension stability. [Copyright &y& Elsevier]

Abstract: New electrically conductive ternary composites were developed by adding 8vol.% of ZrN or ZrB2 to a Si3N4–SiC matrix. During hot pressing, ZrB2 reacted with Si3N4 to form ZrSi2, ZrN, Si and BN whereas added ZrN did not undergo any reactions in the Si3N4–SiC–ZrN composite. The composites modified by ZrN or ZrB2 addition showed a lower resistivity (7×103 Ωcm and 3×10−1 Ωcm) compared to the matrix (3×104 Ωcm). Further studies on the grain size distribution and the volume ratio of conducting and non-conducting phases excluded a percolation network of ZrN and ZrSi2 grains. In fact, doping of SiC grains and modified grain boundaries as a consequence of the formation of liquid phases during sintering are suggested to be the reason for the significantly lower resistivity of materials containing ZrSi2. A decrease in the composite resistivity due to a subsequent heat treatment was obtained for all hot-pressed composites. [Copyright &y& Elsevier]

Abstract: Hexagonal boron nitride/titanium diboride composites are widely used as evaporation boats for aluminium deposition to produce functional and decorative layers on different target materials. The lifetime of such a material is limited mainly by the interaction of the metal with the ceramic substrate, but the corrosion mechanism has still not yet been thoroughly investigated and understood. In this article the corrosion mechanism for the evaporation boats used was investigated using thermodynamic calculations, FESEM, EDX and XRD phase analysis. The analysis showed that hexagonal boron nitride (hBN), which is thermodynamically less stable than TiB2, is passivated during the application process through the formation of AlN surface layers, whereas the thermodynamically more stable TiB2 phase dissolves and Ti-rich components precipitate in cooler regions of the evaporation boats. [Copyright &y& Elsevier]