UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

462 IMMC 2018 | 19th International Metallurgy & Materials Congress

Reactive Spark Plasma Sintering of TaB2 Ceramics

Melis Kaplan, Onuralp Yücel, Filiz Şahin, Gültekin Göller, İpek Akın

Istanbul Technical University, Metallurgical and Materials Engineering Department, 34469, Istanbul

Abstract

High melting point and Vickers hardness, good electrical and thermal conductivities, resistance to corrosion, chemical substances and thermal shocks are the properties that attract attention to tantalum diboride (TaB2) as a high-temperature material. However, poor sinterability and necessity to high temperature consolidation limit the application of monolithic TaB2. External pressure or electrical field require for the consolidation of the TaB2ceramics. In this study, TaB2 was synthesized from reduction of tantalum oxide (Ta2O5) and boron carbide (B4C) powders in situ by the reactive spark plasma sintering (RSPS). Samples that produced were analyzed by using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Densification behavior of the samples were determined by Archimedes method.

1. Introduction

Different boron-containing metal borides are studied widely experimentally or theoretically. These compounds have excellent mechanical properties as well as good thermal and chemical properties. Among the borides, tantalum borides are a group of materials with superior properties such as high melting point, chemical inertness, high hardness, abrasion resistance, good thermal and electrical conductivity [1]. The combination of these properties makes TaB2 a potential tool in terms of cutting tools, high temperature pots and thermal protection components [2].

TaB2 is a transition metal diboride compound which has the hexagonal AlB2 structure and belong to the space group P6 / mmm (space number 191). The unit cell contains three atoms with special positions: X: (0,0,0), B: (1/3, 2/3, 1/2) and B: (2/3, 1/3, 1/2) [3].

There are several studies in the literature regarding the synthesis of TaB2 powder or bulk sample production. Some of these studies were made with pressureless sintering and some with SPS system. The study closest to the scope of the experiment planned to be carried out belongs to You and his team [4]. In You and his team’s study, boro / carbothermal reduction technique is used. The final TaB2 powder synthesis was carried out by vacuum reduction of Ta2O5 with B4C. In the study of different molar ratios and temperature conditions, TaB2

and B2O3 (s) were observed at 1100 for 2 hours and TaB2 + Ta3B4 + TaB at 1550 for 2 hours in the case where the B4C / Ta2O5 molar ratio was 1.57. 100 % TaB2

was synthesized at 1 hour at 1550 in the case where the B4C / Ta2O5 molar ratio was 1.90.

When the works carried out using the SPS system is examined, none of the studies were work with Ta2O5 and B4C starting powders. Ta and amorphous boron were used as starting powders in the study performed by Lukasik and his team [5]. The parameters for the acquiring of TaB2powders produced by the reactive spark plasma method were 1800, 2000 and 2200 ° C for the sintering temperature and 1 to 30 min. for sintering time. In this study, it was aimed to produce TaB2 ceramics with 97 % or higher densification by the reactive spark plasma sintering of Ta2O5 and B4C with respect to equation (1).

𝑇𝑎 𝑂 𝑠 𝐵 𝐶𝑠 𝑇𝑎𝐵 𝑠 𝐵 𝑂 𝑙 𝐶𝑂 𝑔 (1)

According to equation (1), products of the Ta2O5 and B4Creaction are TaB2(s), B2O3(l) and CO(g). This reaction was checked and proved by the Factsage software (Figure 1). Also, experimental stoichiometries and sintering temperatures of the reaction were determined by Factsage software considering thermodynamic calculations. Equilib module of the program was used for the plots.

In Figure 1, Factsage graph was revealed that TaB2(s) was reached the maximum amount at 1550 . At this temperature, CO(g) was produced by the system. At the same time while liquid B2O3 amount decreases during reaction, gas form of B2O3 was formed. In order to observe the effect of B4C amount on the products, Figure 2 was plotted. TaB2 production was started with minimum 1 mole of B4C and increases until reach to 2 moles of B4C. Concurrently, CO gas and B2O3 gas amount show increment. When the system includes 2 moles of B4C, gas release was reached the maximum and then continues with stable amount.

According to literature and Factsage plots, in this study 1550 was preferred as a sintering temperature. For easy understanding, experiments were coded. In the sample codes C corresponds to the composite structure, first four

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numbers define the sintering temperature and last number defines the sintering time (Table 1).

2. Experimental Procedure

2.1. Sample preparation

Commercially available Ta2O5 (Inframat, 99.99%) and B4C (H.C. Starck Corp. Grade HS) were used as starting powders. Both powders were ball milled in ethanol for 24 h. Magnetic stirrer was used for 5 h to provide better dispersion behavior of powders before the ethanol evaporation. In drying oven, powder mixture was dried at 100 for 24 h. Pounding was applied in an agate mortar to get soft and unagglomerated starting powder. Powder mixture was filled in a cylindrical graphite die with a 50

mm height and 50 mm thickness. In order to obtain better conductivity, all the contact zone of the powder with punches and the dies were covered with graphite sheet. The sintering was carried out using a spark plasma sintering (SPS) apparatus (7.40 MK-VII, SPS Syntex Inc.). A uniaxial pressure of 40 MPa and a pulsed direct current (12 ms/on, 2 ms/off) were applied during the entire SPS process under vacuum atmosphere. 1225 and 1550 were used as a sintering temperature with 5 and 10 sintering time.

2.2. Characterization and property measurement

The crystalline phases of the samples were identified by X-ray diffractometer (XRD; MiniFlex, Rigaku Corp.) in the 2 range of 20–90° at scanning rate of 2 °C/min. with CuK radiation. The microstructural characterization was carried out by scanning electron microscope (FESEM; JSM 7000 F, JEOL Ltd.). Archimedes method was employed to determine the sintered density of the samples. Distilled water was taken as the immersion medium.

3. Results and Discussion

Due to composite formation, maximum 94.27 % of relative density was achieved. Increment in sintering time from 5 to 10 min. decreases the density of the sample from 94.27 % to 78.02 % (Table 1). C1550-5 sample, which was sintered at 1550 ° C for 5 minutes, showed partial melting at the sample and cracks were observed in the punches. The sample was not completely removed from the substrate. In order to obtain a target density of 97%, the sintering temperature was increased to 10 min while the sintering temperature in the second production was kept constant at 1550 ° C. However, the relative density value of C1550-10 sample was decreased. Both melting in the sample and cracks in the punches were observed. It has been determined that the melting rate increases with the initial yield and the sample enters the reaction with punches.

Table 1: Relative density of the samples.

Sample Name Relative Density (%) C1550-5 94.27

C1550-10 78.02 C1250-5 39.55

When the displacement curves obtained in the first two tests are examined, it is determined that the initial displacement temperature is ~1350 . It has been decided to reduce the sintering temperature of the experiment to determine the phases present in the sample before the second displacement. In the third experiment carried out, it was decided that the sintering temperature should be 1250

B2O3 (liq)

TaB2 (s)

Figure 1: Factsage graph of temperature versus productmole with respect to 1 mole Ta2O5 and 1.57 mole B4C.

C (s)

TaC (s)

TaB2 (s)

CO (g)

B2O3 (liq)B2O3 (g)

TaB2 (s)

CO (g)

B2O3 (s)

B2O3 (s)

TaC (s)

B2O3 (g)

Gas

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to be lower than the second displacement’s starting temperature. The relative density value of the C1250-5 sample was calculated as 39.55 %.

The reason for the second displacement is thought to be the rapid evaporation of the liquid B2O3 from the outlet of the reaction. It has been reported in literature that the evaporation temperature of B2O3 is 1300 and above [6]. According to Lukasik and his team [5], it is difficult to form TaB2 by reactive sintering method due to the tendency of powders to oxidation and high gas absorption ability. The exothermic reaction B2O3 oxide formed during the heating of the starting powders is sublimed at 1500 .This can lead to sudden increases in gas, resulting in breaks in the punches and mold. In order to avoid this situation, it has been stated that in order to reduce the amount of oxygen in the starting powders, tantalum powders must be subjected to high-energy milling in an argon atmosphere before mixing with boron.

Rapid gas release during the reaction triggers the uncontrollable displacement through the punches. At the same time the powder reacts with the mold and punches. This reaction produces composite structure which includes both TaB2 and TaC in the composition. Main reason of the carbide formation is the diffusion of the carbon from the punches to the sample.

The phase analysis of the C1550-5, C1550-10 and C1250-5 samples were given in Figure 3. As intended, only TaB2(ICCD 00-075-0966) phase did not form. In addition to TaB2, TaO2 (ICCD 00-019-1297) and TaC (ICCD 03-065-0282) phases were also encountered. Three phases exist in the C1550-5 sample (Figure 3 (a)). It was observed that the intensity of the TaC phases decreased in the C1550-10 sample (Figure 3 (b)). However, the amount of TaB2phases is less than the 5 min sintered sample. The result of the phase analysis of the sintering process performed at low temperature in order to learn the phases formed is given in Figure 3 (c). The oxide phase was not observed at the C1250-5 sample. The sample consists only of TaB2 and TaC phases. Compared to the samples sintered at 1550 ° C, the intensity of the carbide peaks is much higher. It is anticipated that lowering the sintering temperature will play an important role in achieving the desired TaB2 phase with a high relative density to both prevent erosion and reduce the amount of formation of carbide phases.

SEM analysis of the fracture surfaces were given in Figure 5. Relative density of the samples was decreased from 94.27 % to 78.02 % which is the result of the increase in porosity (Figure 4 (a) and Figure 4 (b)). In Figure 4 (c), due to insufficient sintering temperature, sintering process was not completed. Grains were not seen clearly and structure is porous. 39.55 % relative density have

evidential value on the microstructure (Figure 4 (c)). XRD results were revealed the composite structure as a conclusion of sintering process. SEM images were supported the XRD analysis. Similar outcome was stated by Lukasik et al. [7], it was concluded that due to proximity of the sintered material to the graphite die and plungers TaC was formed in the structure. According to EDS analysis, boron rich and carbon rich regions were detected. In Figure 4 (a) and (b), sintered bodies were boron rich regions. On the other hand, small bright areas were carbon rich regions.

Figure 3: XRD pattern of the (a) C1550-5, (b) C1550-10 and (c) C1250-5.

Figure 4: SEM images of the fracture surfaces of the sample (a) C1550-5(b) C1550-10 and (c) C1250-5.

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4. Conclusion

In this study, TaB2 was synthesized from the reduction of tantalum oxide (Ta2O5) and boron carbide (B4C) powders in situ by the reactive spark plasma sintering (RSPS). Experimental stoichiometries and sintering temperatures of the reaction were determined by Factsage software considering thermodynamic calculations. According to experimental results, 1550 sintering temperature was not sufficient to achieve 97 % or higher relative density. At 1550 sintering temperature with 5 min sintering time maximum 94. 27 % relative density was obtained. When sintering time was increased to 10 min. relative density was decreased due to increase amount of porosity. This is the result of the both melting and carbide formation due to reaction of the powder with the die and punches.

Acknowledgement

The authors thank to Istanbul Technical University for the financial support through project number: GAP- 40907. Authors are greatly thankful to H. Sezer and B. Yavas for microstructural investigations and SPS experiments.

References

[1] X. Zhang, E. Zhao, Z. Wu, Prediction of new high pressure phase of TaB3: First-principles, J. Alloys Compd. 632 (2015) 37–43. doi:10.1016/j.jallcom.2015.01.144.

[2] W.J. Zhao, Y.X. Wang, Structural, mechanical, and electronic properties of TaB2, TaB, IrB2, and IrB: First-principle calculations, J. Solid State Chem. 182 (2009) 2880–2886. doi:10.1016/j.jssc.2009.07.054.

[3] E. Deligoz, K. Colakoglu, Y.O. Ciftci, Lattice dynamical and thermodynamical properties of HfB2 and TaB2 compounds, Comput. Mater. Sci. 47 (2010) 875–880. doi:10.1016/j.commatsci.2009.11.017.

[4] Y. You, D.W. Tan, W.M. Guo, S.H. Wu, H.T. Lin, Z. Luo, TaB2 powders synthesis by reduction of Ta2O5 with B4C Yang, Ceram. Int. 43 (2017) 897–900. doi:10.1016/j.ceramint.2016.09.193.

[5] J. Laszkiewicz- ukasik, L. Jaworska, P. Putyra, B. Smuk, Reactive Sintering of TaB2 using Spark Plasma Sintering Method, Adv. Sci. Technol. 88 (2014) 54–59. doi:10.4028/www.scientific.net/AST.88.54.

[6] W.M. Guo, Z.G. Yang, G.J. Zhang, Synthesis of submicrometer HfB 2 powder and its densification, Mater. Lett. 83 (2012) 52–55. doi:10.1016/j.matlet.2012.06.012.

[7] J. Laszkiewicz- ukasik, L. Jaworska, P. Putyra, P. Klimczyk, G. Garze , The influence of SPS heating rates on the synthesis reaction of tantalum diboride, Bol. La Soc. Esp. Ceram. Y Vidr. 55 (2016) 159–168. doi:10.1016/j.bsecv.2016.03.001.