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Silicon-carbide materials in recent years have received more attention as a result of the significant expansion of their uses in high-temperature equipment~ In many cases the factor determining the working capacity of the material is its strength at high temperatures~ The investigation of the strength of such materials, in particular self-bonded SiC, has been covered by a number of reports [1-5 etc.]. However, studies at high temperatures have been made either in air [2], or in inert gas [i], and therefore the results are difficult to compare, and it is not possible to make conclusions about the influence of the medium used for testing on the mechanical properties of such materials.Something of an exception is the work described in [3], the authors of which tested selfbonded SiC in argon and in air~ but in this case they examined a limited range of temperature and did not analyse the features of the temperature changes with respect to the material's strength.In [5] it was postulated that oxidation is the main cause of the reduction in strength of self-bonded SiC at high temperatures.At the same time in [6] it was reported that there was some healing of cracks in SiC materials during oxidation, and mention was made of the increase, due to this, in the strength of the specimens with preapplied defects.During a study of the properties of self-bonded silicon carbide it is necessary also to consider the presence in its composition of free silicon and impurities, since these components of ceramics greatly influence the mechanical strength [3], and the oxidation resistance [4].In order to explain the influence of an oxidizing atmosphere on the strength of selfbonded SiC the present authors carried out an investigation of the material similar to that described in [4~ 5], in air and in an atmosphere of argon.In order to carry out the experiments all the specimens were cut from one and the same blank in the form of plates measuring 15 • 190 • 190 mm. The density of the material of the blank was 3.02 g/cm 3. According to x-ray data and chemical analysis, its composition contained ~-and y-SiC, about 13% free silicon, the impurities iron, aluminum, calcium, magnesium, tin, titanium and others in amounts of up to 2%. The study of the oxidation resistance was done on specimens measuring 1.5 • 3 • 15 mm by the periodic weighing method.The fractures and the surfaces of the specimens after testing were studied with the scanning electron microscope JSM-T20.Mechanical rests were made on the specimens with equipment of the type 1958 Y-10-1 and TsD-4 with a displacement rate for the cross pieces of 0.5 mm/min in the range from 20 to 1400~In this case, we used a heating chamber ensuring the possibility of doing the work in air and in argon. To determine the deformability of the specimens a blending meter was fitted in the chamber~ when testing with air, in order to record the relationship between the load applied to the specimen, and its bending. Normally [7] these deformations are determined with respect to the displacement of the ...

Results obtained for a new material, namely, zirconia crystals stabilized by Y203 and CeO2 and containing a technological addition of neodymium oxide, tested at room temperature are described. Special features of the surfaces of the crystal blocks grown and the distribution of the additions introduced into the charge over their height are investigated. The dependences of the amounts of the stabilizing components and the fracture properties of the investigated material on its strength are determined. Certification results for the mechanical characteristics of the material are presented (mean four-point bending strength 1250 MPa, crack resistance in bending of a notched bar 11.43 MPa-m ~n at an elasticity modulus of 366 GPa for the crystallographic direction <100>). The crystals are investigated in the crystallographic plane { 100} by applying Vickers and Knoop indenters, and their hardness at different loads (beginning with 0.1 N) is determined. It is established that zones of phase transformations are formed near the indentations in addition to radial and lateral cracks. Problems of change in the specific features of the mechanical behavior of the crystals with change in the valence of cerium (most experiments were conducted for Ce203 ), with the heat treatment, etc. are considered. The results are analyzed using data of fractographic investigations and the new data obtained in tests of similar crystals partially stabilized by yt~ium and terbium oxides.It was established in the early 1980s [1] that crystals of partially stabilized zirconia possess relatively high mechanical parameters and other advantages that make them applicable for various purposes at temperatures up to 1600°C. However, these materials are not used in industry, although a technology for producing coarse crystals with high strength and crack resistance has been devised [2]. This can be explained by the fact that potential users have insufficient information on these crystals and that it is sometimes impossible to prepare materials with uniform structure and properties in an amount sufficient for practical purposes. In growing the crystals it is difficult to eliminate the appearance of cracklike defects, and in long-term storage they sometimes break spontaneously due to internal stresses. This shows that attention to investigating zirconia crystals for structural applications is insufficient. This especially concerns the interdependence of the technological regimes for preparing them, the phase composition, the structure, the laws governing their mechanical behavior, etc. On the whole, data that would make it possible to improve the production technology for these crystals and realize their potentialities are very scarce. As a continuation of previous work in this field [3], we present results of an investigation of crystals whose charge, in addition to zirconia (ZrO 2 ), contained Y203 and CeO 2 plus neodymium oxide 3 (Nd203 ) as a technological addition. These crystals were denoted Y-Ce-PSZ-K.Materials and methods. The experiments were conduct...

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