Stability of refractory compounds in fused basalt

Summary A study was made of the reaction of titanium, zirconium, niobium, molybdenum, and tungsten carbides, aluminum, zirconium, and silicon nitrides, titanium, zirconium, chromium, and molybdenum borides, molybdenum disilicide, chromic oxide, and refractories based on silicon nitride and carbide with molten basalt at a temperature of 1400°C. It has been established that molybdenum disilicide exhibits the highest stability in molten basalt.Translated from Poroshkovaya Metallurgiya, No. 2 (50), pp. 47–49, February, 1967.     

Surface of powders of silicon nitride produced by self-propagating high-temperature synthesis

Conclusions The surface of a starting silicon powder used in the production of silicon nitride by the SHS method is covered with an oxide film whose thickness varies, depending on the method of comminution and time of storage of the powder, in the range 1–12 nm. During SHS the Si + SiO₂ reaction results in the formation of gaseous SiO, which becomes adsorbed on the cold walls of the reactor and then experiences disproportionation. This lowers the oxygen content of the resultant silicon nitride, but the overall purification effect achieved is small. After comminution, a 1- to 2-mm-thick layer of powdered silicon nitride produced by SHS consists of a silicon oxynitride with an oxygen content of 5–30 wt.%. Milling conditions do not significantly affect the concentration of oxygen inside the nitride particles, which does not exceed 0.4%. To obtain silicon nitride of low oxygen content, it is necessary to employ starting reactants of high purity.Translated from Poroshkovaya Metallurgiya, No. 1(253), pp. 48–54, January, 1984.     

Reaction of cubic boron nitride with titanium iodides

Conclusions The reaction of cubic boron nitride with the gaseous phase of titanium iodides (process duration 0.25–2.0 h, total pressure of titanium iodides 100 Pa) results in the preferential formation of TIN in the temperature range 900–1200°K and of titanium nitride and titanium diboride at 1200–1300°K.Translated from Poroshkovaya Metallurgiya, No. 10(262), pp. 57–59, October, 1984.     

Effect of oxidation on the strength of silicon nitride base reaction sintered ceramics

Conclusions Oxidation of silicon nitride base reaction sintered material at 1400°C with subsequent removal of the oxide layer makes it possible to increase the strength of the material at 20°C by 20%. On the other hand, short-term strength at 1400°C after such treatment is lower than the strength of nonoxidized specimens, yet it is still greater than the strength of oxidized specimens from whose surface the oxide layer was not removed.The increased strength of specimens after oxidation is mainly connected with the fact that defects in the subsurface layers of ceramics are healed by the oxide phase and that impurities are removed from the inner layers. Reduced strength is usually caused by the formation of an oxide layer with high concentration of defects on the surface of the specimens or by considerable internal stresses arising in the specimen.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 40–44, May, 1986.     

Dielectric characteristics of silicon nitride powders in the SHF range

Conclusions The dielectric properties of silicon nitride are strongly affected by the presence of impurities, in particular oxygen. The dielectric permittivity of silicon nitride, calculated from data yielded by measurements at a frequency of 9.5 GHz, is 6.3–7.1, and the tangent of its dielectric loss angle, (5.3–9.7) · 10^–3.Translated from Poroshkovaya Metallurgiya, No. 9(213), pp. 62–70, September, 1980.     

Removal of binder from silicon nitride specimens

Conclusions 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.Translated from Poroshkovaya Metallurgiya, No. 7 (151), pp. 99–101, July, 1975.     

Reaction of a molten mixture of titanium,aluminum, and silicon oxides with hot-pressed silicon nitride

The formation temperature of a liquid phase and the solidification temperature of a molten mixture of Al₂O₃-TiO₂-SiO₂ oxides on a silicon nitride substrate are determined. Data are obtained for the change in kinetics. It is established that the intensity of interaction of molten Al₂O₃-TiO₂-SiO₂ with silicon nitride depends on the oxide mixture composition. With heating there are two possibilities: improvement and worsening of Si₃N₄ crystallite wetting with a liquid phase as well as solidification of the melt. The temperature range where a liquid phase exists for actual materials is about 15°C, which markedly worsens the process of structure formation with Si₃N₄ during sintering.Translated from Poroshkovaya M etallurgiya, No. 5, pp. 39–44, May, 1993.     

Contact reaction of Kh20N80 alloy with boron nitride

Conclusions A study was made of the causes of the contact reaction between Kh20N80 alloy and boron nitride during the sintering of composite compacts in various environments at temperatures of 1170–1330°C. It was established that the contact reaction between Kh20N80 alloy and boron nitride occurs in the following stages:partial dissociation of the boron nitride, with the evolution of free nitrogen, which then reacts with the chromium;reaction of boron with the nickel and chromium, with the formation of boride phases;enrichment of the nickel-base solid solution in chromium and the appearance of a chromium-base solid solution.In sintering in various gaseous environments the temperature at which partial melting begins during the contact reaction falls in the order argon (1330°C) > hydrogen and cracked ammonia (1250°C) > vacuum (1170°C).A 20% Cr-80% Ni alloy - Translator.Translated from Poroshkovaya Metallurgiya, No. 5(161), pp. 31–39, May, 1976.     

DENSE SILICON NITRIDE

Abstract

A method is described for preparing silicon nitride bodies of almost theoretical density. Silicon nitride powder is hot pressed with a small proportion of catalyst, preferably magnesium oxide or nitride, at 1850°C. The bodies have very high flexural strength up to high temperatures, and preliminary measurements of creep rate and thermal shock-resistance indicate that dense silicon nitride should be a suitable material from which to make components required to operate under high stress at temperatures up to ～1200°C.     

Morphological and technological characteristics of silicon nitride powder prepared by self-propagating high-temperature synthesis

Silicon nitride produced by self-propagating high-temperature synthesis is ground in planetary and ball mills successively in both devices. The size distribution of particles, specific surface, shape factor, structure, and bulk density are evaluated. The behavior of powder pelletized with synthetic rubber under compaction and sintering in a nitrogen atmosphere in a charge containing magnesium oxide is studied. Powder after ball milling appears to be the most technologically efficient. Empirical equations are obtained which connect bulk density and shrinkage during sintering of powders with their morphological characteristics. The increase in density during compaction governed by the physicomechanical properties of silicon nitride powder and the plastifier is similar for all of the powders tested.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 16–21, November, 1992.     

取向硅钢高温退火时内氧化层与MgO反应过程研究

对取向硅钢高温退火工艺进行实验室模拟,采用聚焦离子束显微镜（FIB）观察了氧化层中二氧化硅和氧化镁反应的微观形貌演变过程,采用能谱仪（EDS）分析了试样截面近表层Mg、Al、Si等元素的分布规律,最后采用透射电镜（TEM）分析了对成品试样硅酸镁底层的结构特征。结果表明：（1）Mg离子的扩散速度是影响硅酸镁底层反应的主要因素;（2）Mg离子最初沿着二氧化硅颗粒与铁基体之间的界面扩散,逐渐将二氧化硅颗粒包覆;随着温度的进一步升高,Mg离子开始向二氧化硅颗粒内部扩散,并与之反应;（3）随着温度的升高,特别是在AlN分解后,钢基中的Al会逐渐将钉扎部位的硅酸镁（Mg2SiO4）完全转化成为镁铝尖晶石（MgO·Al2O3）。     

Simultaneous nitriding of silicon and aluminum

Conclusions During the high-temperature nitriding of silicon-aluminum mixtures in nitrogen and ammonia the mixture components undergo nitriding separately, first aluminum and then silicon. Aluminum inhibits the nitriding of silicon, shifting the process toward higher temperatures. The inhibition is apparently due to a reaction between the silicon nitride and aluminum, with the formation of aluminum nitride films on the silicon particles. The nitriding of silicon and aluminum in nitrogen results in the formation of a limited solid solution of aluminum in a silicon nitride based on-Si₃N₄.Translated from Poroshkovaya Metallurgiya, No. 7(211), pp. 1–6, July, 1980.     

Hot pressing and vacuum sintering of fine titanium nitride powder

Conclusions Increasing the specific surface of titanium nitride from 18 to 90 m²/g lowers the initial recrystallization temperature of loosely poured powder from 1300 to 600°K. The temperature at which blanks attain practically 100% density in the hot pressing of finely divided titanium nitride (a starting powder particle size of 0.05–0.07 m) is 1600°K, which is 500–700°K below the temperature level of full sintering of relatively coarsegrained powders (a particle size of about 0.5 m). At hot-pressing temperatures above 1800°K a fall in the density of sintered compacts is observed, which is apparently attributable to the beginning of nitrogen evolution from the nitride and also to the formation of microcracks. In vacuum sintering without a plasticizer, crack formation lowering the density of specimens by 3–4% is characteristic of the whole sintering temperature range. The grain size in hot pressing and vacuum sintering is practically the same, being determined chiefly by the sintering temperature and time. At the maximum specimen density the maximum grain size is 20 m.Translated from Poroshkovaya Metallurgiya, No. 12(204), pp. 27–32, December, 1979.The authors wish to thank V. I. Berestenko, T. N. Miller, and D. I. Medvedev for the provision of titanium nitride specimens.     

A model investigation of the sintering of zirconium

Summary A study was made of the welding of iodide zirconium spheres of 2-ram diameter in a purified helium atmosphere (3 atm gauge) and in a vacuum (10^–4–10^–5 mm Hg). It is shown that welding in helium (1265–1810°C occurs mainly as a result of diffusion material transport, whereas welding in a vacuum (1500–1800°C is controlled primarily by an evaporation-condensation mechanism. The data obtained are used to evaluate the self-diffusion and evaporation parameters and to derive equations expressing the self-diffusion coefficient and vapor pressure of zirconium as functions of temperature over the range investigated.Translated from Poroshkovaya Metallurgiya, No. 6(66), pp. 41–46, June, 1968.The authors are indebted to V. V. Khromonozhkin for helpful discussion and to V. G. Osintsev for making available the test materials.     

Preparation of ferric oxide for ferrites based on goethite synthesis by an ammoniacal method

Conclusions It is shown that goethite (yellow ferrie oxide pigment) synthesis by an ammoniacal method offers a suitable means of obtaining, from technical iron vitriol, high-purity ferric oxide with stable properties fulfilling the requirements for ferrite-grade ferric oxide. The ferric oxide is distinguished by its high reactivity after high-temperature (850–900°C) heat treatment. Its physicochemical properties are close to those of 1360 WF grade ferric oxide manufactured by the Bayer Co. of West Germany [5]. The key process parameters in the production of ferric oxide by this method are: goethite synthesis pH 4.0–4.5, synthesis temperature 70–75°C, amount of seed 8–10% of the goethite content at the end of synthesis, and ferric oxide calcining temperature 850–900°C.Translated from Poroshkovaya Metallurgiya, No.4(160), pp. 82–88, April, 1976.     

Nature of strain hardening of PKh23N18 corrosion-resisting powder steel

Conclusions The temperature dependence of the yield stress of PKh23N18 corrosion-resisting powder steel is nonmonotonic. The formation of the maximum on the curve in the range 575–795°K is determined by the interaction between the dislocations and the carbon atoms. Dynamic strain aging of PKh23N18 steel in the temperature range 795–925°K is controlled by the chemical interaction between the split dislocations and chrome atoms.Translated from Poroshkovaya Metallurgiya, No. 1(277), pp. 86–91, January, 1986.     

Investigation of the heat resistance and thermal stability of micaceous ceramic materials with additions of boron nitride

Conclusions On the basis of complex investigations it has been established that Brand UMB-5KT materials, having a high heat resistance and satisfactory thermal stability, can be recommended for use in the flowthrough parts of high-head compressors, with a temperature of the gas flow equal to 600–700°C, and in certain types of turbines with a working temperature up to 950–1000°C.It has been established that the requirements for densified materials are most fully satisfied by Brand UMB-5KT material with 4–8% boron nitride.Special graphite K70/30-3 and aluminum-asbestos-graphite layers have sufficient heat resistance only up to 500°C, and cannot be recommended for long-term operation at higher temperatures.Deceased.Translated from Poroshkovaya Metallurgiya, No. 9 (81), pp. 80–86, September, 1969.     

High-temperature friction of alloys of the TiNx-TiB2 system

Conclusions It has been established that the highest transverse rupture strength and wear resistance combined with the lowest coefficient of friction are shown by alloys of eutectic composition. In alloys of the TiN_0·9-TiB₂ system higher ductility during friction is exhibited by the boride phase, whose substructure experiences greater changes compared with the nitride phase. The greatest deformation during friction characterizes the phases in the TiN_0·73-TiB₂ alloy. The strengthening and strength loss processes at temperatures of 20–400°C are determined by the strength loss processes occurring in the boride phase, and those above 400°C, by the strengthening and strength loss processes taking place in the nitride phase.Translated from Poroshkovaya Metallurgiya, No. 2(242), pp. 70–76, February, 1983.     

Reaction between carbon fibers and nickel through a zirconium nitride or titanium carbide diffusion barrier

Conclusions Carbon fibers in contact with nickel suffer a pronounced loss of strength at temperatures above 600°C as a result of surface attack by the nickel (at temperatures of 800–900°C and treatment times of 10–20 h) or recrystallization (during heating for more than l h at 1000°C). There is no unique correlation between fiber structure and strength. A zirconium nitride coating fails to prevent carbon fibers from reacting with nickel. A titanium carbide coating reduces, even at high temperatures, the strength loss experienced by carbon fibers by decreasing the rate of dissolution of the material of the fibers in nickel and by slowing down the fiber recrystallization process.Translated from Poroshkovaya Metallurgiya, No. 7(175), pp. 32–35, July, 1977.     

High-temperature oxidation of sintered lanthanum hexaboride

Conclusions The intense interaction between sintered LaB₆ with atmospheric oxygen starts, at temperatures close to 700°C, The mechanism of the process and composition of the interaction products change depending on temperature. Oxidation resistance of LaB₆ up to 1200°C is satisfactory. At higher temperatures, catastrophic oxidation of the material caused by active evaporation of boron oxide takes place.Translated from Poroshkovaya Metallurgiya, No. 11(299), pp. 56–59, November, 1987.