Purification of metal powders from oxide films in a jet mill

Conclusions During the jet comminution of metal powders covered with oxide films the latter shear off, and their particles detach themselves from the main volume of material. The amount of oxygen decreases during purification by a factor of 2–17 depending on the oxygen content of the starting material and the latter's hardness and particle size. The removal of oxide films is virtually completed in 5–30 min depending on the properties of the starting powder. For the purification of powders low-pressure (0.5-MPa) compressed air can be employed. During the comminution of powders covered with oxide films, in certain circumstances detached film particles may find their way into the comminuted material.Translated from Poroshkovaya. Metallurgiya, No. 2(230), pp. 1–4, February, 1982.E. A. Filler, of the Special Design and Technology Bureau of the Chemical Engineering Industry, also took part in this work.     

Microstructure formation in wurtzite BN (Geksanit-R) sinterings

Conclusions The loading of wurtzite boron nitride to high static pressures (55–100 kbar) under room-temperature conditions is accompanied by the processes of comminution of polycrystalline powder particles and their monocrystalline grains. Comminution of wurtzite boron nitride particles and grains has been found to take place also during sintering at high pressures and temperatures. The intensity of particle and grain comminution during loading and compression grows with increasing size of the particles in the starting condition. The comminution of particles and grains has a marked effect on structure formation in polycrystalline Geksanit-R alloy sintering.Translated from Poroshkovaya Metallurgiya, No. 8(236), pp. 32–37, August, 1982.     

Reaction between silicon nitride and magnesium oxide during hot pressing

Conclusions Using the methods of microscopical examination, chemical and mass spectrometric analyses, electron probe microanalysis, and weight loss determination, astudy was made of the reaction between technical silicon nitride powder and magnesium oxide. It is shown that the reaction involves not only the formation of forsterite, enstatite, and variable-composition glasses but also diffusion of the main components, with a gradual fall in magnesium concentration over the whole length of the diffusion zone in a model system. In the temperature range 1600–1700°C magnesium atoms drive out of the silicon nitride lattice silicon atoms, which then react with oxygen from the magnesium oxide or air to form silicon monoxide ions in the gaseous phase. The evaporation of the silicon monoxide in the system results in loss of weight, which grows with rise in temperature. The processes of evaporation and dissociation of the starting components, which sharply increase in intensity at 1800°C, make a certain contribution to the loss of weight.Translated from Poroshkovaya Metallurgiya, No. 9(177), pp. 89–96, September, 1977.     

The self-propagating high-temperature synthesis of a nanostructured titanium nitride powder with the use of sodium azide and haloid titanium-containing salt

The use of the “ammonium hexafluorotitanate (NH₄)₂TiF₆-sodium azide NaN₃” system during self-propagating high-temperature synthesis (SHS) allows researchers to obtain nanostructured titanium nitride powder. Nanopowders of silicon, boron, and aluminum nitrides are formed from similar “halogenide of nitrided element-sodium azide” systems by SHS. It is confirmed that the use of compounds rather than pure elements in the starting powder mixtures for SHS makes it possible to substantially decrease the dimensionality of the combustion products and obtain them in the form of nanostructured particles, nanofibers, and nanopowders.     

Recrystallization of silicon nitride powders during hot pressing

Conclusions The hot pressing of comminuted silicon nitride powders enables a uniform fine-grained structure of one and the same mean grain size to be obtained irrespective of the type of starting Si₃N₄, powder. At milling times of more than 100 h no significant decrease in particle size is observed. The recrystallization of milled silicon nitride powders during hot pressing takes place chiefly in the fine fractions appearing during milling. The degree of recrystallization attained is higher with ultrafine active silicon nitride powders (PCS) than with comminuted powders.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 43–47, October, 1982.     

Toxic and fibrogenic effects of plasmochemical silicon nitride

It is found that very finely divided silicon nitride made by plasmochemical synthesis has more pronounced general toxicity and cytotoxicity than coarse material made by nitriding silicon. It is proposed to reduce the maximum permissible concentration for ultrafine silicon nitride powder from 6 mg/m³ to 1 mg/m³.Kiev Medical Institute. Institute of Problems in Materials Science, Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 117–119, January–February, 1994.     

State of oxygen in fine titanium nitride powders

Conclusions Oxygen in fine titanium nitride powders produced by the plasma-chemical method can exist in three forms — replacing the nitrogen in the nitride, forming oxide films, and adsorbed on the surface. In the course of the plasma-chemical synthesis of titanium nitride 1–5 wt. % of oxygen from the starting reactants becomes statistically distributed throughout the particles, forming homogeneous titanium oxynitrides. Contact with humid air results in the formation, on the surfaces of small particles, of thin ( 1-nm) oxide films characterized by activation-type conduction and in adsorption of some oxygen. The composition of the main volume of each particle remains unchanged.Translated from Poroshkovaya Metallurgiya, No. 9 (225), pp. 6–9, September, 1981.     

Effect of nature of the comminution medium on the magnetic properties of SmCo5 alloy powders

Conclusions The production of a powder of the hard magnetic alloy SmCo₅ by mechanical comminution is a complex physicochemical process linked not only with simple mechanical comminution of particles but also with the effect of the comminution medium on the condition of the surfaces of the particles and their magnetic properties. Deformation phenomena induced in the particles by the mechanical action of the milling elements are not, as is currently thought, the dominant factor determining the magnetic properties of the powder. These characteristics can be expected to be strongly influenced by mechanochemical reactions in the disintegration zone linked with the character of the comminution medium. The nature of these reactions and their effect on the condition of the powder particles are not yet clear. To each comminution medium there corresponds (other things being equal) a specific comminution time making it possible to obtain a certain particle size which ensures optimum magnetic properties and is proportional to the viscosity of the medium.Translated from Poroshkovaya Metallurgiya, No. 1(265), pp. 1–5, January, 1985.     

Application of high-energy grinding for the activation of sintering of silicon nitride powder

Grinding of silicon nitride powder of the PS grade with additions of yttrium and aluminum oxides in a high-energy centrifugal-planetary mill is studied, where the grinding is accelerated by using balls to 20g and 40g acceleration. It is shown that the powder dispersion increases from 0.6 to 10–15 m²/g due to the grinding. The ground powders are very active during sintering. The relative density of the material is 97% at 1700°C and a nitrogen pressure of 1 MPa.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 98–101, November, 1992.     

Comminution of high-speed steel powders

Conclusions The best quality of powder is ensured by comminution in a disintegrator. The amount of energy expended in vibratory milling is tens of times larger than that consumed in comminution in a disintegrator. Because of the high oxygen concentration in the powder investigated, its comminution should be performed in a protective atmosphere.Translated from Poroshkovaya Metallurgiya, No. 10(274), pp. 71–74, October, 1985.     

A nondestructive test method for tool nitride ceramic

Measurements have been made on the densities and on the chemical and phase compositions of cutting plates made of Silinit P and Silinit P1. The parameters are correlated with wear tests on the plates on cutting and frictional interaction with steel and cast iron. The spread in the densities of silicon nitride ceramic specimens is related to the presence of inclusions. The wear resistance of the cutting plates is dependent on the inclusion content. This provides a very simple method of nondestructive monitoring for large batches of tool specimens made of silicon nitride ceramic.Institute of Problems in Materials Science, Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 113–116, January–February, 1994.     

Effects of silicon powder structure of nitriding

A study has been made on how the structural state of the silicon affects the nitriding at 1200–1400C. As the initial powder becomes more defective, the nitriding accelerates, and there is an increase in the importance of reactions in the gas phase, with an increase in the proportion of the a modification of silicon nitride, and a tendency for elongated crystals to form.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 1–7, October, 1992.     

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.     

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.     

Surface of particles of TiNx-base powders produced by self-propagating high-temperature synthesis

Conclusions The surface of a powder produced by the nitration of titanium by the SHS method is covered with a titanium oxynitride film of 8- and 15-Å thickness. For a specimen of low total nitrogen content the [N/Ti]_at ratio is much higher in the surface layer than in whole volume of the specimen.Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 1–5, November, 1979.     

Resistance of materials based on silicon nitride to corrosion by solutions of some acids and alkalis

Conclusions Hot-pressed silicon nitride parts of 1.5–4% porosity containing a 10 wt. % activating addition of MgO are resistant to corrosion by nitric acid at room temperature and by 1% NaOH solution at 100°C under steady-state conditions. The presence of 1–20 wt. % CaF₂ added with the aim of improving antifriction properties results in corrosion in nitric acid, obeying the law m²=K, where K 5.5·10^–5· C³ %²/h). The corrosion is due to attack on specimens by hydrofluoric acid forming during the reaction between the CaF₂ and HNO₃.Translated from Poroshkovaya Metallurgiya, No. 1(205), pp. 70–72, January, 1980.     

THE NITRIDING OF SILICON POWDER COMPACTS

Abstract

The reaction-bonding process to prepare silicon nitride by nitriding silicon compacts was studied, and an examination made of the influence of raw material and process variables on the properties of the resulting silicon nitride. The silicon powder grain size and the impurities content were considered as powder variables, and the green density and thermal cycles as process parameters. The examination of green-density effects indicates that, under the experimental conditions, the gas permeation of nitrogen through the silicon compacts was the rate-determining step of the reaction-bonding process. Regarding the effect of nitriding temperature, the final conversion, Si to Si₃N₄, is an increasing function of the temperature in the range 1300–1400°C. As to the composition of silicon nitride obtained, α-phase formation is favoured when oxygen is present as an impurity in silicon powder. Finally, physical, chemical, and thermomechanical tests showed that reaction-bonded silicon nitride has good bending strength (21 kgf/mm²) and can be used in very severe conditions up to 1200°C.     

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.     

Effect of impurities on the properties of silicon nitride materials

Conclusions The presence of oxides in silicon nitride leads to the formation of an intergranular vitreous phase which has an adverse effect on the high-temperature physicomechanical characteristics of constructional silicon nitride ceramics. This phenomenon is particularly pronounced with silicon nitride contaminated with calcium, silicon, and alkaline metal oxides. Future development work on oxidation-resistant constructional materials based on silicon nitride should go in the directions of removal of impurities and use of additions forming solid solutions or refractory compounds with Si₃N₄ and SiO₂. Additions which can be employed for this purpose include oxides of AI, Mg, Y, and some rare-earth elements.Translated from Poroshkovaya Metallurgiya, No. 1(193), pp. 75–80, January, 1979.     

Production of silicon nitride by the acid enrichment of products of combustion of ferrosilicon in nitrogen

Production of silicon nitride by acid enrichment of products of interaction between ferrosilicon and gaseous nitrogen under conditions of self-propagating high-temperature synthesis (SHS) is studied. The effect of the nature of acid, its concentration, agitation of solution, and process temperature is determined. The reaction of the Si₃N₄ + Fe composite powder and the hydrochloric acid solution is found to have a stage behavior. The apparent activation energy of iron passing into the solution is determined. The purity of the produced Si₃N₄ powder is demonstrated to depend on a degree of nitration of SHS products. The chemical and phase composition of the powder and its specific surface are determined.