Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods

Journal Pre-proof Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods Qiu-Yu Liu, Shi-Kuan Sun, Ling-Yong Zeng, Yang You, Wei-Ming Guo, Li-Xiang Wu, Hua-Tay Lin

PII:

S0955-2219(20)30232-6

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2020.03.051

Reference:

JECS 13162

To appear in:

Journal of the European Ceramic Society

Received Date:

2 October 2019

Revised Date:

11 March 2020

Accepted Date:

24 March 2020

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Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods Qiu-Yu Liu1,*, Shi-Kuan Sun2,*, Ling-Yong Zeng1, Yang You3,, Wei-Ming Guo1,,

Li-

Xiang Wu1, Hua-Tay Lin1 1

School of Electromechanical Engineering, Guangdong University of Technology,

Guangzhou 510006, China 2

Department of Materials Science and Engineering, University of Sheffield, Sheffield

S1 3JD, United Kingdom School of Chemical Engineering and Light Industry, Guangdong University of

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3

Technology, Guangzhou 510006, China

Corresponding author. E-mail: [email protected] (Y. You), [email protected] (W.-M. Guo).

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*These authors contribute equally to this paper.

Abstract

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Starting from ZrO2 and boron (molar ratio: 1:4), four ZrB2 powders were synthesized by borothermal reduction method, three of which were designed to introduce minor

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modifications by combining solid solution with Ti and/or water-washing. The sinterability, microstructures, mechanical properties and thermal conductivity were investigated. In comparison with the conventional borothermal reduction, the modified

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methods offered significant improvement in terms of densification of ZrB2 ceramics, particularly the mixture that included water-washing. Owing to the refined particle size and boron residues, ZrB2 ceramics from the modified borothermal reduction which

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included water-washing demonstrated nearly full densification, Vickers hardness of 14.0 GPa and thermal conductivity of 82.5 W/m•K after spark plasma sintering at

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2000oC for 10 min. It was revealed that the properties of ZrB2 ceramics could be enhanced utilizing the proposed minor modification, starting from the same raw materials and adopting the same sintering conditions.

Keywords: ZrB2; Borothermal reduction; Sintering; Microstructure; Mechanical properties.

1. Introduction Among the ultra-high temperature ceramics, zirconium diboride (ZrB2)-based ceramics possess high melting point, high hardness, high thermal conductivity and low density [1-4], which makes ZrB2 the leading candidates for thermal and structural protection applications at temperatures up to 2000oC. The quality control of ZrB2 is mainly achieved by the processing optimization for both powder and ceramic [5,6]. Depending on the processing route, ZrB2 powders were prepared by borothermal reduction [7-9], boro/carbothermal reduction [8,9], sol-gel route [10,11] and self-propagating high-

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temperature synthesis (SHS) [12]. Sol-gel route usually offered ZrB2 products with high purity and ultra-fine particle size. However, the starting materials were highly costly

and the processing was difficult to scale up. Among them, impurities were usually introduced after boro/carbothermal reduction (carbon) and SHS (metal) routes. Due to

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the variation in the raw materials and processes condition used, the synthesized ZrB2 powders possessed different particle size and purity. Consequently, the densification

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and performance of ZrB2-based ceramcis are affected by the poor and notreproducibility quality of powder.[13,14] As a typical method, borothermal reduction

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is regarded to be advantageous as the starting materials are accessible with no carbon or metal evolved in the synthesis process:

(1)

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3 ZrO2 (s) + 10 B (s) = 3 ZrB2 (s) + 2 B2O3 (g)

It was reported that Reaction (1) could be finished at 1000oC [15] but the by-product B2O3 formed and remained in addition to the target ZrB2 phase. To completely remove

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B2O3, a typical temperature above 1500oC was required during borothermal reduction [15]. Besides, the by-product B2O3 would react with boron at high temperature to form

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gaseous boron-rich oxides (B2O2) [16,17]. As a result, boron with excessive amount was added to react with ZrO2 for compensation. Unfortunately, coarse ZrB2 powder was obtained and the existence of B2O3 was proven to promote coarsening ZrB2 powder product [18]. On the basis of borothermal reduction, researchers modified and optimized this method, in order to avoid the negative effects of B2O3 on the particle growth. The modified borothermal reduction methods included, (1) borothermal reduction combined with

solid solution of TiB2 or TaB2: Zr1-xTixB2 powders (x = 0, 1, 2 and 5 mol%) which led to particle size of ~0.40 μm and minimal coarsening effect derived from B2O3 by means of the incorporation of TiB2 [19]; (2) Two-step reduction plus intermediate waterwashing: Guo et al [5] introduced water-washing process of the intermediate after borothermal reduction at 1000oC to eliminate B2O3; After the second step of heattreatment at 1550oC, the oxygen content was reduced and ZrB2 powder possessed a smaller particle size (0.4–0.7 μm). However, the results of the powder synthesis were only reported in Ref. 5 and no investigations on sintering behavior or mechanical

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performance of the sintered bulk was performed. Zou et al conducted a comparative investigation into oxygen content and sinterability of ZrB2 powders prepared by carbo/boro-thermal and borothermal reduction at 1600oC, as well the strength of ZrB2-SiC ceramics [9]. To the best of our knowledge, no studies

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have compared the differences between ZrB2 powder products from the conventional

borothermal reduction and the modified ones, particularly targeting the fabrication of

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ZrB2 ceramics. Pure ZrB2 ceramic is featured by difficult densification due to its strong covalent bonds. It was reported that the relative density reached only ~70% after spark

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plasma sintering of commercial ZrB2 (as received) at 1900oC [20]. In this work, we designed the powder synthesis via the following four methods: (1) borothermal

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reduction (designated as BR); (2) borothermal reduction combined with solid solution with Ti (designated as BRS), (3) borothermal reduction with water-washing (designated as BRW) and (4) borothermal reduction with solid solution and water-washing (denoted

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as BRSW). Fixed variables were the type of ZrO2 and boron raw materials and the molar ratio of ZrO2 (or (Zr,Ti)O2 in the BRS/BRSW method) to boron = 1: 4. The

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influences of characteristics of ZrB2 powders originating from these four methods on the densification behavior, microstructure and performance were investigated aiming at the selection of the most suitable powder process method.

2. Experimental procedure ZrO2 (purity > 99.8%，d50=~1.0 μm, Changsha Xili Nano Grinding Technology Co., Ltd.), amorous boron (> 95.0%, d50=~2.0 μm, Dandong Chemical Research Institute

Co., Ltd.) and TiO2 (> 99.9%, P25, Degussa-Huls, Frankfurt-Main, Germany) were adopted as the raw materials. The regents were weighed according to the molar ratio of ZrO2:B = 1:4 in all methods and 2 mol% TiO2 was selected to substitute ZrO2 in the BRS and BRSW method. The mixtures were mixed by roller mill for 24 hours using Si3N4 as the milling media. The slurries were then dried overnight and pressed into compact (50×50×5 mm3) under the uniaxial pressure of 8.0 MPa for borothermal reduction. The heat-treatment was performed under vacuum (<20 Pa; Zhongshan Kaixuan Vacuum Technology Engineering Co., Ltd.) with ramping rate of 10oC/min to

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the target temperature. The heating profile for the conventional borothermal reduction (BR) and borothermal reduction with solid solution (BRS) method was identical:

ramping to 1000oC and dwelling for 2 hours, and then further increasing to 1550oC and holding for 1 hour. As for borothermal reduction with water washing (BRW) and solid

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solution-water washing (BRSW), the intermediate after heat-treatment at 1000oC for 2 hours was collected and washed using boiling water for 2 hours. The washed

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intermediate was dried and sieved through a 100 mesh screen for further analysis and spark plasma sintering. Spark plasma sintering (SPS; H-HPD 10-FL, FCT Systeme

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Gmbh) was employed to densify the as-prepared powders from these four methods. The sintering was performed at 1900oC and 2000oC for 5 and 10 mins with heating rate was

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150oC/min and the uniaxial pressure of 30 MPa applied at 1200oC in Ar atmosphere. The sintering condition and the corresponding designation is listed in Table 1. The phase assemblage of the as-synthesized powders and SPSed ceramics was

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examined by X-ray diffraction (XRD; D8 Advance diffractometer, Bruker). The morphology of the products was observed by scanning electron microscopy (SEM;

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Nova Nano 430, FEI), equipped with energy-dispersive X-ray spectroscopy (EDS; XMaxN, Oxford). The average particle size was determined from laser diffraction particle size analyzer (Mastersizer 3000, Malvern). The specific surface area (SSA) of the four synthesized ZrB2 powders was also measured by the specific surface area analyzer (SA 3100, Beckman Coulter). The equivalent particle size was determined from SSA assuming that the particle was spherical. The oxygen content of the four types of ZrB2 powders was measured by an oxygen/nitrogen analyzer (TC600, LECO). The

density and the residual porosity was measured based on Archimedes drainage method. The grain size was calculated based on SEM observation on the polished surface. Vickers’ hardness tester (HVS-30/LCD, Temin Optical Instrument Ltd.) was used to determine the hardness of the SPSed ZrB2 ceramics with an indentation loading of 9.8 N and a dwell time of 10 s. The toughness was measured by indentation method using a loading of 98 N for 10 s. The sintered pellets were sectioned into 10×10×1 mm3 for thermal conductivity measurement at 25oC using laser flash method (LFA 447

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NanoFlash, Netzsch).

3. Results and discussion 3.1 Powder characteristics

XRD profiles of ZrB2 powders synthesized by four methods demonstrated pure ZrB2

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formed after borothermal reduction at 1500oC (1100oC for BRW and BRSW) and no

secondary crystalline phase was detected (see Figure S1 in Supporting Information). It

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can be seen in Figure 1 that SEM observation of the powders showed that ZrB2 synthesized by all modified methods possessed finer particle (Fig. 1b-d), compared

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with the conventional borothermal reduction (Fig. 1a). In particular, the ZrB2 particle derived from BRSW method was the finest.

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Table 2 lists the average particle size measured by laser particle size analyzer and surface area methods as well as the oxygen content. It clearly displays the particle refinement by the modified routes. However, the values from laser particle analyzer

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were found to be larger (Table 2) than those observed from Figure 1, mainly because of the agglomeration. The specific surface area of ZrB2 powder synthesized by the

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traditional borothermal reduction method was 0.71 m2/g, and the calculated particle size of ZrB2 powder was 1.39 μm. In contrast, the particle size of ZrB2 powders originating from BRS, BRW and BRSW methods was 0.79 μm, 0.17 μm and 0.14 μm, respectively. The surface area result was in good accordance with SEM observation. As shown in Table 2, the oxygen content was measured to be 0.90 wt% for BR and 0.88 wt% for BRS after heat-treatment at 1500oC. The highest oxygen content was found in BRW (1.98 wt%) and BRSW product (1.50 wt%) since both methods were conducted at

1000oC instead of 1550oC, at such low temperature the removal of by-products by using water-washing was not so complete as the high-temperature processing, and the surface oxygen impurity may not completely be removed.

3.2 Densification behavior Figure 2 displays XRD patterns of ZrB2 ceramics after sintering at 2000oC for 10 mins. Only reflections of ZrB2 were detected, indicating that pure ZrB2 ceramics were obtained after sintering. No obvious difference could be found among these four

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samples. The displacement of four powders during sintering process at 2000oC was plotted as a function of sintering time in Figure 3. The pressure of 30 MPa was applied at 1200oC. For better comparison, the displacement curves were normalized and the highest value

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for y-axis (100%) didn’t imply that the full densification was finished. The

displacement for BR and BRS powders continually increased after holding at 2000oC

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for approximately 5 min and maintained constant afterwards. Simultaneously, the change of displacement for BRW powder stayed unchanged when the sintering

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temperature was held at 2000oC. As for BRSW powder, the displacement stopped at temperature around 1800oC. These differences indicated that BRW and BRSW owned

results.

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the higher sinterability. This will be discussed by combing with the microstructure

The measured density values of the sintered ZrB2 ceramics are listed in Table 3. For the

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ceramics derived from traditional borothermal reduction, the as-sintered ceramics were porous regardless of the sintering conditions. When the sintering temperature increased

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from 1900oC to 2000oC, the relative density value ranged from 64.0% (BR-19-5) to 69.6% (BR-20-10). In comparison with those from BR method, the measured density derived from BRS was overall higher, reaching the values of 89.9% and 91.0% after sintering at 2000oC (listed in Table 3). Provided the same heat-treatment procedure, this indicated the densification was enhanced by doping TiB2 in ZrB2 with only 2 mol% than that originating from BR method. The densification enhancement was evident upon sintering ZrB2 powders obtained by

the BRW method. It is demonstrated in Table 3 that the values of relative density increased from 85.0% to 95.5% as the sintering temperature raised from 1900oC to 2000oC. It further increased to 98.3% when dwelling for 10 min at 2000oC. For BRSW samples, the measured relative density was close to those of BRW ones, apart from the one sintered at 2000oC for 10 min, at which a lower RD value of 94.3% was observed. This would be explained by combining the microstructure results. It was noteworthy to point out that the densification of ZrB2 based on the modified process was significantly higher than the conventional borothermal reduction (RD=54.3% after pressureless

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sintering at 2000oC in Ref. 21).

3.3 Microstructures of sintered bodies

The porosity of BR samples was confirmed by SEM in Figure 4. A considerable

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porosity existed in the sample from borothermal reduction after sintering at 1900oC (Fig. 4a), although the neck growth between grains was observed. Residual porosity of this

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specimen was measured to be 33.8%, as shown in Figure 4a. It was not surprising that the porous microstructure of ZrB2 from conventional borothermal reduction was found.

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It was reported that the relative density reached only ~70% after spark plasma sintering of commercial ZrB2 (Supplier: H.C. Starck, Goslar, Germany and the mean grain size

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was ~2 μm; from Ref. 20) at 1900oC [20]. The density derived from conventional borothermal reduction in this work was comparable with the commercially available ZrB2 powder.

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The microstructural evolution of BRS specimens is exhibited in Figure 5. The porous feature was evident in the sample sintered at 1900oC (residual porosity = 22.3%; seen

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in Fig. 5a), which was consistent with the density measured. For the specimen sintering at 2000oC for 5 min, a relatively denser morphology was observed; pores were mostly found along the triangular grain boundaries (residual porosity = 9.8% in Fig. 5b). However, trapped porosity within the grain was evident in the sample with mean grain size of 5.81 µm (see Table 4) after sintering 2000oC for 10 min (Fig. 5c). This was mainly due to the grain growth in the presence of oxygen impurity and TiB2 at high temperature (and the absence of pinning effect by the secondary phase), leading to the

increase of the proportion of the intragranular pore. It could be deduced that the further improvement on the densification could be achieved by setting longer dwelling time. The distribution of Ti element was observed, as seen in Figure 6. It was found that Ti element uniformly distributed on the surface. It indicated that the solid solution of (Zr0.98Ti0.02)B2 formed and no secondary phase formed, in agreement with XRD results. The fracture surface of ZrB2 ceramics from BRW method is displayed in Figure 7. The porosity of 14.2% is still evident after sintering at 1900oC (Fig. 7a), which well agrees with density measurement (relative density = 85.0%). A much denser morphology was

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observed after sintering at 2000oC for 5 min and 10 min, indicative of the densification enhancement (see in Figure 7b). As listed in Table 4, the mean grain size was measured to be 7.12 µm. Meanwhile, by closer examination and the relevant calculation of Figs.

7b and 7c, black particles were observed as the secondary phase in amount around 4.5

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vol%, and the chemical composition of the black phase was analyzed by EDS. As

shown in Figure 7d, the black phase was identified as boron. Due to the amorphous

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nature, boron was not detected in XRD patterns. Borothermal reduction reaction was interrupted at the temperature of 1000oC, at which B2O3 was present. This caused

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excessive amount of boron remaining within the sample, when starting from the ratio of ZrO2:B=1:4. It should be noted that boron was previously adopted as the oxygen

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remover and would improve the densification of ZrB2 ceramics [21,22]. This could be used to explain the improved densification behavior in the BRW method, in spite of the higher oxygen content found in the powder product. The grain growth is assumed to be

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inhibited by the pinning effect of boron phase distributed at the ZrB2 grain boundaries and triple junctions. As a consequence, transgranular porosity was absent in the

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microstructure. In addition, the grain coarsening caused by oxygen impurity was suppressed due to the fast heating profile of SPS sintering. Similarly, the existence of boron was also observed in BRSW samples, as shown in Figure 8. Compared to the microstructure of BRW samples, the grain size was coarsened in BRSW (mean grain size: 11.23 µm in Table 4). This was likely due to the doping effect of Ti, which accelerated the grain growth. Consequently, intragranular pores were found (residual porosity=4.7% in Table 4 and Figure 8c) and resulted in an

incomplete densification after sintering at 2000oC for 10 min (RD=94.3%). By combining the microstructural observations and density results, we assessed the difficult densification process of ZrB2 powder synthesized by the conventional borothermal reduction. Porous microstructure was observed after sintering at 2000oC or 2100oC. It was meaningful to observe that the densification was improved in the modified methods. It was evidenced that BRS method offered the possibility of density enhancement by means of introducing Ti doping. This was mainly ascribed to the particle refinement of the starting ZrB2 powder. However, the full densification was not

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achieved in BRS method, which was similar as the case of BRSW. On the other hand, densification improvement by BRW method was remarkable. From the above

microstructural analysis, the nearly full densification was confirmed by sintering ZrB2 from BRW process. Apart from the reduced particle size, this was benefitting from the

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dual function of boron as the secondary phase: (1) oxygen remover and (2) pinning

effect. On this basis, the existence of the transgranular pore could be avoided and the

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grain growth could be suppressed. Starting from the identical materials, it was noted that the sinterability of ZrB2 could be much improved by simple modifications on the

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conventional borothermal reduction.

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3.4 Mechanical properties and thermal conductivity The mechanical properties and thermal conductivity of ZrB2 ceramics are listed in Table 4. Due to the porous nature, hardness and toughness was not measured on ZrB2 derived

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from BR. Vickers’ hardness of BRS-20-10 and BRW-20-10 was determined to be 13.0 ± 1.0 GPa and 14.0 ± 0.5 GPa, respectively. The typically Vickers’ hardness of pure

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ZrB2 was reported to be 15.69 ± 1.33 GPa (with the same load of 9.8 N) [23], slightly higher than those in the present work. This may be attributed to be the existence of boron. Hardness of BRSW-20-10 was measured to be 12.0 GPa and this may be as a result of the coarse grain in terms of microstructure and low RD. The toughness of BRS-20-10 and BRW-20-10 were 4.17 ± 0.15 MPa•m1/2 and 4.94 ± 0.19 MPa•m1/2, similar to the reported values [24]. The room-temperature thermal conductivity of ZrB2 ceramics was measured to be 48.1 W/m•K, 72.6 W/m•K, 82.5

W/m•K and 75.2 W/m•K for BR-20-10, BRS-20-10, BRW-20-10 and BRSW-20-10. It could be deduced that ZrB2 originating from water-washing method collected the best combination of performance. The SPSed ZrB2 ceramics derived from the conventional borothermal reduction method displayed the lowest thermal conductivity because of the high porosity, and pore scattering dominated heat transport. The obtained value was close to the ZrB2 ceramics with pores volume fraction of 21~38 vol% [25]. The thermal conductivity of the Ti doped ZrB2 ceramics was slightly higher than that from BR method, but lower than that from water-washing method. This was likely attributed to

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the non-fully densification and crystal defect introduced by Ti-doping [26,27], leading to the formation of lattice strain. As a result, it migh affect both both phonon and

electron transport in ZrB2 and caused the decrease in thermal conductivity [28]. ZrB2

conductivity after sintering was the highest.

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4. Conclusion

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ceramics synthesized by BRW method showed the highest density, hence the thermal

Starting from an oxide to boron molar ratio of (1:4), four ZrB2 powders were

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synthesized by traditional borothermal reduction modified methods, and then consolidated by spark plasma sintering. Synthesis process modifications included: solid

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solution with Ti, water washing and combined solid solution and water washing. The powder characteristics and densification behavior of four synthesized ZrB2 powders revealed that:

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(1) ZrB2 ceramics derived from the conventional borothermal reduction after sintering at the same condition were featured by considerable porosity. ZrB2 powders derived

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from the modified routes demonstrated higher sinterability. (2) Borothermal reduction combined with solid solution enabled ZrB2 ceramics to achieve a relative density of 91.0% after sintering at 2000°C for 10 min. Nevertheless, the transgranular porosity and grain growth up to 5.81 µm penalized the thermal and mechanical properties. Despite the slightly improved densification (RD=94.3%), coarsening of the microstructure and intragranular pores were also observed in the samples from borothermal reduction combined with solid solution and water washing.

(3) Benefitting from the refined particle size of ZrB2 powder and the boron residue, borothermal reduction combined with water-washing displayed the highest sinterability and absence of transgranular pores. It was remarkably observed that nearly full densification with the good combination of the mechanical properties and thermal conductivity was achieved after sintering at 2000oC for 10 min. By means of a simple design within the borothermal reduction, the sinterability and the performance of ZrB2 ceramics was significantly improved.

Declarations of Interest Statement

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With regards to the manuscript titled as ‘Improvement of sinterability and

mechanical properties of ZrB2 ceramics by the modified borothermal reduction

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methods’ submitted to Journal of the European Ceramic Society, the authors declare that they have NO conflict of interest.

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Acknowledgements

This work was financially supported by the Pearl River Nova Program of Guangzhou

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(No. 201710010142), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (No.19ZK0113), National Natural Science Foundation of China (No. 51402055, 51602060 ， 51832002 and U1401247), and

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Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013G061

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and 2014YT02C49).

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densification of Ti-substituted ZrB2-based ceramics, Ceram. Int. 45 (2019) 15749-15753. https://doi.org/10.1016/j.ceramint.2019.05.033. 20. M. Thompson, W.G. Fahrenholtz, G. Hilmas, Effect of starting particle size and oxygen

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content on densification of ZrB2, J. Am. Ceram. Soc. 94 (2011) 429-435. https://doi.org/10.1111/j.1551-2916.2010.04114.x.

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21. W.M. Guo, G.J. Zhang, Z.G. Yang. Pressureless sintering of zirconium diboride ceramics with

boron

additive.

J.

Am.

Ceram.

Soc.

95

(2012)

2470-2473.

https://doi.org/10.1111/j.1551-2916.2012.05307.x

22. X.-G. Wang, W.-M. Guo, G.-J. Zhang, Pressureless sintering mechanism and microstructure of ZrB2–SiC ceramics doped with boron, Scr. Mater. 61 (2009) 177-180. https://doi.org/10.1016/j.scriptamat.2009.03.030. 23. S. Chakraborty, A.R. Mallick, D. Debnath, P.K. Das, Densification, mechanical and

tribological properties of ZrB2 by SPS: Effect of pulsed current, Int. J. Refract. Hard Met. 48 (2015) 150-156. https://doi.org/10.1016/j.ijrmhm.2014.09.004. 24. A. Nisar, S. Ariharan, K. Balani, Establishing microstructure-mechanical property correlation in ZrB2-based ultra-high temperature ceramic composites, Ceram. Int. 43 (2017) 13483-13492. https://doi.org/10.1016/j.ceramint.2017.07.053. 25. H. Yuan, J. Li, Q. Shen, L. Zhang, Preparation and thermal conductivity characterization of ZrB2 porous ceramics fabricated by spark plasma sintering, Int. J. Refract. Hard Met. 36 (2013) 225-231. https://doi.org/10.1016/j.ijrmhm.2012.09.003.

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26. S. Chakraborty, D. Debnath, A.R. Mallick, P.K. Das, Mechanical and thermal properties of hot pressed ZrB2 system with TiB2, Int. J. Refract. Hard Met. 46 (2014) 35-42. https://doi.org/10.1016/j.ijrmhm.2014.05.004.

27. S.M. Sichkar, V.N. Antonov, V.P. Antropov, Comparative study of the electronic structure,

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phonon spectra, and electron-phonon interaction of ZrB2 and TiB2, Phys. Rev. B 87 (2013) 064305. https://doi.org/10.1103/PhysRevB.87.064305.

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28. L.M. Devon, G.F. William, E.H. Gregory. Thermal properties of (Zr,TM)B2 solid solutions with TM = Hf, Nb, W, Ti, and Y, J. Am. Ceram. Soc. 97 (2014) 1552-1558.

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https://doi.org/10.1111/jace.12893.

Figure 1. SEM photographs of four synthesized ZrB2 powders derived from: (a) traditional borothermal reduction (BR); (b) solid solution (BRS), (c) water-washing

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method (BRW) and (d) solid solution with water-washing method (BRSW).

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Figure 2. XRD pattern of ZrB2 ceramic samples: (a) BR-20-10; (b) BRS-20-10; (c)

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BRW-20-10 and (d) BRSW-20-10 after spark plasma sintering at 2000oC for 10 min.

Figure 3. The SPS punch displacement plots as a function of time of four ZrB2 powders during sintering process at 2000oC for 10 min.

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Figure 4. SEM images of ZrB2 ceramics after sintering the powders from the traditional

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borothermal reduction at 1900oC for 5 min (a), 2000oC for 5 min (b) and 2000oC for 10

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min (c). Residual porosity of the sample is listed on the images.

Figure 5. SEM images collected on the fracture surface of ZrB2 ceramics orgininating

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from BRS method after sintering at 1900oC for 5 min (a), 2000oC for 5 min (b) and 2000oC for 10 min (c). Residual porosity of the sample is listed on the images.

Figure 6. Microstructure of the polished surface of BRS after sintering at 2000oC for 10 min and corresponding EDS mapping.

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Figure 7. SEM images of the fracture surface of ZrB2 ceramics derived from ZrB2 powder from BRW method at 1900oC for 5 min (a), 2000oC for 5 min (b) and 2000oC

for 10 min (c). (d) is the EDS spectrum of the black secondary phase with semiquantitative elemental analysis inset. Residual porosity of the sample is listed on the

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images.

Figure 8. SEM images of the fracture surface of ZrB2 ceramics from BRSW method after sintering at 1900oC for 5 min (a), 2000oC for 5 min (b) and 2000oC for 10 min (c). Residual porosity of the sample is listed on the images.

Table 1. Designation of ZrB2 ceramic samples and corresponding processing condition. Table 2. Specific surface area, equivalent particle size and oxygen content of four ZrB2 powders.

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Table 3. Relative density of ZrB2 ceramics after spark plasma sintering. Table 4. Residual porosity, mean grain size, mechanical properties and thermal

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conductivity of ZrB2 ceramics after sintering at 2000oC for 10 min.

Table 1. Designation of ZrB2 ceramic samples and corresponding processing

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condition. *

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Powder synthesis method

Sintering condition (Target

and condition (Target

Label

temperature/dwelling

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temperature/dwelling time)

time)

Borothermal reduction

1900oC/5min

(BR): 1000oC/2 hour and

2000oC/5min

1550oC/1 hour

2000oC/10min

BRS-19-5

Borothermal reduction

1900oC/5min

BRS-20-5

combined with solid

2000oC/5min

BR-19-5

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BR-20-5

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BR-20-10

solution (BRS): 1000oC/2

BRS-20-10

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2000oC/10min

hours and 1550 C/1 hour BRW-19-5

Borothermal

BRW-20-5

with

reduction 1900oC/5min 2000oC/5min

water-washing (BRW) : BRW-20-10

2000oC/10min

o

1000 C/2 hours BRSW-19-5

Borothermal

BRSW-20-5

with

reduction 1900oC/5min 2000oC/5min

water-washing and solid 2000oC/10min

solution (BRSW) :

BRSW-20-10

1000oC/2 hours

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*- The nominal composition of (Zr0.98Ti0.02)B2 was used in the BRS and BRSW powders.

Table 2. Specific surface area, equivalent particle size and oxygen content of four ZrB2 powders.

BR

BRS

BRW

BRSW

d50 (μm) *

3.66

2.26

1.69

1.58

area

(SA; 0.71

m2/g) size

(μm)

1.25

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Surface

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Processing method

5.57

7.23

1.39

0.79

0.17

0.14

Oxygen content (wt%) 0.90

0.88

1.98

1.50

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from SA

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*- Measured by laser particle size analyzer.

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Table 3. Relative density of ZrB2 ceramics after spark plasma sintering.

BR method

BR-19-5

BR-20-5

BR-20-10

Relative density (%)

64.0±0.7

65.9±0.2

69.6±1.4

BRS method

BRS-19-5

BRS-20-5

BRS-20-10

Relative density (%)

76.4±0.3

89.9±0.3

91.0±1.5

BRW method

BRW-19-5

BRW-20-5

BRW-20-10

Relative density (%)

85.0±0.7

95.5±0.6

98.3±0.7

BRSW method

BRSW-19-5

BRSW-20-5

BRSW-20-10

Relative density (%)

91.2±0.1

94.2±0.5

94.3±0.4

Table 4. Residual porosity, mean grain size, mechanical properties and thermal conductivity of ZrB2 ceramics after sintering at 2000oC for 10 min.

Residual

Mean grain Vickers

Thermal Toughness

porosity

size

Hardness

(%)

(μm)

(GPa)

BR-20-10

29.8

3.79±0.52

-

-

48.1

BRS-20-10

7.9

5.81±0.43

13.0±1.0

4.17±0.15

72.6

BRW-20-10

0.6

7.12±0.45

14.0±0.3

4.94±0.19

82.5

4.7

11.23±0.73

12.0±0.5

4.40±0.40

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BRSW-20-

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10

conductivity

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(MPa•m1/2)

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Label

(W/m•K)