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Cold bonding of alumina: Fractured and re-bonding under compression
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Cold bonding of alumina: Fractured and re-bonding under compression Fangming Liua, Wei Dinga, Jin Liua, Duanwei Hea,b, a b
⁎
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610065, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold bonding Alumina Fractured Re-bonding High pressure
Cold bonding of metals necessitates high normal/frictional loads, or ultrahigh-vacuum treated atomically flat ductile surface. In case of ceramics composed of ionic and/or covalent bonds, the high diffusion barrier of ions or atoms gives them a constraint to bond together at room temperature. However, from this investigation, it has been found that alumina crystals could be well bonded at room temperature under high pressure. Also, it has been noticed that if the pressure is capable of yielding and breaking down alumina crystals, it could then re-bond the fractured crystals by reducing the distance of their surface atoms to meet the condition for bonding. Overall, we demonstrate a process for the cold bonding of alumina under high pressure with grain fracturing and rebonding.
1. Introduction Metals are generally described by delocalized bonding between crystalline arrangement of hard spheres (the metal cations) and free electrons moving in the interstices [1]. In the macroscopic cold bonding of metals, atomic diffusion and surface relaxation are considered to be critical driving forces, provided there is sufficient mechanical manipulation to overcome the diffusion barrier for a single metal atom on a metal surface [2–4]. Unlike metallic materials, ceramics are mainly composed of ionic and/or covalent bonds [5], which contribute to their high melting point. Therefore, by mechanical manipulation it is impractical to bond ceramic crystals at room temperature owing to higher diffusion barrier of ions or atoms on the surface of ceramics. Generally, high sintering temperatures (above 2/3 melting point) are indispensable in the fabrication of densified ceramics at atm pressure [6–8]. While the application of pressure (< 500 MPa) in hot isostatic pressing or spark plasma sintering decreased the sintering temperature to about half the melting point [9–11]. Interestingly, high pressure (> 1 GPa) can further decrease the sintering temperature of many high melting transparent ceramics such as alumina [12–14], MgAl2O4 [15–17], YAG [18,19], cubic boron nitride (cBN), and diamond [20,21] to even 1/3 melting point. The spinel powder can be compressed to transparent bulk even at room temperature and 5 GPa [17], and γ-Al2O3 powder compressed at 5.6 GPa and room temperature can yield a Vickers microhardness of 5.7 GPa at an applied load of 50 gf [22]. However, the mechanism of pressure-induced sintering temperature reduction is not precise, and thus the cold bonding behavior of ceramic grains under
⁎
high pressure is further worth investigating. In our study, alumina particles were compressed to a pressure of 20 GPa at room temperature, and the recovered translucent bulk alumina observed to possess a similar Vickers hardness to single-crystal alumina. Through analyzing the microstructures of the samples, it has been noted that the initiation and healing of cracks are considered as dominant factors in the cold bonding of alumina particles under compression. This shows that high pressure (or stress) could break down the alumina crystals, and also capable of making the fractured crystals rebonded well even at room temperature. This phenomenon is different from the cold bonding of metals and could be a critical point in the sintering of ceramics under high pressure and modest temperature. 2. Experimental procedure The bulk alumina sample was fabricated by compressing submicron alumina grains (Xuancheng Jingrui New Material Co., Ltd., China) at room temperature. Fig. 1a presents the X-Ray Diffraction (XRD) pattern of pure α-Al2O3 powder, which shows the absence of impurity peaks. The average grain size of the powder is about 300 nm, as observed from the image of the scanning electron microscope (SEM) (Fig. 1b). The powder dried in vacuum furnace (∼1.5 × 10−3 Pa/800 °C) for 1 h was firstly packaged with tantalum foil (0.025 μm in thickness) and precompressed into discs (1.5 mm in diameter and 1.2 mm in thickness) with a relative density of about 75%, followed by cold compressing in a two-stage multi anvil apparatus based on DS6 × 8 MN cubic press developed at Sichuan University [23–25]. The anvils of eight tungsten
Corresponding author at: Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China. E-mail address: [email protected] (D. He).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.021 Received 24 April 2019; Received in revised form 26 August 2019; Accepted 11 September 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Fangming Liu, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.021
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 1. Cold compression of submicron alumina particles. (a) XRD of starting alumina powder. (b) SEM image of starting powder. (c) A schematic illustration of the octahedral pressure assembly with the third-stage anvil of diamond and WC. (d) Cross-section of the recovered octahedral pressure assembly decompressed at high pressure.
3. Results and discussion
carbide cubes with 4 mm edge length corner truncations were used to generate pressure, and the octahedral semi-sintered magnesia (MgO, about 75% relative density) with 10 mm of edge length served as the pressure medium. The chamber pressure of the sample was estimated by the pressure-induced phase transitions of ZnTe, ZnS, and GaAs [26]. The details of the octahedral pressure cell assembly are shown in Fig. 1c. Diamond (diamond powder sintered at 5.0 GPa/1450 °C with a Co-binder, 1.5 mm in diameter and 1 mm in thickness) and WC (1.5 mm in diameter and 1.2 mm in thickness) discs were used as third-stage anvils to reduce the volume collapse of the pressure transmitting medium significantly. 0.1 mm thick Mo sheet was used to protect WC cube from contacting with WC discs directly. During cold compression, the externally loaded oil pressure was increased to about 35 MPa in 6 h and then released to 1 MPa in 12 h after maintaining for 6 h. Thus, the achieved chamber pressure of the sample could exceed 20 GPa owing to the pressurization of third-stage anvils. The octahedral pressure assembly, which was decompressed at high pressure, is shown in Fig. 2d. It could be seen that the WC discs are cracked while the diamond discs are still intact. The recovered samples were about 2.0 mm in diameter and 0.5 mm in thickness after polishing on one side. The DX-2500 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) was used to identify the phases of starting powder and the hardness of the sample was measured by a Vickers hardness tester (FV-700/B). The density of the sample was measured by the Archimedes method, and the sample film was obtained through Focused Ion Beam (FIB) (FEI Helios Nanolab 600i). Microstructures of the samples were investigated by transmission electron microscope (TEM) (FEI Tecnai G2 F20 S-TWIN) and scanning electron microscope (SEM) (FEI Inspect F50).
Fig. 2a shows the optical image of the sample where cracks could be seen after polishing. The fragment in the white dotted line is translucent as the metallic luster of the Ta package can be distinguished at the bottom of the sample. The translucent state of the sample indicates that the gaps among the first particles were mostly closed through the plastic deformation of individual grain during compression [13,16,19]. The relative density of this sample was about 98%, which is inaccurate due to cracks and Ta package. The Vickers hardness of the sample (inside the black dotted line in Fig. 2a) calculated from the area of Vickers indentations (Fig. 2b and c) is about 17.2 GPa with a loading force of 0.3 kgf, which is comparable with the value of single-crystal alumina, and this demonstrates that the starting alumina grains are well bonded to each other. While removing the Ta package, the sample fractured to fragments, indicating that boundaries between the translucent regions have reduced strength. This might be caused by the high shear stress generated by the non-concentric compression of third-stage anvils, and can be improved by reducing the dimensional error of the assembly. Thus, another sample without cracks was synthesized by using an improved assembly at the same pressure (inset picture in Fig. 2d). The obtained sample has a higher relative density of 99.1% after removing the Ta package but with poor transparency than the sample with cracks. Besides, the Vickers hardness of the sample (Fig. 2d) is about 16.8 GPa (at an applied load of 0.3 kgf) which is consistent with the hardness value of the sample with cracks and can yield a Vickers hardness of 12.6 GPa even at an applied load of 3 kgf. So, the cold compressed alumina powder at 20 GPa has a comparable strength to high pressure, and modest temperature sintered bulk alumina with a micrometer grain size [14], but inferior to that bulks with nano-structured grains compacted at 7.7 GPa/700 °C [12]. Fig. 2e and f 2
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Fig. 2. Microstructures of samples treated at 20 GPa and room temperature. (a) Recovered sample after polishing. (b) Optical image of Vickers indentations of the sample (inside the black dotted line in (a)) at an applied load of 0.3 kg. (c) SEM image of Vickers indentations of the sample at an applied load of 0.3 kg. (d) Optical image of Vickers indentations of the sample without cracks at an applied load of 0.3 kg (inset image is the recovered sample without cracks after polishing). (e) and (f) are the SEM images of fracture the surface of the translucent fragments and the sample without cracks, respectively.
boundary situations appear among the large individual grains. Fig. 3b presents the enlarged view of the square of the dotted white line from Fig. 3a. Along the grain boundary, particles with dozens of nanometers are well bonded, and the joining exhibits a tendency of amorphization as shown in the dotted white line in Fig. 3b. Fig. 3c is the enlarged view of the square of the solid white line from Fig. 3a, which shows an almost amorphous structured boundary between individual grains. The “Y” shape boundaries among three individual large grains are shown as the
present the SEM images of the fractured surface of the translucent fragments and the samples without cracks. The nano-structured grains are the result of pressure-induced grain refinement. The bonding behavior of alumina grains was investigated by TEM using a translucent fragment of a sample. Fig. 3 illustrates the microstructures of the sample with a special grain boundary. The apparent grain size of the sample is about 200 nm, and few pores still exist among particles, as shown in Fig. 3a. During compression, three types of
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Fig. 3. Microstructures of the samples treated at 20 GPa. (a) TEM image of the sample. (b) HRTEM image of the square of the white dotted line from (a). (c) HRTEM image of the square of the solid white line from (a). (d) HRTEM image of the square of the black dotted line from (a). The parallel white lines in b and d represent the crystal orientation of small grains.
the joining of the re-bonded grains shows nanoscale or amorphous structure, which might be due to the support of the fractured nanocrystals for ultra-high pressure again after releasing the stress and further crushed into the amorphous structure. Moreover, the fractured nano-grains at the “Y” boundaries are well bonded, which contributes to its comparable hardness as single-crystal alumina.
white dotted curve in Fig. 3a which is mainly composed of small particles of a few nanometers, as illustrated in Fig. 3d (enlarged view of the square of the black dotted line in Fig. 3a). In Guan’s view, with increasing pressure, the fragmentation of ceramic particles such as diamond comprises of three stages [27]: (i) fracturing of edges and corners, (ii) cracking of the crystal plane, and (iii) refinement of particle disorder. Under the highest load, the maximum stress of the sample reached about twice the chamber pressure of the sample. The static yield strength of alumina is about 15 GPa [28,29], and during cold compression of alumina at 20 GPa, the particles fracture completely or partially and release the deviatoric microscopic stress when the local high shear stress reaches the strength of the individual grains. Then, the fractured particles re-bond since the applied ultra-high pressure is sufficient to shorten the distance of their surface atoms to meet the bonding conditions. It indicates that ultrahigh pressure can break down the alumina particles and re-bond the fractured particles at room temperature. Hence, in the high-pressure sintering process, high temperature is not necessary for grain bonding due to this pressure, which is also responsible for the reduction in the sintering temperature of many high melting point ceramics. Notably,
4. Conclusions In summary, high pressure plays a critical role in the cold bonding of alumina grains. Once the pressure can yield crystals and promote adequate surface atoms to meet the bonding condition, it forms the stable structures at the boundaries, which contribute to its comparable hardness as single-crystal alumina. The outcome from this investigation indicates that the cold bonding of alumina under compression is a process of grain fracturing and re-bonding. This emphasizes the significance of pressure on the reduction in the sintering temperature of high melting point ceramics. It also helps to improve the preparation of bulk ceramic materials (such as alumina, spinel, YAG, WC, cBN, diamond, etc.) to obtain excellent mechanical properties (hardness, 4
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fracture toughness, etc.) or high transparency. It is distinctly different from the cold bonding of metal, and thus offers a new direction in the research of deformation of ceramics under high pressure.
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Acknowledgements This work was supported by the National Key R&D Program of China (No. 2018YFA0305900), and the National Natural Science Foundation of China (Grant No: 51472171 and 11427810). The authors would like to express their gratitude to EditSprings (https://www. editsprings.com/) for the expert linguistic services. References [1] L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1960. [2] G.S. Ferguson, M.K. Chaudhury, G.B. Sigal, G.M. Whitesides, Contact adhesion of thin gold films on elastomeric supports: cold bonding under ambient conditions, Science 253 (1991) 776–778, https://doi.org/10.1126/science.253.5021.776. [3] T. Kizuka, Atomic process of point contact in gold studied by time-resolved highresolution transmission electron microscopy, Phys. Rev. Lett. 81 (1998) 4448–4451, https://doi.org/10.1103/PhysRevLett.81.4448. [4] Y. Lu, J.Y. Huang, C. Wang, S.H. Sun, J. Lou, Cold bonding of ultrathin gold nanowires, Nat. Nanotech. 5 (2010) 218–224, https://doi.org/10.1038/nnano. 2010.4. [5] S. Ciraci, Electronic structure of α-alumina and its defect states, Phys. Rev. B 28 (1983) 982–992, https://doi.org/10.1103/PhysRevB.28.982. [6] J.P. Cheng, D. Agrawal, Y. Zhang, R. Roy, Microwave sintering of transparent alumina, Mater. Lett. 56 (2002) 587–592, https://doi.org/10.1016/S0167-577X (02)00557-8. [7] G. Mata-Osoro, J.S. Moya, C. Pecharroman, Transparent alumina by vacuum sintering, J. Eur. Ceram. Soc. 32 (2012) 2925–2933, https://doi.org/10.1016/j. jeurceramsoc.2012.02.039. [8] X.J. Mao, S.W. Wang, S. Shimai, J.K. Guo, Transparent polycrystalline alumina ceramics with orientated optical axes, J. Am. Ceram. Soc. 91 (2008) 3431–3433, https://doi.org/10.1111/j.1551-2916.2008.02611.x. [9] A. Krell, P. Blank, H. Ma, T. Hutzler, Transparent sintered corundum with high hardness and strength, J. Am. Ceram. Soc. 86 (2003) 12–18, https://doi.org/10. 1111/j.1151-2916.2003.tb03270.x. [10] A.F. Dericioglu, Y. Kagawa, Effect of grain boundary microcracking on the light transmittance of sintered transparent MgAl2O4, J. Eur. Ceram. Soc. 23 (2003) 951–959, https://doi.org/10.1016/S0955-2219(02)00205-4. [11] S. Grasso, H. Yoshida, H. Porwal, Y. Sakka, M. Reece, Highly transparent α-alumina obtained by low cost high pressure SPS, Ceram. Int. 39 (2013) 3243–3248, https:// doi.org/10.1016/j.ceramint.2012.10.012. [12] N. Nishiyama, T. Taniguchi, H. Ohfuji, K. Yoshida, F. Wakai, B.N. Kim, H. Yoshida, Y. Higo, A. Holzheid, O. Beermann, T. Irifune, Y. Sakka, K. Funakoshi, Transparent nanocrystalline bulk alumina obtained at 7.7 GPa and 800 °C, Scripta Mater. 69 (2013) 362–365, https://doi.org/10.1016/j.scriptamat.2013.05.017. [13] F.M. Liu, D.W. He, P.P. Liu, H.K. Wang, C. Xu, S. Yin, W.W. Yin, Y. Li, Plastic deformation and sintering of alumina under high pressure, J. Appl. Phys. 114 (2013) 233504, , https://doi.org/10.1063/1.4844495.
5
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short communication
Cold bonding of alumina: Fractured and re-bonding under compression Fangming Liua, Wei Dinga, Jin Liua, Duanwei Hea,b, a b
⁎
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610065, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold bonding Alumina Fractured Re-bonding High pressure
Cold bonding of metals necessitates high normal/frictional loads, or ultrahigh-vacuum treated atomically flat ductile surface. In case of ceramics composed of ionic and/or covalent bonds, the high diffusion barrier of ions or atoms gives them a constraint to bond together at room temperature. However, from this investigation, it has been found that alumina crystals could be well bonded at room temperature under high pressure. Also, it has been noticed that if the pressure is capable of yielding and breaking down alumina crystals, it could then re-bond the fractured crystals by reducing the distance of their surface atoms to meet the condition for bonding. Overall, we demonstrate a process for the cold bonding of alumina under high pressure with grain fracturing and rebonding.
1. Introduction Metals are generally described by delocalized bonding between crystalline arrangement of hard spheres (the metal cations) and free electrons moving in the interstices [1]. In the macroscopic cold bonding of metals, atomic diffusion and surface relaxation are considered to be critical driving forces, provided there is sufficient mechanical manipulation to overcome the diffusion barrier for a single metal atom on a metal surface [2–4]. Unlike metallic materials, ceramics are mainly composed of ionic and/or covalent bonds [5], which contribute to their high melting point. Therefore, by mechanical manipulation it is impractical to bond ceramic crystals at room temperature owing to higher diffusion barrier of ions or atoms on the surface of ceramics. Generally, high sintering temperatures (above 2/3 melting point) are indispensable in the fabrication of densified ceramics at atm pressure [6–8]. While the application of pressure (< 500 MPa) in hot isostatic pressing or spark plasma sintering decreased the sintering temperature to about half the melting point [9–11]. Interestingly, high pressure (> 1 GPa) can further decrease the sintering temperature of many high melting transparent ceramics such as alumina [12–14], MgAl2O4 [15–17], YAG [18,19], cubic boron nitride (cBN), and diamond [20,21] to even 1/3 melting point. The spinel powder can be compressed to transparent bulk even at room temperature and 5 GPa [17], and γ-Al2O3 powder compressed at 5.6 GPa and room temperature can yield a Vickers microhardness of 5.7 GPa at an applied load of 50 gf [22]. However, the mechanism of pressure-induced sintering temperature reduction is not precise, and thus the cold bonding behavior of ceramic grains under
⁎
high pressure is further worth investigating. In our study, alumina particles were compressed to a pressure of 20 GPa at room temperature, and the recovered translucent bulk alumina observed to possess a similar Vickers hardness to single-crystal alumina. Through analyzing the microstructures of the samples, it has been noted that the initiation and healing of cracks are considered as dominant factors in the cold bonding of alumina particles under compression. This shows that high pressure (or stress) could break down the alumina crystals, and also capable of making the fractured crystals rebonded well even at room temperature. This phenomenon is different from the cold bonding of metals and could be a critical point in the sintering of ceramics under high pressure and modest temperature. 2. Experimental procedure The bulk alumina sample was fabricated by compressing submicron alumina grains (Xuancheng Jingrui New Material Co., Ltd., China) at room temperature. Fig. 1a presents the X-Ray Diffraction (XRD) pattern of pure α-Al2O3 powder, which shows the absence of impurity peaks. The average grain size of the powder is about 300 nm, as observed from the image of the scanning electron microscope (SEM) (Fig. 1b). The powder dried in vacuum furnace (∼1.5 × 10−3 Pa/800 °C) for 1 h was firstly packaged with tantalum foil (0.025 μm in thickness) and precompressed into discs (1.5 mm in diameter and 1.2 mm in thickness) with a relative density of about 75%, followed by cold compressing in a two-stage multi anvil apparatus based on DS6 × 8 MN cubic press developed at Sichuan University [23–25]. The anvils of eight tungsten
Corresponding author at: Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China. E-mail address: [email protected] (D. He).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.021 Received 24 April 2019; Received in revised form 26 August 2019; Accepted 11 September 2019 0955-2219/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Fangming Liu, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.021
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 1. Cold compression of submicron alumina particles. (a) XRD of starting alumina powder. (b) SEM image of starting powder. (c) A schematic illustration of the octahedral pressure assembly with the third-stage anvil of diamond and WC. (d) Cross-section of the recovered octahedral pressure assembly decompressed at high pressure.
3. Results and discussion
carbide cubes with 4 mm edge length corner truncations were used to generate pressure, and the octahedral semi-sintered magnesia (MgO, about 75% relative density) with 10 mm of edge length served as the pressure medium. The chamber pressure of the sample was estimated by the pressure-induced phase transitions of ZnTe, ZnS, and GaAs [26]. The details of the octahedral pressure cell assembly are shown in Fig. 1c. Diamond (diamond powder sintered at 5.0 GPa/1450 °C with a Co-binder, 1.5 mm in diameter and 1 mm in thickness) and WC (1.5 mm in diameter and 1.2 mm in thickness) discs were used as third-stage anvils to reduce the volume collapse of the pressure transmitting medium significantly. 0.1 mm thick Mo sheet was used to protect WC cube from contacting with WC discs directly. During cold compression, the externally loaded oil pressure was increased to about 35 MPa in 6 h and then released to 1 MPa in 12 h after maintaining for 6 h. Thus, the achieved chamber pressure of the sample could exceed 20 GPa owing to the pressurization of third-stage anvils. The octahedral pressure assembly, which was decompressed at high pressure, is shown in Fig. 2d. It could be seen that the WC discs are cracked while the diamond discs are still intact. The recovered samples were about 2.0 mm in diameter and 0.5 mm in thickness after polishing on one side. The DX-2500 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) was used to identify the phases of starting powder and the hardness of the sample was measured by a Vickers hardness tester (FV-700/B). The density of the sample was measured by the Archimedes method, and the sample film was obtained through Focused Ion Beam (FIB) (FEI Helios Nanolab 600i). Microstructures of the samples were investigated by transmission electron microscope (TEM) (FEI Tecnai G2 F20 S-TWIN) and scanning electron microscope (SEM) (FEI Inspect F50).
Fig. 2a shows the optical image of the sample where cracks could be seen after polishing. The fragment in the white dotted line is translucent as the metallic luster of the Ta package can be distinguished at the bottom of the sample. The translucent state of the sample indicates that the gaps among the first particles were mostly closed through the plastic deformation of individual grain during compression [13,16,19]. The relative density of this sample was about 98%, which is inaccurate due to cracks and Ta package. The Vickers hardness of the sample (inside the black dotted line in Fig. 2a) calculated from the area of Vickers indentations (Fig. 2b and c) is about 17.2 GPa with a loading force of 0.3 kgf, which is comparable with the value of single-crystal alumina, and this demonstrates that the starting alumina grains are well bonded to each other. While removing the Ta package, the sample fractured to fragments, indicating that boundaries between the translucent regions have reduced strength. This might be caused by the high shear stress generated by the non-concentric compression of third-stage anvils, and can be improved by reducing the dimensional error of the assembly. Thus, another sample without cracks was synthesized by using an improved assembly at the same pressure (inset picture in Fig. 2d). The obtained sample has a higher relative density of 99.1% after removing the Ta package but with poor transparency than the sample with cracks. Besides, the Vickers hardness of the sample (Fig. 2d) is about 16.8 GPa (at an applied load of 0.3 kgf) which is consistent with the hardness value of the sample with cracks and can yield a Vickers hardness of 12.6 GPa even at an applied load of 3 kgf. So, the cold compressed alumina powder at 20 GPa has a comparable strength to high pressure, and modest temperature sintered bulk alumina with a micrometer grain size [14], but inferior to that bulks with nano-structured grains compacted at 7.7 GPa/700 °C [12]. Fig. 2e and f 2
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Fig. 2. Microstructures of samples treated at 20 GPa and room temperature. (a) Recovered sample after polishing. (b) Optical image of Vickers indentations of the sample (inside the black dotted line in (a)) at an applied load of 0.3 kg. (c) SEM image of Vickers indentations of the sample at an applied load of 0.3 kg. (d) Optical image of Vickers indentations of the sample without cracks at an applied load of 0.3 kg (inset image is the recovered sample without cracks after polishing). (e) and (f) are the SEM images of fracture the surface of the translucent fragments and the sample without cracks, respectively.
boundary situations appear among the large individual grains. Fig. 3b presents the enlarged view of the square of the dotted white line from Fig. 3a. Along the grain boundary, particles with dozens of nanometers are well bonded, and the joining exhibits a tendency of amorphization as shown in the dotted white line in Fig. 3b. Fig. 3c is the enlarged view of the square of the solid white line from Fig. 3a, which shows an almost amorphous structured boundary between individual grains. The “Y” shape boundaries among three individual large grains are shown as the
present the SEM images of the fractured surface of the translucent fragments and the samples without cracks. The nano-structured grains are the result of pressure-induced grain refinement. The bonding behavior of alumina grains was investigated by TEM using a translucent fragment of a sample. Fig. 3 illustrates the microstructures of the sample with a special grain boundary. The apparent grain size of the sample is about 200 nm, and few pores still exist among particles, as shown in Fig. 3a. During compression, three types of
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Fig. 3. Microstructures of the samples treated at 20 GPa. (a) TEM image of the sample. (b) HRTEM image of the square of the white dotted line from (a). (c) HRTEM image of the square of the solid white line from (a). (d) HRTEM image of the square of the black dotted line from (a). The parallel white lines in b and d represent the crystal orientation of small grains.
the joining of the re-bonded grains shows nanoscale or amorphous structure, which might be due to the support of the fractured nanocrystals for ultra-high pressure again after releasing the stress and further crushed into the amorphous structure. Moreover, the fractured nano-grains at the “Y” boundaries are well bonded, which contributes to its comparable hardness as single-crystal alumina.
white dotted curve in Fig. 3a which is mainly composed of small particles of a few nanometers, as illustrated in Fig. 3d (enlarged view of the square of the black dotted line in Fig. 3a). In Guan’s view, with increasing pressure, the fragmentation of ceramic particles such as diamond comprises of three stages [27]: (i) fracturing of edges and corners, (ii) cracking of the crystal plane, and (iii) refinement of particle disorder. Under the highest load, the maximum stress of the sample reached about twice the chamber pressure of the sample. The static yield strength of alumina is about 15 GPa [28,29], and during cold compression of alumina at 20 GPa, the particles fracture completely or partially and release the deviatoric microscopic stress when the local high shear stress reaches the strength of the individual grains. Then, the fractured particles re-bond since the applied ultra-high pressure is sufficient to shorten the distance of their surface atoms to meet the bonding conditions. It indicates that ultrahigh pressure can break down the alumina particles and re-bond the fractured particles at room temperature. Hence, in the high-pressure sintering process, high temperature is not necessary for grain bonding due to this pressure, which is also responsible for the reduction in the sintering temperature of many high melting point ceramics. Notably,
4. Conclusions In summary, high pressure plays a critical role in the cold bonding of alumina grains. Once the pressure can yield crystals and promote adequate surface atoms to meet the bonding condition, it forms the stable structures at the boundaries, which contribute to its comparable hardness as single-crystal alumina. The outcome from this investigation indicates that the cold bonding of alumina under compression is a process of grain fracturing and re-bonding. This emphasizes the significance of pressure on the reduction in the sintering temperature of high melting point ceramics. It also helps to improve the preparation of bulk ceramic materials (such as alumina, spinel, YAG, WC, cBN, diamond, etc.) to obtain excellent mechanical properties (hardness, 4
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fracture toughness, etc.) or high transparency. It is distinctly different from the cold bonding of metal, and thus offers a new direction in the research of deformation of ceramics under high pressure.
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