CC BY
23iSHS 2019
Moscow, Russia
THE KINETICS AND MECHANISMS OF HIGH-TEMPERATURE OXIDATION OF HEAT-RESISTANT MoSi2-MoB-HfB2 CERAMICS PRODUCED BY HYBRID SHS + HP TECHNOLOGY
A. Yu. Potanin*", S. Vorotilo", Yu. S. Pogozhev", P. V. Loginov", and E. A. Levashov"
aNational University of Science and Technology MISIS, Moscow, Russia
*e-mail: a.potanin@inbox.ru
DOI: 10.24411/9999-0014A-2019-10131
Transition metals silicide composites are prospective as high-temperature oxidation and corrosion-resistant structural materials and coatings. They can withstand the mechanical loads at temperatures up to 1700-2000°C in aggressive environments [1]. Among these materials, MoSi2-based composites attract special interest. In particular, they are applied as the heating elements in high-temperature industrial furnaces and exhaust systems of internal combustion engines [2].
The high-temperature oxidation resistance of MoSi2 is based on the formation of glassy SiO2-based protective film, which is nearly impermeable to gasses. However, at the temperatures above 1800°C the formation and evaporation mono-oxide SiO causes the degradation of the SiO2 film and drastically decreases the oxidation resistance of MoSi2.
Alloying by borides TiB2, ZrB2, MoB, HfB2 allows one to enhance the performance of MoSi2-based materials and optimize the protective properties of the oxide films [3, 4]. Formation of a protective borosilicate glass layer (B-SiO2) contributes to efficient healing of microcracks and pores that inevitably emerge in the protective layer during its operation.
The goal of this work was to investigate the influence of the composition and microstructure of the ceramics MoSi2-MoB-HfB2 produced by hybrid SHS + HP technology on the thermal conductivity, mechanisms and kinetics of oxidation in furnace static conditions at 1200 and 1650°C. In the investigated Mo-Hf-Si-B system, the composition of the combustion products was calculated according to the equation (100 - X)(90MoSi2 + 10MoB) + X-HfB2, where X = 0 and 34 at %. Reaction mixtures were prepared using the elemental powders of Mo, Hf, Si, and B. The raw SHS powders were produced according to two schemes: (1) products of the combustion of reaction mixtures Mo-Si-B (X = 0) and Hf + 2B were milled and mixed in a ball mill with HfB2 concentration of 34 at %; (2) products of the combustion of Mo-Hf-Si-B mixture (X = 34%) were ball milled until the micron-sized MoSi2-MoB-HfB2 composite powders were produced. Dense ceramics were produced by hot pressing (HP) of the SHS powders using the DSP-515 SA installation («Dr. Fritsch». Germany) in vacuum at temperature of 1600°C, heating rate of 10 °C/min, pressure of 35 MPa and dwelling time of 10 min.
Figure 1 shows the microstructure of the HP specimens. In the X0 ceramic, MoSi2 (<15 ^m) and MoB (5-10 ^m) grains are present. X34(7) ceramic (Fig. 1b) in addition to MoSi2 and MoB contains faceted HfB2 grains (4-10 |im) and a small number of HfSiO4 inclusions (< 4 |im). In both X0 and X34(7) ceramics, all phases have comparable size and are randomly distributed in the structure; hence these ceramics are denoted as single-level structures (SLS). X34(2) ceramic has a two-level structure (TLS) (Fig. 1c), relatively large MoSi2 grains (<10 |im) are surrounded by the 2-4 |im wide HfB2 layers, which are comprised of elongated HfB2 grains with diameter of ~ 0.3 ^m and length of 0.5-1 ^m. MoB grains (< 3 ^m) are also present in the structure, usually in conjunction with HfB2 grains. This is related to the stages of crystallization in the combustion front of Mo-Hf-Si-B mixture: the most refractory HfB2 forms first, followed by crystallization of MoB on the surface of HfB2 [5].
XV International Symposium on Self-Propagating High-Temperature Synthesis
Addition of hafnium diboride leads to the increase of mass change during the oxidation at 1200°C due to the formation of HfO2 and HfSiO4, which are denser than SiO2. Oxidation of SLS and TLS ceramics produced two-layered oxide films. In the case of SLS ceramic, SiO2-based oxide film was comprised of top 9-12 |im wide Hf-doped amorphous silica layer and bottom 3-5 |im crystalline a-SiO2 layer (Fig. 2b). In the case of TLS ceramic, top layer consisted of crystalline a-SiO2, whereas the bottom layer was formed by HfSiO4 g rains (Fig. 2c).
Fig. 2. SEM images of the fractures of specimens (a) X0, (b) X34(1), and (c) X34(2) oxidized at 1200°C during 30 h.
Figure 3a shows the microstructure of fracture of surface region of X34(1) specimen oxidized at 1650°C during 0.5 h. On the surface of sample, a one-layered SiO2-based 8-10 ^m thick film formed, containing fine Hf-Si-O precipitates. These inclusions were investigated by TEM (Fig. 3b). Elongated dark-grey precipitates are distributed homogeneously in the SiO2 matrix and have the length of 300-400 nm and width of 100-200 nm. By the electron diffraction pattern and energy dispersive X-ray spectrometry analysis, these precipitates were identified as HfSiO4 with body-centered tetragonal lattice. HRTEM (Fig. 3c) demonstrated that the submicron precipitates have no subgrains and are defect-free.
Fig. 3. (a) SEM image of the near-surface zone of fractured specimen X34(1) oxidized at 1650°C during 0.5 h. (b) TEM and (c) HRTEM images of the HfSiO4 precipitate.
Understanding and control of the phase and structure formation of oxide films in single-level structured and two-level structured ceramics open new avenues for the optimization of the
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composition and structure of ceramics in regards to the intended working conditions. Application of SHS is particularly promising in this regard since it allows one to produce a wide spectrum of ceramics with similar phase composition but drastically different microstructures and properties.
The reported study was funded by RFBR and Moscow city Government according to the research proj ect no. 19-38-70013.
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