Georg Grathwohl

Evolution of Porosity by Freeze Casting and Sintering of Sol-Gel Derived Ceramics

Freeze cast of aqueous ceramic powder slurries is described as a versatile process to fabricate complex-shaped ceramic parts. Since freezing of aqueous sols or powder suspensions include the nucleation and growth of ice crystals the evolving microstructure in particular the pore characteristics which are left behind after elimination of the solvent can be controlled by the freezing process. The freezing kinetics have then to be used to manifest the conditions for the formation of the intended porosity. The temperature profile in the freezing slurry was measured and calculated, in particular the movement of the freezing front through the slurry was determined. The results show that a homogeneous microstructure is reached in the surface region of the consolidated part. Individual ice crystals are detected within a distance of some hundred micrometers from the surface. The final pores are dendritic in shape with an elliptical cross section. The pores can grow up to several millimeters in length under the process conditions used in this study. The limits of freeze-sensitive slurry compositions should be investigated in further studies and the approach should be followed to increase the porosity by additional foaming steps.

Freeze gelation: a new option for the production of biological ceramic composites (biocers)

Abstract Freeze gelation was used for immobilization and conservation of living microorganisms in inorganic solids. The irreversible sol–gel transition of a mixture of colloidal silica, ceramic powder and biocomponent by freezing (freeze gelation or freeze casting) enables the production of low-cost, porous, crack-free green bodies with nearly zero shrinkage in which microorganisms are immobilized safely. First investigations to the survival rate, activity and storage ability will be presented for biocers with Bacillus sphaericus and Saccharomyces cerevisiae under the selected freeze and drying conditions. The structure of these new biocers were characterized by SEM micrographs, physical adsorption and mercury porosimetry.

Microanalytical investigation of sintered SiC

The microstructures and compositions of Al- and B-doped pressureless sintered SiC-materials from four different sources were investigated by the combined usage of several microanalytical techniques. Besides the fundamental ceramography and chemical analysis the methods of Auger electron spectroscopy (AES), wavelength dispersive analysis of X-rays (WDX), microautoradiography and scanning transmission electron microscopy (STEM) were used. In transgranular fractured surface regions the stoichiometric Si/C relation was found by AES. The grain boundaries, however, are enriched with C and O, and also partly with B and Al. Additives and impurities are distributed in an inhomogeneous manner; the heterogeneous inclusions are very differently sized from less than 0.1μm (STEM) to 10 to 20μm (WDX on polished specimens) and 200μm (AES on fracture surfaces). These results reveal the need for improving the production process.

Creep of silicon nitride-titanium nitride composites

The effect of particulate TiN additions (0–50 wt%) on creep behaviour of hot-pressed (5 wt%Y2O3 + 2 wt%Al2O3)-doped silicon nitride (HPSN)-based ceramics was studied. Creep was measured using a four-point bending fixture in air at 1100–1340 °C. At 1100 °C, very low creep rates of HPSN with 0–30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200 °C the creep rate is slightly higher, and at 1300 °C the creep rate is increased by three orders of magnitude compared to 1100 °C and rupture occurs after a few hours under creep conditions. It was established that the formation of a TiN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter rupture times of the samples. Activation energies of 500–1000 kJ mol−1 in the temperature range of 1100–1340 °C at 100 MPa, and stress exponentsn⩽4 in the stress range 100–160 MPa at 1130–1200 °C were calculated. Possible creep mechanisms and the effect of oxidation on creep are discussed.

Advanced Evaluation of Push‐In Data for the Assessment of Fiber Reinforced Ceramic Matrix Composites

Fiber reinforced ceramics are ready to be used in wide spread applications due to their unique combination of typical ceramic properties and enhanced fracture toughness. This outstanding combination is only achievable if appropriate fiber–matrix interfaces are ascertained. The interfacial state can favourably be measured by the fiber push-in test. This method is reviewed in this paper and further elaborated in order to derive quantitative prediction of the composite performance from fiber–matrix interface bonding and friction data. The problem of overestimating the frictional forces due to the Poisson effect is overcome by means of analysing the hysteresis area from two load–unload-cycles. Significant dependence of interface friction on fiber diameter is detected as it is predicted taking into account the effect of fiber roughness on the interface clamping stress. A statistical procedure is applied to determine the critical debond stress which is the upper limit to prevent brittle fiber failure.

Evaluation of interface parameters in push-out and pull-out tests

Abstract Push-out and pull-out tests with single-fibre model composites are performed to evaluate the frictional parameters of the interface. Analysis of the experimental data according to models based on the assumption of an ideal cylindrically shaped fibre does not satisfy all observed phenomena. Thus, a model is proposed to include the effects of roughness interaction during fibre sliding. The predictions of the model fit the experimental results very well.

Oxidation of yttria- and alumina-containing dense silicon nitride ceramics

Abstract The oxidation behaviour of Y2O3 + Al2O3-doped Si3N4-based ceramics was investigated in air up to 1500°C under isothermal and non-isothermal conditions. The oxidation resistance of materials is strongly dependent on the content of additives, secondary phases and processing methods, as evident from the behaviour of five materials of the same starting composition prepared by hot-pressing and gas-pressure sintering. Gas-pressure sintered ceramics containing crystalline yttrium-silicon oxynitrides in the intergranular phase showed instability at intermediate temperatures and lower oxidation resistance than hot-pressed ceramics prepared from the same powder mixtures. Hot-pressed ceramics exhibited excellent resistance to oxidation by air at temperatures up to 1400–1450°C, because of the formation of a protective oxide layer. A decrease of oxidation rates with the increase of additives content was found at temperatures below 1300°C, while the reverse was the case for all materials above 1400°C.

A push-out technique for the evaluation of interfacial properties of fiber-reinforced materials

Abstract The friction stress and the shear strength of the interfaces in fiber-reinforced composites can be calculated from load-displacement diagrams as determined in push-out tests. In this study a fiber push-out instrument was developed, which is based on the extension of a piezo-translator connected with a hard-metal tip. Both the indentation process and the acquisition of data were carried out under computer control. Poisitioning of the indenter tip over the fiber axis was achieved using a video system with a macro-objective, allowing in-situ observation of the fiber push-out process. The examined material is a single-fiber composite consisting of a SiC fiber in an aluminosilicate glass matrix. The results of the push-out experiments and the calculation of the interfacial shear strength and the interfacial friction stress are presented and compared to data described in the literature.

Effects of grain size and specimen geometry on the transformation and R-curve behaviour of 9Ce-TZP ceramics

Transformation and R-curve behaviour have been investigated in 9 mol% Ce-TZP ceramics with different grain sizes. Both single-edge notched beam and short double-cantilever beam specimens were tested to measure the crack-resistance curves. The size and shape of the transformation zone not only depend on grain size, but are also strongly influenced by the specimen geometry. This different transformation behaviour has led to different crack-resistance curves. These experimental results are discussed in terms of the thermodynamics of transformation, the effect of autocatalytic transformation, and fracture mechanics.

Fibre-reinforced SiC-matrix composites: Microstructure, interfaces and mechanical properties

Abstract Two fibre-reinforced SiC-materials containing SiC- and C-fibres, respectively, are characterized by evaluation of their microstructures, especially by microanalysis of the interfaces between fibre and matrix and by their mechanical properties. The interfaces show a layered structure consisting of C-layers and an intermediate SiO 2 -layer in the case of C-fibre and SiC-fibre reinforced ceramics, respectively. C-layers are also found to interrupt the SiC-matrix, then discriminating the individual chemical vapour infiltration (CVI) sequences during the formation of the matrix. Both interlayer phenomena can be effective in provoking favourable pull-out processes during mechanical loading of the composites. While the C-fibre reinforced ceramic reveals an increase in strength when being tested in vacuum at elevated temperatures, the strength of the SiC-fibre reinforced ceramic decreases drastically in the high-temperature test in air, both being compared to room-temperature conditions. This loss of strength is caused by the destruction of the interfacial C-layers due to oxidation processes.