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.     

Wettability of aluminum nitride by molten metals

The sessile drop method has been used with a vacuum of 2.10^–3 Pa to examine the wettability of aluminum nitride by fourteen molten pure metals as well as the effects of adding chromium, vanadium, niobium, tantalum, and titanium to the liquids on the wetting angle in systems containing aluminum nitride and liquid tin, copper, and germanium. Aluminum nitride is wetted only by molten silicon and aluminum. Out of the elements examined, titanium is the most adhesion-active for this ceramic. The results are examined from the viewpoint of thermodynamic wetting theory.Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 74–78, May—June, 1996. Original article submitted June 29, 1994.     

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.     

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.     

Properties of composite powder coatings based on iron and ultrafine silicon nitride

Conclusions Composite-powder plasma coatings based on iron and ultrafine silicon nitride consist of a structure made up of iron matrix with inclusions of solid particles of silicon-containing compounds.The coatings have a high wear-resistance (wear rate 6.10^–11 m per l m of tracking) and are not inferior to coatings made of thermally active PN85Yul5 powders.The coatings material has a low cost, good workability with traditional methods, and demonstrates excellent working properties in actual conditions for friction units.Translated from Poroshkovaya Metallurgiya, No. 3(339), pp. 65–68, March, 1991.     

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.     

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.     

A method of determination of the dielectric characteristics of very fine silicon nitride powders

Conclusions In the area of audio and radio frequencies the sample preparation method has a significant influence on the results of measurements of the dielectric parameters of powders. The minimum distortions of the data are observed in investigation of powder dispersed in transformer oil.In the area of audio and radio frequencies the dielectric properties are determined primarily by the surface condition of the powder particles.In the shf range the dielectric characteristics are determined by polarization of the volume of the powder particle. This range is preferable for measurement of the dielectric parameters of very fine silicon nitride powders since the results of such measurements correlate with the degree of contamination of the Si₃N₄ powders while the surface condition of the particles (with the exception of ultrafine particles) has practically no influence on them.Translated from Poroshkovaya Metallurgiya, No. 9(261), pp. 76–82, September, 1984.     

Quality control of hot-pressed silicon nitride dielectric materials

Conclusions Exceeding an optimum temperature in the pressing of silicon nitride parts leads to the appearance of conducting inclusions (SiC and free Si) in specimens, while pressing at temperatures below the optimum temperature increases the porosity of specimens and hence the amount of water absorbed by them. With rising conducting inclusion content the electrical conductivity, , and tan of a material grow and its E_br falls. Increasing porosity has, by increasing water absorption, a similar effect. The coefficient of losses at a frequency of 1 kHz can be employed as a criterion for assessment of the quality of dielectric materials based on silicon nitride.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 53–59, March, 1982.     

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.     

Some mechanical properties of silicon carbide fibers

Conclusions Electroerosion of silicon carbide fibers in contaminated mercury electrical contacts decreases their strength by between one-half and two-thirds. During prolonged holding under load at room temperature the creep of silicon carbide fibers is close to zero, while at 1173°K l/l 10^–7 l/day. X-ray structural analysis of silicon carbide fibers annealed for (1.8–10.8)·10³ sec in a hydrogen atmosphere at 1273–1773°K and for (3.6–18)·10³ sec in a nitrogen atmosphere at 1773–1973°K revealed no recrystallization in (4–5)·10^–7-m-thick layers of polycrystalline SiC deposited from a gaseous phase. A correlation was found between the strength of silicon carbide fibers and the number of flashes forming during their fracture.Translated from Poroghkovaya Metallurgiya, No. 1(253), pp. 55–59, January, 1984.     

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.     

Structural special features and infrared spectra of absorption of ultradisperse boron nitride

Conclusions The ultradisperse powders of boron nitride with the mean particle size of 5.3–66 nm showed marked anisotropic deformation of the crystal lattice as a result of the structural stresses which are caused by the dimensional factor and are distributed nonuniformly through the volume of the particle.The turbostratic strain leads mainly to weakening of the interlayer bonds and shifts the maximum of the band of the interplanar oscillations to lower frequency by 17 cm^–1 for the specimen with a dispersion of 5.3 nm. In addition to this, the turbostratic strain causes asymmetric widening of the band of the intraplanar oscillations in the short-wave wing of the curve and displaces the maximum of this band by 20 cm^–1 to the short-wave range (for the specimen with a dispersion of 5.3 nm).Examination by the IR spectral method showed that the interatomic bonds shorten on the average by 10^–4 nm within the limits of the flat lattices. The investigations revealed the regular nature of the removal of the strain of the crystal lattice from the ultradisperse boron nitride during vacuum heat treatment accompanied by sintering of the powder.Translated from Poroshkovaya Metallurgiya, No. 1(301), pp. 85–91, January, 1988.     

Electrophysical properties of a porous silicon carbide ceramic

The electrical resistance and thermal electromotive force have been measured for porous silicon carbide resistance materials. It has been established that the range of controlled variation in the electrical resistance is 5·10^–2 to 1·10⁴ · cm, and that the composition of the conducting SiC phase and the concentration of added dielectric constituents have a decisive influence on the electrical characteristics examined.Material Science Problems Institute, Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 85–89, April, 1993.     

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.     

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.     

Salient features of thermal expansion of multilayer cermet composites

The linear thermal expansion coefficient (LTEC) of multilayer brazed cermet composites made of alternating plates of Si₃N₄ and Cu (ZrO₂ and Cu) has been studied. The coefficient was observed to be anisotropic parallel and perpendicular to the plane of brazing. In the first case the coefficient of the composite was practically equal to that of silicon nitride and in the second case reached or at times even exceed that of copper. Moreover the LTEC was considerably higher than the value calculated for the composite on the basis of the additivity principle, the assumption being that the copper and silicon nitride expand independently. The anisotropy observed in the LTEC of the composite can be attributed to the anisotropy of copper expansion in such a material. The linear dimension of the same sample, heated repeatedly, was observed to decrease in the lateral direction; this is due to plastic deformation of the copper and the resultant thinning of the sample.Institute of Problems in Material Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 43–46, July–August, 1996.     

An abrasive material based on silicon nitride for grinding unhardened steels

Conclusions A study was made of the strength of abrasive grains of silicon nitride and of the wear resistance of silicon nitride material under conditions simulating the operation of abrasive grains in a grinding wheel. It is shown that in grain strength the new silicon nitride material is superior to diamond, Kubonit, and El'bor and inferior to Geksanit-A, while in wear resistance in the microcutting of unhardened steel it is comparable with Geksanit-A. On the basis of an electron microscopical examination of the surface structure of a worn microtool a mechanism is proposed for the wear of a material based on silicon nitride in the grinding of unhardened steel. A material based on silicon nitride could be successfully used in abrasive tools for the rough grinding of unhardened ferrous alloys.Translated from Poroshkovaya Metallurgiya, No. 5(173), pp. 34–37, May, 1977.     

Determination of the dielectric characteristics of powders at superhigh frequencies

Conclusions The measurement method described enables the dielectric characteristics of powder objects in the shf range to be determined with errors of not more than ±2% in and not more than ±15% in tan. Conversion of results of measurements on the dielectric parameters in wide ranges of particle sizes and values of can best be performed with formula (9). To obtain silicon nitride materials characterized by a low level of dielectric losses, it is necessary to reduce to a minimum their impurity content and ensure that their composition approximates as closely as possible to stoichiometric.Translated from Poroshkovaya Metallurgiya, No. 3(207), pp. 65–72, March, 1980.