- Email: [email protected]
Strengthening of alumina ceramics under cold compression
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Strengthening of alumina ceramics under cold compression Fangming Liua,∗, Jiawei Zhangb, Pingping Liua, Qihuang Denga, Duanwei Heb,∗∗ a Chongqing Key Laboratory of Extraordinary Bond Engineering and Advance Materials Technology (EBEAM), School of Materials Science and Engineering, Yangtze Normal University, Chongqing, 408100, PR China b Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 Cold compression Strengthen Hardness Plastic deformation
High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al2O3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure.
1. Introduction High pressure can largely decrease the sintering temperature of high melting point ceramics like alumina [1–4], MgAL2O4 [5,6], cubic born nitride (cBN) [7–9] and even diamond [10,11]. Through high pressure sintering, nano-sized twinning or stacking faults were produced inside grains, and bulk ceramics with excellent mechanical and thermal properties can be obtained. While, The potential of pressure alone on the densification of ceramic was not shown to the fullest since the temperature was considered as an important process parameter in the sintering of ceramics [12]. Researchers had tried to fabricate densified ceramics by means of cold sintering (below 300 °C) process under external uniaxial pressure of 100–500 MPa [13–17]. During the sintering process, inorganic powder was compressed with the assistance of a transient liquid phase (like water) which is introduced to wet the graingrain interfaces to dissolve the sharp edges of the grains and realize mass transport. Although the obtained sample can reach a maximum density values approaching 100% theoretical [15], but it has poor contribution to the mechanical properties of the samples. Costa [18] successfully produced γ-Al2O3 bulks by compressing γ-Al2O3 powder with 13 nm grain size using a toroidal-type apparatus at 4.5 GPa and room temperature. The obtained translucent sample with a relative density of 80%, yields a vickers microhardness of 4.5 GPa at an applied load of 0.1 kgf. Recently, Wollmershauser [19] obtained nano-crystalline transparent spinel by compressing 30 nm MgAl2O4 powder under
∗
5 GPa after vacuum treatment. The sample yields a Vickers hardness of 182HV200, which greater than mild steel but poorly compared to the normal high temperature sintered bulks. The high pressure suppressed grain growth but failed to improve the mechanical properties of cold sintered samples. In this work, we investigated the influence of pressure alone on the densification of alumina particles. Densified alumina bulks with unexpected hardness were fabricated by compressing submicron alumina powder under ultra-high pressure at room temperature. The microstructure and hardness of the samples imply that the applied high pressure can refine grains and produce micro-structures inner the grains at room temperature, which contribute to grain boundary strengthening and enhancement of hardness of the alumina ceramics. 2. Experimental procedure Cold compressing experiments of alumina powder were carried in a two-stage large volume multi-anvil apparatus based on the DS6 × 8 MN cubic press with a standard 10/4 assembly [20,21]. Eight tungsten carbide cubes with 4 mm edge length corner truncations were used as the second stage anvils to generate pressure, and the semi-sintered octahedral magnesia with 10 mm edge length was served as the pressure transmitting medium. Fig. 1 shows the diagram of the pressure assembly for cold compression. The diamond discs were used as thirdstage anvils and the sample chamber pressure was estimated by the well
Corresponding author. No. 16 Juxian Road, Fuling District, Chongqing City, 408100, China. Corresponding author. No. 24 South Section, First Ring Road, Chengdu City, Sichuan Province, 610065, China. E-mail addresses: [email protected] (F. Liu), [email protected] (D. He).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.257 Received 15 August 2019; Received in revised form 19 September 2019; Accepted 26 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Fangming Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.257
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 1. The schematic illustration of the pressure assembly for cold compression of alumina powder.
known pressure-induced phase transitions of ZnTe, ZnS, GaAs [22]. The α-Al2O3 powder (0.25 μm, Shanghai Aladdin Biochemical Technology Co., Ltd.) was firstly dried in vacuum furnace (~1.5 × 10−3 Pa, 600 °C) for 1 h to get rid of impure gases, and then packaged with Mo foil and pre-compressed into discs with relative density about 65%. During the experiment, the green bodies were compressed to 9–18 GPa in 3–6 h at room temperature, and then decreased to room pressure in 8–12 h after maintaining for 6 h. The recovered samples were firstly treated by aqua regia to remove Mo package for further X-ray diffraction (XRD) tests, and then polished on both sides for hardness tests. The samples after polishing treatment were elliptical discs with about 2.0 mm in diameter and 0.8 mm in thickness. The XRD (DX2500, Cu Kα, λ = 0.15406 nm) tests were carried out to analysis the composition of starting material and the phase of the samples was checked by the micro focus X-ray diffractometer (SmartLab 9 KW) with Cu Kβ radiation (λ = 0.154 nm) with same test parameters. The hardness of samples was measured by a Vickers hardness tester (FV-700/B). And the fracture toughness (KIC) of the samples was determined by using Anstis' expression based on Vickers indentation crack length [23]. Relative density of samples were determined by Archimede's method. Microstructures of samples were investigated by scanning electron microscope (SEM, FEI Inspect F50) and transmission electron microscope (TEM, JEOL2100F).
Fig. 2. XRD patterns of starting powder and recovered samples after cold compression. (a) Starting powder. (b) Cold compressed at 9 GPa. (c) Cold compressed at 15 GPa. (d) Cold compressed at 18 GPa. The inset image shows enlarged broadening peaks of (012), (104), (110) and (113) of samples treated at 9 GPa and 18 GPa.
at 18 GPa (Fig. 3d), most of the pores was excluded due to plastic deformation of individual grains, and the obtained translucent bulk alumina has a relative density of about 98.9%. Interestingly, there is no apparent further refinement in particle size showing in the SEM images of fracture surface of samples treated under 15 GPa and 18 GPa, implying that grain refinement mainly occurred at lower pressure. So, the further broadening of X-ray diffraction peaks of samples under higher pressure was manly attributed to strain broadening. Fig. 4 illustrates the SEM images of polished surface of the samples decompressed at various pressures. As is shown in Fig. 4a, large size pores exist in the sample treated under 9 GPa which contributes to the poor relative density. With the pressure increased to 15 GPa (Fig. 4b), the apparent pores were largely eliminated through plastic deformation of grains which obviously enhanced the densification of the sample. The grain boundary of the sample became blurry, indicating that grains were well bonded. While individual small grains can still be distinguished as is shown in the white dotted line area in Fig. 4b. When increasing the pressure to 18 GPa (Fig. 4c), the sample shows similar microstructure to Fig. 4b, and pores exist in the sample were further eliminated. The microstructure evolution of the samples in Fig. 4 is in consistent with the result in Fig. 3, which illustrates the effect of pressure alone on the densification process of samples. TEM tests were carried out to further analyze the microstructures of the cold compressed sample. Fig. 5 presents the TEM and High-Resolution TEM (HRTEM) images of starting alumina powder and sample cold compressed under 18 GPa. The initial grains in Fig. 5a shows a particle size of about 300 nm which is in consistent with the observations in SEM in Fig. 3a. Few nanometers grains exists in the starting powder as is shown in the circular dashed line in Fig. 5a. The HRTEM image (Fig. 5b) of the grain shows no obvious crystal defects, implying that the initial particles were well crystallized. Fig. 5c illustrated TEM image of sample treated under 18 GPa at room temperature. The grain boundaries among individual large particles marked as black dashed and solid line square in Fig. 5c were in amorphous structure or composed of well bonded nano-structured grains as is shown in Fig. 5d and f, respectively. The mechanism of grain refinement should be as follows: During the compression, the boundary stress generated by contact of particles is very high at the beginning of compaction. The particles will be firstly rearranged through grain rotation or sliding. At this stage, destruction and plastic deformation at the particle contacts might happened at the particle contacts. This process eliminated large size
3. Results and discussion The XRD patterns of starting alumina powder and recovered samples are shown in Fig. 2. The obvious peak broadening in the high pressure treated samples should be attributed to simultaneous small grain size and strain [24,25]. Under ultra-high pressure, grain refinement and the common sources of strain like dislocations, stacking faults, twinning, microstresses, etc. can be easy produced [25]. As the pressure increases, the broadening of the diffraction peaks is more pronounced as is shown in the inset image in Fig. 2. Note that, the diffraction peaks of the samples in Fig. 2 show unsymmetrical broadening with right shoulders, so we considered that the shell of grains should be compressed [26,27]. Fig. 3 presents SEM images of starting powder and samples compressed at various pressures. The particles size of starting powder illustrated in Fig. 3a is about 300 nm. When applying pressure increased to 9 GPa, the appearance of nano-sized particles is the result of grain refinement as is shown in Fig. 3b. This phenomenon is in consistent with the result of XRD analysis i.e. small grain size can contribute to diffraction peak broadening [24,25]. While, many large sized pores existed in this sample (arrows in Fig. 3b) which is responsible for the poor relative density of about 96.2%. With the pressure increasing to 15 GPa (Fig. 3c), the relative density of the sample reaches to 98.1% owing to the reduction of pores. When the sample was cold compressed 2
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 3. SEM images of starting powder and samples treated at various pressures. (a) Starting powder. (b) Cold compressed at 9 GPa. (c) Cold compressed at 15 GPa. (d) Cold compressed at 18 GPa. The inset image shows the polished translucent sample.
deformation provides contact and adhesion between clean surfaces in the cold welding process of particles [28,29]. The above results were in agreement with the XRD and SEM analysis, and should be responsible for the diffraction peak broadening and densification of alumina samples under cold compression. The Vickers hardness of cold compressed samples as a function of applied load is shown in Fig. 6. The sample treated under 9 GPa yields a poor Vickers hardness of 7.5 GPa at an applied load of 29.4 N. With the pressure increasing to 15 GPa, the improvement of the hardness is mainly due to the reduction of pores and enhancement of bonding at grain boundaries. When the sample was treated at 18 GPa, the Vickers hardness further increased to an unexpected value of 13 GPa at an applied of 29.4 N which is comparable to the coarse particles sintered samples [4], but inferior to those sintered with submicron grains at the similar loads [1,3,30,31]. And the calculated fracture toughness of the sample is about 3.4 MPa m1/2, which is inferior to that as purchased polycrystalline Al2O3 (3.9 MPa m1/2), but superior to alumina single
pores in the green bodies. Then, with the increasing punch pressure, grain destruction and plastic deformation dominated the densification process owing to the ultra-high boundary stress. And the fragmentation of alumina particles at this stage was mainly at grain edges and corners because there is no obvious changes in the size of individual large grains comparing with initial powder as is shown in Fig. 5c. When the contact area among particles grows to a critical size, the plastic deformation and destruction ceased because the boundary stress was exceeded by the yield stress. Thus individual large grains were packaged by fractured nano-particles and further grain refinement (maily transgranular fracture) requires an ultra-higher pressure. So, the higher pressures in this experiment were mainly responsible for the densification of samples and the bonding of fracture grains. The grain boundaries equipped with nano-structured grains was responsible for the improvement of sample strength. In addition, pressure induced lattice distortion (or stacking faults) occurred inner individual grains (white solid line square in Fig. 5c) as is shown in Fig. 5f. These plastic
Fig. 4. SEM images of polished surface of samples treated at various pressures. (a) Cold compressed at 9 GPa. (b) Cold compressed at 15 GPa. (c) Cold compressed at 18 GPa. 3
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 5. TEM images of starting alumina powder and sample compressed under 18 GPa at room temperature. (a) TEM image of starting alumina powder, circular dashed line shows nano-particles. (b) HRTEM image of starting alumina particle. (c) TEM image of sample treated under 18 GPa. (d) HRTEM image of the black solid line square in (c), white parallel lines represent crystal direction and white dotted line shows amorphous structure. (e) HRTEM image of the black dashed line square in (c), white parallel lines represent crystal direction and white dotted line shows amorphous structure. (f) HRTEM image of the white solid line square in (c).
crystal (2.1 MPa m1/2 for sapphire) [23], and alumina compactions with high hardness (Hv = 25.3 GPa, KIC = 2.9 MPa m1/2) [3]. We considered that the modest fracture toughness of the sample is owing to the well bonded nano-particles among individual large grains. As is known to us that the yield strength of alumina is about 15 GPa at room temperature [32–34]. So, when applied pressure exceeded the strength of alumina, the initial of plastic deformation will maximum decrease the pores among particles and enhance hardness of the sample since the hardness depends strongly on plastic deformation [35]. Besides, ultra-high pressure crushed alumina particles to nanosized grains which could be considered as the cement to bond individual large grains, thus combining plastic deformation of grains to strengthen the samples.
4. Conclusions In summary, translucent alumina bulks with Vickers hardness of 13 GPa (29.4 N load) and relative density of 98.9% can be fabricated by compressing sub-micron alumina grains under 18 GPa and room temperature. The microstructure and hardness analysis reveal that ultrahigh pressure can refine the alumina grains during the cold compression process, and promote plastic deformation of individual grains when the applied pressure exceeds the yield strength of alumina. The refined grains can be used as cement to bond coarse particles to strengthen grain boundary under higher pressure, thus to effectively reduce the voids and strengthen the sample. The results illustrate the uniqueness of pressure in the densification of alumina powder at room temperature, which is important for the study of the sintering behavior 4
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
Fig. 6. Vickers hardness of cold compressed samples as a function of applied load. Inset image shows the Vickers indentations of the sample treated under 18 GPa.
[17]
of ceramics under high pressure and the preparation process of highperformance ceramics.
[18]
Declaration of competing interest
[19]
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. No conflict of interest exists in the submission and the manuscript has been approved by all authors for publication. We would like to declare that the work described was original research that has not been published or under consideration for publication elsewhere, in whole or in part.
[20]
[21]
[22]
Acknowledgements [23]
This work was supported by the Projects of Fuling District Science and Technology Commission (FLKJ, 2018BBA3060), the Project of Yangtze Normal University (2017KYQD131), and the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN201801412 and KJQN201901413).
[24]
References
[26]
[25]
[1] R.S. Mishra, C.E. Lesher, A.K. Mukherjee, High-pressure sintering of nanocrystalline γ-Al2O3, J. Am. Ceram. Soc. 79 (1996) 2989–2992 https://doi.org/10.1111/j.11512916.1996.tb08741.x. [2] N. Kuskonmaz, N. Can, A. Can, L. Sigalas, Sintering behaviour of nano-crystalline γAl2O3 powder without additives at 2-7 GPa, Ceram. Int. 37 (2011) 437–442 https:// doi.org/10.1016/j.ceramint.2010.09.026. [3] 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, Scr. Mater. 69 (2013) 362–365 https://doi.org/10.1016/j.scriptamat.2013.05.017. [4] F.M. Liu, D.W. He, Q. Wang, W. Ding, J. Liu, Y.J. Liu, Bimodal transparent alumina ceramics prepared with micro/nano-particles under high pressure, Scr. Mater. 122 (2016) 54–58 https://doi.org/10.1016/j.scriptamat.2016.05.017. [5] T.C. Lu, X.H. Chang, J.Q. Qi, X.J. Luo, Q.M. Wei, S. Zhu, K. Sun, J. Lian, L.M. Wang, Low-temperature high-pressure preparation of transparent nanocrystalline MgAl2O4 ceramics, Appl. Phys. Lett. 88 (2006) 213120–213121 3 https://doi.org/ 10.1063/1.2207571. [6] Y.T. Zou, D.W. He, X.K. Wei, R.C. Yu, T.C. Lu, X.H. Chang, S.M. Wang, L. Lei, Nanosintering mechanism of MgAl2O4 transparent ceramics under high pressure, Mater. Chem. Phys. 123 (2010) 529–533 https://doi.org/10.1016/j.matchemphys. 2010.05.009. [7] G.D. Liu, Z.L. Kou, X.Z. Yan, L. Lei, F. Peng, Q.M. Wang, K.X. Wang, P. Wang, L. Li, Y. Li, W.T. Li, Y.H. Wang, Y. Bi, Y. Leng, D.W. He, Submicron cubic boron nitride as hard as diamond, Appl. Phys. Lett. 106 (2015) 121901-1-4 https://doi.org/10. 1063/1.4915253. [8] V.L. Solozhenko, O.O. Kurakevych, Y. Le Godec, Creation of nanostuctures by extreme conditions: high-pressure synthesis of ultrahard nanocrystalline cubic boron
[27]
[28] [29] [30]
[31]
[32]
[33] [34] [35]
5
nitride, Adv. Mater. 24 (2012) 1540–1544 https://doi.org/10.1002/adma. 201104361. Y.J. Tian, B. Xu, D.L. Yu, Y.M. Ma, Y.B. Wang, Y.B. Jiang, W.T. Hu, C.C. Tang, Y.F. Gao, K. Luo, Z.S. Zhao, L.M. Wang, B. Wen, J.L. He, Z.Y. Liu, Ultrahard nanotwinned cubic boron nitride, Nature 493 (2013) 385–388 https://doi.org/10. 1038/nature11728. Q. Huang, D.L. Yu, B. Xu, W.T. Hu, Y.M. Ma, Y.B. Wang, Z.S. Zhao, B. Wen, J.L. He, Z.Y. Liu, Y.J. Tian, Nanotwinned diamond with unprecedented hardness and stability, Nature 510 (2014) 250–253, https://doi.org/10.1038/nature13381. J. Liu, G.D. Zhan, Q. Wang, X.Z. Yan, F.M. Liu, P. Wang, L. Lei, F. Peng, Z.L. Kou, D.W. He, Superstrong micro-grained polycrystalline diamond compact through work hardening under high pressure, Appl. Phys. Lett. 112 (2018) 061901-1-5 https://doi.org/10.1063/1.5016110. M.N. Rahaman, Sintering of Ceramics, CRC Press Taylor & Francis Group, Boca Raton, 2008. H.Z. Guo, A. Baker, J. Guo, C.A. Randall, Protocol for ultralow-temperature ceramic sintering: an integration of nanotechnology and the cold sintering process, ACS Nano 10 (2016) 10606–10614 https://doi.org/10.1021/acsnano.6b03800. J.P. Maria, X. Kang, R.D. Floyd, E.C. Dickey, Cold sintering: current status and prospects, J. Mater. Res. 32 (2017) 3205–3218 https://doi.org/10.1557/jmr.2017. 262. S. Funahashi, J. Guo, H.Z. Guo, K. Wang, A.L. Baker, K. Shiratsuyu, C.A. Randall, Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics, J. Am. Ceram. Soc. 100 (2017) 546–553 https://doi.org/ 10.1111/jace.14617. J.P. Ma, X.M. Chen, W.Q. Ouyang, J. Wang, H. Li, J.L. Fang, Microstructure, dielectric, and energy storage properties of BaTiO3 ceramics prepared via cold sintering, Ceram. Int. 44 (2018) 4436–4441 https://doi.org/10.1016/j.ceramint.2017. 12.044. X.P. Jiang, G.S. Zhu, H.R. Xu, L. Dong, J.J. Song, X.Y. Zhang, Y.Y. Zhao, D.L. Yan, A. Yu, Preparation of high density ZnO ceramics by the cold sintering process, Ceram. Int. 45 (2019) 17382–17386 https://doi.org/10.1016/j.ceramint.2019.05. 298. T.M.H. Costa, M.R. Gallas, E.V. Benvenutti, J.A.H. da Jornada, Study of nanocrystalline γ-Al2O3 produced by high-pressure compaction, J. Phys. Chem. B 103 (1999) 4278–4284 https://doi.org/10.1021/jp983791o. J.A. Wollmershauser, B.N. Feigelson, S.B. Qadri, G.R. Villalobos, M. Hunt, M.A. Imam, J.S. Sanghera, Transparent nanocrystalline spinel by room temperature high-pressure compaction, Scr. Mater. 69 (2013) 334–337 https://doi.org/10. 1016/j.scriptamat.2013.05.014. F. L Wang, D.W. He, L.M. Fang, X.F. Chen, Y.J. Li, W. Zhang, J. Zhang, A.L. Kou, F. Peng, Design and assembly of split-sphere high pressure apparatus based on the hinge-type cubic-anvil press, Acta Phys. Sin. 57 (2008) 5429–5434 [in Chinese] https://doi.org/10.7498/aps.57.5429. W.D. Wang, D.W. He, H.K. Wang, F.L. Wang, H.N. Dong, H.H. Chen, Z.Y. Li, J. Zhang, S.M. Wang, Z.L. Kou, F. Peng, Research on pressure generation efficiency of 6-8 type multianvil high pressure apparatus, Acta Phys. Sin. 59 (2009) 3107–3115 [in Chinese] https://doi.org/10.7498/aps.59.3107. A. Onodera, A. Ohtani, Fixed points for pressure calibration above 100 kbars related to semiconductor‐metal transitions, J. Appl. Phys. 51 (1980) 2581–2585 https:// doi.org/10.1063/1.327984. G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshalla, Critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538 https://doi.org/10.1111/j.11512916.1981.tb10320.x. G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22–31 https://doi.org/10.1016/0001-6160(53) 90006-6. T. Ungár, Microstructural parameters from X-ray diffraction peak broadening, Scr. Mater. 51 (2004) 777–781 https://doi.org/10.1016/j.scriptamat.2004.05.007. B. Palosz, E. Grzanka, S. Gierlotka, S. Stel'makh, R. Pielaszek, U. Bismayer, J. Neuefeind, H.P. Weber, W. Palosz, Diffraction studies of nanocrystals: theory and experiment, Acta Phys. Pol., A 102 (2002) 57–82 https://doi:10.12693/APhysPolA. 102.57. S.M. Wang, D.W. He, Y.T. Zou, J.J. Wei, L. Lei, Y.J. Li, J.H. Wang, W.D. Wang, Z.L. Kou, High-pressure and high-temperature sintering of nanostructured bulk NiAl materials, J. Mater. Res. 24 (2009) 2089–2096 https://doi:10.1557/JMR.2009. 0227. W.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, London, 1964. E.Y. Gutmanas, A. Rabinkin, M. Roitberg, Cold sintering under high pressure, Scr. Metall. 13 (1979) 11–15 https://doi.org/10.1016/0036-9748(79)90380-6. R. Apetz, M.P.B. Van Bruggen, Transparent alumina: a light-scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486 https://doi.org/10.1111/j.1151-2916.2003. tb03325.x. S. Ghanizadeh, S. Grasso, P. Ramanujam, B. Vaidhyanathan, J. Binner, P. Brown, J. Goldwasser, Improved transparency and hardness in α-alumina ceramics fabricated by high-pressure SPS of nanopowders, Ceram. Int. 43 (2017) 275–281 https://doi.org/10.1016/j.ceramint.2016.09.150. T.J. Ahrens, W.H. Gust, E.B. Royce, Material strength effect in the shock compression of alumina, J. Appl. Phys. 39 (1968) 4610–4616 https://doi.org/10.1063/ 1.1655810. D.E. Munson, R.J. Lawrence, Dynamic deformation of polycrystalline alumina, J. Appl. Phys. 50 (1979) 6272–6282 https://doi.org/10.1063/1.325766. C. Meade, R. Jeanloz, Yield strength of Al2O3 at high pressures, Phys. Rev. B. 42 (1990) 2532–2535 https://doi.org/10.1103/PhysRevB.42.2532. J. Haines, J.M. Leger, G. Bocquillon, Synthesis and design of superhard materials, Annu. Rev. Mater. Res. 31 (2001) 1–23 https://doi.org/10.1146/annurev.matsci. 31.1.1.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Strengthening of alumina ceramics under cold compression Fangming Liua,∗, Jiawei Zhangb, Pingping Liua, Qihuang Denga, Duanwei Heb,∗∗ a Chongqing Key Laboratory of Extraordinary Bond Engineering and Advance Materials Technology (EBEAM), School of Materials Science and Engineering, Yangtze Normal University, Chongqing, 408100, PR China b Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 Cold compression Strengthen Hardness Plastic deformation
High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al2O3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure.
1. Introduction High pressure can largely decrease the sintering temperature of high melting point ceramics like alumina [1–4], MgAL2O4 [5,6], cubic born nitride (cBN) [7–9] and even diamond [10,11]. Through high pressure sintering, nano-sized twinning or stacking faults were produced inside grains, and bulk ceramics with excellent mechanical and thermal properties can be obtained. While, The potential of pressure alone on the densification of ceramic was not shown to the fullest since the temperature was considered as an important process parameter in the sintering of ceramics [12]. Researchers had tried to fabricate densified ceramics by means of cold sintering (below 300 °C) process under external uniaxial pressure of 100–500 MPa [13–17]. During the sintering process, inorganic powder was compressed with the assistance of a transient liquid phase (like water) which is introduced to wet the graingrain interfaces to dissolve the sharp edges of the grains and realize mass transport. Although the obtained sample can reach a maximum density values approaching 100% theoretical [15], but it has poor contribution to the mechanical properties of the samples. Costa [18] successfully produced γ-Al2O3 bulks by compressing γ-Al2O3 powder with 13 nm grain size using a toroidal-type apparatus at 4.5 GPa and room temperature. The obtained translucent sample with a relative density of 80%, yields a vickers microhardness of 4.5 GPa at an applied load of 0.1 kgf. Recently, Wollmershauser [19] obtained nano-crystalline transparent spinel by compressing 30 nm MgAl2O4 powder under
∗
5 GPa after vacuum treatment. The sample yields a Vickers hardness of 182HV200, which greater than mild steel but poorly compared to the normal high temperature sintered bulks. The high pressure suppressed grain growth but failed to improve the mechanical properties of cold sintered samples. In this work, we investigated the influence of pressure alone on the densification of alumina particles. Densified alumina bulks with unexpected hardness were fabricated by compressing submicron alumina powder under ultra-high pressure at room temperature. The microstructure and hardness of the samples imply that the applied high pressure can refine grains and produce micro-structures inner the grains at room temperature, which contribute to grain boundary strengthening and enhancement of hardness of the alumina ceramics. 2. Experimental procedure Cold compressing experiments of alumina powder were carried in a two-stage large volume multi-anvil apparatus based on the DS6 × 8 MN cubic press with a standard 10/4 assembly [20,21]. Eight tungsten carbide cubes with 4 mm edge length corner truncations were used as the second stage anvils to generate pressure, and the semi-sintered octahedral magnesia with 10 mm edge length was served as the pressure transmitting medium. Fig. 1 shows the diagram of the pressure assembly for cold compression. The diamond discs were used as thirdstage anvils and the sample chamber pressure was estimated by the well
Corresponding author. No. 16 Juxian Road, Fuling District, Chongqing City, 408100, China. Corresponding author. No. 24 South Section, First Ring Road, Chengdu City, Sichuan Province, 610065, China. E-mail addresses: [email protected] (F. Liu), [email protected] (D. He).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.257 Received 15 August 2019; Received in revised form 19 September 2019; Accepted 26 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Fangming Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.257
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 1. The schematic illustration of the pressure assembly for cold compression of alumina powder.
known pressure-induced phase transitions of ZnTe, ZnS, GaAs [22]. The α-Al2O3 powder (0.25 μm, Shanghai Aladdin Biochemical Technology Co., Ltd.) was firstly dried in vacuum furnace (~1.5 × 10−3 Pa, 600 °C) for 1 h to get rid of impure gases, and then packaged with Mo foil and pre-compressed into discs with relative density about 65%. During the experiment, the green bodies were compressed to 9–18 GPa in 3–6 h at room temperature, and then decreased to room pressure in 8–12 h after maintaining for 6 h. The recovered samples were firstly treated by aqua regia to remove Mo package for further X-ray diffraction (XRD) tests, and then polished on both sides for hardness tests. The samples after polishing treatment were elliptical discs with about 2.0 mm in diameter and 0.8 mm in thickness. The XRD (DX2500, Cu Kα, λ = 0.15406 nm) tests were carried out to analysis the composition of starting material and the phase of the samples was checked by the micro focus X-ray diffractometer (SmartLab 9 KW) with Cu Kβ radiation (λ = 0.154 nm) with same test parameters. The hardness of samples was measured by a Vickers hardness tester (FV-700/B). And the fracture toughness (KIC) of the samples was determined by using Anstis' expression based on Vickers indentation crack length [23]. Relative density of samples were determined by Archimede's method. Microstructures of samples were investigated by scanning electron microscope (SEM, FEI Inspect F50) and transmission electron microscope (TEM, JEOL2100F).
Fig. 2. XRD patterns of starting powder and recovered samples after cold compression. (a) Starting powder. (b) Cold compressed at 9 GPa. (c) Cold compressed at 15 GPa. (d) Cold compressed at 18 GPa. The inset image shows enlarged broadening peaks of (012), (104), (110) and (113) of samples treated at 9 GPa and 18 GPa.
at 18 GPa (Fig. 3d), most of the pores was excluded due to plastic deformation of individual grains, and the obtained translucent bulk alumina has a relative density of about 98.9%. Interestingly, there is no apparent further refinement in particle size showing in the SEM images of fracture surface of samples treated under 15 GPa and 18 GPa, implying that grain refinement mainly occurred at lower pressure. So, the further broadening of X-ray diffraction peaks of samples under higher pressure was manly attributed to strain broadening. Fig. 4 illustrates the SEM images of polished surface of the samples decompressed at various pressures. As is shown in Fig. 4a, large size pores exist in the sample treated under 9 GPa which contributes to the poor relative density. With the pressure increased to 15 GPa (Fig. 4b), the apparent pores were largely eliminated through plastic deformation of grains which obviously enhanced the densification of the sample. The grain boundary of the sample became blurry, indicating that grains were well bonded. While individual small grains can still be distinguished as is shown in the white dotted line area in Fig. 4b. When increasing the pressure to 18 GPa (Fig. 4c), the sample shows similar microstructure to Fig. 4b, and pores exist in the sample were further eliminated. The microstructure evolution of the samples in Fig. 4 is in consistent with the result in Fig. 3, which illustrates the effect of pressure alone on the densification process of samples. TEM tests were carried out to further analyze the microstructures of the cold compressed sample. Fig. 5 presents the TEM and High-Resolution TEM (HRTEM) images of starting alumina powder and sample cold compressed under 18 GPa. The initial grains in Fig. 5a shows a particle size of about 300 nm which is in consistent with the observations in SEM in Fig. 3a. Few nanometers grains exists in the starting powder as is shown in the circular dashed line in Fig. 5a. The HRTEM image (Fig. 5b) of the grain shows no obvious crystal defects, implying that the initial particles were well crystallized. Fig. 5c illustrated TEM image of sample treated under 18 GPa at room temperature. The grain boundaries among individual large particles marked as black dashed and solid line square in Fig. 5c were in amorphous structure or composed of well bonded nano-structured grains as is shown in Fig. 5d and f, respectively. The mechanism of grain refinement should be as follows: During the compression, the boundary stress generated by contact of particles is very high at the beginning of compaction. The particles will be firstly rearranged through grain rotation or sliding. At this stage, destruction and plastic deformation at the particle contacts might happened at the particle contacts. This process eliminated large size
3. Results and discussion The XRD patterns of starting alumina powder and recovered samples are shown in Fig. 2. The obvious peak broadening in the high pressure treated samples should be attributed to simultaneous small grain size and strain [24,25]. Under ultra-high pressure, grain refinement and the common sources of strain like dislocations, stacking faults, twinning, microstresses, etc. can be easy produced [25]. As the pressure increases, the broadening of the diffraction peaks is more pronounced as is shown in the inset image in Fig. 2. Note that, the diffraction peaks of the samples in Fig. 2 show unsymmetrical broadening with right shoulders, so we considered that the shell of grains should be compressed [26,27]. Fig. 3 presents SEM images of starting powder and samples compressed at various pressures. The particles size of starting powder illustrated in Fig. 3a is about 300 nm. When applying pressure increased to 9 GPa, the appearance of nano-sized particles is the result of grain refinement as is shown in Fig. 3b. This phenomenon is in consistent with the result of XRD analysis i.e. small grain size can contribute to diffraction peak broadening [24,25]. While, many large sized pores existed in this sample (arrows in Fig. 3b) which is responsible for the poor relative density of about 96.2%. With the pressure increasing to 15 GPa (Fig. 3c), the relative density of the sample reaches to 98.1% owing to the reduction of pores. When the sample was cold compressed 2
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 3. SEM images of starting powder and samples treated at various pressures. (a) Starting powder. (b) Cold compressed at 9 GPa. (c) Cold compressed at 15 GPa. (d) Cold compressed at 18 GPa. The inset image shows the polished translucent sample.
deformation provides contact and adhesion between clean surfaces in the cold welding process of particles [28,29]. The above results were in agreement with the XRD and SEM analysis, and should be responsible for the diffraction peak broadening and densification of alumina samples under cold compression. The Vickers hardness of cold compressed samples as a function of applied load is shown in Fig. 6. The sample treated under 9 GPa yields a poor Vickers hardness of 7.5 GPa at an applied load of 29.4 N. With the pressure increasing to 15 GPa, the improvement of the hardness is mainly due to the reduction of pores and enhancement of bonding at grain boundaries. When the sample was treated at 18 GPa, the Vickers hardness further increased to an unexpected value of 13 GPa at an applied of 29.4 N which is comparable to the coarse particles sintered samples [4], but inferior to those sintered with submicron grains at the similar loads [1,3,30,31]. And the calculated fracture toughness of the sample is about 3.4 MPa m1/2, which is inferior to that as purchased polycrystalline Al2O3 (3.9 MPa m1/2), but superior to alumina single
pores in the green bodies. Then, with the increasing punch pressure, grain destruction and plastic deformation dominated the densification process owing to the ultra-high boundary stress. And the fragmentation of alumina particles at this stage was mainly at grain edges and corners because there is no obvious changes in the size of individual large grains comparing with initial powder as is shown in Fig. 5c. When the contact area among particles grows to a critical size, the plastic deformation and destruction ceased because the boundary stress was exceeded by the yield stress. Thus individual large grains were packaged by fractured nano-particles and further grain refinement (maily transgranular fracture) requires an ultra-higher pressure. So, the higher pressures in this experiment were mainly responsible for the densification of samples and the bonding of fracture grains. The grain boundaries equipped with nano-structured grains was responsible for the improvement of sample strength. In addition, pressure induced lattice distortion (or stacking faults) occurred inner individual grains (white solid line square in Fig. 5c) as is shown in Fig. 5f. These plastic
Fig. 4. SEM images of polished surface of samples treated at various pressures. (a) Cold compressed at 9 GPa. (b) Cold compressed at 15 GPa. (c) Cold compressed at 18 GPa. 3
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
Fig. 5. TEM images of starting alumina powder and sample compressed under 18 GPa at room temperature. (a) TEM image of starting alumina powder, circular dashed line shows nano-particles. (b) HRTEM image of starting alumina particle. (c) TEM image of sample treated under 18 GPa. (d) HRTEM image of the black solid line square in (c), white parallel lines represent crystal direction and white dotted line shows amorphous structure. (e) HRTEM image of the black dashed line square in (c), white parallel lines represent crystal direction and white dotted line shows amorphous structure. (f) HRTEM image of the white solid line square in (c).
crystal (2.1 MPa m1/2 for sapphire) [23], and alumina compactions with high hardness (Hv = 25.3 GPa, KIC = 2.9 MPa m1/2) [3]. We considered that the modest fracture toughness of the sample is owing to the well bonded nano-particles among individual large grains. As is known to us that the yield strength of alumina is about 15 GPa at room temperature [32–34]. So, when applied pressure exceeded the strength of alumina, the initial of plastic deformation will maximum decrease the pores among particles and enhance hardness of the sample since the hardness depends strongly on plastic deformation [35]. Besides, ultra-high pressure crushed alumina particles to nanosized grains which could be considered as the cement to bond individual large grains, thus combining plastic deformation of grains to strengthen the samples.
4. Conclusions In summary, translucent alumina bulks with Vickers hardness of 13 GPa (29.4 N load) and relative density of 98.9% can be fabricated by compressing sub-micron alumina grains under 18 GPa and room temperature. The microstructure and hardness analysis reveal that ultrahigh pressure can refine the alumina grains during the cold compression process, and promote plastic deformation of individual grains when the applied pressure exceeds the yield strength of alumina. The refined grains can be used as cement to bond coarse particles to strengthen grain boundary under higher pressure, thus to effectively reduce the voids and strengthen the sample. The results illustrate the uniqueness of pressure in the densification of alumina powder at room temperature, which is important for the study of the sintering behavior 4
Ceramics International xxx (xxxx) xxx–xxx
F. Liu, et al.
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
Fig. 6. Vickers hardness of cold compressed samples as a function of applied load. Inset image shows the Vickers indentations of the sample treated under 18 GPa.
[17]
of ceramics under high pressure and the preparation process of highperformance ceramics.
[18]
Declaration of competing interest
[19]
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. No conflict of interest exists in the submission and the manuscript has been approved by all authors for publication. We would like to declare that the work described was original research that has not been published or under consideration for publication elsewhere, in whole or in part.
[20]
[21]
[22]
Acknowledgements [23]
This work was supported by the Projects of Fuling District Science and Technology Commission (FLKJ, 2018BBA3060), the Project of Yangtze Normal University (2017KYQD131), and the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN201801412 and KJQN201901413).
[24]
References
[26]
[25]
[1] R.S. Mishra, C.E. Lesher, A.K. Mukherjee, High-pressure sintering of nanocrystalline γ-Al2O3, J. Am. Ceram. Soc. 79 (1996) 2989–2992 https://doi.org/10.1111/j.11512916.1996.tb08741.x. [2] N. Kuskonmaz, N. Can, A. Can, L. Sigalas, Sintering behaviour of nano-crystalline γAl2O3 powder without additives at 2-7 GPa, Ceram. Int. 37 (2011) 437–442 https:// doi.org/10.1016/j.ceramint.2010.09.026. [3] 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, Scr. Mater. 69 (2013) 362–365 https://doi.org/10.1016/j.scriptamat.2013.05.017. [4] F.M. Liu, D.W. He, Q. Wang, W. Ding, J. Liu, Y.J. Liu, Bimodal transparent alumina ceramics prepared with micro/nano-particles under high pressure, Scr. Mater. 122 (2016) 54–58 https://doi.org/10.1016/j.scriptamat.2016.05.017. [5] T.C. Lu, X.H. Chang, J.Q. Qi, X.J. Luo, Q.M. Wei, S. Zhu, K. Sun, J. Lian, L.M. Wang, Low-temperature high-pressure preparation of transparent nanocrystalline MgAl2O4 ceramics, Appl. Phys. Lett. 88 (2006) 213120–213121 3 https://doi.org/ 10.1063/1.2207571. [6] Y.T. Zou, D.W. He, X.K. Wei, R.C. Yu, T.C. Lu, X.H. Chang, S.M. Wang, L. Lei, Nanosintering mechanism of MgAl2O4 transparent ceramics under high pressure, Mater. Chem. Phys. 123 (2010) 529–533 https://doi.org/10.1016/j.matchemphys. 2010.05.009. [7] G.D. Liu, Z.L. Kou, X.Z. Yan, L. Lei, F. Peng, Q.M. Wang, K.X. Wang, P. Wang, L. Li, Y. Li, W.T. Li, Y.H. Wang, Y. Bi, Y. Leng, D.W. He, Submicron cubic boron nitride as hard as diamond, Appl. Phys. Lett. 106 (2015) 121901-1-4 https://doi.org/10. 1063/1.4915253. [8] V.L. Solozhenko, O.O. Kurakevych, Y. Le Godec, Creation of nanostuctures by extreme conditions: high-pressure synthesis of ultrahard nanocrystalline cubic boron
[27]
[28] [29] [30]
[31]
[32]
[33] [34] [35]
5
nitride, Adv. Mater. 24 (2012) 1540–1544 https://doi.org/10.1002/adma. 201104361. Y.J. Tian, B. Xu, D.L. Yu, Y.M. Ma, Y.B. Wang, Y.B. Jiang, W.T. Hu, C.C. Tang, Y.F. Gao, K. Luo, Z.S. Zhao, L.M. Wang, B. Wen, J.L. He, Z.Y. Liu, Ultrahard nanotwinned cubic boron nitride, Nature 493 (2013) 385–388 https://doi.org/10. 1038/nature11728. Q. Huang, D.L. Yu, B. Xu, W.T. Hu, Y.M. Ma, Y.B. Wang, Z.S. Zhao, B. Wen, J.L. He, Z.Y. Liu, Y.J. Tian, Nanotwinned diamond with unprecedented hardness and stability, Nature 510 (2014) 250–253, https://doi.org/10.1038/nature13381. J. Liu, G.D. Zhan, Q. Wang, X.Z. Yan, F.M. Liu, P. Wang, L. Lei, F. Peng, Z.L. Kou, D.W. He, Superstrong micro-grained polycrystalline diamond compact through work hardening under high pressure, Appl. Phys. Lett. 112 (2018) 061901-1-5 https://doi.org/10.1063/1.5016110. M.N. Rahaman, Sintering of Ceramics, CRC Press Taylor & Francis Group, Boca Raton, 2008. H.Z. Guo, A. Baker, J. Guo, C.A. Randall, Protocol for ultralow-temperature ceramic sintering: an integration of nanotechnology and the cold sintering process, ACS Nano 10 (2016) 10606–10614 https://doi.org/10.1021/acsnano.6b03800. J.P. Maria, X. Kang, R.D. Floyd, E.C. Dickey, Cold sintering: current status and prospects, J. Mater. Res. 32 (2017) 3205–3218 https://doi.org/10.1557/jmr.2017. 262. S. Funahashi, J. Guo, H.Z. Guo, K. Wang, A.L. Baker, K. Shiratsuyu, C.A. Randall, Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics, J. Am. Ceram. Soc. 100 (2017) 546–553 https://doi.org/ 10.1111/jace.14617. J.P. Ma, X.M. Chen, W.Q. Ouyang, J. Wang, H. Li, J.L. Fang, Microstructure, dielectric, and energy storage properties of BaTiO3 ceramics prepared via cold sintering, Ceram. Int. 44 (2018) 4436–4441 https://doi.org/10.1016/j.ceramint.2017. 12.044. X.P. Jiang, G.S. Zhu, H.R. Xu, L. Dong, J.J. Song, X.Y. Zhang, Y.Y. Zhao, D.L. Yan, A. Yu, Preparation of high density ZnO ceramics by the cold sintering process, Ceram. Int. 45 (2019) 17382–17386 https://doi.org/10.1016/j.ceramint.2019.05. 298. T.M.H. Costa, M.R. Gallas, E.V. Benvenutti, J.A.H. da Jornada, Study of nanocrystalline γ-Al2O3 produced by high-pressure compaction, J. Phys. Chem. B 103 (1999) 4278–4284 https://doi.org/10.1021/jp983791o. J.A. Wollmershauser, B.N. Feigelson, S.B. Qadri, G.R. Villalobos, M. Hunt, M.A. Imam, J.S. Sanghera, Transparent nanocrystalline spinel by room temperature high-pressure compaction, Scr. Mater. 69 (2013) 334–337 https://doi.org/10. 1016/j.scriptamat.2013.05.014. F. L Wang, D.W. He, L.M. Fang, X.F. Chen, Y.J. Li, W. Zhang, J. Zhang, A.L. Kou, F. Peng, Design and assembly of split-sphere high pressure apparatus based on the hinge-type cubic-anvil press, Acta Phys. Sin. 57 (2008) 5429–5434 [in Chinese] https://doi.org/10.7498/aps.57.5429. W.D. Wang, D.W. He, H.K. Wang, F.L. Wang, H.N. Dong, H.H. Chen, Z.Y. Li, J. Zhang, S.M. Wang, Z.L. Kou, F. Peng, Research on pressure generation efficiency of 6-8 type multianvil high pressure apparatus, Acta Phys. Sin. 59 (2009) 3107–3115 [in Chinese] https://doi.org/10.7498/aps.59.3107. A. Onodera, A. Ohtani, Fixed points for pressure calibration above 100 kbars related to semiconductor‐metal transitions, J. Appl. Phys. 51 (1980) 2581–2585 https:// doi.org/10.1063/1.327984. G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshalla, Critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538 https://doi.org/10.1111/j.11512916.1981.tb10320.x. G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22–31 https://doi.org/10.1016/0001-6160(53) 90006-6. T. Ungár, Microstructural parameters from X-ray diffraction peak broadening, Scr. Mater. 51 (2004) 777–781 https://doi.org/10.1016/j.scriptamat.2004.05.007. B. Palosz, E. Grzanka, S. Gierlotka, S. Stel'makh, R. Pielaszek, U. Bismayer, J. Neuefeind, H.P. Weber, W. Palosz, Diffraction studies of nanocrystals: theory and experiment, Acta Phys. Pol., A 102 (2002) 57–82 https://doi:10.12693/APhysPolA. 102.57. S.M. Wang, D.W. He, Y.T. Zou, J.J. Wei, L. Lei, Y.J. Li, J.H. Wang, W.D. Wang, Z.L. Kou, High-pressure and high-temperature sintering of nanostructured bulk NiAl materials, J. Mater. Res. 24 (2009) 2089–2096 https://doi:10.1557/JMR.2009. 0227. W.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, London, 1964. E.Y. Gutmanas, A. Rabinkin, M. Roitberg, Cold sintering under high pressure, Scr. Metall. 13 (1979) 11–15 https://doi.org/10.1016/0036-9748(79)90380-6. R. Apetz, M.P.B. Van Bruggen, Transparent alumina: a light-scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486 https://doi.org/10.1111/j.1151-2916.2003. tb03325.x. S. Ghanizadeh, S. Grasso, P. Ramanujam, B. Vaidhyanathan, J. Binner, P. Brown, J. Goldwasser, Improved transparency and hardness in α-alumina ceramics fabricated by high-pressure SPS of nanopowders, Ceram. Int. 43 (2017) 275–281 https://doi.org/10.1016/j.ceramint.2016.09.150. T.J. Ahrens, W.H. Gust, E.B. Royce, Material strength effect in the shock compression of alumina, J. Appl. Phys. 39 (1968) 4610–4616 https://doi.org/10.1063/ 1.1655810. D.E. Munson, R.J. Lawrence, Dynamic deformation of polycrystalline alumina, J. Appl. Phys. 50 (1979) 6272–6282 https://doi.org/10.1063/1.325766. C. Meade, R. Jeanloz, Yield strength of Al2O3 at high pressures, Phys. Rev. B. 42 (1990) 2532–2535 https://doi.org/10.1103/PhysRevB.42.2532. J. Haines, J.M. Leger, G. Bocquillon, Synthesis and design of superhard materials, Annu. Rev. Mater. Res. 31 (2001) 1–23 https://doi.org/10.1146/annurev.matsci. 31.1.1.