Specifics of structure formation and properties of composite ceramics in the Si3N4 — Al2O3 system

An investigation of the performance characteristics of silicon nitride ceramics due to the introduction of hollow and solid microspheres of aluminum oxide is continued. The highest fracture toughnessK _Ic , equal to 4.5 MPa · m^1/2, for the same ultimate bending strength of 300 – 500 MPa is attained in a ceramics based on an SHS matrix with hollow microspheres. This is associated with inhomogeneities of the microstructure, namely, the presence of coarse spherical pores and elongated prismatic grains that can hamper the propagation of cracks and thus affect considerably the fracture toughness. This material possesses a maximum fraction of closed pores and is lighter than its analogs. The use of microspheres of Al₂O₃ as pore-forming structural cells decreases the density of the material by 5 – 10%. The efficiency of the developed composite materials is shown by tests of armor plates. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 7, pp. 7 – 10, July, 1996.     

Effect of Nitrides of Refractory Metals on the Properties of Sintered Molybdenum and Tungsten

The effect of additives of titanium, zirconium, hafnium, and niobium in an amount of 2% on the technological and mechanical properties of sintered molybdenum and tungsten is considered. The changes in the chemical and phase compositions of pressed specimens after sintering in a flow of hydrogen are described. The temperature ranges of the effective hardening of molybdenum and tungsten by nitride additives are established. The composition with the best parameters of ductility and strength in tensile tests is determined.     

Electric-spark alloying of steels with chromium carbide base alloys

Conclusions It is shown that, of the Cr₃C₂-Fe cermets investigated, KKhZh-50 alloy is the best alloying electrode material. Alloying should be performed under soft conditions (E_pulse ^–2 J). The quality of the reinforced layer depends on the pulse energy and the base and electrode materials. The continuity of the layer increases and its surface roughness and thickness decrease with increasing pulse energy. Anode erosion and alloyed layer formation are affected by the secondary structure on the electron surface. For KKhZh-50 alloy its formation decreases anode erosion and the cathode weight gain, while for KKhNF-15 alloy, by contrast, it increases them. It is proposed that, to increase the corrosion resistance of such coatings (by about two orders), 65G steel should be alloyed in two stages, first with KKhZh-50 cermet and then with nickel.Translated from Poroshkovaya Metallurgiya, No, 4(304), pp. 1–6, April, 1988.     

Effect of Reinforcement of a Silicon Nitride Matrix by Silicon Carbide Whiskers

A fine-grain high-density tough matrix is a general prerequisite for synthesis of high-strength and crack-resistant composite materials, as well as its self-reinforcement with extended grains of β-Si₃N₄ and reinforcement with β-SiC crystals. Both approaches are used simultaneously in the present work. It is shown that the reinforcement of a silicon nitride matrix based on ultradisperse powder compositions by SiC whiskers of grade TWS provides hot-pressed ceramics with a high ultimate bending strength (950 MPa) and crack resistance (10 MPa · m^1/2). Reinforcement by fine or coarse whiskers increases the crack resistance by 30% with respect to monolithic Si₃N₄. Composites reinforced by TWS silicon carbide whiskers preserve their high strength up to 1500°C.