Synthesis and property characterization of ternary laminar Zr2SB ceramic

In this paper, Zr2SB ceramic with purity of 82.95 wt% (containing 8.96 wt% ZrB2 and 8.09 wt% zirconium) and high relative density (99.03%) was successfully synthesized from ZrH2, sublimated sulfur, and boron powders by spark plasma sintering (SPS) at 1300 °C. The reaction process, microstructure, and physical and mechanical properties of Zr2SB ceramic were systematically studied. The results show that the optimum molar ratio to synthesize Zr2SB is n(ZrH2):n(S):n(B) = 1.4:1.6:0.7. The average grain size of Zr2SB is 12.46 µm in length and 5.12 µm in width, and the mean grain sizes of ZrB2 and zirconium impurities are about 300 nm. In terms of physical properties, the measured thermal expansion coefficient (TEC) is 7.64×10−6 K−1 from room temperature to 1200 °C, and the thermal capacity and thermal conductivity at room temperature are 0.39 J·g−1·K−1 and 12.01 W·m−1·K−1, respectively. The room temperature electrical conductivity of Zr2SB ceramic is measured to be 1.74×106 Ω−1·m−1. In terms of mechanical properties, Vickers hardness is 9.86±0.63 GPa under 200 N load, and the measured flexural strength, fracture toughness, and compressive strength are 269±12.7 MPa, 3.94±0.63 MPa·m1/2, and 2166.74±291.34 MPa, respectively.

1 Introduction  M n+1 AX n (MAX) phase (n = 1-3) is a kind of ternary layered compounds, in which the M element is the early transition metal, the A element is the main group element (12)(13)(14)(15)(16), and the X element is B, C, or N. MAX phase was first proposed by Jeitschko et al. [1] in the 1960s. Barsoum [2] synthesized the bulk Ti 3 SiC 2 ceramics through hot pressing and reported on a range layers are interleaved with the A element layers [3,7], while the M 6 B unit in the MAB phase has a completely different structure of trigonal prism. Both MAB phase and MAX phase have layered structure, but their atomic structure and bonding are different. Especially, there are B-B bonds in the MAB phase, but not in the MAX phase. The known MAB phases include MAlB (M = Mo, W) phase, M 2 AlB 2 (M = Mn, Cr, Fe) phase, M 5 Si 3 B x (M = Cr, Hf) phase [8][9][10], etc. Like MAX phase, MAB phase also has a series of interesting properties.
Most initial studies of MAX phase boride started from the corresponding carbide MAX phase. Chakrabotry et al. [7] simulated the properties of V 2 AlC-V 2 AlB and judged that the introduction of boron can effectively enhance the ductility. At present, there are mature studies on the synthesis, characterization, and calculation of M 2 SC (M = Ti, Zr, Nb, Hf) [11][12][13][14][15][16][17][18][19][20][21]. Based on the research of M 2 SC, Ali et al. [22] simulated the lattice structure and physical properties of M 2 SC and M 2 SB (M = Zr, Hf, Nb), identified that M 2 SB phases have dynamical stability, and revealed the application prospect of Zr 2 SB in reducing solar absorption coating. Rackl et al. [23,24] were the first to synthesize the MAX phase Nb 2 S(B,C) and Nb 2 SB powders by solid phase reaction methods and applied similar processes to synthesize Hf 2 SB and Zr 2 SB phases. They reported that the substitution of boron for carbon could increase the cell size and reduce the deformation of Nb 6 (B,C) octahedron. Qin et al. [25] successfully synthesized compact and pure Nb 2 SB for the first time and tested a series of mechanical and physical properties.
To date, the literature on MAX phase boride, especially M 2 SB phase has focused mainly on crystal structure, electron distribution, and physical properties, and little is known about the mechanical strength. To tackle this void, dense Zr 2 SB ceramic was rapidly prepared by spark plasma sintering (SPS) and explored a series of physical and mechanical properties.
The powder mixtures with different amount of substance ratio were weighed on an electronic scale (accurate to 10 −4 g) and ball-milled on a rotating machine at 50 r/min for 12 h. These powder mixtures were loaded into a Ø20 mm cylindrical graphite mold and sintered in an SPS furnace (SPS-20T-10, Chenhua Technology Co., Ltd., China) at preset heating rate and pressure. In this preset sintering step, the heating rate below 700 ℃ was set as 50 /min and the pressure was set as ℃ 20 MPa; the heating rate above 700 was set as ℃ 10 /min and the pressure was set as 30 MPa. After ℃ sintering, the sample was cooled to 900 at a rate of ℃ 50 /min and then cooled with the furnace.
℃ The phase composition was determined by X-ray diffractometer (D8 Advance, Bruker, Germany) with a Cu Kα radiation (λ = 1.54178×10 −10 m) source and the scanning speed was set to 0.02 (°)/step. The fracture surface of bulk Zr 2 SB ceramic was observed by a field emission scanning electron microscope (Apreo 2C, Thermo Fisher Scientific, Czech Republic). Energy dispersive spectroscope (EDS) was also used to determine the composition of fracture surfaces.
The thermal expansion coefficient (TEC) was tested in argon environment using a thermal expansion analyzer (L75HD1600C, Netzsch, Germany) with a temperature range of 200-1200 and a heating rate ℃ of 5 /min. The sample size was 4 mm × 4 mm × ℃ 15 mm. The thermal properties were tested in vacuum with a laser thermal conductivity meter (LFA467, Netzsch, Germany) in the temperature range of 25-1200 . The sample size was Ø12.5 ℃ mm × 3 mm. The conductivity of Zr 2 SB was measured at room temperature by a resistivity tester (FT-300A1, Ningbo Rooko Instrument Co., Ltd., China) with a sample size of 1 mm × 1 mm × 10 mm.
Vickers hardness of Zr 2 SB ceramic under 1-200 N loads was tested by a micro-hardness tester (HVS-1000ZA, Wanheng Corp., China and HVS-50, Lianer Corp., China) and measured five times for each load point.

1 Reaction process
To study the reaction process, this work used commercial purchased zirconium hydride, sublimated sulfur, and boron powders as initial materials to investigate the reaction path. It should be pointed out that the reason for choosing ZrH 2 as the initial material is that the powder mixtures of zirconium, sulfur, and boron are very easy to undergo self-propagating high-temperature synthesis (SHS) process, resulting in uncontrollable reaction. As the hydride of zirconium, the hydrogen atoms occupy the octahedral gap of zirconium atoms, so it is difficult for sulfur and boron to react with the hydride of zirconium at low temperature, which inhibits the occurrence of SHS process. . For ease of ℃ analysis, the phase compositions of the samples at each temperature are summarized in Table 1.  Results show that ZrH 2 in the initial material is decomposed into ZrH 1.66 below 800 , and finally ℃ completely decomposed into zirconium at 1000 . ℃ Sulfides begin to form above 650 , and the formed ℃ sulfides include Zr 0.75 S and ZrS 0.67 . The content of sulfides reaches the maximum value at 900 and ℃ disappears completely at 1100 . The impu ℃ rity phases zirconium and ZrB 2 coexist at 900 . With the increase ℃ of temperature from 900 to 1200 , the content of ℃ zirconium decreased gradually, while the content of ZrB 2 increased gradually. Zr 2 SB also appeared at 900 and reached the highest purity a ℃ t 1200 . As ℃ the temperature increased from 1200 to 1300 , the ℃ phase composition was almost the same, but it was observed that the relative density of the samples became higher at 1300 . Therefore, it is believed that the most ℃ suitable sintering temperature for Zr 2 SB is 1300 . At ℃ 1400 , the content of ZrB ℃ 2 increases abnormally. At the same time, it was observed that the metallic shiny solid adhered to the punches of graphite mold, which resulted in the loss of part of the sample. According to the XRD analysis, it is believed that the main component of shiny solid is zirconium. This means that the decomposition temperature of Zr 2 SB is above 1400 . ℃ It should be mentioned that the sublimated sulfur would suffer gasification loss at normal temperature, and the sulfur would also react with the hydrogen released by the decomposition of ZrH 2 during heating. The loss of sulfur would result in a portion of ZrB 2 and zirconium remained after sintering the powder mixed at standard stoichiometry. Therefore, the sulfur proportion was further adjusted to confirm the appropriate sulfur ratio and the corresponding process under the experimental conditions.

2 Formula modification
According to the reaction path studied above, a holding 828 J Adv Ceram 2022, 11(5): 825-833 www.springer.com/journal/40145 temperature lower than 800 can ℃ not only increase the formation of zirconium sulfide, but also control the formation of impurities (zirconium boride), thereby improving the purity of the obtained samples. Therefore, sintering temperature was set to 1300 and kept at ℃ 700 for 10 m ℃ in to adjust the proportion of sulfur to explore the most appropriate relative proportion. A series of relative proportions were selected according to the impurity content, and the results are shown in It is obvious that with the increase of sulfur addition, zirconium and ZrB 2 in the sample have a significant downward trend and reach the minimum value at n(ZrH 2 ):n(S):n(B) = 2:1.6:1. With the increase of sulfur proportion, the decreasing trend of zirconium and ZrB 2 gradually slowed down. This phenomenon is due to the addition of excess sulfur. The increase in the vapor pressure of volatile sulfur in the mold makes the reversible reaction of hydrogen and sulfur more complete, and more sulfur escapes in the form of hydrogen sulfide.
When the proportion was 2:1.7:1, it was observed that when the temperature raised about 300 , a large ℃ amount of gas escaped from the mold, and the furnace vacuum dropped rapidly. This is because the volatile sulfur and hydrogen released by the decomposition of zirconium hydride make the pressure in the mold too high, and the gas escapes from the gap of the mold. As a result, the sulfurs involved in the reaction decrease and the content of zirconium and ZrB 2 in the sample increase abnormally.
Based on the results of these experiments, it can be confirmed that zirconium sulfides play an important role in the formation of Zr 2 SB and the loss of sulfur is the main reason for the existence of impurity phases ZrB 2 and zirconium. As a rapid sintering method, SPS [26][27][28][29][30][31] can control the escape of sulfur and purify samples as much as possible, but the sintering of Zr 2 SB is still difficult. In our experiment, the best relative ratio of sulfur under these experimental conditions was 1.6. However, since there are still a large amount of zirconium and ZrB 2 in the sample, we would like to adjust the ratio of zirconium and boron to obtain higher purity samples.
To further purify the sample, the formula of zirconium was first adjusted. Under the condition of a sintering temperature of 1300 and relative sulfur ratio of 1.6, ℃ the results obtained by adjusting the addition amount of ZrH 2 are shown in Figs. 3(a)-3(f). Figure 3 depicts the gradual decrease of zirconium in the sample as the content of ZrH 2 decreases. Specifically, when the relative ratio of ZrH 2 is 1.6, the peak of zirconium almost disappears. When the relative ratio of ZrH 2 is 1.5, the peak of zirconium disappears completely, and the peak of ZrS 0.67 appears. The simultaneous existence of ZrS 0.67 and ZrB 2 proves that ZrB 2 does not react with zirconium sulfide to form Zr 2 SB, which is an impurity phase generated during the reaction. This also indicates that zirconium sulfide may be an ideal precursor of Zr 2 SB, but due to the limited experimental conditions, this method has not been applied in this paper. Meanwhile, the content of ZrB 2 increases obviously with the decrease of ZrH 2 . This is because the excess of boron leads to a strong tendency to form ZrB 2 . Therefore, the boron proportion It can be seen from Fig. 4 that the ratio of zirconium hydride to boron has a great impact on the purity of the sample, and the best molar ratio of n(ZrH 2 ):n(B) is 2:1. Therefore, the adjustment of these two raw materials was coordinated, and finally obtained the purest sample with n(ZrH 2 ):n(S):n(B) = 1.4:1.6:0.7. The XRD data of the obtained samples are shown in Fig. 5, and the pattern is similar to that obtained by Rackl and Johrendt [24]. In present work, the broader diffraction peaks of Zr 2 SB are probably associated with the finer grain size. Based on their work, zirconium was further  Comparison between the observed (black cross) and calculated (red curve) XRD patterns of Zr 2 SB ceramic. The blue curve represents the difference between the observed and calculated XRD patterns. Green, purple, and pink marks are the peak positions of Zr 2 SB, ZrB 2 , and zirconium, respectively. incorporated into the Rietveld refinement, which is shown in Fig. 5. According to the Rietveld refinement results, the sample is composed of 82.95 wt% Zr 2 SB, 8.96 wt% ZrB 2 , and 8.09 wt% zirconium.
The crystallographic data of Zr 2 SB are listed in Table 2. The lattice constants obtained in this paper for Zr 2 SB are a = b = 3.52084×10 −10 m and c = 12.30501× 10 −10 m, which are close to the data in the literature [24].

3 Microstructure characterization
The density of Zr 2 SB sample measured by the Archimedes' method is 6.13 g·cm −3 , and the theoretical density including impurity content is 6.19 g·cm −3 , corresponding to the densification of 99.03%. Figure 6 is the SEM image and the EDS results of the flexural fracture surface of Zr 2 SB. The EDS results confirm that the n(Zr):n(S) ratio of the larger grain in this sample is about 1.85, which is within the allowable error range. Thus, it is believed that the larger grain in the sample is Zr 2 SB. 30 crystal grains with clear boundaries on the concave fracture surface were selected for grain size calculation. The grain size of Zr 2 SB is larger than that of Nb 2 SB (6 μm in length and 3.6 μm in width) sintered with SPS, with an average length of 12.46 μm and an average width of 5.12 μm. The impurity phases ZrB 2 and zirconium are distributed at the grain boundaries of lath shaped Zr 2 SB.
Intergranular and transgranular fracture occurred in Zr 2 SB grains. Many transgranular fractured Zr 2 SB grains with obvious layered characteristics can be observed on both concave and convex fractures. Meanwhile, pits left after the grains fall off due to intergranular fracture can be observed on the concave fracture, and the corresponding detached complete grains can be observed on the convex surface. Intergranular fracture exists in the ZrB 2 and zirconium impurity grains. And due to the  low strength of the impurity grain boundary, the cracks expand rapidly at these positions, resulting in that the bending strength and fracture toughness test results of this sample will be slightly lower than the actual values.

4 Physical property evaluation
The red curve in Fig. 7 is the thermal expansion curve of Zr 2 SB measured at 25-1200 , and the black line ℃ is the result of linear fitting. The equation is α TEC,1200 ℃ = -0.01147+0.00078337T with the R-square of 0.99417. The average TEC of Zr 2 SB is calculated as 7.64×10 −6 K −1 , which is between those of Zr 2 SC (8.8×10 −6 K −1 ) [21] and Nb 2 SB (7.1×10 −6 K −1 ) [25].  . Compared with Nb ℃ 2 SB, the thermal diffusion coefficient of Zr 2 SB is smaller in the same temperature range, and maintains at a very low value. Figure 9 shows the temperature dependence of thermal conductivity and heat capacity of Zr 2 SB ceramic sintered by SPS. The thermal conductivity of Zr 2 SB increases rapidly from 12.0 W·m −1 ·K −1 at room temperature to 30.7 W·m −1 ·K −1 at 800 . As the ℃ temperature continues to rise, the change trend slows down obviously and tends to a constant value of 34.2 W·m −1 ·K −1 at 1200 . The thermal conductivity ℃ of Zr 2 SB at room temperature is lower than those of Nb 2 SB (13.79 W·m −1 ·K −1 ) [25] and Zr 2 SC (38 W·m −1 ·K −1 at 100 ) [21]. However, as the temperature increases, ℃ the thermal conductivity of Zr 2 SB (16.7 W·m −1 ·K −1 ) , the heat capacity of Zr ℃ 2 SB (0.58 J·g −1 ·K −1 ) exceeds that of Zr 2 SC (0.53 J·g −1 ·K −1 ). Also, at high temperatures, the heat capacity of Zr 2 SB (0.78 J·g −1 ·K −1 at 1200 ) is significantly higher than ℃ that of Zr 2 SC (0.50 J·g −1 ·K −1 at 1000 ). Compared ℃ with Zr 2 SB and Zr 2 SC, the heat capacity of Nb 2 SB is relatively low (0.36 J·g −1 ·K −1 at room temperature and 0.49 J·g −1 ·K −1 at 800 ). ℃ In addition, the room temperature electrical conductivity of Zr 2 SB ceramic is measured to be 1.74×10 6 Ω −1 ·m −1 , which is nearly 50% higher than the room temperature electrical conductivity of Nb 2 SB (1.17×10 6 Ω −1 ·m −1 ), and slightly lower than the room temperature electrical conductivity of Ti 2 SC (1.85×10 6 Ω −1 ·m −1 ) [12]. As zirconium is located on the left side of niobium in the periodic table, it has stronger metallicity, which also leads to better conductivity of Zr 2 SB than that of Nb 2 SB. Compared with Ti 2 SC, zirconium has stronger metallicity than titanium, but Zr-B bond has covalent properties to some extent, which results in lower conductivity of Zr 2 SB [24]. Table 3 lists a series of physical and mechanical properties of Zr 2 SB sintered by SPS, and compares these properties with that of Nb 2 SB [25] and Zr 2 SC [21] Figure 10 depicts the relationship between the hardness of Zr 2 SB and the indentation load. The Vickers hardness is 14.39±0.43 GPa at 1 N load. As the pressure increases, the hardness gradually decreases and reaches 9.86±0.63 GPa at 200 N load. At a load of 10 N, the hardness of Zr 2 SB is 12.55±0.72 GPa, which is higher than that of Nb 2 SB (11.89±0.37 GPa), Zr 2 SC (6.4 GPa), and Ti 2 SC (6.7 GPa). As shown in Fig. 10, due to the high hardness and low fracture toughness of Zr 2 SB, cracks appear at the four corners of the indent. The overall indentation is complete without grain extrusion, and there are no crack extensions at the four corners which will affect the indentation size. Therefore, Zr 2 SB, due to its high hardness value corresponding to its good mechanical properties, is currently one of the MAX phases that are most promising to develop into functioal-structural ceramics at present.

Conclusions
Highly dense boron containing MAX phase Zr 2 SB was successfully prepared by SPS, and the corresponding reaction process, microstructure, and physical and mechanical properties of the samples were investigated. The following results are obtained: 1) The optimum molar ratio to synthesize Zr 2 SB is n(ZrH 2 ):n(S):n(B) = 1.4:1.6:0.7 with the process of holding time of 10 min at 700 and 10 min at ℃ 1300 and 30 MPa. The obtained sample has a purity ℃ of 82.95 wt%, a relative density of 99.03%, and mean grain size of 12.46 μm in length and 5.12 μm in width.
3) The flexural strength, fracture toughness, and compressive strength are determined to be 269±12.7 MPa, 3.94±0.63 MPa·m 1/2 , and 2166.74±291.34 MPa, respectively. The Vickers hardness is determined to be 9.86±0.63 GPa at 200 N. Excellent physical and mechanical properties give Zr 2 SB the prospect as a functional-structural ceramic.