CC BY
64iSHS 2019
Moscow, Russia
RECENT ADVANCES AND APPROACHES IN SHS OF HIGH-TEMPERATURE MATERIALS (OVERVIEW)
E. A. Levashov
National University of Science and Technology MISIS, Moscow, 119049 Russia
e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10082
Development of next-generation spacecraft raises the problem of searching for materials resistant to high-enthalpy oxidative flows of gas containing abrasive particles. These materials are used in supersonic aerospace vehicles [1] with sharp leading edges and fairings, as well as in the critical section of nozzle clusters in solid propellant engines of spacecraft. Zirconium diboride ZrB2 is among promising materials: it is characterized by melting point > 3000°C [2], high strength, fracture toughness, wear resistance, and thermal stability. High thermal conductivity ensuring rapid heat transfer away from the surface contacting the oxidative gas flow is an important advantage of ZrB2-based composites. Elaboration of efficient methods for fabrication of erosion-resistant and ultrarefractory materials based on HfC (Tmelt = 3900°C), TaC (3800°C), HfB2 (3380°C), ZrB2 (3200°C), TaB2 (3200°C), and NbB2 (3050°C) is a topical problem. Solid solutions have a higher melting point than individual compounds. Thus, the melting point of single-phase carbide (Ta,Hf)C with 20% HfC dissolved in it is ~ 3950°C. In addition to the high melting point, solid solutions are characterized by extreme correlations between hardness, coefficient of thermal expansion (CTE), or thermal conductivity and solid solution composition. A combination of high thermal conductivity and low CTE determines material's thermal shock resistance.
Therefore, synthesis of single-phase solid solutions (Ta,Zr)C, (Ta,Hf)C, (Ta,Zr)B2, and (Ta,Hf)B2 is an independent task. These compounds can be fabricated using various methods: by heating a mixture of powders, reducing an oxide mixture by hydrogen, precipitating from solutions, by microwave synthesis, sol-gel synthesis, or SHS.
Elemental and magnesium-thermal SHS, hot pressing, and SPS were used to produce ultrahigh temperature ceramics (UHTCs) based on the single-phase solid solutions of diborides (Ta,Zr)B2, (Ta,Hf)B2 [3-7] and carbides (Ta,Zr)C, (Ta,Hf)C [8-11]. The kinetics and the mechanism of combustion and structure formation in the Ta-Zr-C, Ta-Hf-C, Zr-Ta-B, Hf-Ta-B, Mo-Si-B, Zr-B-Si, Zr-B-Si-C, and Ta-Si-C systems were studied. Mechanical activation (MA) or reaction mixtures was shown to play a crucial role in production of singlephase solid solutions (Ta,Hf)C and (Ta,Zr)C. For example, synthesis after MA of Ta-Hf-C mixtures gave rise to single-phase carbide (Ta,Hf)C with the lattice parameter a = 0.4487 nm, which corresponded to 18 at % of dissolved HfC, while the content of HfO2 impurity oxide was < 1%. SHS powders of solid solutions are excellent raw materials for consolidation by such methods as HP, HIP, and SPS, which gave rise to materials with the relative density up to 98%.
It was explained [12] that the consolidation of powders of UHTCs such as ZrB2 [13], TiB2 [14], ZrB2-ZrC-SiC [15] is made easier when using powders prepared by SHS, instead of other commercially available powders prepared by alternative methods. Mishra et al. [13] ascribed such peculiarity to the high defect concentration in SHS powders generated by the severe heating and cooling rate conditions in combustion wave.
To enhance the thermal shock, UHTCs were strengthened by whiskers/fibers of SiCf or Cf [12]. In case of the HfB2-SiCf system, the best ceramic composition produced by SHS + SPS has a hardness (HV) = 21.6 GPa and fracture toughness (K1c) = 6.2 MPa*m1/2 [12]. However,
SiC fibers break into fragments at consolidation temperatures of 2000°C and higher; some silicides melt down. Therefore, the temperature of SPS should be around 1800°C or lower.
MoSi2, ZrSi2, and TaSi2 silicides are fruitfully used as additives to diborides- and carbides-based composition in order to increase oxidation resistance [4, 16-20]. MoSi2 is characterized by excellent oxidation resistance and can remain operable during 2000 h at 1923 K. Oxidation of ZrSi2 gives rise to the ZrSiOVSiO2 glassy phase with an effect of self-healing of defects and cracks. SHS of heterophase powders ZrB2-MoSi2-ZrSi2 and HfB2-MoSi2-HfSi2 was employed to obtain UHTCs with 1-2 p,m boride grains and 2-4 p,m silicide grains. The kinetics and mechanism of high-temperature oxidation of dense heterophase ZrSi2-MoSi2-ZrB2 ceramics at a temperature of 1650°C were studied [20]. The kinetic oxidation curve is described by the power-law function, which indicates that the evolutionary changes in the structure of the resulting oxide films significantly affect the course of the oxidation process. The oxidation mechanism involves formation of a multi-layered structure of heterogeneous oxide film, partial dissociation of the ZrSiO4 phase, and formation of secondary MoB and Mo5Si3 compounds. The influence of the content of ZrSi2, MoSi2, and ZrB2 phases on the structural and morphological features of the formed oxide films and the efficiency of their protective action were studied. Silicon is reduced and zirconium is simultaneously oxidized to ZrO2 in the ZrSi2-ZrSiO4 system at temperatures above 1620°C in the absence of oxygen or in low-oxygen environment.
Vorotilo et al. [18] studied two schemes for fabricating the composite powders ZrB2-TaB2-TaSi2 by SHS that are discussed in this study: (1) elemental synthesis in the Zr-Ta-B and Ta-Si mixtures followed by mixing of the combustion products, and (2) elemental synthesis in the Zr-Ta-Si-B mixture. The macrokinetic features of combustion of the Zr-Ta-Si-B mixtures, the mechanism of structure and phase formation in the combustion wave, and the structure and properties of the combustion products were discussed. Primary crystals of tantalum and zirconium diborides were formed in the preheating zone as a result of gas-phase mass transfer of boron onto the surface of metal particles. In the combustion zone, melting of Si-B eutectic and Zr particles took place, followed by the formation of borides and silicides of tantalum and zirconium. In the zones of post-combustion and secondary structure formation, zirconium diboride partially interacted with tantalum diboride and formed (Ta,Zr)B2 solid solution. HP ceramics with the relative density of 98% fabricated using scheme 2 have a specific microgradient grain structure and significantly higher hardness and Ku as compared to the similar composite prepared using scheme 1.
Investigation of the macrokinetic features of elemental synthesis in the Mo-Hf-Si-B mixtures, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics, was carried out in [19]. Two routes were also used for fabrication of composite powders in the MoSi2-HfB2-MoB system:
(1) synthesis using Mo-Si-B and Hf-B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo-Hf-Si-B mixture. Although the particle size distribution and phase composition of SHS powders were similar for both routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo-Hf-Si-B mixture allows one to produce hierarchically ordered ceramics with improved hardness up to 17.6 GPa and K1c up to 7.2 MPa m1/2.
The produced UHTCs exhibited high resistance against the impact of high-enthalpy oxidative gas flow. Sequences of chemical and structural transformations were investigated. At 3000°C, the rate of thermochemical corrosion of carbide-based UHTCs was 15-20% lower as compared to that of the basic composites, whereas the boride ceramics based on (Ta,Zr)B2, (Ta,Hf)B2 demonstrated decomposition enthalpy up to 390 kJ/g, which is an order of magnitude higher than the enthalpy of analogous materials [7]. During firing tests, the
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linear rate of thermochemical erosion of dense UHTCs of single-phase solid solution (Ta,Hf)C, (Ta,Zr)C produced by SPS and HP was lower than that of industrial analogues by 15-20% [9]. The hierarchically structured TaSi2-SiC ceramics reinforced with discrete SiC and Si3N4 fibers were synthesized. The mechanism of formation of the TaSi2 + SiC ceramics was revealed [21]. The microstructure becomes significantly finer-grained in the post-combustion zone as SiC nanocrystallites (20-40 nm in size) are formed inside the TaC, Ta5Si3, and TaSi2 grains. Some SiC and TaSi2 grains crystallize from silicon based melt, while crystallization of the remaining SiC and TaSi2 grains occurs via the solid-phase transformation mechanism. The final product has a hierarchical two-level structure with high hardness of 19.1 GPa and fracture toughness Kic = 6.7 MPa*m1/2.
As an example of application, the TaSi2 + SiC ceramic were applied in magnetron sputtering of high-temperature coatings [22]. Coatings consisted of the amorphous phase with the inclusion of nanoparticles of fcc solid solution Ta(Si,C,N), were characterized by hardness of 26 GPa, Young modulus of 268 GPa, thermal stability up to 800°C, and friction coefficient of
0.2.at 800°C in air. Low friction coefficient was caused by formation of a thin oxide layer consisting of TaSixOy nanofibers in the contact zone.
This work was carried out with financial support from the Russian Science Foundation under project no. 19-19-00117.
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