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ISSN 1062 8738, Bulletin of the Russian Academy of Sciences: Physics, 2009, Vol. 73, No. 10, pp. 1377­1379. © Allerton Press, Inc., 2009. Original Russian Text © K.N. Filonov, V.N. Kurlov, N.V. Klassen, V.M. Samoilov, A.N. Vodovozov, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 10, pp. 1460­1462.

New Shaped Ceramics Based on Silicon Carbide
K. N. Filonova, V. N. Kurlova, N. V. Klassena, V. M. Samoilovb, and A. N. Vodovozovc
a

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia b State Research Institute of Graphite, Elektrodnaya ul., Moscow, 111524 Russia c OOO Galf, Moscow, Russia e mail: kurlov@issp.ac.ru

Abstract--New techniques have been developed for producing inexpensive shaped SiC ceramics with certain structure and porosity for a wide variety of applications. These techniques are based on the interaction of sil icon melt with carbon from a previously pressed blank of definite composition (carbon, silicon carbide, organic bond) and porosity. DOI: 10.3103/S1062873809100165

Silicon carbide (SiC) based ceramics features high mechanical stability at high temperatures, wear resis tance, low thermal expansion coefficient, high oxida tion resistance at temperatures below 1500°C, high oxidation resistance, biocompatibility, corrosion resis tance, radiation stability, high hardness, and thermal conductivity [1­3]. Because of unique physicochemi cal properties, SiC ceramics is widely useful in mechanical engineering, as well as nuclear power, defense, metallurgical, food, chemical, and petroleum industries. Possible applications include friction pairs, dry gas dynamic seals, plain journal bearings rated for use in severe conditions of abrasive and chemically active environments at high temperatures, heating ele ments, die blocks, spray nozzles, thermocouple cases, and structural elements of rotary engines and turbo charged engines. There are various ways of obtaining SiC materials: interaction of a silicon melt with porous graphite to produce the so called siliconized graphite [4, 5], agglomeration of SiC powder at high (>2100°C) tem peratures in the presence or absence of activators [6, 7], hot pressing (application of pressure in sintering speeds up significantly the material compaction) [7, 8], and production of a reaction sintered (self bonded) SiC [9]. Each of these approaches features nevertheless a number of processing constraints (com plexity, high power consumption, impossibility of obtaining complex shapes, etc), due to which the characteristics of resulting SiC ceramic materials often fail to satisfy the up to date requirements. For the most part these materials are heteroge neous compositions in which individuals SiC grains are cemented by adhesives of different composition and physicochemical properties [9]. These adhesives are necessary to facilitate the production of materials and articles or reach intended physical properties or operational performance. In the latter case the proper ties of the material depend on the amount of the phase

components, their sizes, type of distribution, and nature of interphase interactions. In turn, the above factors depend on the process particularities in pro duction of particular materials. We have developed an economical method for pro ducing multifunctional shaped SiC ceramics. This method relies on the interaction of silicon melt with carbon (siliconization), which is contained in a previ ously shaped blank of certain geometry, composition (carbon, silicone carbide and organic adhesive), and porosity (Fig. 1). The deviation of siliconized article sizes from preset values does not exceed 1%; i.e., the mechanical treatment necessary for refining articles is minimized, additionally reducing their cost. This method has an important advantage: the con tent and porosity of primary blank can be varied in a wide range, depending on the available raw materials, ratio of the components, and the ways for their mixing and compaction. To prepare the input components, we have developed a dry mixing technique. This approach is more flexible and environmentally friendly, because it excludes toxic phenolic adhesives. Varying the gas environment composition and the temperature and duration of thermal treatment of sil iconized articles, one can control the content of resid ual silicon and carbon in the articles, which in turn helps to obtain articles with widely controllable ther

Fig. 1. Sequence of structural SiC ceramics production: (left) SiC­carbon blanks, (middle) siliconized blanks, and (right) final articles after mechanical treatment.

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(a)

200 µm

(b)

200 µm

Fig. 2. Structure of SiC ceramics (a) with high content of residual silicon and presence of carbon (black inclusions) and (b) with low content of residual silicon (bright areas).

mal conductivity, electrical conductivity, chemical resis tance in aggressive environments, morphology, and other important engineering characteristics. Thus, for specific applications, the density of the new ceramics described here may vary from 1.8 to 3.15 g cm­3. The ceramics with the residual carbon and silicone removed by deliberate thermal treatment features better chemical resistance at high temperatures. On the other hand, articles con taining residual carbon exhibit the best antifriction properties. Examination of the morphology and elementary composition of specimen was carried out using a mod ern Carl Zeiss Supra 50 VP field emission scanning microscope with a high resolution (1.3 nm at an accel erating voltage of 20 kV and working distance of 2 mm). The microscope is equipped with several sec ondary electron detectors: a standard Everhart­ Thornley secondary electron detector, a low vacuum secondary electron detector to examine nonconduct ing specimen, and an In lens detector to obtain high resolution images.

Figure 2 shows the microstructure of SiC ceramics with high (>20%) and low (<2%) residual silicon con tents. The SiC ceramics with a high free silicon con tent (Fig. 2a) was prepared from the initial specimen obtained by wet mixing of carbon and silicon carbide particles with sizes from few microns to 100 µm. To obtain the ceramics with a low silicon content (Fig. 2b) with a small deviation from stoichiometric SiC, we used dry mixing of nanosized powders with subsequent compressing blanks to a density of 1.95 ­ 2.05 g cm­3. The blank siliconization yielded SiC ceramics specimen with a density up to 3.15 g cm­3, having a much smaller grain size with a higher struc tural uniformity. A significant advantage of the SiC ceramics devel oped is its high resistance to thermal shock; in this parameter it is considerably superior to the commer cial SiC ceramics. This was verified by comparative tests of the articles made of three types of SiC ceram ics: the hot pressed Hexoloy ceramics (Saint Gobain (France, USA) Production) [8], silit produced in Rus sian, and the ceramics in question. The specimen were heated in air in a resistor furnace to 1200° and then immediately quenched into cold water. For Hexoloy and silit exhibited crack formation in water in the first cycle, whereas the new ceramics articles showed no signs of failure after ten heating­cooling cycles. The combination of thermal shock resistance and high temperature chemical resistance substantively widens the possibilities for practical applications of new ceramics. For example, the replacement of conven tional ceramic heaters in high temperature (1500° or more) air furnaces will make unnecessary energy and time consuming stages of smooth heating and cooling these furnaces. Indeed, for conventional ceramic heaters the heating and cooling rates should not exceed 150 K/h to prevent them from cracked. At the same time, the electric heaters made of the novel SiC ceramics can withstand repeated heating and cooling cycles from room temperature to 150° for times of 3­5 s, with subseguent instantaneous switch ing off the feed current. The possibility of controlling crystallite sizes and pores in articles made of the novel SiC ceramics in a wide range allows one to substantially increase their radiation resistance. One of the reasons for this increase is that the time of radiation defect emergence to the grain surface for small grains (which decreases proportionally to the square of the grain linear size) turns out to be smaller that the time interval between two successive events of trapping of ionized particle by a given grain. As a result, various nanostructured ceramics can stably operate in reactor cores with radi ation flows of 1013 or more particles per square centi meter per second. The SiC materials obtained far surpass in basic physical and mechanical characteristics the available siliconized graphite. Their bending resistance (~400 MPa) and Young modulus (up to 430 HPa) are
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the same or better than the corresponding characteris tics of hot pressed silicon carbide of Hexoloy type, the latter being produced by a much more complicated and expensive technology (the price of hot pressed sil icone carbide articles is approximately an order of magnitude higher than that of similar articles made of our ceramics). CONCLUSIONS We have developed methods for producing shaped articles from multifunctional and inexpensive SiC ceramics. Our approach is based on interaction of sil icone melt with carbon from a previously pressed blank of definite composition (carbon, silicon carbide, organic adhesive) and porosity. The indisputable merit of the developed ways for producing SiC ceramics is the simplicity and low cost of equipment, availability of raw materials, and capa bility of controllable variation of the ceramics compo sition and structure in a wide range, depending on the particular application. As a result, one can obtain per formance specifications significantly exceeding those of commercial SiC ceramics. ACKNOWLEDGMENTS This work was supported in part by the Russian Foun dation for Basic Research, project no. 08 03 00105, and

the Program "Fundamentals of Basic Research of Nanotechnology and Nanomaterials" of the Presid ium of the Russian Academy of Sciences. REFERENCES
1. Watari, K., J. Ceram. Soc. Jpn., 2001, vol. 109, p. S7. 2. Jang, B.K. and Sakka, Y., J. Alloys Compd., 2008, vol. 463, p. 493. 3. Schwetz, K.A., Handbook of Ceramic Hard Materials, Riedel, R., Ed., Weinheim, Germany: Wiley, 2000, p. 683. 4. Anikin, L.P., Kostikov, V.I., and Kravetskii, G.A., Graphite Based Structural Materials, Moscow: Metal lurgiya, 1970, p. 143. 5. http://www.advtech.ru/niigrafit/prod/graf.htm 6. Geguzin, Ya.E., Fizika spekaniya (Physics of Sinter ing), Moscow: Nauka, 1967. 7. Somiya, S. and Inomata, Y., Silicon Carbide Ceramic, Berlin: Springer, 1991, p. 305. 8. http://www.hexoloy.com 9. Gnesin, G.G., Karbidokremnievye materialy (Silicon Carbide Materials), Moscow: Metallurgiya, 1977.

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