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Ceramic matrix composites
No full textThe 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
Stabilization of cubic zirconia by aluminum nitride
No full textCubic 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
Oxidation-kinetics of reaction-sintered silicon-carbide
Get PDFThe 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
Properties of reaction bonded silicon-nitride obtained from slip cast preforms
No full textFabrication of silicon preforms of high green density (> 1.2 g/cm3) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59-85% alpha-Si3N4, 7-22% beta-Si3N4 and 7-23% Si2N2O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (alpha-Si3N4) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000-degrees-C or 1400-degrees-C or show slight increase up to 1000-degrees-C or 1200-degrees-C. In some cases a little dip around 1200-degrees-C, then a sharp increase in MOR up to 1400-degrees-C was observed. K(ic) values are almost constant up to 1000-degrees-C, and thereafter increase sharply. Pore size distribution, existence of Si2N2O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sintering and properties of sialons derived from kaolin
No full textThe sintering behavior of βâ²-sialon (Z = 2) derived from kaolin (AlâOâ{center dot}2SiOâ{center dot}2HâO) without additives is described. The refractoriness of the product increased with increasing nitrogen content. A product of 74% theoretical density was obtained after sintering at 1,700°C a synthesized product having 79.17% theoretical Nâ content. A fully dense product could not be made. The MOR of sintered specimens prepared from samples containing 79.17% theoretical nitrogen content retained it room-temperature strength up to 1,350°C after showing a brief increase to 1,000°C. The total creep strain for the same sample conducted at a load of 18 MN {center dot} mâ»Â² and at a temperature of 1,300°C was 0.06% in 20 h and 2.72% at 1,400°C in 30 h. The oxidation curve increased initially and leveled thereafter. The maximum weight gain at 1,350°C after 72 h was 4 mg/cm². The activation energy was calculated to be 47 kcal/mol. The thermal-expansion coefficient was 2.65 à 10â»â¶/°C from room temperature to 1,000°C. Corrosion in molten aluminum and fluoride baths is described
The comparative behaviour and some parameters on the carbothermal reduction and nitridation of aluminosilicates
No full textA few natural aluminosilicates with variable Al: Si ratios of 1:2, 1:1 and 2:1 have been subjected to carbothermal reduction and nitridation process. Beta'-Sialon with different z-values and in different amounts are formed associated with alpha-Al2O3. The degree of substitution in beta'-sialon is always lower than that predicted theoretically. Among all the aluminosilicates under study, kaolin is the best producer of sialon. The reactivity of the reduction and nitridation process increases with increasing Al:Si ratio of the starting raw aluminosilicates. The presence of iron is essential, which possibly acts as nitrogen carrier
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