CC BY
58iSHS 2019
Moscow, Russia
ADVANCED ULTRA-HIGH-TEMPERATURE COMPOSITES BASED ON
SHS-SINGLE PHASE SOLID SOLUTION (Hf,Ta)B2
V. V. Kurbatkina*", E. I. Patsera", and E. A. Levashov"
aNational University of Science and Technology MISIS, Moscow, 119049, Russia
*e-mail: vvkurb@mail.ru
DOI: 10.24411/9999-0014A-2019-10076
Development of advanced composites for high-temperatures applications are important trend in materials science. Design of such materials will enable the creation of new types of energy-conversion engines, high-velocity transportation and aerospace vehicles [1-3].
Ones of the most prospective materials in this regard are composites based on HfB2 (Tm = 3653 K), ZrB2 (3473 K), TaB2 (3473 K), NbB2 (3323 K) [4], which possess high strength, fracture toughness, resistance against the wear and thermal shock. Another important advantage of the boride composites is their high thermal conductivity, which ensures the efficient heat sink from the surface which contacts with the high-temperature oxidative gas. Solid solutions possess higher melting points as compared to the individual constituents. For example, singlephase carbide (Ta,Hf)C, containing 20% HfC, has the melting point of 4223 K. Non-linear dependence on the composition of the solid solution is characteristic for hardness, coefficient of thermal expansion (CTE), thermal conductivity. The combination of high thermal conductivity and low CTE defines the resistance of the material against thermal shocks [1-3].
Synthesis of single-phase compounds based on refractory borides is a relevant scientific and technological task. Various routes are known for the production of complex carbides and borides, including the heating of powder mixtures, carbon reduction of oxide mixture, solution precipitation, microwave synthesis, sol-gel synthesis, hot pressing, etc. SHS offers unique possibilities for the synthesis of UHTCs. Hybrid technologies SHS+HP, SHS+SPS and reactive HP and SPS are capable of producing the dense ultra-high temperature materials with unique structure and properties [3, 5, 6].
This work considers the problems of obtaining single-phase solid solutions based on hafnium and tantalum diborides (Hf,Ta)B2 by the SHS method [7, 8].
In the Hf-Ta-B system, the phase formation mechanisms differ from the previously investigated Ta-Zr-B system [6], which is related to the contact melting which occurs in the Hf-B system at temperature 2153 K and B content < 0.8 wt %. Contact melting occurs corresponding to the eutectic reaction: P-Hf + HfB ^ Liq with the formation of a melt containing solid particles HfB + P-Hf. At temperature 2365 K, boron melts and nearly simultaneously the primary crystals of HfB2 and TaB2 are formed, followed by the melting of hafnium (2505 K) and active chemical interaction in melt following the formation of solid solution based on the tantalum and hafnium diborides [7, 8]. Due to the high melting temperature and high heat loss, the post-combustion zone is relatively narrow. Concentration homogenization takes place in the post-combustion zone. However, the high porosity of the synthesis products and low diffusion rates do not allow the formation of equilibrium phases to be completed. A significant difference in grain composition and size is retained in the synthesis products. However, the synthesized powders are successfully employed for the subsequent sintering by HP and SPS. Chemical uniformity is achieved during the sintering.
SHS powders in Hf-Ta-B systems were employed for the hot pressing and spark plasma sintering of dense composites with residual porosity below 2-3%. Two produced composites based on (Hf,Ta)B2 possessed superior high hardness 60-70 GPa and elastic modulus 584 GPa.
XV International Symposium on Self-Propagating High-Temperature Synthesis
The measured heat conductivity of the HfB2-based solid solutions (Hf,Ta)B2 was 53 Wt/m-K, which is above the heat conductivity for TaB2, but below the heat conductivity of HfB2. Heat conductivity of two-phase specimen was 30 Wt/m-K. The measured values of heat conductivity are 10% higher for (HfTa)B2 than for (Zr,Ta)B2, and are comparable to the ZrB2-25% SiC ceramic.
Refractory silicides are often used as the alloying additive for the betterment of the heat resistance of boride and carbide-based ceramics. In particular, in this work, Ti5Si3 and TaSi2 were added to the (HfTa)B2 solid solution. SHS in the Hf-Ti-Ta-Si-B system was studied. Combustion temperature and velocity were measured, and the sequence of phase formation in the combustion front was investigated. Optimal conditions for the force SHS-pressing were established without the "chemical furnace" use, which allows the one-step production of dense Hf-Ti-Ta-Si-B ceramic with relative density 94-95%. Synthesized specimens consist of two complex solid solutions: (Hf,Ti,Ta)B2 and (Hf,Ti,Ta)5Si3B. Diboride crystals with the size of 1-2 |im are embedded in the borosilicide matrix. Mechanical properties of such composites consisted of (Hf,Ti,Ta)B2 and (Hf,Ti,Ta)5Si3B were studied. The microhardness of the (Hf,Ti,Ta)B2 is almost three times higher as compared to the (Hf,Ti,Ta)5Si3B. In a similar fashion, elastic modulus for diboride solid solution was 3 times higher as compared with (Hf,Ti,Ta)5Si3B.
Specimens in system Hf-Ti-Ta-Si-B sintered by various techniques (HP, SPS, force SHS-pressing) have identical phase composition, which includes two complex solid solutions diboride-based (Hf,Ti,Ta)B2 and borosilicide-based (Hf,Ti,Ta)5Si3B.
For the first time, the temperature dependencies of the heat capacity, thermal diffusivity, thermal conductivity, coefficient of thermal expansion were measured for the composites (Hf,Ti,Ta)B2 + (Hf,Ti,Ta)5Si3B sintered by HP and SPS. Thermal conductivity for (initial mixture (Hf, Ta)B2 + 35% Ti5Si3) ceramics produced by hot pressing at 1700oC was 24 Wt/m K; for the same composition produced by SPS, the conductivity was 23 Wt/m K. The CTE for the hot-pressed specimens at 600oC in the height direction was 14.5-10-6 K-1, in the diameter direction - 7.0-10-6 K-1. With an increase in the temperature till 900oC, these values increase up to 18.5 and 10.1-10-6 K-1, correspondingly.
Oxidation mechanism for ceramics (Hf,Ti,Ta)5Si3B + (Hf,Ta,Ti)B2 at 1100oC was studied. Mass change during oxidation was approximated as logarithmic functions for all specimens to reveal the common oxidation mechanism.
This work was conducted with the financial support of the Russian Ministry of Science and High Education in the framework of state assignment no. 11.1207.2017/nH.
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