Self-Propagating High Temperature Synthesis of TiB₂–MgAl₂O₄ Composites

Abstract

Metal borides are widely used as heat-insulating materials, however, the range of their application in high-temperature conditions with oxidative medium is significantly restricted. To improve the thermal stability of structural materials based on titanium boride, and to prevent the growth of TiB₂ crystals, additives based on alumina-magnesia spinel with chemical resistant and refractory properties have been used. The aim of this work is to study the structure of TiB₂ with alumina-magnesia spinel additives obtained by self-propagating high-temperature synthesis (SHS). TiB₂ structure with uniform fine-grained distribution was obtained in an MgAl₂O₄ matrix. The material composition was confirmed by X-ray diffraction analysis (DRON-3M, filtered Со kα-emission), FTIR spectroscopy (Thermo Electron Nicolet 5700, within the range of 1300–400 cm⁻¹), and scanning electron microscopy (Philips SEM 515). The obtained material represents a composite, where the particles of TiB₂ with a size of 5 µm are uniformly distributed in the alloy of alumina-magnesia spinel.

1. Introduction

Self-propagating high-temperature synthesis (SHS) is used to develop new technologies for the production of refractory nonmetallic composite materials with defined properties. In spite of the fact that metal carbides and borides are widely used as insulation materials, the range of their application in oxidative mediums at high temperatures is very restricted. To increase the refractory properties of metal carbides and borides, alumina-magnesia spinel MgAl₂O₄ with the melting temperature of 2105 °C, which corresponds to the high level of refractoriness [1], is used as an additive.

Magnesium and aluminothermic synthesis is widely used for the production of refractory ceramic materials, e.g., with the use of metallothermic reduction in a TiO₂–MgO–Al₂O₃–Al system, the refractory materials based on MgAl₂O₄ and titanium carbonitrides are obtained [2]. High-strength porous ceramic material, containing in its composition MgAl₂O₄, TiB₂, TiO₂, Al₄B₂O₆, and Mg₂B₂O₅ was obtained in a TiO₂–B₂O₃–Al system with MgO additives. This material can be used as a catalyst at temperatures of 600 °C–700 °C in an open atmosphere [3]. Moreover, aluminum is widely used in the synthesis of composite materials. In the structure of composites, the intermetallic matrices from both TiAl/Ti₃Al and MgAl₂O₄ are incorporated [4]. In all of the abovementioned works, MgAl₂O₄ is synthesized in the form of particles.

Another method of heat-resistant composite production is through titanium diboride synthesis from its elements with the use of chemical-resistant and refractory alumina-magnesia spinel (MgAl₂O₄). This method allows decelerating high-temperature solid-phase oxidative reactions in the process of material exploitation.

The aim of this work is to study the phase composition and microstructure of a TiB₂ + MgAl₂O₄ heat-resistant composite obtained by self-propagating high-temperature synthesis with MgAl₂O₄ additives of different concentrations.

At high temperatures (~3000 °C), spinel melts and spreads along the surface of TiB₂ grains, forming the matrix that protects the TiB₂ grain surface with the spinel.

2. Materials and Methods

To prepare reaction mixtures, dried in a vacuum at temperature of 200 °C for 2 h, titanium powders (TPP-8, JSC “Avisma”; titanium composition ~96 wt %; particle size < 160 µm), amorphous boron (B-99A-TU-6-02-585-75), and alumina-magnesia spinel (TU 6-09-01-136) were used. Four mixtures of different compositions were prepared: (1) 90% (Ti + 2B) + 10% MgAl₂O₄; (2) 75% (Ti + 2B) + 25% MgAl₂O₄; (3) 60% (Ti + 2B) + 40% MgAl₂O₄; (4) 50% (Ti + 2B) + 50% MgAl₂O₄. Powders were thoroughly mixed to obtain homogenous blends. Then, from the obtained mixtures, porous (40–45%) cylindrical particles were formed with a diameter of 20 mm and a length of 30–32 mm by using a hydraulic press. Self-propagating high-temperature synthesis was conducted in a constant pressure setup in argon atmosphere at a pressure of ~6 atm. Samples ignition was carried out using an ignition mixture of powders (Ti + 2B) with the help of a tungsten filament, which was supplied with a short-term electrical impulse. The maximal combustion temperature was detected by the tungsten-rhenium thermocouple BP5-BP20 with a diameter of 100 µm. Temperature registration was conducted with the use of an analog-to-digital converter LA-20USB connected with a personal computer.

The compositions of the obtained materials were proved by X-ray phase analyses (Dron-3M, filtered Co kα-emission, Saint Petersburg, Russia), IR spectroscopy (FTIR spectrometer Nicolet-5700, Thermo Electron Corporation, Atkinson, USA). Measurements were carried out using an add-in device of scattering reflection in KBr at a frequency interval of 1300–400 cm⁻¹. To study the microstructure, an optical microscope (Axiovert 200M, OM, Karl Zeiss, Germany) and a scanning electron microscope (SEM-515, Philips, Amsterdam, The Netherlands) were used.

3. Results and Discussion

Among gas-free systems, the Ti-B system is characterized by the highest exothermicity. For a powder mixture with the ratio of components Ti:B = 1:2 the adiabatic temperature of combustion is Tad = 3190 K [5]. Alumina-magnesia spinel MgAl₂O₄ is inert in relation to the mixture Ti-2B. In

Table 1. Physicochemical properties of compounds.

Compound	Melting Temperature, °C	Density, g/cm3	−ΔH°form, kJ/mol
MgAl2O4	2135	3.8	2307.8
TiB2	2850	4.45–4.50	293.3
MgTiO3	1680	3.91	1573.6
α–Al2O3	2045	3.99	1675.0

, the physicochemical properties of spinel are presented [6,7].

Figure 1 shows the combustion thermogram of the TiB₂ (75 wt %) + MgAl₂O₄ (25 wt %) system. The maximal combustion temperature is 2300 °C, which is higher than the spinel melting temperature. Synthesis was conducted layer-by-layer in the steady state combustion conditions. Similar combusting conditions were observed for Ti + 2B + xCu and Ti + 2B + xFe systems. Depending on their content, different metal alloys partially or fully surround particles of titanium borides [8,9].

Studies on the microstructure of the composite blends based on TiB₂ with different MgAl₂O₄ compositions showed that, depending on the amount of added spinel, the composite structure change (Figure 2). If the amount of added MgAl₂O₄ is <10%, the grains of titanium diboride in the microstructure of the composite are partially surrounded by a solidified alloy of MgAl₂O₄ (Figure 2a). The best results were obtained at a spinel composition of 25%. The fine-grain microstructure from TiB₂ grains (light crystals) was observed, which is fully surrounded by spinel (dark areas). When 40% MgAl₂O₄ was added to the blend during the synthesis, the formation of a non-homogeneous structure was observed. The structure contains areas with the fine-grained titanium diboride and adjusting areas from alumina-magnesia spinel (Figure 2c).

When 45% MgAl₂O₄ is added to the composite, the mixture does not burn in this case, because MgAl₂O₄ is inert.

Complete information on the structure of the product formed during SHS can be obtained by analyses of fracture surfaces, studied with scanning electron microscopy. Figure 3 shows the microstructure of fractures of SHS ceramic samples based on titanium diboride with the addition of 25% MgAl₂O₄ (Figure 3a,b), and 0% MgAl₂O₄ (Figure 3c,d).

As can be seen from Figure 3, the addition of 25% MgAl₂O₄ leads to the decreasing of TiB₂ crystals (~2 µm), which are surrounded by a solidified alloy of alumina-magnesia spinel. The microstructure of the SHS sample with Ti + 2B composition is formed by large TiB₂ faceted crystals.

Figure 4 shows the diffraction patterns of TiB₂ composites with different amounts of spinel. X-ray diffraction analyses showed that in the composition of alumina-magnesia spinel, there is 12 wt % of MgAl₂O₄. Figure 4 shows that spinel is identified in the composite containing 25 wt % of MgAl₂O₄, though, metallographically the spinel is identified at 10 wt % of MgAl₂O₄.

The composite with the fine-grained microstructure containing 25 wt % of MgAl₂O₄ was studied by FTIR spectroscopy. Figure 5 shows the FTIR spectrum of MgAl₂O₄, TiB₂–MgAl₂O₄ composite, corundum, and TiB₂.

Figure 5 (pattern 1) shows that alumina-magnesia spinel has two different absorption bands with maximums at 692.0 cm⁻¹ and 540.0 cm⁻¹, related to the tetrahedral coordinated magnesium MgO₄ and octahedral coordinated aluminum of AlO₆. The small peak in the frequency range of 800–900 cm⁻¹ proves the presence of Al₂O₃ in spinel content. Irregularity of the spinel structure leading to a change of binding force in the cation sub-lattice is identified by the emergence of an absorption band at 558.7 cm⁻¹ [10].

FTIR spectrum of composite (TiB₂ + 25 wt % of MgAl₂O₄) consists of numerous absorption bands typical for titanium diboride, spinel, and corundum (pattern 2).

According to the burning thermogram for the 75 wt % TiB₂ + 25 wt % MgAl₂O₄ system, the burning temperature is 2300 °C. Therefore, MgAl₂O₄ is partially decomposed with corundum formation.

$${\mathrm{MgAl}}_2{\mathrm{O}}_4\stackrel{T}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3+\mathrm{MgO}$$

Pattern 3 shows the FTIR spectrum of gray corundum. Along with absorption bands at 639.0 cm⁻¹, 605.8 cm⁻¹, and 456.9 cm⁻¹ typical for octahedral coordinated aluminum AlO₆ in α–Al₂O₃, there are absorption bands at 1089.1 cm⁻¹, 797.9 cm⁻¹, and 778.7 cm⁻¹, related to the tetrahedral coordinated aluminum AlO₄ [11]. The same absorption bands are observed in the composite spectrum.

It is well known that α–Al₂O₃ contains aluminum atoms which are octahedrally coordinated by oxygen [10,12]. According to the literature data [13], the gray color of corundum is caused by the presence of aluminous spinel AlOAl₂O₃. This spinel was identified during the electrocorundum synthesis in reducing medium [13]. The melting temperature of spinel is 1980 °C [1].

At the interference level, the absorption bands at 940.2 cm⁻¹, 727.2 cm⁻¹, and 507.8 cm⁻¹ are observed. They can be referred to MgTiO₃ [12]. The formation of MgTiO₃ is possible during the synthesis at the phase boundary between TiB₂ and MgAl₂O₄.

Oxygen and MgO can be borrowed during the thermal decomposition of spinel. In this case, aluminum is moved from an octahedral coordination to a tetrahedral one with the formation of both MgTiO₃ and aluminous spinel with an intensive absorption band at 1089.1 cm⁻¹. It is well-known [14] that at high temperatures over Al₂O₃, the gas phase is formed as a result of thermal dissociation. The gas phase contains aluminum sub-oxides Al₂O and AlO⁻.

$${\mathrm{Al}}_2{\mathrm{O}}_3\stackrel{T}{\to }{\mathrm{Al}}_2\mathrm{O}\uparrow +{\mathrm{O}}_2\uparrow$$

Aluminum sub-oxides can also participate in the formation of aluminous spinel AlOAl₂O₃.

2Al₂O₃ + Al₂O + O → 2AlOAl₂O₃

The FTIR spectrum of this composite (pattern 2) represents the envelope line along the spectrum of alumina-magnesia spinel. The overlap of numerous bond oscillation frequencies, related to the TiB₂, corundum, aluminous spinel, and MgTiO₃, is observed.

Studies showed that the obtained composite consists of TiB₂ fine grains, which are homogeneously distributed in the alumina-magnesia matrix containing α–Al₂O₃. Traces of MgTiO₃ and aluminous spinel are also present in the composite.

According to the literature data [5], 12 mol % of MgO and 85.5 mol % of Al₂O₃ can be dissolved in alumina-magnesia spinel. In

Table 2. Eutectic melting temperatures in the MgO–Al₂O₃ system.

Chemical Compounds in Eutectics	Al2O3 Composition, wt %	Melting Temperature, °C
MgO, MgAl2O4	55	1995
MgAl2O4, Al2O3	98	1920

, the eutectic melting temperatures in the MgO–Al₂O₃ system are presented.

Melting temperatures of TiB₂, α–Al₂O₃, MgTiO₃, and MgAl₂O₄ as well as their eutectics are presented in Table 1 and Table 2. As can be seen from Table 2, all values of the melting temperatures are very high, which proves that the obtained ceramic material is refractory.

4. Conclusions

It was shown that structure with a homogeneous fine-grained distribution of TiB₂ grains was obtained by using 25 wt % of MgAl₂O₄.

The formed surface layer of MgAl₂O₄ on the grains boundary of TiB₂ serves as a blocking protection from titanium diboride oxidation and prevents the growth of TiB₂ crystals.

A partial decomposition of spinel occurred during the composite synthesis. This is proved by the presence of MgTiO₃ and corundum traces in the composite, which were identified by FTIR spectroscopy.