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Results of a study of the microstructure of ceramic matrix composites (CMC) in the SiC -C f system with high crack resistance and strength tested under conditions of unidirectional infiltration of molten high-purity silicon by methods of electron and optical microscopy and fractography are reported.Crack resistance has been and continues to be an issue of major concern in ceramic technology considering that highmelting nonmetallic materials share a common shortcoming -brittleness over a wide temperature range. To remedy the situation, one will have to develop a structural design focused on increasing resistance to brittle failure [1,2].Fibrous reinforcement provides a route towards increasing the resistance of ceramic matrix composites (CMC) to failure. CMC display a nonlinear strain behavior and retain carrying capacity under heavy loading conditions, unlike unreinforced ceramics prone to catastrophic failure when subjected to maximum loading. Promising reinforcing elements are carbon fibers. They display a low density, high strength, high elastic modulus, and low plasticity; furthermore, they have a tendency to soften with temperature [3,4].In our studies, major attention was focused on the development of CMC in the SiC -C f system produced by infiltration of silicon into fibrous carbon preforms. Advantages of these composites are the low density (1.8 -2.3 g/cm 3 ), ability to sustain high working temperatures (2000°C and even higher), and moderate cost, which opens up new fields of use in advanced areas of technology, especially aircraft and aerospace engineering.In this study, we used carbon ribbons and fibers based on PAN-fibers (available from domestic manufacturers) [5]; their characteristics are given in Table 1.The tests were carried out on specimens prepared by infiltrating high-purity silicon (semiconductor grade) into a C -C f semifinished product which was a prepreg containing 25 to 57% fibrous carbon elements arranged into a unidirectional pattern. The binder was a bakelite-based varnish containing powdered silicon, boron, and pyrocarbon.The microstructure of the composite materials was studied by scanning electron microscopy (JSM-35 CF microscope) and optical microscopy (Neophot-30 metallurgical microscope). The local CMC chemical composition was studied by electron probe x-ray microanalysis (EPXMA) using an ASID-4D scanning attachment of the JEM-100CX electron microscope interfaced to a KEVEX energy-dispersive x-ray spectrometer. The materials were analyzed for composition by x-ray phase analysis methods using a DRON-6 diffractometer. The mechanical characteristics were measured using a 9024 DP-100/1500 unit.The microstructure of a material is known to be related to many of the material's properties, including its strength and crack resistance; therefore in studying CMC specimens, our attention was focused on the following structural features: buildup of a dense defect-free matrix; achieving a uniform distribution of the reinforcing fibrous elements; separating the fibers by a sandwiched matrix la...

UDC 666.775Ultradispersed compositions of silicon nitride-yttrium oxide, silicon nitride-magnesium oxide, and mixtures of them with hexagonal boron nitride were used as the original powders for production of constructional ceramic materials.Various production methods used in the Special Design Office for Inorganic Materials of the Institute of Inorganic Chemistry of the Latvian Academy of Sciences and providing high chemical, phase, and grain size uniformity of the ultradispersed compositions were used. For example, the uniformity of nitrogen, free silicon, and yttrium content reaches 0.1-0.2 wt. %. The granulated original powders have a volume weight of 0.4-0.7 g/cm 3, which makes it possible to produce large parts up to 250 mm in weight and 2-2.5 kg in weight. The hot pressing method was developed on equipment designed and built in Technology Okhtinsk Scientific and Production Union. The set of equipment includes three presses with capacities of 0.05, 0.i, and 0.25 MN.In the silicon nitride-yttrium oxide system OTM-906 and OTM-914 materials were developed. They differ from one another in the content of sintered addition to the original raw material, which determines the differences in physico-mechanical, thermophysical, and special properties. The OTM-914 material exceeds the OTM-906 in the level of working temperatures, hightemperature strength, and crack resistance (Fig. i).The high service properties of the materials developed are the result of the optimum microstructure and phase composition, for which the presence of primarily elongated grains of hexagonal 8-SiBN ~ surrounded by refractory crystalline yttrium silicate phases is characteristic (Fig. 2).

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