Silicon nitride materials produced by the hot-pressing and reactive sintering methods find wide application in engineering. At high temperatures, however, their strength sharply falls, and consequently, as constructional materials, they can only be u sed up to 1200~ With hot-pressed materials, the fall in strength is due to a softening of a vitreous phase which is invariably present in them in the form of grain interlayers, while with reactively sintered materials it is attributable to the presence of uncombined silicon and to diffusional mobility, made possible by their fine microstructure and uneven porosity. It follows therefore that in attempts to produce reactively sintered materials suitable for operation at temperatures higher than 1200~ efforts should be concentrated on decreasing their porosity, lowering their uncombined silicon content, and increasing their grain size. In this connection, the work described below was undertaken with the aim of studying the effect of additions of rare-earth metal oxides on the microstructure and strength of a reactively sintered material of composition SigN4 + SiC. The reason why this particular topic was chosen for investigation was that, according to literature data, additions of rare-earth metals [I] and of silica [2] increase the degree of nitration of silicon. Apart from this, as is shown in [3], the presence of such oxides as LabOr, Ce203, and CeO2 in hot-pressed Si3N ~ promotes the formation of crystals of elongated shape. As a starting material for the production of specimens, a silicon powder of 5- to 20-~ particle size was used. The amount of iron, which activates the nitration of silicon [4], in the powder was ~2v/0. * Specimens were prepared by the thermoplastic casting technique [5]. The variable parameters were the silicon and silicon carbide contents, the SiC crystal size, and the amounts of additions of yttrium, lanthanum, cerium, europium, and gadolinium oxides. Sintering was performed in a nitrogen atmosphere at a temperature of 16001700~

An experimental study was made of the process of densification of zirconium diboride—niobium alloys containing 80, 60, and 40 mole% ZrB2. It was established that the highest density is attained after pressing for 8–10 min at temperatures of 2000–2100°C under a pressure of 18.4 MPa.

A study was made of the effects of temperature and holding time upon the process of binder removal from silicon nitride specimens prepared by the thermoplastic slip-casting technique. It was established that, to obtain high-quality blanks, treatment should be performed under isothermal conditions at a temperature of 80–100°C, which ensures that a large proportion of the binder used is removed through the liquid phase. In the manufacture of parts calling for close size limits, it is necessary to allow for the appreciable shrinkage exhibited by castings during the binder removal operation.

: The problem of creating reliable materials for the electrodes of open cycle MHD generators is still incompletely resolved. The accumulation of experimental data on the interaction of refractory electrode materials with aggressive flows is one of the pressing problems in seeking the necessary compositions, technology of separation and means of protection against electrode destruction. Carbides and borides of transition metals from groups IV-VI of the periodic system, having high melting points, high hardness, high temperature- strength, wear resistance, high thermal and electrical conductivity, are of considerable practical interest in high temperature technology, in particular when they are used as the electrode materials in MHD equipment. The work studies the behavior of niobium and titanium carbides, and zirconium boride in a flow of air plasma.

A study was made of the effects of compaction pressure and carbide content on the ejection force for the systems Fe-ZrC, Fe-NbC, Fe-Mo2C, Co-ZrC, Co-NbC, Co-Mo2C, Ni-ZrC, Ni-NbC, and Ni-Mo2C. It is demonstrated that the Pe/Pc ratio nonlinearly increases with rise in compaction pressure. With increase in carbide content, the value of Pe/Pc diminishes.

A study was made of the conditions of preparation of highly dispersed copper-base composite powders by the process of reduction of basic copper carbonate with hydrogen in the presence of 0.1–16 vol.% of TiC, ZrC, NbC, Cr3C2, Mo2C, and WC carbides. It is shown that virtually complete reduction is achieved in 2 h at 170–200°C, the resulting powder particles being < 1μ in size.

A study was made of the influence of zirconium, niobium, and molybdenum carbides on the magnitude of radial elastic after-effect in the cold pressing of cermets with iron, cobalt, and nickel binders. It was demonstrated that, with increase in compaction pressure and carbide content, the elastic after-effect increases.