REACTION-SINTERED CARBIDE-NITRIDE SYSTEMST. P. Markholiya, I. I. Kozelkova, T. M. Bragina, and L. M 9 Aksel'rod UDC 666.762.85.046.4 In order to develop high-temperature technology and to intensify metallurgical and chemical processes, it is necessary to create materials possessing high strength and wear resistance in combination with thermal shock resistance and corrosion resistance in aggressive media. In this context, composite materials containing carbides and nitrides [1-3] are important. However, it is known that obtaining the products from the high-temperature (refractory) materials according to the hot-pressing technique is costly and that pressureless sintering does not ensure the required strength levels in most cases [4, 5]. The technology of producing high-temperature materials according to the method of reaction sintering is quite promising.We studied composite materials (systems) based on boron carbide containing boron nitride and silicon carbide that were obtained in a dried and decontaminated nitrogen atmosphere (the content of the impurities was less than 3 vol.%). Table 1 shows the composition of the Nos. i-3. The base of the charge consists of amorphous boron or boron carbide. Besides these constituents, silicon and quartz glass additives were introduced into the Nos. 1 and 2 bodies. Specimens were compacted using bakelite as a temporary binder and were fired at 1450~ in a nitrogen atmosphere using a furnace available at the Semiluksk Refractories Plant 9The phase composition was controlled according to x-ray structural analysis; the analysis was carried out on a DRON-2,0 diffractometer using Cu Ks-radiation and a nickel filter. ,,,= zooo Fig. i.7590
Silicon carbide and boron carbide are used in various fields of industry owing to their high levels of physicochemical properties and hardness.In the absence of special additives, these materials are usually sintered at elevated (2000~ and above) temperatures.The materials based on silicon carbide in which binding agents form at relatively low temperatures (1500~ with the participation of a gaseous medium (atmosphere) are finding wide application.In view of this, it was of interest to explore the possibility of using a similar technological route for boron carbide also. For this purpose, we studied the behavior of boron carbide during heat treatment in a carbonaceous particulate charge consisting of a mixture of coke and quartz sand.It is known that the gaseous phase in the C--Si02 system consists of CO, C02, and SiO [i]; besides this, the gaseous phase of the aforementioned charge contains nitrogen and traces of oxygen.The information available on the behavior of boron carbide in various gaseous media is quite limited.In the atmosphere of pure oxygen [2], oxidation of boron carbide starts at 600~ and the oxidation rate increases at 800-i000~ thereafter, it decreases and at 1200-1300~ it abruptly increases once again due to the formation of liquid B20s and its vaporization. Besides this, the formation of volatile lower oxides of boron is also possible.
Im Kontakt mit SnTe steigt die Aktivität der Eisenmetalle in der Reihenfolge Fe ‐ Co ‐ Ni.
Boron carbide is one of the promising materials because of its excellent hardness, chemical inertness, and other properties [i]. However, being bonded by strong covalent bonds, boron carbide does not sinter well. The manufacture of articles from it using hot-pressing methods and high-temperature (0.8-0.9Tmelt) is fraught with significant technological difficulties [2]. Therefore, it is important to produce a material capable of reaction sintering at medium temperatures and to study its structure.On the basis of the established data we suggest that when boron carbide is heated in a charge consisting of a mixture of coke and quartz sand [3], that secondary phases may form which make the material more dense. The effect of the gas phase in these processes must be particularly emphasized. Thus, in the interaction between boron carbide and C02, B203 and carbon monoxide may be liberated [4]. In turn, the interaction between B203 and carbon and nitrogen may lead to the formation of boron nitride [5] and secondary boron carbide [6]. In the presence of CO, B203 is reduced to free boron [2,7], which later takes part in a reaction with nitrogen [8] and carbon [9]. Moreover, in the presence of SiO (gas) it is possible for the carbon to be transformed into silicon carbide [i0].The present study deals with the structure of materials based on boron carbide obtained by reaction sintering and also with an elucidation of the part played in this process by the addition of boron.The studies were carried out using electron microscopy and x-ray phase analysis. As the starting materials we used boron carbide (GOST 5744-74) and amorphous boron (TU 6-02-923--74).The specimens were prepared using as a temporary binder a bakelite-type phenol-formaldehyde resin, The heat treatment was carried out in a carbon-containing charge at a temperature of -1600~The specimens obtained from boron carbide without additives were black and loose, With added boron, the specimen was sintered and two zones were formed: a dark one 0.5 mm thick and a pale-grey core.Judging from the data of the x-ray phase analysis both forms of the material had the same qualitative composition and contained carbide plus boron nitride, silicon carbide, boric oxide, and carbon in the form of graphite.A study of chips of the specimens using scanning electron microscopy made it possible to distinguish different types of structures reflecting the special features in the development of the processes in which the secondary phases are formed. The inner zone has a similar structure. However, a decrease in the bubble formations and a slight increase in the proportion of fine-grained component are noted. The amount of the latter component is insufficient to strengthen the material. This confirms also the character of chips: destruction occurs along the binder without involving the grains of the original boron carbide.All-Union Institute of Refractories. bet, 1986.
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