Oxidation-kinetics of reaction-sintered silicon-carbide

The oxidation kinetics of reaction-sintered silicon carbide has been studied over the temperature range 1200-degrees to 1350-degrees-C. The material has a bulk density of 3.00 g/cm3 and the unreacted Si content is 22.5% (v/v). The activation energy for oxidation is 28.75 +/- 2.61 kcal/mol. It is proposed that the diffusion of oxygen through the growing oxide film is the rate-controlling process

Preparation and characterization of silicon nitride-silicon carbide composites

Silicon nitride-silicon carbide (Si3N4-SiC) composites were prepared by varying the percentage of silicon nitride at temperatures of 1350 to 1450°C. The mechanical and thermal properties of these composites were determined. The modulus of rupture of the composites increases with increase of temperature whereas the thermal expansion decreases. Composites with 10% and 50% Si3N4 have modulus of rupture of 49 and 86 MPa at 1400°C and thermal expansion coefficients (25°–1000°C) of 4·4 × 10−6 and 3·2 × 10−6°C−1 respectively

Stabilization of cubic zirconia by aluminum nitride

Cubic ZrO2 is stabilized at room temperature by the addition of AIN. The percentage of c-ZrO2 in the mixture of c-ZrO2 and m-ZrO2 increases linearly with the addition of up to 20 mol% AIN and decreases thereafter. Stabilization becomes complete at 50 mol% AIN. The lattice spacing of the cubic phase gradually expands up to 20 mol% AIN. A displacement reaction ZrO2+ AIN → ZrN+Al2O3 occurs above 20 mol% AIN and is completed at 50 mol% AIN

Ceramic matrix composites

The present state of the knowledge of ceramic-matrix composites have been reviewed. The fracture toughness of present structural ceramics are not enough to permit design of high performance machines with ceramic parts. They also fail by catastrophic brittle fracture. It is generally believed that further improvement of fracture toughness is only possible by making composites of ceramics with ceramic fibre, particulate or platelets. Only ceramic-matrix composites capable of working above 1000 °C has been dealt with keeping reinforced plastics and metal-reinforced ceramics outside the purview. The author has discussed the basic mechanisms of toughening and fabrication of composites and the difficulties involved. Properties of available fibres and whiskers have been given. The best results obtained so far have been indicated. The limitations of improvement in properties of ceramic-matirx composites have been discussed. replace costly superalloys but also for use in those areas where even the superalloys did not perform

Sintering and properties of SiAlONs without externally added liquid-phase

β′-Sialon has the general formula Si6-ZAlZOZN8-Z The synthesis of pure β′-sialon with three different Z values of 0.5, 1.0, and 2.0 in the system Si3N4-AlN-Al2O3-SiO2 has been reported without the addition of a foreign sintering aid. A small shift in the composition toward the SiO2 corner has been made in each case. The Z=1.0 and 2.0 sialon can be sintered almost to full density while that with Z=0.5 sialon is difficult even with a higher amount of excess oxide addition. A packing bed of Si3N4 and SiO2 in a weight ratio of 7:3 was found to be most useful. The coefficient of linear thermal expansion of Z=1.0 sialon is 2.2°10-6/°C (25° to 1000°C). The room-temperature modulus of rupture value can be retained up to 90% of the value at 1400°C. A similar trend has also been observed in the KlC value with temperature. The steady-state flexural creep rate varies from 0.5°10-6 to 2.5°10-6 h-1 in the temperature and load ranges of 1200° to 1300°C and 100 to 250 MPa

Reaction sequences in the synthesis of silicon-nitride from quartz

Carbothermal reduction and nitridation of pure silica have been performed to synthesize pure Si3N4 powder. Iron was used to serve as a catalyst. The reduction reaction was studied with respect to different parameters such as temperature, soaking time and gas flow rate, etc. Almost pure Si3N4 powder with predominating beta-phase could be synthesised at 1540-degrees-C. Below this temperature SiC and Si2N2O are the associated phases. SiC is unstable to Si3N4 in nitrogen atmosphere and disappears at 1540-degrees-C or under a longer holding at 1440-degrees-C. There is a critical flow rate of the nitrogen gas of 1.32 x 10(4) litre m-2 h-1, above which the yield on nitridation almost flattens. It has been suggested through thermodynamic arguments that the Si3N4 formation at 1440-degrees-C takes place through the intermediate formation of SiC and Si2N2O as follows: SiO2(s) + 2SiC(s) + 2N2(g) = Si3N4(s) + 2CO(g) SiO2(s) + Si2N2O(s) + 3C(s) + N2(g) = Si3N4(s) + 3CO(g) At 1540-degrees-C the following reaction may predominate: 3SiO2(s) + 6C(s) + 2N2(g) = Si3N4(s) + 6CO(g

Wear of nitrogen ceramics and composites in contact with bearing steel under oscillating sliding condition

The wear of nitrogen based ceramics in contact with bearing steel balls (SAE 52100) was investigated under reciprocating sliding conditions at a velocity of 0.1 m/s, and 20 N, 40 N and 100 N load. Three types of ceramics were studied: viz. hot pressed silicon nitride (:HPSN) sintered with selected liquid in the system yttria-aluminium nitride-silica and composites of HPSN with BN and TIG; SiAlON formulated with different amounts of alumina and silica; and alumina-titanium nitride composite having 60mol% TIN. Among the HPSM composites, HPSN 15 vol% TiC had the lowest average wear factor (K) of 2.0 x 10(-5) mm(3)/m/N. The SiAlONs, in general, had the highest K and this increased with increasing O concentration. The lowest K value for SiAlON ceramic was 6.9 x 10(-5) mm(3)/m/N. The high K value of SiAlON is attributed to O substitution, which promotes adhesive wear resulting in mutual material transfer. The lowest wear of all the ceramics tested was found for the alumina-titanium nitride composite, the wear factor being of the order of 4.4 x 10(-7) mm(3)/m/N, which is one order of magnitude lower than the other nitrogen ceramics. In the case of HPSN and alumina-titanium nitride composites, abrasive wear and wear due to microfracture were the main wear mechanisms. Wear of the steel ball was lowest when in contact with HPSN-TiC composites, being almost equal to that of the ceramic, whereas the alumina-titanium nitride composite wore away the steel almost 20 times faster than the ceramic and will thus be unsuitable as a counterface material for 52100 steel. (C) 1997 Elsevier Science Limited and Techna S.r.l

Effect of crystal orientation, structure and dimension on vickers microhardness anisotropy of β-, α-Si3N4, α-SiO2 and α-SiC single crystals

Variation of Vickers microhardness on planes parallel to VMH(||C) and perpendicular to the C-axis VMH(IC) of hexagonal β-Si3N4, α-Si3N4, α-SiO2 (α-quartz) and α-SiC(6H) single crystals has been studied. The VMH (||C) VMH (||C) ratios have been correlated with a c ratios, a and c being unit cell parameters of the single crystals. VMH(||C) was found to be independent of orientation of Vickers indenter on basal plane (0001) of α-SiC, while VMH(||C) was found to vary with Vickers indenter direction on prismatic plane (10-10) of α-SiC. VMH(||C) of β-Si3N4 single crystals was found to depend on the aspect ratio of β-Si3N4, while VMH(||) was found to be independent of the dimension of basal plane. VMH(||C) of α-SiC single crystals was also found to be independent of the dimension of basal plane of α-SiC. © 1982

The effect of nitrogen-content on the sintering behavior and properties of SiAlON prepared from kaolin

The sintering of powders with variable nitrogen content prepared from the carbothermal reduction and nitridation of kaolin has been studied. With increasing nitrogen uptake in the starting powder, the refractoriness and the fusion point increase. The coefficient of linear thermal expansion decreases linearly with the nitrogen content of the sintered product. The product with 80% theoretical nitrogen has good oxidation resistance. The oxidation rate is parabolic, and the product contains mullite, cristobalite, and a glass. The X-sialon phase oxidises preferentially to -sialon