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31HIERARCHICALLY-STRUCTURED HIGH-TEMPERATURE ZrB2-MoB-MoSi2 CERAMICS PRODUCED BY DIFFERENT SHS ROUTES AND SUBSEQUENT HP
Yu. S. Pogozhev"*, M. V. Lemesheva", A. Yu. Potanin", S. I. Rupasov", V. I. VershinnikovA, and E. A. Levashov"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, 142432 Russia *e-mail: yspogozhev@mail.ru
DOI: 10.24411/9999-0014A-2019-10125
The creation of new generation of power-generating units, high-temperature internal combustion engines, supersonic aircrafts, and reusable space vehicles greatly increases the interest to high-temperature ceramic materials used to manufacture thermal protection systems for gliders and air flow ducts in propulsion systems [1]. Such high-temperature ceramics is crucial for aerospace engineering, chemical and metallurgical facilities [2].
At present time, the choice of structural materials to be used in aerospace industry under high-temperature oxidation conditions is limited to SiC- and Si3N4-based composites, oxide ceramics, superalloys based on Ni, Cr, Fe, and carbon-carbon composites with protective SiC coatings. The temperature range of application of SiC- and Si3N4-based composites is limited to 1600oC. However, they exhibit low stability to thermal cycling, low resistance against water vapor-induced corrosion, a strong decrease of oxidation resistance at presence of cations of alkali and alkaline earth metals salts, and deterioration of mechanical properties upon intensive oxidation [3]. The operational temperature of superalloys is lower than 1000oC. They cannot be used at higher temperatures due to low creep and oxidation resistance. Carbon based materials possess excellent mechanical properties in a wide temperature range, but above 600oC they undergo to rapid oxidation.
Currently, the next generation of high-temperature hierarchically structured ceramics with high oxidation resistance and excellent mechanical properties at elevated temperatures is being developed. Refractory transition metal disilicides and molybdenum disilicide MoSi2, in particular, are widely used as basis of such high-temperature ceramics. MoSi2 is characterized by excellent resistance to high-temperature oxidation up to 1600°C [4, 5] and less susceptible than SiC to deterioration upon exposure to water vapor and alkali metal cations. However, MoSi2 possess poor mechanical properties: low plasticity at temperatures less than 1000°C, low strength, and creep resistance at temperatures greater 1250°C, and also characterized by low oxidation resistance in the range of 500-700°C due to oxide layer destruction (pesting phenomena) [6]. That's why the single-phase MoSi2 material cannot be used as a high-temperature structural material for specified purposes. Alloying of MoSi2-based composites with transition metal borides, like MoB, HfB2, and ZrB2, prevents the intense oxidation at low temperature range and also increase hardness and strength of the composite and improve its creep resistance and thermal cycling stability [7-9]. In [9]. it was shown that the composition 90% MoSi2-10% MoB (at %) has the best resistance to high-temperature oxidation. Further improvement of the performance characteristics of MoSi2-based ceramics is possible when doping with HfB2 and ZrB2 borides, which have similar mechanical and thermophysical properties [10, 11]. However, from an economic point of view, the most appropriate use is ZrB2.
Self-propagating high-temperature synthesis (SHS) is an effective way of obtaining MoSi2-MoB-ZrB2 ceramics from both elemental powders and less expensive oxide raw materials
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[12-14]. To obtain similar high-density ceramics are mainly used methods of hot pressing (HP) and spark plasma sintering (SPS), as well as the hybrid technologies SHS + HP (reactive HP) and SHS + SPS (reactive SPS) [15, 16]. However, the use of hybrid methods is associated with some problems related to significant differences in the melting points of the initial components and the necessity of high purity elemental powders application. The technology of forced SHS-pressing is devoid of these drawbacks and allows to obtain similar ceramics with a hierarchical structure and level of residual porosity of 1.5-3.0%. However, this does not always satisfy the requirements for such materials. To obtain high-density MoSi2-MoB-ZrB2 ceramics, it is advisable to use a combined two-stage method, when the heterophase SHS-powders obtained at the 1st stage is consolidated using the HP or SPS methods on the 2nd stage. SHS-powders have improved sinterability compared to powders obtained by traditional methods. The increased concentration of defects in SHS powders is a consequence of extreme heating and cooling rates (2000-20000 K-min-1) during the combustion wave propagation [17]. Two different SHS routs can be used to produce SHS-powders of MoSi2-MoB-ZrB2 ceramics: direct synthesis from elemental powders and magnesiothermic reduction of oxide raw.
The present study suggests a complex technology for the production of high-density MoSi2-MoB-ZrB2 ceramics, which includes two stages: 1) synthesis of heterophase SHS-powders using both above mentioned SHS routes; 2) consolidation of SHS-powders by HP technology.
Experimental studies of the combustion process kinetics were carried out during direct elemental synthesis of the heterophase SHS powders. Unconventional dependences of combustion temperature (Tc) of the reactionary mixtures on the initial temperature (T0) of the SHS-process were observed. The increase in 7 from 25 to 450oC leads to a proportional reduction of combustion temperature (Tc) by 80-130oC in dependence of the mixture composition. Probably this is due to the zirconium oxidation with an increase in T0, as evidenced by the self-heating of the mixture before the initiation of combustion. At the same time combustion rate (Uc) of the mixture calculated for the formation of 40% ZrB2 monotonically increases from 1 to 3 cm/s as the T0 increases. The Uc(T)) dependence for the reactionary mixture calculated for the formation of 20% ZrB2 has an extreme nature. With an increase in T0 to 140°C, the Uc increases by more than 2 times, and further growth of T0 leads to its slight decrease. Apparently, this is due to changes in the combustion mechanism or the transition of chemical reactions of MoSi2 and ZrB2 formation occurring in this heterogeneous system from the merge mode to the separated mode [18]. The amount of heat released as a result of chemical reactions in the elemental reactionary mixture containing the greatest amount of zirconium and boron is 1654 J/g.
Heterophase SHS powders obtained by both SHS routes (direct elemental synthesis and magnesiothermic reduction) are characterized by homogeneous and fine structure (Fig. 1a).
(a) (b)
Fig. 1. (a) Typical structure of the SHS powders in MoSi2-MoB-ZrB2 obtained by direct elemental synthesis and (b) its fractional composition.
Particles of the resulting composite SHS powders have polyhedral shape closed to spherical shape, and consist of 50-70% MoSi2, 20-40% ZrB2, and 5-10% MoB in dependence of the initial mixture composition. Also there is a small amount of ZrO2 in powders composition less than 1%. Differential curves of granulometric composition of the obtained powders are similar for both SHS routes and demonstrate normal particle size distribution with only one maximum. The average particles size of the obtained SHS powders is about 5 |im with a maximum size less than 25 |im.
Consolidated ceramic samples by HP using obtained heterophase SHS powders have homogeneous dense structure (Fig. 2).
(a) (b)
Fig. 2. (a) Microstructure of the MoSi2-MoB-ZrB2 ceramic samples obtained by HP and (b) corresponding element distribution maps.
The phase composition of hot-pressed samples is almost identical to the phase composition of SHS-powders. The structure of compact ceramics is represented by a two-phase matrix consisting of MoSi2 and ZrSi2 grains. Light-gray MoB inclusions are located on the grain boundaries. Elements distribution maps demonstrate a high uniformity of Mo, Si, and Zr distribution over the samples volume. The relative density of the obtained ceramic samples is 98.9-99.3%, and their thermal conductivity varies in the range of 32-36 W/(m K) in dependence of the ZrB2 content.
Heterophase MoSi2-MoB-ZrB2 ceramics is a promising structural material for the creation of heat-loaded structural elements for aerospace engineering.
The research was carried out under the financial support of the Russian Science Foundation in the framework of project no. 19-19-00117.
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