Search

Tissue engineering is an attractive approach to heal bony defects. However, three-dimensional cell-scaffold constructs display uneven oxygen supply resulting in inhomogeneous tissue quality. We assessed different strategies to improve oxygen supply in vitro. Scaffolds with differing inner surface were seeded with preosteoblastic cells and cultivated either statically or in perfusion bioreactors. Oxygen concentration and pH were measured in the center of the scaffolds. An inductive feedback mechanism was build to increase bioreactor pump speed according to the oxygen concentrations measured within the scaffolds. While pH remained stable, oxygen concentration decreased significantly under static conditions within the cell-seeded scaffolds. Reducing the scaffolds' inner surface as well as increasing perfusion speeds in bioreactors resulted in improved oxygen supply. We conclude that improving oxygen supply to three dimensional culture systems for bone tissue engineering is feasible in an automated manner. Culture conditions have to be adapted to each cell-scaffold system individually.

The chemical composition of calcium phosphate (CaP) materials for the regenerative therapy of large bone defects is similar to that of bone. Additionally, calcium phosphates show an excellent biocompatibility. Besides the support of defect healing calcium phosphate implants should be completely degraded within an adequate time period to be replaced by newly formed bone. Although degradation of CaP-implants occurs mainly by dissolution of the material, it is important to characterize the osteoclastic resorption as well, which is involved in native bone remodeling. The degradation of bone substitutes made of calcium phosphate ceramics is influenced by various parameters, such as defect size and localization, the general health situation, and age of the patient, but also material properties are important. Especially, the calcium phosphate composition is crucial for the degradation behavior of a calcium phosphate material. Additionally, at the cellular level the micro- and macroporosity, including interconnecting pores, influences both, the dissolution and the osteoclastic resorption. In our study, three different calcium phosphate materials (hydroxyapatite, tricalcium phosphate, and a biphasic calcium phosphate) and two different geometries (dense 2D samples and porous 3D scaffolds) are compared regarding their dissolution and resorption behavior. The results show, that the dissolution of CaP-ceramics, as examined by the incubation in a degradation solution, depends mainly on the calcium phosphate phase but also on the porosity of the implant. Regarding the resorption, cell proliferation and differentiation of a monocytic cell line as well as the formation of resorption lacunas are analyzed. Cell proliferation is comparable on all phase compositions. Cell differentiation and resorption, however, are influenced by the calcium phosphate phase composition and by the implant porosity as well. By understanding these two mechanisms of degradation, bone substitute materials and, as a result, the bone regeneration of large bone defects using CaP-ceramics can be improved.

In the last few years new fabrication methods, called rapid prototyping (RP) techniques, have been developed for the fabrication of hydroxyapatite scaffolds for bone substitutes or tissue engineering applications. With this generative fabrication technology an individual tailoring of the scaffold characteristics can be realised. In this work two RP techniques, a direct (dispense-plotting) and an indirect one (negative mould technique), are described by means of fabricating hydroxyapatite (HA) scaffolds for bone substitutes or bone tissue engineering. The produced scaffolds were characterised, mainly regarding their pore and strut characteristics. By these data the performance of the two fabrication techniques was compared. Dispense-plotting turned out to be the faster technique while the negative mould method was better suited for the fabrication of exact pore and strut geometries.

In this study the degradation behaviour of pure hydroxyapatite (HA), pure tricalcium phosphate (β-TCP) and four biphasic calcium phosphate ceramics was studied to gain information about the influence of the phase composition on this property with the aim to tailor individually adapted bone substitute materials. The chemical dissolution of each ceramic composition was measured by its release of calcium ions into a buffered solution. With decreasing HA content in the ceramics the degradation rate increased. Cell experiments were carried out with stimulated osteoclast-like RAW 264.7 cells. Using biochemical, genetic and microscopic analysis, the differentiation of the cells on the ceramic samples was studied. The monocytic precursor cells differentiated into osteoclast-like cells on all ceramics. The strongest cell differentiation into osteoclast-like cells was found on ceramics with HA/β-TCP ratios of 80/20, 60/40 and 40/60. Cells on these ceramics had many nuclei and the largest cell size. As a result of resorption, lacunas were found on all ceramics except β-TCP. All these experimental results proved the influence of the phase composition on degradation and resorption of calcium phosphate ceramics. Biphasic calcium phosphate ceramics with HA/β-TCP ratios of 80/20 and 60/40 exhibited the most promising properties to serve as synthetic bone substitute materials because for integration in the physiological bone remodeling process the implanted bone substitute materials should have optimized dissolution and resorption properties.

Bone remodelling is a coupled process of bone formation and resorption. This process is physiologically controlled and involves the synthesis of bone matrix by osteoblasts and bone resorption by giant cells called osteoclasts. It includes a complex interaction of cells, with specific chemokine signalling. In order to study bone remodelling in vitro, we cultivated two precursor cellline types in a model coculture system on synthetic hydroxyapatite ceramic (HA) for 14 days. The monocytes have the capacity to differentiate into osteoclast-like cells, and the bone marrow stromal cells can differentiate into osteoblast-like cells. This coculture was used to analyse the in vitro cell interaction between monocytes and stromal cells. Furthermore, the attachment of the bone marrow stromal cells to the resorbed HA-surfaces was studied. In this in vitro study we demonstrated osteoclast-like differentiation and bone marrow stromal growth in a coculture system on a synthetic bone substitute material. Under optimal conditions, HA can be resorbed and bone marrow cells can grow into the lacunas to form new bone. These results give important cues for the adjustment of synthetic bone substitute materials for optimal remodelling behaviour.

The aim of this study is to analyse the influence of differently fabricated HA-scaffolds on bone marrow stromal cells. Therefore, three methods were used (a polyurethane (PU)-replica technique, the dispense-plotting and a negative mould technique) to produce porous hydroxyapatite (HA) ceramics. The different HA-scaffolds were then cultivated with an osteoblastic precursor cell line. In our study, highest cell proliferation and differentiation was achieved by using (PU)-replica technique. However, this study shows also that all three types of scaffolds are suitable for tissue engineering applications and as bone substitute material. The knowledge about the influence of pore size and geometry on the cell behaviour will help to tailor scaffolds, by different 3D fabrication methods, for the needs of tissue engineering laboratories or patients.

Bulk SiCN glasses, derived from polymer precursors with tailored architectures, were characterized upon pyrolysis at 1000 °C and subsequent annealing at 1400 °C and 1540 °C. The characterization tools employed were high-resolution transmission electron microscopy (HRTEM) imaging and energy-filtered selected area electron diffraction (EF-SAED). Main emphasis of this work was to verify as to whether the intrinsic amorphous structure of polymer-derived bulk materials exhibits different structural features upon pyrolysis, that depend on the functionalities of the pre-ceramic polymer, and whether such characteristic features are maintained upon high-temperature anneal. HRTEM imaging did not reveal any pronounced variation between the different SiCN glass structures after the organic–inorganic transition at 1000 °C or even upon heat treatment at 1400 °C. However, EF-SAED analysis showed differences in the amorphous structure of the four polymer-derived SiCN glasses studied even upon thermolysis at 1000 °C. The evolution of the electron diffraction intensity profiles of the EF-SAED patterns after subsequent thermal treatment at elevated temperatures showed clear evidence of structural rearrangements within the SiCN glass networks upon annealing at temperatures exceeding the pyrolysis temperature. The experimental results provide evidence that the metastable glass structure of polymer-derived SiCN ceramics depends on the starting polymer employed and, in addition, that the intrinsic glass architecture affects the thermal stability (onset of crystallization) of the amorphous matrix.

The effect of surface modification of polymer filler on the static mechanical properties of acrylic bone cement was studied. The surface of polymer beads was modified with carboxylic and amino groups by photochemical reaction with azide compounds. Monomer modifiers (maleic anhydride, methacrylic acid and p-aminostyrene) are attached to the functionalized surface of polymer beads. Functional allyl groups, which are capable of the graft polymerisation reaction, are attached to the surface via photochemical reaction with N-(2-nitro-4-azidophenyl)-N-(-propen) amine. This approach to bone cement provides the additional covalent bonds between the polymer beads and the inter-bead matrix. The static mechanical properties of bone cements containing modified polymer beads were investigated and compared with the static mechanical properties of unmodified cements. The absolute values of compressive strength for the modified and unmodified cements were found to be similar. An increase in flexural strength for the modified cements (dry and after water storage) was observed. The structure of the surface functional groups affects the methyl methacrylate grafting resulting in a higher value of flexural strength for the maleic anhydride- and p-aminostyrene-modified cements. The scanning electron microscopy examination of the fracture surface of the cement samples showed an improvement of the adhesion between the beads and the matrix after modification.

Hydroxyapatite ceramics for biomedical applications are often used in form of coatings to increase the biocompatibility of metallic implants, e.g. the femoral shaft of a hip prosthesis. A new method to produce such layers is electrophoretic deposition (EPD). The aim of this study was to investigate the influence of different powders and process parameters on the quality of the electrophoretically deposited thin HA layers. In order to attain layers with a thickness less than 50 μm, it is essential to use fine powders with particle sizes in the sub-micron range, and to stabilize these powders in suspension. We show that commercially available hydroxyapatite powders, which usually have a strong tendency to agglomerate, can be milled to particle sizes smaller than 500 nm by adding an organic dispersant. When such milled powders are used together with optimized process parameters, thin and homogenous layers can be produced by EPD. Using either aqueous or non-aqueous suspensions, hydroxyapatite layers can be deposited directly onto a metallic hip shaft, or onto a polymeric membrane, producing thin free-standing ceramics used for special biomedical applications.

Hydroxyapatite ceramics for biomedical applications are used in form of layers, granulates and bulk materials. A new method to produce layers or bulk materials is the electrophoretic deposition. To achieve layers with a thickness between 5 and 50 m the use of a fine powder with particle size in the sub micron range is essential. In this work we show that commercially available hydroxyapatite powders, which usually have a strong tendency to agglomerate, can be milled to particle sizes smaller than 500 nm. Using one of these powders and optimizing the parameters for the electrophoretic deposition thin, homogenous layers were produced.

The influence of sintering parameters at an early stage of densification on the evolution of a bimodal microstructure in Si 3 N 4 ceramics was investigated. Commonly two different methods are pursued to design a bimodal Si 3 N 4 microstructure: (i) annealing at a later sintering stage ( T > 1850 °C) initiating β-Si 3 N 4 grain growth via Ostwald ripening and (ii) seeding with β-Si 3 N 4 nuclei, which abnormally grow during the liquid-phase sintering process. In this study, a third and novel method to design Si 3 N 4 microstructures by affecting intrinsic nucleation phenomena at an early sintering stage is presented. In order to study the influence of sintering parameters on β-Si 3 N 4 nuclei formation during the early stage of densification, temperature and pressure were systematically changed. Starting from identical green bodies (identical processing and doping), the variation of the sintering parameters affected intrinsic β-Si 3 N 4 nucleation. This procedure allows variation in the fineness of the matrix as well as in the number and dimension of the large elongated β-Si 3 N 4 grains embedded in the matrix. Since identical green bodies are used as starting material, the resulting microstructure can easily be tailored toward corresponding application needs.

Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) are of great interest due to their potential application as bone-replacement materials. In particular, composites made of a mixture of these Ca-phosphates revealed improved mechanical properties; however, the reason for this improvement is unknown. Future development and properties enhancement of such bioceramics is linked to the possibility to characterise their particular microstructure. In this context, the ability to quickly identify individual grains of HA and β-TCP within these composites will allow acquiring information about the phase distributions and the phase-boundary microstructure. The aim of the present study is, therefore, to demonstrate that electron energy-loss spectroscopy (EELS) can be successfully employed to differentiate between individual grains of HA and β-TCP. In particular, the analysis of the near-edge structure of the oxygen K-ionisation edge allows detection of a characteristic signal at ca. 536 eV that can be employed as an identification tool for HA. EELS investigations were performed first on as-received and calcined (1000 °C) HA and β-TCP powders and subsequently on pure bulk HA and β-TCP samples sintered at 1250 °C. Finally, this method was successfully applied to a HA/β-TCP (50/50 wt.%) composite sintered at 1250 °C.

Sol-gel-derived mullite ceramics were processed by pressureless sintering at 1600°, 1650°, and 1700°C for 4 h. Microstructural and microchemical characterization of the mullite materials was performed using transmission electron microscopy, in conjunction with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy (EELS). Apart from mullite grain diameter and triplepocket size, no major microstructural changes were observed with increasing sintering temperature. Residual glass was present at triple pockets and along two-grain junctions. Not all grain boundaries revealed the presence of a continuous amorphous intergranular film. Clean interfaces were observed only at boundaries with one grain parallel to the [001] orientation (low-energy configuration). Quantitative EELS analysis of mullite grains and glass pockets revealed only small changes in composition with increasing sintering temperature; i.e., the alumina:silica ratio slightly increased for mullite and glass. The analysis implied that mullite with this relatively high aluminum content would not be stable adjacent to residual glass. However, a stable glass-mullite system has been proposed, because impurity cations were detected within glass pockets, which suggested a slight shift of the subsolidus line (glass-mullite/ mullite) to a higher amount of alumina. Energy-loss nearedge structure studies of the Si-L2,3 edge revealed a similar near-edge structure for the mullite, residual glass, and quartz. Thus, SiO4 tetrahedra were thought to be the main building units of the glass contained in sintered mullite.

The microstructure evolution of different precursors-derived SiCN ceramics upon exposure at temperatures ranging between 1000 and 1400 °C has been investigated by means of transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), X-ray energy dispersive spectroscopy (XEDS), Raman spectroscopy and Neutron scattering. The results of the present study indicate that these materials remain structurally amorphous and can, therefore, be considered stable against devitrification within this temperature range. However, the amorphous microstructure of these ceramics does undergo a short-range rearrangement. It is important to note that SiCN precursors with very similar chemical composition but different starting precursor architecture result in very similar amorphous network structures after exposure to 1400 °C. The presence of few localized crystalline regions, composed of Si3N4 crystals or turbostratic carbon features is explained in terms of gas-phase reactions occurring at residual closed pores between 1000 and 1400 °C.

Monolithic mullite with low sintering shrinkage was synthesized from polymer/filler blendsMthat is, siloxane/alumina (siloxane/Al2O3) (and siloxane/aluminum (siloxane/Al)) mixtures. The synthesis was based on a reaction-bonding process of amorphous silica, which formed when the siloxane was oxidized, with Al2O3 (or oxidized aluminum filler) at temperatures >1250°C. Thermodynamic calculations were used to calculate the phase composition at equilibrium. Thermoanalytical, infrared-spectroscopic, and microscopic techniques were applied to reveal the microstructural evolution. The corresponding volume changes were used to evaluate the linear shrinkage, based on the quantitative phase assemblage.

Monolithic polymer-derived Si-C-N ceramics were processed by blending an oligomeric Si-C-N precursor (liquid polysilazane) with 70 vol% of crosslinked or pyrolyzed Si-C-N powder particles, which were obtained from the same liquid precursor preheated at 300° or 1000°C, respectively. Powder compacts subsequently were annealed at 300°C to crosslink the liquid precursor acting as a binder between the powder particles, thus yielding monolithic green bodies. Heat treatment at 1540°C was performed to initiate crystallization in the various samples. Microstructure development and, in particular, crystallization behavior were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and preliminary nuclear magnetic resonance (NMR) spectroscopy. The material containing 300°C polymer powder (with oligomeric binder, also crosslinked at 300°C) revealed a homogeneous amorphous microstructure after exposure to temperatures of 1540°C. In contrast, the specimen containing powder particles preheated at 1000°C exhibited a high volume fraction of SiC crystallites within regions that were previously filled by the binder; however, the Si-C-N powder particles themselves remained amorphous. SEM observations as well as XRD studies showed the formation of idiomorphic SiC and Si3N4 crystallites on specimen surfaces as well as along internal crack walls. This finding suggested that vapor-phase reactions at the surface were involved in the formation of crystalline phases at temperatures >1250°C. Moreover, NMR spectroscopy data indicated a phase separation process, implying structural rearrangement prior to crystallization.

During tribological testing of hydrogenated amorphous carbon coatings (a-C:H) on silicon carbide ceramics, a friction coefficient of 0.06 was observed under dry sliding conditions, which is thought to represent a lower boundary value for the given experimental setups. Based on chemical and thermal analyses of the coating material, a structural model of the as-deposited amorphous coating was modified taking simplified quantum chemical and statistical arguments into consideration. The tribological behavior of the coating could be rationalized by this modified structure model: Tribological loading leads to a partial release of internal stresses stored in the coating. This process results in an increase of the local mobility of atoms, which can be seen as the driving force for initial structural rearrangements within the coating. Proof for the occurrence of such rearrangements has been provided by electron energy-loss spectroscopy (EELS) measurements, monitoring the local sp3/sp2 hybridization ratio, of the coating before and after tribological tests.

The excess carbon of various polysilazane precursors with varying carbon contents was investigated using 13C MAS NMR, Raman and ESR spectroscopies, and microwave conductivity measurements. Microstructure characterization was investigated using TEM. The collected data were compared with those from a previous study on the same precursor. This study focused on the distribution and appearance of the free-carbon phase. Although the spectroscopic techniques showed no differences in the structure of the free-carbon phase, a clear distinction between the various precursors was found using microwave conductivity and high-resolution TEM imaging.

Polyvinylsilazane, as a precursor for Si-C-N ceramics, was prepared by ammonolysis of functionalized chlorosilanes. Pyrolysis under inert atmospheres at Tp= 1000°C led to an amorphous Si-C-N-(H) ceramic. Further heat treatment caused the transformation to the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterizing the solid intermediates via solid-state magic angle spinning nuclear magnetic resonance spectroscopy. Moreover, Raman spectroscopy, electron spin resonance spectroscopy, and chemical analysis were used. Based on these methods, a comprehensive picture of the formation and behavior of the free-carbon phase present in polymer-derived ceramics was obtained.

Fine silicon powders with a mean particle size of 0.5 μm were used to prepare aqueous suspensions for electrophoretic deposition (EPD). A cellulose film membrane was proved to be a cost-efficient alternative material to the dialysis hoses with excellent properties for the EPD-process. Several organic additives were tested to stabilise the suspension against sedimentation effects, to increase the electrophoretic mobility of particles and to optimise the mechanical properties of the dried green bodies. Mainly the electrokinetic sonic analysis (ESA) was used to characterise the influence of the additives and several parameters like solids content and ionic strength on the thickness of the electrical double layer. A computer-controlled and monitored EPD led to a dense and homogeneous packing of the powder and contributes to the understanding of deposition mechanisms. Even severe machining of the dried green bodies was possible. Using the planetary milled silicon powder the small spaces between the carbon (C-) or silicon carbide (SiC-) monofilaments of the fibre fabrics were infiltrated homogeneously by the silicon particles.

Electrical properties of monolithic, amorphous SiCxNyHz-ceramics derived from 1,3,5-trimethyl-1,3,5-trivinylcyclotrisilazane via polymer pyrolysis were investigated from room temperature to 400 °C using impedance spectroscopy. Depending on the pyrolysis temperature Tp, the d.c. conductivity varies up to 8 orders of magnitude. The temperature dependence of samples pyrolysed at low temperatures (Tp=700–1200 °C) follows a Mott law, whereas samples pyrolysed at high temperature (Tp=1400 °C) show an Arrhenius dependence. Structural changes during pyrolysis were characterized by solid state magic angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy and X-ray diffractometry. NMR and especially Raman measurements indicate the formation of sp2-carbon atoms, which rearrange towards graphitic-like domains with increasing pyrolysis temperature. This observation can explain the related increase of the d.c.-conductivity.

Two silicon nitride powders prepared by diimide precipitation and direct nitridation were doped with 5 vol.% yttria using (i) a commercial Y2O3 powder and (ii) a soluble Y-organometallic compound as additive. The aim of this work was a direct comparison between those two doping methods. Homogeneity of additive distribution was characterized by Fourier transform infra-red spectroscopy (FT-IR), photo electron spectroscopy (XPS) and transmission electron microscopy (TEM). Depending on the doping technique used, no major difference in sintering behavior and microstructure evolution were monitored. However, chemical doping results in (i) an enhanced densification rate in the early stage of sintering, (ii) a slight shift of the densification rate maximum (ds/dt) to higher temperatures, and (iii) in a slightly finer final microstructure. Note that doping via organometallic precursors can only be successfully applied, when the powder particle surface reveals the presence of silanol groups, as in case of diimide starting powders. Electron microscopy, Organometallic dopants, Sintering, Si3N4

For improved quality and machinability of ceramic green bodies the use of organic additives is necessary. During the debinding process the polymers are thermally decomposed and partially emitted as ecotoxicologically relevant components. The aim of this study was to determine and to quantify these compounds during pyrolysis with the ultimate goal of reducing the emission of undesirable pyrolysis products. Four organic polymers frequently used in ceramics industry were investigated: polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and latex. The decomposition experiments were carried out using a tube furnace. Cylindrical green bodies, mainly Al 2 O 3 but also SiC, containing a common amount of 5 mass % of organic additives were pressed and heated. To gather the different volatile decomposition products, a branched sampling system with different adsorbents was developed. After elution of these adsorbents separation and quantification of the pyrolysis products were performed using GC/MS and HPLC. The main pyrolysis products of the three aliphatic organic additives are low molecular mass aldehydes in particular the potentially carcinogenic form-, acet-, and crotonaldehyde in the range of mg/g. According to the used polymer, the concentration of aromatic compounds varies over a wide range from μg/g to mg/g. While the pyrolysis of PEG results in nearly undetectable concentrations of aromatics, mono-, di-, and trimers of ethylene glycol, 2-methyl-1,3-dioxolan and 1,4-dioxan are formed in concentrations in the range of mg/g. The formation of the potentially carcinogenic 1,4-dioxan could be of toxicological relevance.

Different polymeric precursors with varying carbon contents were prepared by ammonolysis of functionalised chlorosilanes. Pyrolysis under inert atmospheres at 1000 °C led to amorphous Si–C–N–(H) ceramics. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterising the solid intermediates via solid state magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR). In addition, Raman spectroscopy, electron spin resonance spectroscopy (ESR), microwave conductivity measurements and chemical analysis were employed. Combination of all these methods provides a comprehensive picture of the formation and of the behaviour of the free-carbon phase present in the polymer-derived ceramics.

Processing of two precursor-derived SiCN ceramic monoliths was performed employing different liquid polymers, which were obtained by ammonolysis of functionalized chlorosilanes. Preparation of monolithic samples was performed by mixing liquid polysilazane with cross-linked SiCN-powder particles, derived from the same precursors by heat treatment at 300 °C, and subsequent annealing upon pyrolysis at temperatures exceeding 1000 °C to initiate crystallization. Characterization of the polymer-derived ceramics was performed after pyrolysis at 1000 °C and, in particular, after annealing at temperatures ranging from 1400 to 1540 °C. Transmission electron microscopy was conducted in order to study the devitrification/thermal stability of the corresponding bulk SiCN glasses. Depending on the functionalities of the SiCN precursor and the processing conditions, different microstructures were obtained. The material prepared from precursor A showed crystallization of large α-Si3N4 grains within the overall homogeneous amorphous bulk material after exposure at 1540 °C for 6 h in nitrogen atmosphere. In contrast, the ceramic monoliths derived from SiCN precursor B remained completely amorphous, with no indication of local nucleation or crystallization. It is thought that devitrification of these polymer-derived glasses is promoted by local rearrangements of the glass network within the amorphous bulk. In addition, the role of the excess free carbon, commonly present in polymer-derived SiCN ceramics,­on the thermal stability is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

A model system, composed of powder blends of amorphous isomorphic silica spheres, being 500 nm in diameter, and also monosized crystalline α-Al 2 O 3 powder, was investigated. Two different particle sizes of the corresponding alumina powder were employed: 300 nm and 2 μm. This particular assembly enabled a distinction between amorphous silica and crystalline alumina merely by their difference in particle morphology. The powder blends were sintered at temperatures between 1400 and 1700 C and the microstructure evolution was characterized by scanning (SEM) and transmission electron microscopy (TEM). It is worth noting that upon annealing at 1700°C, both microstructures were indistinguishable. However, depending on the Al 2 O 3 particle size, different conversion mechanisms were monitored. When using the 300 nm Al 2 O 3 powder, fast dissolution of alumina into the coalesced silica glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 μm Al 2 O 3 particles, however, resulted in the formation of two Al-containing glasses (phase separation into a Si- and Al-rich glass). In this case, the transformation to mullite can be rationalized by the conversion of the metastable Al-rich transient glass into mullite, which forms an epitaxial, single crystalline coating on the host Al 2 O 3 particle. Therefore, depending on the initial Al 2 O 3 particle size, mullite formation follows either the stable or metastable phase diagram.

Polymeric precursors with tailored structures were prepared from functionalised chlorosilanes. Pyrolysis under inert atmospheres led to amorphous Si–C–N–(H) ceramics at 1000°C. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes associated with crosslinking, pyrolysis and crystallisation were studied by characterising the solid intermediates between 300 and 1600°C applying 29 Si and 13 C solid state nuclear magnetic resonance (NMR) spectroscopy. In addition, Fourier transformed infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermoanalytical techniques and density measurements were employed. The combination of these methods and the use of polymeric precursors with tailored structures pointed to a correlation of the polymer architecture with the structure of the amorphous ceramic material.

Different liquid polymers in the system SiCN with tailored structures were prepared by ammonolysis from functionalized chlorosilanes. Crosslinking to an unmeltable polymer with initiators at low temperatures and subsequent ceramization were studied applying 29 Si solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with Fourier transformed infrared (FTIR) spectroscopy and thermoanalytical techniques. Microstructure development, in particular, the devitrification of the corresponding bulk polymer-derived SiCN glasses was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preparation of monolithic samples was performed by mixing liquid polysilazane with SiCN-powder particles, derived from the same precursors by heat treatment at 300°C, and subsequent annealing at temperatures exceeding 1000°C to initiate crystallization. Depending on the functionalities of the SiCN-precursor and the processing conditions, different microstructures were obtained. The material prepared from the HVNG precursor revealed a homogeneous amorphous micro structure with only a small fraction of crystallized spherical inclusions after exposure at 1540°C for 6 h in nitrogen atmosphere. In contrast, investigating ceramic monoliths derived from another SiCN precursor, a different crystallization sequence was observed. The material derived from the HPS precursor showed crystallization of large α-Si3N4 grains within the bulk. As will be discussed in detail, devitrification of these polymer-derived glasses is promoted by local rearrangements and possible phase separations within the amorphous bulk. Moreover, local decomposition and residual porosity can affect the crystallization behavior, which strongly differs depending on the polymer employed. In addition to the crystallization phenomena observed, different oxidation response was monitored for the two SiCN ceramics discussed here. Moreover, fracture strength and hardness data were recorded, which, however, did not substantially differ between the polymer-derived ceramics investigated.

Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary-phase chemistry. Si3N4 grain growth was dominated by diffusion-controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack-propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary-phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.

Carbon and SiC fiber-reinforced ceramic matrix composites were prepared via infiltration of fiber preforms using the polymer infiltration technique and polymer pyrolysis. Suitable silazane (SiCN) precursors with appropriate thermosetting behavior, viscosity and ceramic yield were synthesized, starting from functionalized chlorosilanes. Microstructural development and fracture behavior was studied after various infiltration and pyrolysis cycles. Residual stresses induced during processing were evaluated. Mechanical and thermo-physical properties of the composites with polymer-derived matrix, i.e. 3-pt bending strength and thermal expansion coefficients (CTE), were measured dependent on reinfiltration cycles and fiber orientation. The oxidation resistance was investigated. Specific pyrolyzed samples were infiltrated via silicon melts in order to enhance corrosion and wear resistance.