Yu. G. Gogotsi

Synthesis and properties of ceramics in the SiC — B4C — MeB2 system

The effect of impurities and additives of titanium and zirconium borides on the structure and mechanical properties of SiC — B4C ceramics over a broad temperature range has been investigated. The ceramics was fabricated by hot pressing without a protective medium. Introduction of borides is accompanied by improvement in all the studied mechanical properties at room temperature, and the nature of hardening of the ceramics is practically independent of the type of SiC powders used. At high temperatures, the mechanical behavior of the ceramics is determined by the impurity composition: the ceramics obtained using abrasive powders loses strength beginning at 600°C, while using powders with decreased impurity content makes it possible to preserve the strength of the material up to a temperature of 1400°C.

Influence of oxidation on the strength of hot-pressed silicon nitride ceramic at high temperatures

Silicon nitride-base materials are widely used for production of parts of various high temperature equipment. In the development of such materials primary attention is devoted to providing the necessary level of mechanical properties at the operating temperature [i, 2]. The rules of failure and the temperature relationships of strength of pure single phase polycrystalline materials were investigated in [3, 4]. In the low-temperature zone the ss which is dependent upon brittle fracture processes, remains constant or changes insignificantly. In the next temperature zone there is formed a peak in strength related to relaxation of stress raisers as the result of plasticity of the grain boundary phases or the bodies of the grains of the matrix. In the third zone significant loss of strength caused by creep occurs.

Mechanical properties and special features of the structure of materials based on boron carbide

The authors employ destructive tensile testing in conjunction with x-ray diffraction, scanning electron microscopy, and electron microprobe analysis to determine correlations between the fracture properties and microhardness of boron carbide ceramics and their microstructure. Other properties studied for signs of correlation include tensile strength, limiting strain, and elasticity moduli. Results are depicted in tables and micrographs.

The effect of oxidation in the presence of salts on the strength of silicon nitride base ceramics

The authors assess the effects of oxidation on the elasticity, ultimate high-temperature strength, and stress-strain behavior of a silicon-nitride-based ceramic used in gas turbine engines. Using an combination of chemical, mechanical and ultrasonic techniques, they comparatively test the corrosive effects of sodium chlorides, sodium sulfates, seawater, and the vanadium pentoxide found in the exhaust gases of low-quality fuels on the ceramic, as well as the kinetics of the corrosion reaction itself.

High-temperature oxidation of sintered lanthanum hexaboride

ConclusionsThe intense interaction between sintered LaB6 with atmospheric oxygen starts, at temperatures close to 700°C, The mechanism of the process and composition of the interaction products change depending on temperature. Oxidation resistance of LaB6 up to 1200°C is satisfactory. At higher temperatures, catastrophic oxidation of the material caused by active evaporation of boron oxide takes place.

Effect of oxidation on the strength of silicon nitride base reaction sintered ceramics

ConclusionsOxidation of silicon nitride base reaction sintered material at 1400°C with subsequent removal of the oxide layer makes it possible to increase the strength of the material at 20°C by 20%. On the other hand, short-term strength at 1400°C after such treatment is lower than the strength of nonoxidized specimens, yet it is still greater than the strength of oxidized specimens from whose surface the oxide layer was not removed.The increased strength of specimens after oxidation is mainly connected with the fact that defects in the subsurface layers of ceramics are healed by the oxide phase and that impurities are removed from the inner layers. Reduced strength is usually caused by the formation of an oxide layer with high concentration of defects on the surface of the specimens or by considerable internal stresses arising in the specimen.

Influence of batch preparation method on the structure and properties of ceramics in the Al2O3-ZrO2-Y2O3 system

Unfortunately, existing publications covering research on compositions in the AI203ZrO2-Y=O 3 system contain limited information on the relationships between the starting properties of the materials, determined by the methods used to prepare them, the structure and properties of the final products. In the first case this refers to powders produced by foreign industries. The aim of the present work was to study the effect of various methods of obtaining materials on the final properties and structure of the ceramics~

Effect of oxygen on the strength of reaction-bonded silicon nitride

ConclusionsWhen obtaining reaction-bonded Si3N4, drying of the silicon powder or the charge at ≥400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.Addition of small quantities of SiO2 to the reaction-bonded Si3N4 improves the high-temperature strength to some extent although it affects the room temperature strength adversely.

Structure and properties of hot-pressed material based on silicon nitride

Phase analysis was performed on specimens and the starting materials, and microscopic studies were performed in a transmission electron microscope with an attachment for scanning electron microscopy and X-ray microanalysis. Ultimate strength was determined with threepoint bending, and crack resistance was determined by indentation in accordance with [4] and also from specimen fractures as described in [5]; Vickers hardness measurements were made.

Oxidation resistance and strength of hot-pressed ceramics based on silicon nitride

The authors investigate the metallurgical effects of boron nitride, magnesium nitride, and magnesium oxide additions to silicon nitride on the corrosion and heat resistance and the fracture behavior and ultimate strength of the silicon nitride-based ceramic. The phase interactions between the constituents are also investigated. Oxidation experiments are conducted as well as mathematically represented.