Structure and properties of nichrome-boron nitride powder materials for tribotechnical pairs

Translated from Poroshkovaya Metallurgiya, No. 6(330), pp. 47–51, June, 1990.     

Thermophysical properties of materials based on silicon nitride

The heat capacity and thermal conductivity of materials based on Si₃N₄ are investigated for the first time. The temperature dependence of the thermal diffusivity of the composites studied is calculated. The influence of structural changes on the nature of the thermophysical properties of materials based on -Si₃N₄.Institute of Material Sciences, Problems, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Pokrytiya, Nos. 3/4, pp. 43–47, March–April, 1995.     

Strength and thermophysical properties of porous silicon nitride materials

Conclusions A slurry casting process has been developed for the production of porous silicon nitride specimens of 40–60%. It has been established that the strength and thermal conductivity of such specimens sharply decrease with increasing porosity. However, porous Si₃N₄ possesses fairly high resistance to oxidation in air, which suggests the possibility of employing it as a heat insulating material.Translated from Poroshkovaya Metallurgiya, No. 11(263), pp. 67–70, November, 1984.     

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.     

Manufacture of silicon nitride parts and properties of silicon nitride

Conclusions A study was made of the manufacture of parts from silicon nitride powder and of some properties of this compound. It was established that the composition of the material in which parts are packed during sintering exerts a strong influence on the strength properties and other characteristics of the resulting silicon nitride. Explanations are offered for the low microhardness of this material and the mechanism of its erosion wear.Translated from Poroshkovaya Metallurgiya, No. 4 (112), pp. 53–56, April, 1972.     

Effect of dimensional laser machining on the structure and properties of silicon nitride

Translated from Poroshkovaya Metallurgiya, No. 4(328), pp. 85–89, April, 1990.     

Dielectric characteristics of sintered silicon nitride materials

We have studied and summarized data on structure formation and dielectric characteristics of Si₃N₄-based materials obtained by hot pressing and activated sintering. We have measured the dielectric characteristics in the frequency range 1 kHz to 10 MHz. We have established that the level of the dielectric characteristics of the materials is significantly affected by the content of highly dispersed Si₃N₄ powder obtained by plasmochemical synthesis in the original mix. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 22–26, March–April, 2000.     

Degradation of rail-steel structure and properties of the surface layer

The change in the structure–phase states and defect substructure of the rail surface after prolonged operation (passed tonnage of 500 and 1000 million t) is studied by optical microscopy, by scanning and transmission electron diffraction microscopy, and by measurement of the microhardness and tribological characteristics. It is found that the wear rate increases by a factor of 3.0 and 3.4 after passed tonnage of 500 and 1000 million t, respectively, while the frictional coefficient is reduced by a factor of 1.4 and 1.1, respectively. After 500 million t, the cementite plates break down completely, and rounded cementite particles (10–50 nm) are formed. After 1000 million t, the initial stage of dynamic recrystallization is noted. Possible explanations of the observations are discussed. Two competing processes may occur in rail operation: (1) fragmentation of the cementite particles, with their subsequent entrainment in the ferrite grains or plates (in the pearlite structure); (2) fragmentation and subsequent solution of the cementite particles, with transfer of the carbon particles to dislocations (Cottrell atmospheres) and transportation of carbon atoms by dislocations within the ferrite grains (or plates), culminating in the formation of cementite nanoparticles.     

Statistical investigation of the strength of silicon nitride materials

Conclusions In a statistical investigation into the strength of a material based on silicon nitride the scatter of strength values was found to be described with satisfactory accuracy by the Gaussian and Weibull distributions. The speed of load application to specimens and the rigidity of the testing machine employed have no effect on the scatter of strength values of this material. Machining and preloading of specimens substantially reduce the scatter of strength values recorded.Translated from Poroshkovaya Metallurgiya, No. 2(218), pp. 80–88, February, 1981.     

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.