English

• 1. INTRODUCTION

  Binary borides have excellent properties such as high temperature resistance, corrosion resistance and good chemical stability, which enable them to be high-temperature materials^{[1, 2]}. However, due to their inherent brittleness, the application of binary borides is greatly restricted^[3-5]. In recent years, with the development of MAX phase materials, the preparation of ternary-layered borides (MAB) has aroused a lot of interest^[6]. For the MAX materials, M and A represent the transition metal and the third or fourth main group element (usually referring to Al or Si) respectively, while X is a carbon or nitrogen atom^{[7, 8]}. MAB is similar, where B exclusively refers to boron element. The MAB material is a class of ternary transition metal (M) borides with atomic lamination, in which the metal-boride (M–B) layer composed of coplanar BM₆ triangular prisms is separated by single or double Al atomic layers^[9-12]. Due to its special crystal structure, MAB material possesses integrated characteristics of metal and ceramic simultaneously, including good electrical conductivity, thermal conductivity^{[9, 10]}, high damage tolerance^[11] and mechanical processability^[12]. Fe₂AlB₂ is one of the most important MAB materials, of which the raw materials are quite accessible.

  At present, Fe₂AlB₂ is mainly synthesized by hot pressing or arc-melting method, which is complicated and expensive. The as-product is in bulk state leading to low purity, which hinders its research and application. To develop the method of large-scale production of high-purity Fe₂AlB₂, a simple solid-state synthesis was investigated in this work. In addition, the weak bonding between adjacent layers of metal boride and Al in Fe₂AlB₂ could yield Al and FeB in situ at relatively high temperature. Zirconium diboride (ZrB₂) has attracted more attention due to wide-range applications such as high temperature structural ceramics, cutting tools and wear-resistant parts, but it is difficult to obtain high density even under extremely high sintering temperature (~2000 ℃) with outer pressure as assistance. The in-situ liquid Al from decomposition of Fe₂AlB₂ has good wettability, which can accelerate mass transfer, remove the oxides from the surface of ZrB₂ (such as ZrO₂, B₂O₃ or amorphous Zr–B–C–O) and promote densification. Meanwhile, the produced FeB has good compatibility with the ZrB₂ matrix. Herein, the sintering of ZrB₂ with Fe₂AlB₂ additive was performed to obtain highly dense ceramic materials for the first time.

  2. EXPERIMENTAL

  2.1 Preparation of Fe₂AlB₂ powder

  Commercially available Al (99.99%, 3μm), Fe (99.99%, 3μm) and B (≥95%, 5μm) powders (Aladdin Chemical Co., China) were used as received. In a typical synthesis, raw materials were accurately weighed according to a certain ratio, and mixed via ball milling at 250 rpm for 24 hours. After drying and sieving, the obtained powder was uniaxially pressed into a disk of 30 mm diameter and 4 mm thickness, and then cold isostatically pressed under 150 MPa. The green body was placed in a BN crucible and heated in a tube furnace from 1130 to 1170 ℃ for different duration time. As a comparison, the parallel preparation was also carried out without cold isostatic pressing. The product was crushed in the agate mortar and soaked in HCl/NaOH solution under ultrasonic stirring for purification. Finally, the obtained powder was washed and collected by filtration.

  2.2 Sintering of ZrB₂ ceramic with Fe₂AlB₂ additive

  ZrB₂ (99.99%, 3 μm, Aladdin Chemical Co., China) and different contents (25, 30 and 35 wt%) of Fe₂AlB₂ were mixed by ball-milling in ethanol media for 24 h at 250 rpm. The slurry was dried at 80 ℃ and screened through a 150-mesh sieve. The powder mixture was compacted in a graphite die with the diameter of 25 mm and hot pressing sintered under a pressure of 35 MPa at different temperature for 120 min in Ar atmosphere.

  2.3 Characterization

  The phase compositions were examined by powder X-ray diffraction (XRD, Mini Flex600, Rigaku, Japan) using CuKα radiation with a scan step width of 0.02°. Calc. Fe₂AlB₂ refers to the theoretical diffraction pattern derived from the result of crystal simulation. The microstructures were investigated using a scanning electron microscope (SEM, Sigma300, Carl Zeiss Microscopy Ltd, Germany and Su-8010, Hitachi, Japan). The density was measured by the Archimedes method. Vickers hardness (HV) was measured under the loads of 9.8 N for 15s on a polished surface. The direct crack measurement (DCM) method was used to evaluate fracture toughness, based on an equation suitable for the ratio of crack length to diagonal length larger than 2^[13]. Thermogravimetric analysis (TGA, STA 449F3, Netzsch, Germany) was performed in N₂ atmosphere at a heating rate of 10 ℃/min.

  3. RESULTS AND DISCUSSIONS

  3.1 Characterization of Fe₂AlB₂ powder

  The phase composition of product was characterized by XRD analysis. As shown in Fig. 1, Fe₂AlB₂ is the predominant phase with the trace of FeB, Al₂O₃ and Al₁₃Fe₄ after being calcined at 1130 ℃. It has been widely recognized that excess Al is necessary to achieve high conversion of MAB phase, considering the volatilization of Al at high temperature. The formation of FeB impurity was suppressed by increasing Al content. For the product with the Al ratio of 1.7, FeB has been completely eliminated. Meanwhile, the unwanted Al-rich intermetallic, identified as Al₁₃Fe₄, is difficult to avoid. The Al element evaporated during the sintering process at high temperature. Aluminum is liable to react with oxygen even at extremely low oxygen partial. The existence of Al₂O₃ originates from the oxide on the surface of Al powder or the aluminothermic reduction of oxide in other reagents. Fortunately, the above two byproducts can be easily removed by treating with acid or alkaline solution. The XRD patterns of the products with the ratio of 1.5Al/2Fe/2B calcined from 1130 to 1170 ℃ are shown in Fig. 2. Fe₂AlB₂ is the main phase with a bit of FeB and Al₁₃Fe₄ at 1130 ℃^[14]. With the increase of temperature to 1150 ℃, these two byproducts are further consumed to form ternary boride and FeB almost disappears. For that of 1170 ℃, the peak intensity of Fe₂AlB₂ decreases dramatically, while those of FeB, Al₂O₃ and Al₁₃Fe₄ increase. It can be deduced that the ternary boride decomposes at higher temperature. The optimal reaction temperature is set to 1150 ℃ for this system. This method can be extended to the preparation of other ternary borides, and the related work is in progress.

  Figure 1

  

  Figure 1.  XRD patterns of products prepared from compacted raw materials at 1130 ℃

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  Figure 2

  

  Figure 2.  XRD patterns of products prepared from compacted raw materials of 1.5Al/2Fe/2B at different temperature

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  The uncompacted reactants have been used for the synthesis as a comparison. The raw powders were directly poured into BN crucible for calcining after ball-milling. The reaction temperature was selected as 1150 ℃ and the ratio of Al content was 1.3 and 1.5, respectively. As shown in Fig. 3, Al and FeB are the predominant phases, while a small amount of Al₂O₃ coexists. There is no target product Fe₂AlB₂, even with various Al contents. It is noteworthy that there is no Al₁₃Fe₄, which as the important intermediate can react with FeB to yield ternary boride^[14]. From a thermodynamic point of view, the reaction of Fe and B is easier than that of Al. However, this difference in reactivity can be overcome in the pressed green body, where the mass transfer distance is greatly shortened. When Al and Fe melt, they can infiltrate and combine with each other to form an alloy phase. Thus, the pre-compression step of raw materials is very important for the synthesis of Fe₂AlB₂. Such a feature is different from the well-known MAX phase, which can be synthesized from powdered reactants^[15].

  Figure 3

  

  Figure 3.  XRD patterns of products prepared from powdered raw materials at 1150 ℃

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  In order to obtain Fe₂AlB₂ with high purity, the crude product was soaked in diluted HCl to remove the impurity phases. The effect of acid concentration and dwelling time on purification has been investigated. The grated product was treated with 1.0 and 1.5 mol/L HCl for 10 and 20 min, respectively. As shown in Fig. 4, Fe₂AlB₂ has totally decomposed into FeB, Fe₂B and AlB₁₂ after etching by 1.5 mol/L HCl, although Al₂O₃ and Al₁₃Fe₄ have transferred into soluble AlCl₃ and FeCl₃. Under such high concentration, the dwelling time of 10 min also caused the decomposition similar to the 20 min process. When the HCl concentration is reduced to 1.0 mol/L, the impurities of Al₂O₃ and Al₁₃Fe₄ can be eliminated exactly. Fe₂AlB₂ also reacts with low concentration HCl but at a much slower rate, so Fe₂AlB₂ with just a trace of FeB can be achieved^[16]. When the dwell time lasts to 20 min, the crystallinity decreases and a small amount of FeB and Fe₂B appear. Therefore, the low concentration and short dwelling time are more advantageous, when HCl solution is preferred.

  Figure 4

  

  Figure 4.  XRD patterns of products treated with HCl solution

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  Alkaline solution can also dissolve Al₂O₃ and react slowly with iron and aluminum metals. In present work, the purification of crude product with NaOH solution is carried out and the effect of concentration and dwelling time is studied. As shown in Fig. 5, when the soaking time is 10 min, most intense reflections of the impurities between 40~45^o disappears gradually with the increase of NaOH solution concentration. At the same time, a small diffraction peak appears around 37^o, corresponding to the FeB phase. Furthermore, we fixed the NaOH solution concentration to 1.0 mol/L and adjusted the processing time (Fig. 6). When the etching time exceeds 20 min, all the impurity peaks disappear, and no iron boride phase exists. Therefore, low concentration NaOH solution with long immersion time has better results, which can effectively remove impurities and avoid decomposition of Fe₂AlB₂.

  Figure 5

  

  Figure 5.  XRD patterns of products treated with NaOH solutions

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  Figure 6

  

  Figure 6.  XRD pattern of products treated with 1.0 mol/L NaOH solution for different times

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  3.2 Thermal stability and microstructure of Fe₂AlB₂

  The thermal stability of Fe₂AlB₂ has been studied in N₂ atmosphere (Fig. 7). The TG curve is stable before 150 ℃, after which the trace weight reduction (about 0.3 wt%) can be attributed to the volatilization of adsorbed water. The small amount of mass increase (about 0.4 wt%) between 200 and 1000 ℃ is due to the partial nitridation of Al component. As the temperature arises, the decomposition and nitridation of Fe₂AlB₂ increase, leading to a rapid weight increase, with a sharp endothermic peak at 1245 ℃. However, only 2% weight gains until 1300 ℃, indicating that Fe₂AlB₂ has good thermal stability and can be used as a potential high temperature material.

  Figure 7

  

  Figure 7.  TG-DTA curves of Fe₂AlB₂ in N₂ atmosphere

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  Previous reports mostly focus on bulk materials^[17], while the micro-morphology of MAB powder has not been investigated yet. In the present work, a high-resolution observation of the ferromagnetic Fe₂AlB₂ powder is carried out for the first time, through the scanning electron microscopy (SEM) equipment with external objective lens. Fig. 8a shows the SEM image of as-prepared Fe₂AlB₂ powder with a particle size of 20~30 μm, where the irregular shape with obvious polyhedron outline can be seen. The grinding trace of the particle can be attributed to the weak basal planes, making it easy to damage during the milling process. The layered characteristic of Fe₂AlB₂ is illustrated in Fig. 8b, which can be explained by the crystalline structure, similar to those of MAX phases. After the treatment with NaOH solution, there is no obvious change in morphology, in which the polyhedron edges and laminated structure are still clear (Fig. 8c). This means that the reaction between NaOH and Fe₂AlB₂ is relatively mild, without destroying the microstructure. For the sample treated with HCl solution, the morphology changes greatly, where the particle size decreases, the corners become round and the edges disappear gradually (Fig. 8d). The layers can still be observed, while the surface becomes rough with the attachment of fine particles. This can be attributed to the easier etching of Al atomic layer in the acid solution.

  Figure 8

  

  Figure 8.  SEM images of Fe₂AlB₂. (a) and (b) As-prepared Fe₂AlB₂; (c) treated with NaOH solution; (d) treated with HCl solution

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  3.3 Composition, microstructure and properties of ZrB₂ ceramic sintered with Fe₂AlB₂

  To investigate the effect of Fe₂AlB₂ as an additive on densification, microstructure and properties of ZrB₂ ceramic, the hot-pressing process at the relative low temperature (1250~1500 ℃) has been carried out. Fig. 9 shows the XRD patterns of the recants and as-sintered ZrB₂ ceramics with 25 wt% Fe₂AlB₂ at different temperature. The diffraction peaks of ZrB₂ are predominant, while those of Fe₂AlB₂ are not obvious, because the X-ray diffraction of Fe₂AlB₂ is much weaker than that of ZrB₂. As the sintering temperature increases, the diffraction intensity of Fe₂AlB₂ decreases with the appearance and increase of FeB phase. Because the Fe₂AlB₂ in the composite ceramic will decompose in a high temperature environment, in which the Al element was volatile, the FeB impurity in the composite ceramic at high temperature increases, while reducing the temperature, the MAB phase in composite ceramics increases. When sintering at 1500 ℃, Fe₂AlB₂ is almost completely decomposed. The composition of samples with different amounts of Fe₂AlB₂ sintered at 1250 ℃ is shown in Fig. 10. ZrB₂ is distinguished as the main phase, and several additional diffraction peaks are observed due to Fe₂AlB₂ and FeB, respectively. The Fe₂AlB₂ content in the raw material is proportional to the residual amount in the sintered body. Therefore, by adjusting the amount of additive and the sintering temperature, the phase composition of product can be modified conveniently.

  Figure 9

  

  Figure 9.  XRD patterns of ZrB₂-25% Fe₂AlB₂ ceramics sintered at different temperature

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  Figure 10

  

  Figure 10.  XRD patterns of ceramics sintered at 1250 ℃ with different Fe₂AlB₂ contents

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  The fracture surfaces of ZrB₂-25/30 wt% Fe₂AlB₂ sintered at 1250 ℃ are shown in Fig. 11. No obvious pores can be observed, confirming the high density. Besides the polyhedral grains, few layered components can be observed and identified as residual Fe₂AlB₂. Fe₂AlB₂ and FeB adjacent to ZrB₂ grains play an important role in joining and improve the densification. The grain size tends to decrease with the increase of Fe₂AlB₂ content, which suggests the in situ formation of FeB and composition can effectively inhibit the grain growth. The fracture mode of monolithic ZrB₂ is dominantly transgranular, leading to the low fracture toughness. Due to the volatilization of the Al element in the high-temperature environment, the internal grains of the composite ceramic are rearranged. For present ZrB₂ with Fe₂AlB₂ additive, due to the different thermal expansion coefficients of ZrB₂, Fe₂AlB₂ and FeB, residual stress can be produced at the grain boundaries. When cracks appear, they tend to be deflected and to propagate along the grains rather than through them. Such intergranular fracture costs more energy to prolong the route expansion, so the fracture toughness of the material can be enhanced. In addition, according to the Hall­Petch relationship, the hardness and toughness could increase with the decrease of grain size due to the fine-grain reinforcing^[18].

  Figure 11

  

  Figure 11.  SEM images of fracture surfaces for ceramics with different Fe₂AlB₂ contents sintered at 1250 ℃: a 25 wt%; b 30wt%

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  Table 1 summarizes the mechanical properties of ZrB₂ sintered with different amounts of Fe₂AlB₂. The increase of Fe₂AlB₂ composition can promote the densification of composite ceramics. When the mass fraction of Fe₂AlB₂ is 10%, the relative density is only 84.5%, and the presence of Fe₂AlB₂ can promote the sintering of composite ceramics. With the increase of additive, both the harness and fracture toughness have been improved. The high harness and fracture toughness of 22.3 GPa and 5.78 MPa·m^1/2 respectively are obtained for ZrB₂ sintered with 35 wt% Fe₂AlB₂.

  Table 1

  

  Table 1.  Mechanical Properties of the ZrB₂-Fe₂AlB₂ Ceramics

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  Material	Relative density (%)	Vickers hardness (GPa)	Fracture toughness (MPa·m1/2)
  ZrB2-10%Fe2AlB2	84.5	4 ± 2	-
  ZrB2-25%Fe2AlB2	96.0	18 ± 0.6	4.64 ± 0.2
  ZrB2-30%Fe2AlB2	95.6	19 ± 0.5	5.23 ± 0.3
  ZrB2-35%Fe2AlB2	96.2	22 ± 0.3	5.78 ± 0.5

  4. CONCLUSION

  Fe₂AlB₂ was successfully synthesized from compacted Fe, Al and B with molar ratio of 2:1.5:2 at 1150 ℃ for 30 min. The product was further purified by ultrasonic vibration in 1.0 mol/L NaOH solution for 15 min. Using Fe₂AlB₂ as a sintering additive, high dense ZrB₂ ceramics were obtained by hot pressing at low temperature (1250 ℃). Its relative density is up to 96.2 %, while the hardness and fracture toughness are 22 ± 0.3 GPa and 5.78 ± 0.5 MPa·m^1/2, respectively.

  
  
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  Figure 1  XRD patterns of products prepared from compacted raw materials at 1130 ℃

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  Figure 2  XRD patterns of products prepared from compacted raw materials of 1.5Al/2Fe/2B at different temperature

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  Figure 3  XRD patterns of products prepared from powdered raw materials at 1150 ℃

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  Figure 4  XRD patterns of products treated with HCl solution

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  Figure 5  XRD patterns of products treated with NaOH solutions

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  Figure 6  XRD pattern of products treated with 1.0 mol/L NaOH solution for different times

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  Figure 7  TG-DTA curves of Fe₂AlB₂ in N₂ atmosphere

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  Figure 8  SEM images of Fe₂AlB₂. (a) and (b) As-prepared Fe₂AlB₂; (c) treated with NaOH solution; (d) treated with HCl solution

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  Figure 9  XRD patterns of ZrB₂-25% Fe₂AlB₂ ceramics sintered at different temperature

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  Figure 10  XRD patterns of ceramics sintered at 1250 ℃ with different Fe₂AlB₂ contents

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  Figure 11  SEM images of fracture surfaces for ceramics with different Fe₂AlB₂ contents sintered at 1250 ℃: a 25 wt%; b 30wt%

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  Table 1.  Mechanical Properties of the ZrB₂-Fe₂AlB₂ Ceramics

  Material	Relative density (%)	Vickers hardness (GPa)	Fracture toughness (MPa·m1/2)
  ZrB2-10%Fe2AlB2	84.5	4 ± 2	-
  ZrB2-25%Fe2AlB2	96.0	18 ± 0.6	4.64 ± 0.2
  ZrB2-30%Fe2AlB2	95.6	19 ± 0.5	5.23 ± 0.3
  ZrB2-35%Fe2AlB2	96.2	22 ± 0.3	5.78 ± 0.5

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