The structure and the electrical properties of porous and dense liquid-phase sintered silicon carbide ceramics (LPS-SiC), containing yttria and alumina additives, have been studied. The electrical resistance of LPS-SiC-materials varies in a wide range. This paper is focused on the influence of porosity on the electrical resistance of the sintered SiC. The porosity was controlled by the SiC grain size. Porous LPS-SiC materials were found to have substantially lower electrical resistance in comparison to dense materials of the same type. The structure of the materials was investigated by XRD and FESEM. The porous materials consist of large grains due to the coarser starting powders in comparison to the dense materials. This results in a reduction of the dissolved and re-precipitated fraction of the SiC during sintering. Using the in-lens SEM mode, the high conductivity of the formed rims of the SiC grains could be shown. These observations reveal that the rim volume of dense LPS-SiC is much more extended than the rims of porous materials showing the higher conductivity.

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Silicon nitride ceramics are used under conditions were high strength, hardness and wear resistant is necessary. The increasing use of Si 3 N 4 -ceramics as ceramic ball bearings and valves in chemical apparatus demand an understanding of the relations between microstructure and corrosion behaviour of silicon nitride ceramics in different environments. The stability of the ceramics against corrosion in acids and bases is mostly controlled by the stability of the grain boundary. Therefore, the influence of different rare earth ions and the influence of the crystallisation of the grain boundary on the stability in sulphuric acid were investigated. The results showed that the nature of the rare earth has a minor influence on the corrosion behaviour. The stability against corrosion of the ceramics with amorphous grain boundaries is mostly controlled by the amount of the SiO 2 in the grain boundary. The crystallisation of the grain boundary can improve or not change the corrosion behaviour depending on the amount of crystalline grain boundary phases and the composition stability of the residual amorphous grain boundary phase. The results are discussed in the context with previous investigation and some systematisation of the corrosion behaviour of Si 3 N 4 in acids and bases is given.

The influence of additions of molybdenum disilicide (MoSi2) on the microstructure and the mechanical properties of a silicon nitride (Si3N4) material, with neodymium oxide (Nd2O3) and aluminum nitride (AIN) as sintering aids, was studied. The composites, containing 5, 10, and 17.6 wt% MoSi2, were fabricated by hot pressing. All materials exhibited a similar phase composition, detected by X‐ray diffractometry. Up to MoSi2 additions of 10 wt%, mechanical properties such as strength, fracture toughness, or creep at 1400°C were not affected significantly, in comparison to that of monolithic Si3N4. The oxidation resistance of the composites, in terms of weight gain, degraded. After 1000 h of oxidation at 1400° and 1450°C in air, a greater weight gain (by a factor of approximately three) was obtained, in comparison to that of the material without MoSi2. Nevertheless, after 1000 h of oxidation, the degradation in strength of the composites was considerably less severe than that of the material without MoSi2. An additional layer was formed, caused by processes at the surface of the Si3N4 material, preventing the formation of pores, cracks, or glassy‐phase‐rich areas, which are common features of oxidation damage in Si3N4 materials. This surface layer, containing Mo5Si3 and silicon oxynitride (Si2ON2), was the result of reactions between MoSi2, Si3N4, and the oxygen penetrating by diffusion into the material during the hightemperature treatment.