High strength and high porosity YB₂C₂ ceramics prepared by a new high temperature reaction/ partial sintering process

1. Introduction

Porous ultrahigh temperature ceramics (UHTCs) of transition metal borides, nitrides or carbides are potential candidates as reusable thermal protection materials of transpiration cooling system in scramjet engine [[1], [2], [3]], due to their high melting point (>3000 ℃), low density, solid-state stability, good oxidation resistance, high surface area, good thermal shock resistance, excellent corrosion resistance and low thermal conductivity [[4], [5], [6], [7], [8], [9], [10]]. Thus, extensive research efforts have been dedicated to exploring and developing high strength and high porosity UHTCs. Previous works mainly focused on manufacturing porous ZrB₂, HfB₂, HfC, TiC ceramics using the green bodies formed by mixing transition metal borides or carbides powders with sintering additive and pore-forming agent, and then pressureless sintered at high temperatures. Sani et al. [11] prepared porous ZrB₂, HfB₂ and HfC ceramics with porosities of 13%, 39% and 33%, respectively. Medri et al. [12] synthesized porous ZrB₂ foam with porosity of >70% and compressive strength of <4.8 MPa. Kobashi et al. [13] fabricated porous TiC/Ti composite with porosity of 13% - 50% and compressive strength of 165-17 MPa by self-propagating high- temperature synthesis. The porous products prepared by these methods generally have either low porosity or low strength due to the following reasons. (1) Strong covalent bond and the oxide layer on the surface of raw powders cause hard-to-sinter for conventional porous UHTCs, which leads to low strength. (2) In the sintering process, large volume shrinkage occurs, which results in low porosity. (3) There are no formation and escape of gases during the sintering process, which leads to low porosity.

To obtain high strength and high porosity porous UHTCs, it is significant to develop a novel method to prepare porous UHTCs through one-step sintering. Herein, a novel and simple method combining a gas-releasing in-situ reaction and the partial sintering process is developed. The generated gases contribute to the development of porous structure and block the shrinkage during sintering process, which can increase the porosity of porous UHTCs [14,15]. The strength can be increased through the partial sintering process [15]. The structure of porous UHTCs prepared by this method is homogeneous, while the pore size and porosity can be controlled by adjusting the green density and the amount of reaction gases [16,17]. This novel method is simple and highly-efficient, which is expected to increase both the porosity and strength of porous UHTCs.

In our previous works, a similar method was used to prepare porous UHTCs through designing in-situ reactions. Wang et al. [14] designed the following reaction:

(1)Yb₂O₃+3B₄C = 2YbB₆+3CO

to synthesize porous YbB₆ ceramics. Guo et al. [15] designed the following reaction:

(2)7Y₂O₃+15B₄C = 14YB₄+2B₂O₃+15CO

to fabricate the porous YB₄ ceramics. In these works, due to formation and escape of CO and/or B₂O₃ gases, high porosity is achieved and large volume shrinkage is prevented.

In this work, we aim to prepare porous YB₂C₂ ceramics using the in-situ reaction/partial sintering approach. YB₂C₂ is selected as the backbone material due to the fact that it possesses the unique combination of properties, such as low density (4.35 g/cm³), good damage tolerance, good electrical conductivity, high melting point, excellent chemical stability [18,19]. However, previous works on YB₂C₂ ceramics mainly focused on the synthesis of powder and dense bulk, and there was no report on the preparation of porous YB₂C₂ ceramics. Reckeweg and DiSalvo synthesized YB₂C₂ powders using elemental Y, B and C as starting materials by the following solid-state reaction [20]:

(3)Y+2B+2C = YB₂C₂

Obviously there is no gas phase released in Eq. (3). Zhao et al. [18] prepared bulk YB₂C₂ ceramics from YH₂, B₄C and graphite powders by the following reaction:

(4)2YH₂+B₄C+3C = 2YB₂C₂+2H₂

In this process, the H₂ was released before the formation of YB₂C₂ so that it cannot be used to prepare porous YB₂C₂ ceramics. In order to prepare porous YB₂C₂ ceramics, the following gas-releasing in-situ reaction was designed in this work:

(5)Y₂O₃+4BN+7C = 2YB₂C₂+2N₂+3CO

Moreover, compared to our previous works, reaction (5) can release more molar volume gases. It is thus expected that the porous YB₂C₂ ceramics prepared by this method will have high porosity and high strength.

To achieve such a goal, in this work, porous YB₂C₂ ceramics were fabricated from Y₂O₃, BN and C powders by a combination of the gas-releasing in-situ reaction (5) and partial sintering process for the first time. The advantages of this method are as follows: (1) Relatively low-cost for the synthesis of high purity YB₂C₂; (2) A simple process without adding any pore-forming or sintering agent; (3) Relatively low synthesis temperature and short synthesis period; (4) Controlled porosity, radical shrinkage and compressive strength of the porous YB₂C₂ ceramics. To demonstrate the advantages of this new method and the as-prepared porous YB₂C₂, the reaction path was investigated first, and the phase composition, microstructure, porosity, radical shrinkage and compressive strength of as-prepared porous YB₂C₂ ceramics were studied.

2. Experimental

2.1. Materials preparation

To prepare porous YB₂C₂ ceramics using the in-situ reaction/partial sintering process, Y₂O₃ powders (99.99%, <300 nm, Rare Chemical Technology Co., Ltd, Huizhou, China), BN powders (99%, <500 nm, Sanxing Ceramic Materials Co. Ltd., Gongyi, China) and graphite powders (99%, -100 mesh, Tianyuan, China) were selected as raw materials following reaction (5). The molar ratio of mixed Y₂O₃/BN/Cpowders was 1/4/7. The reactant powders were ball-milled in absolute ethyl alcohol with agate balls for 16 h. After ball milling, the powder mixtures were dried in an oven and then sieved through a -120 mesh screen to obtain homogeneous reactants. The mixed powders were uniaxially pressed into columnar compacts (Φ50 mm) with different green densities of 0.63, 0.76, 0.83, 0.91, 1.14, 1.27, 1.39 and 1.47 g/cm³ by adding different pressures, with a dwell time of 5 min. Then these green bodies were sintered in vacuum under ambient pressure at 1800 ℃ for 1 h. Finally, the porous YB₂C₂ ceramics with different porosities were synthesized after sintering.

2.2. Characterization methods

After high temperature sintering, the as-prepared porous YB₂C₂ ceramics remained in intact shape without cracks and other defects, e.g., the as-prepared porous YB₂C₂ ceramics possess excellent shape stability. The density of porous YB₂C₂ ceramics was determined geometrically, by measuring the volume and weight of five identical samples and averaging the data to ensure the accuracy. And the porosity (P) of porous YB₂C₂ ceramics was calculated according to Eq. (6):

(6)P = (1-ρ/ρ′) ×100%

where ρ is the sintered density of as-prepared porous YB₂C₂ samples, ρ′ is the theoretical density of YB₂C₂ (4.35 g/cm³). In addition, in order to characterize the shrinkage behavior of as-prepared porous YB₂C₂ ceramics, the radical shrinkage (S) was evaluated by Eq. (7):

(7)S = (1-d/d′) ×100%

where d is the diameter of as-prepared porous YB₂C₂ samples, d′ is the diameter of green bodies.

The phase composition was identified by X-ray diffraction analysis (XRD, D8 advanced, Bruker, Karlsruhe, Germany) using CuKα radiation (λ = 1.54178 Å). Microstructure was observed in a scanning electron microscope (SEM, Apollo 300, CamScan, Cambridge, UK). The compressive strength was measured using a universal testing machine (DZS-III, China Building Material Test and Certification Center, Beijing, China). The tested samples were rectangular bars with a dimension of 5 mm × 5 mm × 10 mm. The crosshead speed was 0.5 mm/min. Five samples with the same density were used to obtain the average compressive strength.

3. Results

3.1. Reaction synthesis of YB₂C₂

In order to synthesize porous YB₂C₂ ceramics, the reaction path needs to be investigated first. Fig. 1 displays the XRD patterns of samples heated at 1500-1800 ℃ for 1 h. For the sample heated at 1500 ℃, Y₂O₃, BN and C can be found, which indicates that the reaction temperature should be higher than 1500 ℃. When the temperature increases to 1600 ℃, in addition to Y₂O₃, BN and C, a small amount of YB₂C₂ can be detected, which suggests that the formation of YB₂C₂ starts at 1600 ℃. When the temperature increases to 1700 ℃, the main phase is YB₂C₂ and the peck of C (002) can be seen. When the temperature increases to 1800 ℃, only YB₂C₂ is found, and all the diffraction peaks in the XRD pattern correspond to pure YB₂C₂ phase (according to JCPDF card# 27-0971). Based on the results of XRD, it is determined that YB₂C₂ is synthesized at 1800 ℃ by reaction (5).

3.2. Crystal structure of YB₂C₂

In the above section, the as-synthesized YB₂C₂ was identified as YB₂C₂ with P4₂/mmc symmetry according to the JCPDF card # 27-0971. It is known that there are two possible structures of YB₂C₂: one with P4₂/mmc symmetry [21,22], the other with P4/mbm symmetry [20], as shown in Fig. 2. The main difference is the bonding environment of B₂C₂ layers. In the P4₂/mmc symmetried YB₂C₂, each C atom is bonded to one C atom and two B atoms, and each B atom is bonded to one B atom and two C atoms. While in the P4/mbm symmetried YB₂C₂, each C atom is bonded to three B atoms and vice versa.

To make sure that the as-synthesized YB₂C₂ has the P4₂/mmc symmetry, Fig. 3 compares the simulated and the experimental XRD patterns of YB₂C₂. The simulated XRD data are generated from the Reflex code in Materials Studio software suit (Accelrys Software Inc., San Diego, CA, USA) using CuKα radiation of λ = 1.5418 Å, step size of 0.02°, and Pseudo-Voigt peak shape profile. From Fig. 3, one can see that the main difference between the simulated XRD patterns of two structures is the presence of (101) peak at 2θ = 26.69° in P4₂/mmc symmetrized YB₂C₂, while it is absent in P4/mbm symmetrized YB₂C₂. It is also seen that all the diffraction peaks of the experimental XRD pattern are consistent with the simulated XRD pattern with P4₂/mmc symmetry. Therefore, it is determined that the as-prepared YB₂C₂ exhibits P4₂/mmc symmetry.

3.3. Porosity and radical shrinkage control of porous YB₂C₂ ceramics

After determining the crystal structure, the green density, sintered density, porosity, radical shrinkage and compressive strength of as-prepared porous YB₂C₂ ceramics are tested, as listed in Table 1. Fig. 4 displays the photographs of green bodies and as-prepared porous YB₂C₂ ceramics. The samples are dark brown in color and remain in intact shape without cracks and other defects, which manifests that the as-prepared porous YB₂C₂ ceramics possess excellent shape stability.

The sintered density, porosity and radical shrinkage are important parameters for porous ceramics. The sintered density and porosity of as-synthesized porous YB₂C₂ ceramics are recorded as a function of green density, as shown in Fig. 5(a). It can be seen that the sintered density and porosity show opposite relationships with the green density. When the green density increases from 0.63 to 1.47 g/cm³, the sintered density increases from 1.08 to 1.85 g/cm³, whereas, the porosity decreases from 75.26% to 57.17%. It indicates that the green density plays a major role in controlling the sintered density and porosity of porous YB₂C₂ ceramics. The radical shrinkage has a critical effect on controlling the size of products. Fig. 5(b) reveals the relationship between radical shrinkage and green density of as-synthesized porous YB₂C₂ ceramics. As the green density increases from 0.63 to 1.47 g/cm³, the radical shrinkage of porous YB₂C₂ decreases from 26.93% to 12.37%. The radical shrinkage has an inverse relationship with the green density, which indicates that the radical shrinkage can be controlled by changing the green density.

Based on the above results, it has come to light that the green density has an important influence on the sintered density, porosity and radical shrinkage of porous YB₂C₂ ceramics. As the porosity (P), sintered density (ρ) and radical shrinkage (S_r) are linearly dependent on green density (ρ₀), we fitted the following linear equations for P, ρ and S_r.

(8)P=-18.74ρ₀ + 85.96

(9)ρ = 0.82ρ₀ + 0.61

(10)S_r=-18.01ρ₀ + 38.29

where ρ₀ is the green density, P is the porosity of as-prepared porous YB₂C₂ ceramics, ρ is the sintered density of as-prepared porous YB₂C₂ ceramics, S_r is the radical shrinkage of as-prepared porous YB₂C₂ ceramics. Therefore, the shape and density of porous YB₂C₂ ceramics can be controlled through changing green density.

3.4. Microstructure of porous YB₂C₂ ceramics

Fig. 6 shows the porous skeleton microstructure of as-synthetized porous YB₂C₂ ceramic with the sintered density of 1.51 g/cm³. According to the matter transport by surface diffusion or evaporation-condensation process, the particles are bonded to form a porous structure [16]. From Fig. 6(a), the grains have two kinds of micro-morphologies; i. e, small grains (2-5 μm) and large lamellar grains (10-20 μm). The smaller grains are formed in the in-situ reaction and the large lamellar grains are inherited from graphite. It can be seen that the thick necks are formed between particles, and several small particles are sintered together to develop into large particles. In the heating process, the generated CO and N₂ prevent the densification of porous YB₂C₂ ceramics. And the formation and escape of gases can lead to the formation of a large number of connected pores in the structure. Meanwhile, neck growth between grains occurs. The bridging of particles is developed through necks in the sintering process. Jayaseelan et al. [23] and Yang et al. [24] proved that forming necks between particles could result in relatively high strength compared to conventional porous materials. Therefore, it is expected that the as-prepared porous YB₂C₂ ceramics have high strength. In addition, from Fig. 6(b), the as-synthesized porous YB₂C₂ ceramics exhibit anisotropic microstructure, because the large lamellar particles are oriented preferentially in the plane perpendicular to the forming direction.

3.5. Compressive behavior of porous YB₂C₂ ceramics

The compressive strength is an important parameter for porous ceramics. As shown in Table 1, the compressive strengths of samples with porosity of 75.26% -57.17% are in the range of 9.32-34.78 MPa. Fig. 7 compares the porosity and compressive strength of porous YB₂C₂, YbB₆, ZrB₂ and YB₄ ceramics. For all the materials, the compressive strength decreases with increasing porosity. Since the porosity is dependent on the green density, the compressive strength can be controlled by changing the green density. The compressive strength of the porous ZrB₂ ceramics with porosity of 52.1% prepared by Landi et al. [3] is 18.4 MPa. The compressive strengths of porous YbB₆ ceramics with porosity of 72.5%-58.7% prepared by Wang et al. [14] are in the range of 9.12-21.3 MPa. While those of porous YB₄ ceramics with porosity of 69.6%-60.9% prepared by Guo et al. [15] are in the range of 7.3-19.9 MPa. It is obvious that the as-prepared porous YB₂C₂ ceramics have better mechanical properties, compared to porous YbB₆, YB₄ and ZrB₂ ceramics.

4. Discussion

4.1. Pore forming mechanism

The porosity is an important parameter for porous UHTCs. The porosities of reported porous UHTCs are usually less than 50% [3,11,13]. However, the porosity of porous YB₂C₂ ceramics can be controlled in a high level from 57.17% to 75.26%, which indicates that the gas-releasing in-situ reaction/partial sintering process is effective in increasing the porosity of porous ceramics. According to reaction (5), 2 mol of N₂ and 3 mol of CO are released form solid mixtures of 1 mol of Y₂O₃, 4 mol of BN and 7 mol of C. The gases produced in this reaction replace pore-forming agent to aggregate particles and expand the porous structure [14,15]. To better understand the pore-forming process, a schematic illustration for the preparation of porous YB₂C₂ ceramics is shown in Fig. 8, which illustrates the effects of gas produced in in-situ reaction and the evolution of the porous structure during the preparation of porous YB₂C₂. The representative microstructures of green body and sintered sample are also included in Fig. 8. In the green body, both the number and size of pores are lower. In the sintered samples, however, the porosity increases substantially. We can thus conclude that the generation and escape of gases result in the formation of a large number of pores in the preparation process.

4.2. Anisotropic mechanical property

The as-prepared porous YB₂C₂ ceramics have anisotropic microstructure with a majority of large lamellar grains arranged in the plane perpendicular to the forming direction, as shown in Fig. 6. The anisotropy was induced during the dry pressing technique, leading to graphite grains orientated perpendicular to the pressing direction. The microstructure of as-prepared porous YB₂C₂ ceramics is similar to porous short-fibrous ceramics, such as carbon fibrous composites [25], mullite fibrous ceramics [26]. If structure units are preferentially oriented in one direction, the porous short-fibrous ceramics will be stronger in this direction, but weaker in other directions [[26], [27], [28]]. Similarly, it can be infer that the as-prepared porous YB₂C₂ ceramics will exhibit anisotropy in mechanical properties.

To confirm the anisotropic mechanical properties, compression test was carried out in different directions. Fig. 9(a) compares the results of compression test in different loading directions, wherein σ_∥ is the strength tested in the direction parallel to the forming direction, and σ_⊥ is the strength tested in the direction perpendicular to the forming direction. Significant difference between σ_∥ and σ_⊥ clearly indicates that the as-synthesized porous YB₂C₂ ceramics exhibit anisotropy in mechanical properties. To disclose the mechanism underpins the anisotropic properties, the representative stress-strain curves of samples with the porosity of 57.75% tested in the direction (∥) and (⊥) are compared in Fig. 9(b). In the initial stage, the stress increases with strain in the same trend. After reaching a maximum, the trends of the two curves are different. σ_⊥ decreases sharply with the increasing of strain, which indicates that the failure is caused by permanent damage [25], i.e., a catastrophic fracture. In contrast, σ_∥ decreases slowly with the increasing of strain, which indicates that the failure is caused by accumulated damage [29].

In order to further verify the different compression processes, the changes of microstructure in the compression tests were investigated by SEM. In the porous microstructure, skeleton network is composed of large lamellar particles, and interstices in the skeleton are filled with small particles, as shown in Fig. 10(a, b). In the compression tested in the ∥ direction, the interstices are narrowed under pressure. With the increase of load, the porous structure collapses layer by layer and gradually becomes dense, as shown in Fig. 10(c, d). The fracture mode is generally interlayer failure with cracks propagating along interlayer regions, resulting in complete failure of the sample together with a large amount of dust debris. This is a gradual process, not a catastrophic fracture process. In the compression tested in the ⊥ direction, skeleton network is subjected to pressure load. With the increase of the load, the porous structure gradually becomes dense. Upon further loading, the structure formed by small particles disintegrates, resulting in cracking. And cracks expand rapidly along the direction parallel to large lamellar particles orientation, as shown in Fig. 10(e, f).

4.3. Anisotropic microstructure

The anisotropic microstructure of as-prepared porous YB₂C₂ ceramics is shown in Fig. 6. The grains have two kinds of micro-morphologies; i. e, small grains (2-5 μm) and large lamellar grains (10-20 μm). The morphology of large lamellar grains is inherited from graphite. The plate like graphite grains are arranged in the plane perpendicular to the forming direction, which causes the anisotropic microstructure. In order to further confirm the origin of anisotropy, carbon black is selected as the carbon source instead of graphite to prepare porous YB₂C₂ ceramics. The microstructure of as-prepared porous YB₂C₂ ceramics with the sintered density of 1.53 g/cm³ using carbon black as the carbon source is shown in Fig. 11. It is seen that the microstructure exhibits no anisotropy, the grains possess uniform size (∼1 μm) and the microstructure is homogeneous. This is different from the anisotropic microstructure of porous YB₂C₂ ceramics synthetized from graphite. Therefore, the microstructure of porous YB₂C₂ ceramics can be controlled by selecting different carbon sources. In addition, compression test was carried out. Fig. 12 compares the compressive strength tested in the direction parallel to forming direction of porous YB₂C₂ ceramics prepared using different carbon sources, wherein σ_G is the strength of samples using graphite, and σ_C is the strength of samples using carbon black. Significant difference between σ_G and σ_C clearly indicates that the as-synthesized porous YB₂C₂ ceramics prepared by using graphite as carbon source exhibits high strength.

5. Conclusion

A new and simple method for preparing porous YB₂C₂ ceramics with high porosity and high compressive strength is presented in this work, which is a combination of gas-releasing in-situ reaction and partial sintering process. In this method, the mixed powders with a molar ratio of Y₂O₃/BN/C = 1/4/7 are pressed into green bodies, and then porous YB₂C₂ ceramics are prepared through sintering at 1800 ℃ for 1 h in vacuum. The as-prepared porous YB₂C₂ ceramics possess low density of 1.08-1.85 g/cm³, high porosity of 57.17%-75.26 %. And the density, porosity, radical shrinkage and compressive strength can be controlled by changing green density. Based on the results of XRD, the as-prepared YB₂C₂ is phase-pure YB₂C₂ and has P4₂/mmc symmetry. Investigation on microstructure reveals that the as-synthesized porous YB₂C₂ ceramics using graphite as carbon source exhibit anisotropy in microstructure. The compressive tests reveal that the as-synthesized porous YB₂C₂ ceramics using graphite as carbon source possess excellent mechanical properties and show anisotropy in the mechanical behavior. In the direction parallel to the forming direction, the samples with porosity of 75.26%-57.17% possess compressive strength of 9.32-34.78 MPa. And in the direction perpendicular to the forming direction, the samples with porosity of 72.40%-54.02% possess compressive strength of 17.47-98.57 MPa. These unique properties indicate that porous YB₂C₂ ceramics can be used as potential reusable thermal protection materials of transpiration cooling system in scramjet engine.

Acknowledgments

This work was supported by the National Natural Science Foundation of China under Grant Nos. U1435206 and 51672064, and by the Beijing Municipal Science & Technology Commission under Grant No. D161100002416001.

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