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47COMBUSTION SYNTHESIS AND CONSOLIDATION OF (Zr/Hf)B2-(Zr/Hf)Si2-MoSi2 POWDER CERAMICS FOR HIGH-TEMPERATURE PROTECTIVE COATINGS
M. V. Lemesheva*", Yu. S. Pogozhev", A. Yu. Potanin", S. I. Rupasov", V. I. VershinnikovA, and E. A. Levashov"
^National University of Science and Technology MISIS, Moscow, 119049 Russia bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: lemescheva.margarita@yandex.ru
DOI: 10.24411/9999-0014A-2019-10080
Nowadays, superhigh-temperature ceramic materials that can be effectively used under severe thermal loads are of great importance in materials science research. Such materials must withstand extreme operating temperatures above 2000°C, resist erosion, evaporation and oxidation. The most Promising candidates for this kind of applications are diborides ZrB2 and HfB2, which are characterized by high melting points (3246 and 3380°C) [1, 2], thermal conductivity (57.9 and 104 W/(m • K)) [1-3], hardness (21 and 28 GPa) [2, 4, 5], modulus of elasticity (450 and 480 GPa) [2], as well as a low coefficient of thermal expansion (5.9 and 6.3-7.15 x 10-6 K-1) [3, 5, 6]. The use of pure ZrB2 and HfB2 ceramics is limited due to the evaporation of B2O3 above 1100°C. The release of porous oxides ZrO2 and HfO2 occurs as a result, which leads to the penetrating oxidation of the material [7-9]. In order to improve heat resistance, SiC, MoSi2, ZrSi2, etc. are included into the ZrB2 and HfB2 ceramics. The presence of boron and silicon in ceramics leads to the formation of borosilicate glass during the oxidation process, which not only effectively resists high-temperature oxidation, but also has the effect of self-healing of defects and cracks [2, 7]. In addition, such additives have a positive effect on the compactibility of ceramics in the process of consolidation [10, 11]. Effective methods for producing such ceramics are self-propagating high-temperature synthesis and hot pressing (HP) [12, 13].
In the present work, composite powders with varying the concentration of the main elements (Zr, Hf, Mo, Si, B) were obtained by magnesium thermal reduction from oxides. The consolidation of the powders was carried out using the HP method in a graphite mold at a pressure of 30 MPa and a process temperature of 1200-1250°C. The microstructure and phase composition of the obtained powders and compact ceramics were studied by scanning electron microscopy, energy dispersive spectroscopy and X-ray phase analysis. Investigations of the fine structure were carried out on a transmission electron microscope. The granulometric composition of the powders was determined on a laser particle size analyzer. The density of the consolidated samples was measured by hydrostatic weighing and helium pycnometry. Residual porosity was calculated from the relative density values. Vickers hardness HV was measured on an automated hardness tester with a load of 10 kg.
The phase composition of the synthesized composite powders includes zirconium and hafnium diborides ZrB2/HfB2, silicides ZrSi2/HfSi2, ZrSi, and also MoSi2, with different ratio between them in dependence of the reactionary mixture composition. Synthesized SHS powders consists of polyhedral shape particles and characterized by a homogeneous, compositional, and finely dispersed structure (Fig. 1). The average particle size of the powders was 5-9 p,m. Each powder particle is a microcomposite consisting of boride and silicide grains 1-2 and 2-4 |im in size, respectively.
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Fig. 1. Typical morphology, granulometric composition, and fine structure of SHS powders in the MeIV-MeVI-B-Si system.
The phase composition of the ceramic samples consolidated by HP is similar to powders and also represented by silicides and borides of transition metals. The microstructure of compact materials consisted of rectangular-shaped ZrB2/HfB2 grains from 3 to 10 p,m in length and ZrSi2/HfSi2 and MoSi2 inclusions in the form of large irregular-shaped grains. Compact ceramics had a low residual porosity of 1.1-1.7%, hardness of 11-12 GPa, thermal conductivity of 62-87 W/(m-K) and specific oxidation rate of (1.3-2.9)-105 mg/(cm2-s).
Fig. 2. Microstructure of compact samples obtained by hot pressing of SHS powders: (a) ZrB2-ZrSi2-MoSi2; (b) HfB2-HfSi2-MoSi2.
Ceramic SHS powders were tested as precursors for the preparation of protective coatings on thermally loaded carbon-based structural elements operating in high-enthalpy oxidizing gas flows.
The reported study was funded by RFBR according to the research project no. 18-08-00269.
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