Mathias Herrmann

Silicon Nitride/Silicon Carbide Nanocomposite Materials: II, Hot Strength, Creep, and Oxidation Resistance

The mechanical properties of Si3N4/SiC nanocomposite materials that contained nanosized intercrystalline SiC dispersions that originated from different starting powders and were made via different fabrication routes were studied in the temperature range of 1400°-1550°C. The strength retained at 1400°C was between 70% and ∼100% of the room-temperature strength. Both creep and oxidation resistance were very high and were comparable to or better than those of the best Si3N4-based materials published previously. The effect of SiC particles on the creep properties can be understood in terms of a recent model of dilatational creep; however, the model invokes a series of microstructural, micromechanical, and chemical modifications.

The effect of processing conditions, amount of additives and composition on the microstructures and mechanical properties of α-SiAlON ceramics

Abstract A series of samples with yttrium α-SiAlON compositions and different amounts of additive has been fabricated from α-Si 3 N 4 , AlN, Al 2 O 3 and Y 2 O 3 starting powders, using gas pressure sintering and three different sintering procedures. One series of samples was heated up to 1825°C and then held for 3 h, another group of samples was held at a lower temperature (1500 or 1600°C) for 1 h and then heated up to 1825°C and held for 3 h. The results of investigations using scanning electron microscopy showed the effect of composition and sintering procedure on the morphology of α-SiAlON grains. It was found that the amount of elongated grains increased with increasing amount of liquid phase. The mechanical tests showed that all of the samples exhibited HV10 values in the range of 1800–1976 kg/mm and K IC values in the range of 3.9–6.3 MPam 1/2 .

Preparation and application of cellular and nanoporous carbides

A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review.

Platinum induced crosslinking of polycarbosilanes for the formation of highly porous CeO2/silicon oxycarbide catalysts

A new synthesis scheme for the formation of porous CeO2/Pt-polycarbosilane composites using inverse microemulsions is presented. Aqueous hexachloroplatinic acid was used as a hydrosilylation catalyst causing crosslinking of allyl groups in a liquid polycarbosilane (PCS). The resulting polymers are temperature stable and highly porous. The Pt catalyst content and post-treatment of the polymer can be used to adjust the porosity. For the first time hydrophobic polymers with specific surface areas up to 896 m2/g were obtained by catalytic crosslinking of polycarbosilanes. Ceria nanoparticles 2–3 nm in diameter are well dispersed in the PCS matrix as proven using high resolution electron microscopy. Porosity of the hydrophobic materials could be increased up to 992 m2/g by adding divinylbenzene in the oil phase. Pyrolyses at 1200–1500 °C and post-oxidative treatment at various temperatures produce porous ceramic structures with surface areas up to 423 m2/g. X-Ray diffration investigations show that the crystallinity of the SiC matrix can be controlled by the pyrolysis temperature. Post-oxidative treatments cause silicon oxycarbide formation. Structure and morphology of the polymeric and ceramic composites were investigated using 29Si MAS NMR, FESEM, FT-IR and EDX techniques. The temperature programmed oxidation (TPO) of methane shows a high catalytic activity of CeO2/Pt-SiC(O) composites lowering the onset in the TPO to 400–500 °C.

Corrosion of silicon nitride materials in acidic and basic solutions and under hydrothermal conditions

Abstract Silicon nitride ceramics with different amounts and compositons of the grain boundary phases are produced by gas pressure sintering and the corrosion behaviour was analysed in acids, bases and under hydrothermal conditions. The investigation shows, that the corrosion is strongly affected by the SiO 2 content in the grain boundary and the amount of the additives. The results indicate that the grains and the thin films between the grains are very stable in acids and bases but solves partially under hydrothermal conditions. The corrosion behaviour of the investigated materials with a relatively stable grain boundary phase in acids can be described by the same kinetic laws as those used to describe the corrosion of glasses. An interfacial reaction is assumed to be rate-controlling. The less stable material with Y 2 O 3 /Al 2 O 3 additives can be described by the corrosion behaviour of oxide nitride glasses only at the beginning. For higher corrosion depths, the formation of protective barriers consisting of a hydrated SiO 2 network is assumed to occur.

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.

Microstructure and properties of thermally sprayed silicon nitride-based coatings

The preparation of thermally sprayed, dense, Si3N4-based coatings can be accomplished using composite spray powders with Si3N4 embedded in a complex oxide binder matrix. Powders with excellent processability were developed and produced by agglomeration (spray drying) and sintering. Optimization of the heat transfer into the powder particles was found to be the most decisive factor necessary for the production of dense and well-adhering coatings. In the present work, different thermal spray processes such as detonation gun spraying (DGS), atmospheric plasma spraying (APS) with axial powder injection, and high-velocity oxyfuel spraying (HVOF) were used. The coatings were characterized using optical and scanning electron microscopy (SEM), x-ray diffraction (XRD), and microhardness testing. The wear resistance was tested using a rubber wheel abrasion wear test (ASTM G65). In addition, thermoshock and corrosion resistances were determined. The microstructure and the performance of the best coatings were found to be sufficient, suggesting the technical applicability of this new type of coating.

Effect of long-term oxidation on creep and failure of Si3N4 and Si3N4/SiC nanocomposites

Abstract The high-temperature mechanical behaviour of an Si 3 N 4 /SiC nanocomposite and its monolithic Si 3 N 4 reference material was studied after long-term oxidation treatments intended to simulate future operating conditions in a severe environment. Creep and failure at elevated temperature were significantly affected, in the direction of increased brittleness. The transition stress between the ductile range present at low stresses and the brittle range existing at high stresses was shifted to distinctly lower values. The creep resistance in the low-stress range was increased by the oxidation treatment. The failure time under a given stress was drastically reduced; this was attributed to an increased sensitivity to subcritical crack growth. The failure stress for a given failure time was decreased by about half. The phenomena are explained in terms of a purification of the intergranular phase and by the formation of surface defects and of a uniformly distributed pore population.

Colour of gas-pressure-sintered silicon nitride ceramics Part I. Experimental data

Gas-pressure sintering is the most widely used process for manufacturing high-performance silicon nitride ceramics. The interaction between the sintering atmosphere and the silicon nitride ceramics is often featured by an inhomogeneous grey coloration of the near surface area and a grey coloration of the bulk material. In Part I of this work the dependence of the colour of the material on the sintering parameters was investigated experimentally. It was shown that silicon inclusions having sizes ranging from a few nm up to several μm cause the coloration of the material. Gas pressure, temperature and sintering time influence the formation of the inhomogeneous grey coloration.

High Temperature Oxidation and Corrosion of Silicon-Based Non-Oxide Ceramics

The present study is focused on the oxidation behavior of nonoxide silicon-based ceramics. Various Si{sub 3}N{sub 4} and SiC ceramics were examined after long term oxidation tests (up to 5,000 h) at 1,500 C in ambient air. The damage mechanisms were discussed on the basis of a comprehensive chemical and microstructural analysis of the materials after the oxidation tests. The diffusion of oxygen into the material and its further reaction in the bulk of the material were found to be the most critical factors during long term oxidation treatment at elevated temperatures. However, the resulting damage in the microstructure of the materials can be significantly reduced by purposeful microstructural engineering. Using Si{sub 3}N{sub 4}/SiC and Si{sub 3}N{sub 4}/MoSi{sub 2} composite materials provides the possibility to improve the high temperature stability.