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YAG nanoeutectic composite ceramics by a modified laser surface processing
Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society 34 (2014) 739–744
In situ fabrication of highly-dense Al2O3/YAG nanoeutectic composite ceramics by a modified laser surface processing Haijun Su a,b,∗ , Jun Zhang a , Weidan Ma a , Kaichen Wei a , Lin Liu a , Hengzhi Fu a , Shien-Ping Feng b , A.K. Soh c a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China b Department of Mechanical Engineering, The University of Hong Kong, Hong Kong c School of Engineering, Monash University Sunway Campus, Malaysia Received 11 September 2013; received in revised form 30 September 2013; accepted 1 October 2013 Available online 8 November 2013
Abstract Alumina-matrix eutectic in situ composite ceramics present excellent high-temperature mechanical properties, which have been considered as promising next-generation ultra-high temperature structural materials. A modified laser surface processing is developed to in situ fabricate highlydense Al2 O3 /YAG bulk nanoeutectic ceramics with large size and homogeneous three-dimensional network of nanoeutectic microstructure by introducing two-side remelting and high-temperature preheating. The crack and porosity are avoided, and the eutectic structure achieves a good continuous growth between two solidified layers. The eutectic phases show sharp interface bonding with a defined orientation relationship. The dislocations and crack deflection at high-density phase interfaces importantly contribute to the enhanced fracture toughness. © 2013 Elsevier Ltd. All rights reserved. Keywords: Rapid solidification; Composites; Laser surface processing; Ceramics; Fracture toughness
1. Introduction In order to meet the increase of inlet temperature of gas turbines and improve its thermal efficiency as well as reduce the pollution emissions such as CO2 or NOx , the development and design of new ultra-high temperature structural materials with superior high-temperature performance and longevity above 1600 ◦ C in oxidizing environment have been greatly focused on all over the world.1–3 Directionally solidified oxide eutectic ceramic composite (DSOEC), a newly developed ultra-high temperature structural material in recent few decades, has attracted extensive attention due to its excellent mechanical property at high temperatures.4–7 Thus, it has been considered as one of the most potential alternatives to current traditional superalloys or sintered ceramics applied in the aerospace field.
∗ Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China. Tel.: +86 29 88492228; fax: +86 29 88494080. E-mail addresses: [email protected], [email protected] (H. Su).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.014
In DSOEC, it is known that the eutectic phases can be simultaneously grown from melt, and the well-aligned growth arrays can be produced by controlling thermal flow direction. The directionally solidified eutectics not only combine the unique characteristic of each component material but also enhance the phase interface bonding, so that greatly improve the properties of the composite.8,9 Of its numerous exciting properties, the one material property that stands out is DSOEC’s outstanding high temperature strength stability from room temperature up to high temperatures (near to its melting point). Especially, the DSOEC can availablely serve in the aggressive oxidizing environments without coating as compared with other non-oxide materials.4,10–13 Furthermore, highly homogenous and very fine microstructures based on eutectic structures having strongly bonded phases and high-density interphase areas can be obtained and tailored by rapid solidification or directional solidification technique with high temperature gradient, which can further improve the properties of materials.14–18 However, up to date, it is still very little that directional solidification or rapid solidification is applied to produce DSOEC composites in the industrial practice.4–16 One of the main reasons is that the conventional directional solidification technique,
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such as Bridgman method, can prepare large eutectic ceramic rods, but the preparation procedure needs expensive crucibles (high purity Mo or Ir) and the crucibles can easily react with ceramic melt, leading to the melt pollution and high preparation cost due to non-reuse of crucibles.4,5,10,11 Moreover, the eutectic interphase spacing is large (10–20 m) due to its low temperature gradient (<100 K/cm) and slow solidification rate, resulting in low production efficiency and reduced properties.6 Additionally, as noted, the laser floating zone remelting with high temperature gradient (>1000 K/cm) can produce eutectic ceramics with nanostructure and excellent properties, but it is only limited to small sample diameter (<1 cm).2,8,12 It is thus highly desirable to develop new preparation techniques for the oxide eutectic ceramic with large size, fine microstructure, high preparation efficiency and low cost to catch up its wide application. Laser surface zone remelting is recently developed as a rapid solidification technique to prepare oxide eutectic ceramic composites with advantages of ultra-fine microstructure, free of crucible and high solidification rate. However, it is only focused on small eutectic plates with hundreds of microns in thickness so far.19–22 Herein, we report a modified laser surface (MLS) processing to in situ prepare rapidly solidified Al2 O3 /YAG nanoeutectic ceramic rods having nearly full density and large size by introducing two-side remelting and high-temperature preheating. The homogenous microstructure and enhanced fracture toughness are obtained. The microstructure characteristic, interface structure, orientation relationship and toughening mechanism are investigated. 2. Experimental procedure High purity (>4 N) Al2 O3 and Y2 O3 nano-powders were homogeneously mixed according to the eutectic composition (82 mol% Al2 O3 /18 mol% Y2 O3 ) by wet ball milling. The mixed powders were die pressed to form rectangular precursor plates [65 mm × (6–10) mm × (3–5) mm] at room temperature and sintered at 1500 ◦ C for 2 h to improve the handing strength. The modified laser surface processing of the precursor plate was performed in vacuum by using a laser surface atmosphere melting furnace.16,22 A high-purity N2 gas was filled into the chamber with a flow rate of 5–10 L/min in order to prevent from the formation of porosity. Two superimposed precursor plates were firstly located on the laser worktable with a 5-axis and 4direction coupled motion, and then the back of the plates was preheated to 1200 ◦ C by the furnace for 30 min. After that, an incident CO2 laser with beam diameter of 10 mm and power of 200–600 W from the laser window on the furnace up-surface started to scan and melt the upper surface of the superimposed precursor samples along the sample axis with different rates. When the upper surface of the samples was completely remelted, the samples were overturned to restart the second remelting of the sample back surface, as schematically shown in Fig. 1. The laser scanning rate was 20–400 m/s dependent on the sample thickness. The density of MLS-processed specimens was measured by using Archimedes’s method. The microstructure was analysed
Fig. 1. Sketch diagram of the modified laser surface (MLS) processing technique to prepare the Al2 O3 /YAG bulk eutectic ceramics.
using scanning electron microscopy (SEM, JSM-5800), energy disperse spectroscopy (EDS, Link-Isis) and X-ray diffraction (XRD, Rigakumsg-158). The phase interface and orientation relationship were investigated by transmission electron microscopy (TEM, JEM-200CX) and high resolution transmission electron microscopy (HRTEM, JEM-2100F). The hardness and fracture toughness were determined by the Vickers indentation technique with load of 9.8 N for 15 s and an average value of 10 measurements was reported. The indentation size and crack length were measured by SEM. The hardness and fracture toughness were calculated according to the equation proposed by Antis23 as reported in previous literatures.24,25 3. Results and discussion Fig. 2 shows the macroscopic photograph of the rapidly solidified Al2 O3 /YAG bulk eutectic ceramic grown by the MLS-processing at the scanning rate of 20 m/s. The obtained eutectic ceramic presents an approximate rod shape due to the melt surface tension with the diameter up to 8 mm and the length of 60 mm. The diameter and size are almost 2–4 times larger
Fig. 2. The photograph of the grown Al2 O3 /YAG eutectic composite rod by the modified laser surface processing.
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Fig. 3. Typical microstructures of the cross-section of the MLS-processed Al2 O3 /YAG eutectic ceramics rod grown at the scanning rate of 40 m/s: (a) low magnification; (b) high magnification of second remelting layer; (c) high magnification of first remelting layer and (d) microstructure of the interface between two remelting layers. The inset of (d) is the high magnification of interface layer microstructure. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
than that of the eutectic ceramic rod availablely obtained by the melt growth method without using cubicle such as laser floating zone or laser surface remelting in the previous reports.3,8,19 It indicates that the MLS-processing can remarkably enhance the thickness of solidified layer and achieve the rapid preparation of large bulk eutectic ceramics by introducing the laser two-side remelting. The sample diameter is dependent on the precursor shape and size as well as laser energy density and scanning rate. During the MLS-processing, the eliminations of cracks due to rapid cooling or residual stress as well as the inhibition of gas porosity introduced during melting process are the keys of influencing the ceramic melt preparation. In our case, the MLS-processed eutectic ceramic rod displays glossy surface with white color, and no crack or porosity is found on the surface and inside of the sample. It demonstrates that no impurity is introduced, and the growth defects such as porosity and crack have been well avoided, which is contributed by the high temperature preheating and second two-side laser remelting for effectively relaxing thermal stress and healing cracks. The surface finish increases with the increase of scanning rates, but tiny cracks sometimes still appear when the scanning rate is larger than 200 m/s. The specimens exhibit a density of nearly 100% which is superior to the previously reported laser selective melted Al2 O3 /ZrO2 ceramic26 and spark plasma sintered oxide eutectic ceramic.27 There is an overall trend of decreasing
density with increasing scanning rate. When the laser scanning rate is over 400 m/s, the composite density tends to be below 99%. Fig. 3 shows the typical cross-section microstructures of the MLS-processed Al2 O3 /YAG eutectic ceramics rod grown at the scanning rate of 40 m/s. As shown in Fig. 3(a), the composite ceramic rods present dense and ultra-fine microstructure without cracks and defects. There is an obvious solidification interface layer between both the remelting layers. As seen, the microstructure from the outside to middle interface layer of the sample is very homogeneous, which is superior to the previously reported eutectic ceramics fabricated by the single side laser remelting method.3,19,25 The homogenous microstructure from top/bottom to middle is mainly ascribed to the superheating treatment of second remelting and to no defect (pore or impurity) disturbance in the solidification interface front during the melt growth process.18,28 The component phases, ␣-Al2 O3 and YAG, are identified by XRD and EDS, and no other crystalline phases or amorphous phases are detected. The magnified microstructures of the top and bottom of the rod are depicted in Fig. 3(b) and (c). The two phases show well-coupled eutectic growth with an interpenetrating three-dimension network nanostructure aligned along the scanning direction (red arrow in Fig. 3b). As seen in the yellow arrow in Fig. 3(c), the morphology is completely different from the common regular eutectic structure in metal alloys such
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Fig. 4. TEM micrograph showing the eutectic phase interface of the MLS-processed Al2 O3 /YAG eutectic rod (a) and the selected area electron diffraction patterns at the Al2 O3 /YAG phase interface (b).
as NiAl-Mo eutectic,29 indicating a complex irregular eutectic characteristic is mainly dominated by the branch growth of YAG phase due to its stronger faceted growth habit than Al2 O3 .3 The domain width of eutectic phases decreases with increasing the laser scanning rate, and it can be refined below 100 nm when the rate is over 400 m/s due to the high temperature gradient (∼104 K/cm) and large cooling rate (∼400 K/s) for laser zone remelting.20 During eutectic solidification, the nucleation undercooling T increases with cooling rate V, resulting in the decrease of eutectic spacing λ, namely, λ ∝ 1/V.29 The volume fractions of the eutectic phases determined by the SEM images are 44.6% and 55.4% for Al2 O3 and YAG respectively, which is in agreement with the theoretical calculation from the eutectic phase diagram. It is also in accordance with the volume fraction range of theoretical prediction for the irregular lamellar eutectic classification based on Hunt–Jackson theory.30,31 Moreover, by careful examination, it is found that the eutectic microstructure of the second remelting layer (Fig. 3b) is slightly finer than that of the first remelting layer (Fig. 3c), which of similar phenomena was also found in the eutectic ceramic by laser direct forming.16 It demonstrates that the two-side laser second superheating can effectively refine the eutectic spacing. In the interfacial transition zone, a coarse coupled band eutectic structure (Fig. 3d) with the width of about 40 m and strong faceted growth habit appears. The width of the band eutectic decreases with increasing the laser power and decreasing the laser scanning rate. Additionally, no sintered multicrystalline ceramic structure can be found in the interface zone, revealing that the two remelting layers successfully achieve the continuous eutectic growth by frequent branching (yellow arrows in Fig. 3d) of faceted Al2 O3 and YAG phases which produce complex irregular eutectic structure. During the growth of eutectic composites, the solidification process does not only affect the microstructure but also influences the preferred growth orientations.31,32 In directionally eutectics, the microstructures (lamellae or rods) usually are composed of single crystal phases preferentially growing along a defined crystallographic direction. Fig. 4 shows the phase interface TEM structure and electron diffraction pattern of selected area perpendicular to the rod axes of the MLS-processed Al2 O3 /YAG eutectic ceramic. It is clearly observed that the phase interface is clean and planar without intermediary phases detected, showing sharp morphology
with well interface matching. According to Bragg diffraction law, the electron diffraction analysis at the phase interface center (Fig. 4b) determines that the MLS-processed Al2 O3 /YAG eutectic shows well-defined crystallographic direc¯ YAG and tion and relationship: [1 0 1¯ 0] Al2 O3 //[1 0 1] (0 0 0 1)Al2 O3 //(1 2 1) YAG, which are similar to that of Mazerolles et al. by optical floating zone melting.32 Thus, the preferred crystallographic growth directions for Al2 O3 and YAG ¯ and [101], ¯ respecphase during the MLS-processing are [1010] tively. The minimum interfacial energy is produced aligned above directions and the configuration at the interface is also illustrated by the HRTEM image, as shown in Fig. 5. It is found that the arrays of atomic planes of both phases at interface are not corresponding but there is a crystal lattice misfit calculated to be 6.17% belonging to the half-coherent phase interface. The inverse Fourier transform at interface (inset of Fig. 5) further reveals that the accommodation between two phase structures is adjusted by the dislocations along the interface.
Fig. 5. The high-resolution transmission electron micrograph of the Al2 O3 /YAG phase interface. The inset is the inverse Fourier transform from the digitized image of the HRTEM interface.
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Fig. 6. The variations of mechanical properties of the MLS-processed Al2 O3 /YAG eutectic ceramic with the increase of laser scanning rate (a) and the crack propagation paths of the indention (b).
Fig. 6(a) shows the variations of typical mechanical properties with the increase of laser scanning rates for the MLS-processed Al2 O3 /YAG eutectic ceramics. The interaction between the fine microstructure and cracks nucleated from the corners of Vickers indentations was analysed by SEM. At least ten valid tests were carried out on each sample. The results are comparable to the previously reported eutectic ceramics determined using the indentation method. The indentation cracks belong to Median cracks determined by the polished methods.17 It is indicated that the hardness increases rapidly with increasing laser scanning rate in the low rate range, but decreases when the rate is over 100 m/s, because the high scanning rate causes the reduced thickness of solidified eutectics and generates more defects such as micro-cracks or pores. Although the fracture toughness shows an increase with the scanning rate, it almost keeps constant when the rate is over 100 m/s. The maximum hardness and fracture toughness obtained by the optimized laser scanning rate of 100 m/s are 18.7 GPa and 3.7 MPa m1/2 , respectively. The fracture toughness of eutectic is obviously higher than those of monocrystal Al2 O3 (2.4 MPa m1/2 ),33 YAG (3.0 MPa m1/2 )33 and Al2 O3 /YAG binary eutectic (2.0 MPa m1/2 ),24 and is close to that of Al2 O3 /ZrO2 binary eutectic (4.8 MPa m1/2 )34 measured also by using the indentation method. The enhanced mechanical properties are primarily attributed to refined micro/nanostructured eutectic microstructures on the one hand.3,32 The ceramic fracture is a process of crack propagation and growth, in which the cracks inevitably produce interaction with the interphases. In previous reports for the Al2 O3 /YAG binary eutectic,6,24 the straight indentation crack normally propagates directly across the Al2 O3 and YAG phases, leading to material fracture. As shown in the arrows in Fig. 6(b), for the MLS-processed Al2 O3 /YAG composite, the crack bridging and deflection at interface are clearly observed, indicating more fracture energy can be absorbed. The crack is finally stopped by the fine phase interface shown by red arrow in Fig. 7, and therefore the fracture toughness is effectively increased. The eutectic ceramic presents typical transgranular cracking propagation. Moreover, the considerable linear dislocations formed
Fig. 7. The TEM showing the dislocation distribution in the Al2 O3 /YAG eutectic phases.
in the Al2 O3 phase as compared with the YAG phase can also contribute to the toughness improvement,9 as shown in Fig. 7. 4. Conclusions Rapidly solidified Al2 O3 /YAG nanoeutectic ceramic rods with large size (diameter >6 mm) and nearly full density are produced by a modified laser surface processing method. The laser two-side surface remelting and high temperature preheating effectively enhance the sample size, avoid crack generation and achieve continuous growth between two solidified layers. The composite presents a homogeneous three-dimensional network with the irregular eutectic structure consisting of micro/nano structured Al2 O3 and YAG phases with a defined ¯ YAG orientation relationship of [1 0 1¯ 0] Al2 O3 //[1 0 1] and (0 0 0 1)Al2 O3 //(1 2 1) YAG. The improvement of fracture
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toughness is mainly ascribed to the microstructure refinement by rapid solidification, crack bridging and deflection induced at phase interfaces as well as the high density dislocations produced in the YAG phases. Acknowledgements This work was supported by the National Natural Science Foundation of China (51272211, 51002122), Provincial Natural Science Foundation of Shaanxi Province, the China Postdoctoral Science Foundation (2012M51028), the Hong Kong Scholars Program Fund (XJ2011030), Research Grants Council of the Hong Kong Special Administrative Region (HKU 717011E), and the 111 Project (B08040). References 1. Waku Y, Sakum T. Dislocation mechanism of deformation and strength of Al2 O3 -YAG single crystal composites at high temperatures above 1500 ◦ C. J Eur Ceram Soc 2000;20:1453–8. 2. Orera VM, Pe˜na JI, Oliete PB, Merino RI, Larrea A. Growth of eutectic ceramic structures by directional solidification methods. J Cryst Growth 2013;360:99–104. 3. Zhang J, Su HJ, Song K, Liu L, Fu HZ. Microstructure, growth mechanism and mechanical property of Al2 O3 -based eutectic ceramic in-situ composites. J Eur Ceram Soc 2011;31(7):1191–8. 4. Waku Y, Nakagaw N, Wakamoto T, Ohtsubo H, Shimizu K, Kohtoku Y. A ductile ceramic eutectic composite with high strength at 1873 K. Nature 1997;389:49–52. 5. Hirano K. Application of eutectic composites to gas turbine system and fundamental fracture properties up to 1700 ◦ C. J Eur Ceram Soc 2005;25(8):1191–9. 6. Llorca J, Orera VM. Directionally solidified eutectic ceramic oxides. Prog Mater Sci 2006;51:711–809. 7. Pastor JY, Martín A, Molina-Aldareguía JM, Llorca J, Oliete PB, Larrea A, et al. Superplastic deformation of directionally solidified nanofibrillar Al2 O3 -Y3 Al5 O12 -ZrO2 eutectics. J Eur Ceram Soc 2013;33(13/14):2579–86. 8. Oliete PB, Pe˜na JI, Larrea A, Orera VM, Llorca J, Pastor JY, et al. Ultra-high-strength nanofibrillar Al2 O3 -YAG-YSZ eutectics. Adv Mater 2007;19:2313–8. 9. Mazerolles L, Perriere L, Lartigue-Korinek S, Parlier M. Creep behavior and related structural defects in Al2 O3 -Ln2 O3 (ZrO2 ) directionally solidified eutectics (Ln = Gd, Er, Y). J Eur Ceram Soc 2011;31: 1219–25. 10. Waku Y, Nakagawa N, Ohtsubo H, Mitani A, Shimizu K. Fracture and deformation behaviour of melt growth composites at very high temperatures. J Mater Sci 2001;36:1585–94. 11. Waku Y, Sakata S, Mitani A, Shimizu K, Ohtsuka K, Hasebe M. Microstructure and high-temperature strength of Al2 O3 /Er3 Al5 O12 /ZrO2 ternary melt growth composite. J Mater Sci 2005;40:711–7. 12. Pastor JY, LLorca J, Martín A, Pe˜na JI, Oliete PB. Fracture toughness and strength of Al2 O3 -Y3 Al5 O12 and Al2 O3 -Y3 Al5 O12 -ZrO2 directionally solidified eutectic oxides up to 1900 K. J Eur Ceram Soc 2008;28(12):2345–51. 13. Perrière L, Valle R, Mazerolles L, Parlier M. Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites. J Eur Ceram Soc 2008;28(12):2301–8.
14. Zhao ZM, Zhang L, Song YG, Wang WG, Wu J. Microstructures and properties of rapidly solidified Y2 O3 doped Al2 O3 /ZrO2 composites prepared by combustion synthesis. Scripta Mater 2006;55:819–22. 15. Su HJ, Zhang J, Deng YF, Liu L, Fu HZ. A modified preparation technique and characterization of directionally solidified Al2 O3 /Y3 Al5 O12 eutectic in-situ composites. Scripta Mater 2009;60:362–5. 16. Su HJ, Zhang J, Liu L, Eckert J, Fu HZ. Rapid growth and formation mechanism of ultrafine structural oxide eutectic ceramics by laser direct forming. Appl Phys Lett 2011;99:221913. 17. Rios CT, Contieri RJ, Souza SA, Cremasco A, Hayama AOF, Caram R. Fracture toughness of a directionally solidified Al-Nb-Ni ternary eutectic. Mater Des 2012;33:563–8. 18. Fu XS, Chen GQ, Zu YF, Luo JT, Zhou WL. Microstructure refinement of melt-grown Al2 O3 /YAG/ZrO2 eutectic composite by a new method: melt superheating treatment. Scripta Mater 2013;68:731–4. 19. Larrea A, Fuente GF, Merino RI, Orera VM. ZrO2 -Al2 O3 eutectic plates produced by laser zone melting. J Eur Ceram Soc 2002;22: 191–8. 20. Su HJ, Zhang J, Wang HF, Song K, Liu L, Fu HZ. Effect of solidification path on the microstructure of Al2 O3 -Y2 O3 -ZrO2 ternary oxide eutectic ceramic system. J Eur Ceram Soc 2012;32:3137–42. 21. White RM, Kunkle JM, Polotai AV, Dickey EC. Microstructure and hardness scaling in laser-processed B4 C-TiB2 eutectic ceramics. J Eur Ceram Soc 2011;31:1227–32. 22. Yu JZ, Zhang J, Su HJ, Song K, Liu L, Fu HZ. Fabrication and characterization of Al2 O3 /Y3 Al5 O12 eutectic in situ composite ceramics by double side laser zone remelting method. J Inorg Mater 2012;27:843–8. 23. Anstis GR, Chantikul P, Lawn BR, Marshall DB. A critical evaluation of indentation techniques for measuring fracture toughness: I. direct crack measurements. J Am Ceram Soc 1981;64:533–8. 24. Pastor JY, Llorca J, Salazar A, Oliete PB, Francisco I, Pe˜na JI. Mechanical properties of melt-grown alumina-yttrium aluminum garnet eutectics up to 1900 K. J Am Ceram Soc 2005;88(6):1488–95. 25. Su HJ, Zhang J, Cui CJ, Liu L, Fu HZ. Rapid solidification behaviour of Al2 O3 /Y3 Al5 O12 (YAG) binary eutectic ceramic in-situ composites. Mater Sci Eng A 2008;479(1/2):380–8. 26. Yves-Christian H, Jan W, Wilhelm M, Konrad W, Reinhart P. Net shaped high performance oxide ceramic parts by selective laser melting. Procedia Phys 2010;5:587–94. 27. Harada Y, Ayabe K, Uekawa N, Kojima T, Kakegawa K, Kim SJ. Formation of GdAlO3 -Al2 O3 composite having fine pseudo-eutectic microstructure. J Eur Ceram Soc 2008;28(15):2941–6. 28. Gu DD, Hagedorn YC, Meiners W, Meng GB, Batista RJS, Wissenbach K, Poprawe R. Densification behavior, microstructure evolution, and wear performance of selective laser melting processed commercially pure titanium. Acta Mater 2012;60:3849–60. 29. Bei H, George EP. Microstructures and mechanical properties of a directionally solidified NiAl-Mo eutectic alloy. Acta Mater 2005;53:69–77. 30. Hunt JD, Jackson KA. Binary eutectic solidification. Trans AIME 1966;236:843–52. 31. Echigoya J. Structure of interface in directionally solidified oxide eutectic systems. J Eur Ceram Soc 2005;25:1381–7. 32. Mazerolles L, Piquet N, Trichet MF, Perrière L, Boivin D, Parlier M. New microstructures in ceramic materials from the melt for high temperature applications. Aero Sci Technol 2008;12:499–505. 33. Ochiai S, Ueda T, Sato K, Hojo M, Waku Y, Nakagawa N, et al. Deformation and fracture behavior of an Al2 O3 /YAG composite from room temperature to 2023 K. Comp Sci Tech 2001;61:2117–28. 34. Pastor JY, Poza P, Llorca J, Pe˜na JI, Merino RI, Orera VM. Mechanical properties of directionally solidified Al2 O3 -ZrO2 (Y2 O3 ) eutectics. Mater Sci Eng A 2001;308:241–9.
ScienceDirect Journal of the European Ceramic Society 34 (2014) 739–744
In situ fabrication of highly-dense Al2O3/YAG nanoeutectic composite ceramics by a modified laser surface processing Haijun Su a,b,∗ , Jun Zhang a , Weidan Ma a , Kaichen Wei a , Lin Liu a , Hengzhi Fu a , Shien-Ping Feng b , A.K. Soh c a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China b Department of Mechanical Engineering, The University of Hong Kong, Hong Kong c School of Engineering, Monash University Sunway Campus, Malaysia Received 11 September 2013; received in revised form 30 September 2013; accepted 1 October 2013 Available online 8 November 2013
Abstract Alumina-matrix eutectic in situ composite ceramics present excellent high-temperature mechanical properties, which have been considered as promising next-generation ultra-high temperature structural materials. A modified laser surface processing is developed to in situ fabricate highlydense Al2 O3 /YAG bulk nanoeutectic ceramics with large size and homogeneous three-dimensional network of nanoeutectic microstructure by introducing two-side remelting and high-temperature preheating. The crack and porosity are avoided, and the eutectic structure achieves a good continuous growth between two solidified layers. The eutectic phases show sharp interface bonding with a defined orientation relationship. The dislocations and crack deflection at high-density phase interfaces importantly contribute to the enhanced fracture toughness. © 2013 Elsevier Ltd. All rights reserved. Keywords: Rapid solidification; Composites; Laser surface processing; Ceramics; Fracture toughness
1. Introduction In order to meet the increase of inlet temperature of gas turbines and improve its thermal efficiency as well as reduce the pollution emissions such as CO2 or NOx , the development and design of new ultra-high temperature structural materials with superior high-temperature performance and longevity above 1600 ◦ C in oxidizing environment have been greatly focused on all over the world.1–3 Directionally solidified oxide eutectic ceramic composite (DSOEC), a newly developed ultra-high temperature structural material in recent few decades, has attracted extensive attention due to its excellent mechanical property at high temperatures.4–7 Thus, it has been considered as one of the most potential alternatives to current traditional superalloys or sintered ceramics applied in the aerospace field.
∗ Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China. Tel.: +86 29 88492228; fax: +86 29 88494080. E-mail addresses: [email protected], [email protected] (H. Su).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.014
In DSOEC, it is known that the eutectic phases can be simultaneously grown from melt, and the well-aligned growth arrays can be produced by controlling thermal flow direction. The directionally solidified eutectics not only combine the unique characteristic of each component material but also enhance the phase interface bonding, so that greatly improve the properties of the composite.8,9 Of its numerous exciting properties, the one material property that stands out is DSOEC’s outstanding high temperature strength stability from room temperature up to high temperatures (near to its melting point). Especially, the DSOEC can availablely serve in the aggressive oxidizing environments without coating as compared with other non-oxide materials.4,10–13 Furthermore, highly homogenous and very fine microstructures based on eutectic structures having strongly bonded phases and high-density interphase areas can be obtained and tailored by rapid solidification or directional solidification technique with high temperature gradient, which can further improve the properties of materials.14–18 However, up to date, it is still very little that directional solidification or rapid solidification is applied to produce DSOEC composites in the industrial practice.4–16 One of the main reasons is that the conventional directional solidification technique,
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such as Bridgman method, can prepare large eutectic ceramic rods, but the preparation procedure needs expensive crucibles (high purity Mo or Ir) and the crucibles can easily react with ceramic melt, leading to the melt pollution and high preparation cost due to non-reuse of crucibles.4,5,10,11 Moreover, the eutectic interphase spacing is large (10–20 m) due to its low temperature gradient (<100 K/cm) and slow solidification rate, resulting in low production efficiency and reduced properties.6 Additionally, as noted, the laser floating zone remelting with high temperature gradient (>1000 K/cm) can produce eutectic ceramics with nanostructure and excellent properties, but it is only limited to small sample diameter (<1 cm).2,8,12 It is thus highly desirable to develop new preparation techniques for the oxide eutectic ceramic with large size, fine microstructure, high preparation efficiency and low cost to catch up its wide application. Laser surface zone remelting is recently developed as a rapid solidification technique to prepare oxide eutectic ceramic composites with advantages of ultra-fine microstructure, free of crucible and high solidification rate. However, it is only focused on small eutectic plates with hundreds of microns in thickness so far.19–22 Herein, we report a modified laser surface (MLS) processing to in situ prepare rapidly solidified Al2 O3 /YAG nanoeutectic ceramic rods having nearly full density and large size by introducing two-side remelting and high-temperature preheating. The homogenous microstructure and enhanced fracture toughness are obtained. The microstructure characteristic, interface structure, orientation relationship and toughening mechanism are investigated. 2. Experimental procedure High purity (>4 N) Al2 O3 and Y2 O3 nano-powders were homogeneously mixed according to the eutectic composition (82 mol% Al2 O3 /18 mol% Y2 O3 ) by wet ball milling. The mixed powders were die pressed to form rectangular precursor plates [65 mm × (6–10) mm × (3–5) mm] at room temperature and sintered at 1500 ◦ C for 2 h to improve the handing strength. The modified laser surface processing of the precursor plate was performed in vacuum by using a laser surface atmosphere melting furnace.16,22 A high-purity N2 gas was filled into the chamber with a flow rate of 5–10 L/min in order to prevent from the formation of porosity. Two superimposed precursor plates were firstly located on the laser worktable with a 5-axis and 4direction coupled motion, and then the back of the plates was preheated to 1200 ◦ C by the furnace for 30 min. After that, an incident CO2 laser with beam diameter of 10 mm and power of 200–600 W from the laser window on the furnace up-surface started to scan and melt the upper surface of the superimposed precursor samples along the sample axis with different rates. When the upper surface of the samples was completely remelted, the samples were overturned to restart the second remelting of the sample back surface, as schematically shown in Fig. 1. The laser scanning rate was 20–400 m/s dependent on the sample thickness. The density of MLS-processed specimens was measured by using Archimedes’s method. The microstructure was analysed
Fig. 1. Sketch diagram of the modified laser surface (MLS) processing technique to prepare the Al2 O3 /YAG bulk eutectic ceramics.
using scanning electron microscopy (SEM, JSM-5800), energy disperse spectroscopy (EDS, Link-Isis) and X-ray diffraction (XRD, Rigakumsg-158). The phase interface and orientation relationship were investigated by transmission electron microscopy (TEM, JEM-200CX) and high resolution transmission electron microscopy (HRTEM, JEM-2100F). The hardness and fracture toughness were determined by the Vickers indentation technique with load of 9.8 N for 15 s and an average value of 10 measurements was reported. The indentation size and crack length were measured by SEM. The hardness and fracture toughness were calculated according to the equation proposed by Antis23 as reported in previous literatures.24,25 3. Results and discussion Fig. 2 shows the macroscopic photograph of the rapidly solidified Al2 O3 /YAG bulk eutectic ceramic grown by the MLS-processing at the scanning rate of 20 m/s. The obtained eutectic ceramic presents an approximate rod shape due to the melt surface tension with the diameter up to 8 mm and the length of 60 mm. The diameter and size are almost 2–4 times larger
Fig. 2. The photograph of the grown Al2 O3 /YAG eutectic composite rod by the modified laser surface processing.
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Fig. 3. Typical microstructures of the cross-section of the MLS-processed Al2 O3 /YAG eutectic ceramics rod grown at the scanning rate of 40 m/s: (a) low magnification; (b) high magnification of second remelting layer; (c) high magnification of first remelting layer and (d) microstructure of the interface between two remelting layers. The inset of (d) is the high magnification of interface layer microstructure. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
than that of the eutectic ceramic rod availablely obtained by the melt growth method without using cubicle such as laser floating zone or laser surface remelting in the previous reports.3,8,19 It indicates that the MLS-processing can remarkably enhance the thickness of solidified layer and achieve the rapid preparation of large bulk eutectic ceramics by introducing the laser two-side remelting. The sample diameter is dependent on the precursor shape and size as well as laser energy density and scanning rate. During the MLS-processing, the eliminations of cracks due to rapid cooling or residual stress as well as the inhibition of gas porosity introduced during melting process are the keys of influencing the ceramic melt preparation. In our case, the MLS-processed eutectic ceramic rod displays glossy surface with white color, and no crack or porosity is found on the surface and inside of the sample. It demonstrates that no impurity is introduced, and the growth defects such as porosity and crack have been well avoided, which is contributed by the high temperature preheating and second two-side laser remelting for effectively relaxing thermal stress and healing cracks. The surface finish increases with the increase of scanning rates, but tiny cracks sometimes still appear when the scanning rate is larger than 200 m/s. The specimens exhibit a density of nearly 100% which is superior to the previously reported laser selective melted Al2 O3 /ZrO2 ceramic26 and spark plasma sintered oxide eutectic ceramic.27 There is an overall trend of decreasing
density with increasing scanning rate. When the laser scanning rate is over 400 m/s, the composite density tends to be below 99%. Fig. 3 shows the typical cross-section microstructures of the MLS-processed Al2 O3 /YAG eutectic ceramics rod grown at the scanning rate of 40 m/s. As shown in Fig. 3(a), the composite ceramic rods present dense and ultra-fine microstructure without cracks and defects. There is an obvious solidification interface layer between both the remelting layers. As seen, the microstructure from the outside to middle interface layer of the sample is very homogeneous, which is superior to the previously reported eutectic ceramics fabricated by the single side laser remelting method.3,19,25 The homogenous microstructure from top/bottom to middle is mainly ascribed to the superheating treatment of second remelting and to no defect (pore or impurity) disturbance in the solidification interface front during the melt growth process.18,28 The component phases, ␣-Al2 O3 and YAG, are identified by XRD and EDS, and no other crystalline phases or amorphous phases are detected. The magnified microstructures of the top and bottom of the rod are depicted in Fig. 3(b) and (c). The two phases show well-coupled eutectic growth with an interpenetrating three-dimension network nanostructure aligned along the scanning direction (red arrow in Fig. 3b). As seen in the yellow arrow in Fig. 3(c), the morphology is completely different from the common regular eutectic structure in metal alloys such
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Fig. 4. TEM micrograph showing the eutectic phase interface of the MLS-processed Al2 O3 /YAG eutectic rod (a) and the selected area electron diffraction patterns at the Al2 O3 /YAG phase interface (b).
as NiAl-Mo eutectic,29 indicating a complex irregular eutectic characteristic is mainly dominated by the branch growth of YAG phase due to its stronger faceted growth habit than Al2 O3 .3 The domain width of eutectic phases decreases with increasing the laser scanning rate, and it can be refined below 100 nm when the rate is over 400 m/s due to the high temperature gradient (∼104 K/cm) and large cooling rate (∼400 K/s) for laser zone remelting.20 During eutectic solidification, the nucleation undercooling T increases with cooling rate V, resulting in the decrease of eutectic spacing λ, namely, λ ∝ 1/V.29 The volume fractions of the eutectic phases determined by the SEM images are 44.6% and 55.4% for Al2 O3 and YAG respectively, which is in agreement with the theoretical calculation from the eutectic phase diagram. It is also in accordance with the volume fraction range of theoretical prediction for the irregular lamellar eutectic classification based on Hunt–Jackson theory.30,31 Moreover, by careful examination, it is found that the eutectic microstructure of the second remelting layer (Fig. 3b) is slightly finer than that of the first remelting layer (Fig. 3c), which of similar phenomena was also found in the eutectic ceramic by laser direct forming.16 It demonstrates that the two-side laser second superheating can effectively refine the eutectic spacing. In the interfacial transition zone, a coarse coupled band eutectic structure (Fig. 3d) with the width of about 40 m and strong faceted growth habit appears. The width of the band eutectic decreases with increasing the laser power and decreasing the laser scanning rate. Additionally, no sintered multicrystalline ceramic structure can be found in the interface zone, revealing that the two remelting layers successfully achieve the continuous eutectic growth by frequent branching (yellow arrows in Fig. 3d) of faceted Al2 O3 and YAG phases which produce complex irregular eutectic structure. During the growth of eutectic composites, the solidification process does not only affect the microstructure but also influences the preferred growth orientations.31,32 In directionally eutectics, the microstructures (lamellae or rods) usually are composed of single crystal phases preferentially growing along a defined crystallographic direction. Fig. 4 shows the phase interface TEM structure and electron diffraction pattern of selected area perpendicular to the rod axes of the MLS-processed Al2 O3 /YAG eutectic ceramic. It is clearly observed that the phase interface is clean and planar without intermediary phases detected, showing sharp morphology
with well interface matching. According to Bragg diffraction law, the electron diffraction analysis at the phase interface center (Fig. 4b) determines that the MLS-processed Al2 O3 /YAG eutectic shows well-defined crystallographic direc¯ YAG and tion and relationship: [1 0 1¯ 0] Al2 O3 //[1 0 1] (0 0 0 1)Al2 O3 //(1 2 1) YAG, which are similar to that of Mazerolles et al. by optical floating zone melting.32 Thus, the preferred crystallographic growth directions for Al2 O3 and YAG ¯ and [101], ¯ respecphase during the MLS-processing are [1010] tively. The minimum interfacial energy is produced aligned above directions and the configuration at the interface is also illustrated by the HRTEM image, as shown in Fig. 5. It is found that the arrays of atomic planes of both phases at interface are not corresponding but there is a crystal lattice misfit calculated to be 6.17% belonging to the half-coherent phase interface. The inverse Fourier transform at interface (inset of Fig. 5) further reveals that the accommodation between two phase structures is adjusted by the dislocations along the interface.
Fig. 5. The high-resolution transmission electron micrograph of the Al2 O3 /YAG phase interface. The inset is the inverse Fourier transform from the digitized image of the HRTEM interface.
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Fig. 6. The variations of mechanical properties of the MLS-processed Al2 O3 /YAG eutectic ceramic with the increase of laser scanning rate (a) and the crack propagation paths of the indention (b).
Fig. 6(a) shows the variations of typical mechanical properties with the increase of laser scanning rates for the MLS-processed Al2 O3 /YAG eutectic ceramics. The interaction between the fine microstructure and cracks nucleated from the corners of Vickers indentations was analysed by SEM. At least ten valid tests were carried out on each sample. The results are comparable to the previously reported eutectic ceramics determined using the indentation method. The indentation cracks belong to Median cracks determined by the polished methods.17 It is indicated that the hardness increases rapidly with increasing laser scanning rate in the low rate range, but decreases when the rate is over 100 m/s, because the high scanning rate causes the reduced thickness of solidified eutectics and generates more defects such as micro-cracks or pores. Although the fracture toughness shows an increase with the scanning rate, it almost keeps constant when the rate is over 100 m/s. The maximum hardness and fracture toughness obtained by the optimized laser scanning rate of 100 m/s are 18.7 GPa and 3.7 MPa m1/2 , respectively. The fracture toughness of eutectic is obviously higher than those of monocrystal Al2 O3 (2.4 MPa m1/2 ),33 YAG (3.0 MPa m1/2 )33 and Al2 O3 /YAG binary eutectic (2.0 MPa m1/2 ),24 and is close to that of Al2 O3 /ZrO2 binary eutectic (4.8 MPa m1/2 )34 measured also by using the indentation method. The enhanced mechanical properties are primarily attributed to refined micro/nanostructured eutectic microstructures on the one hand.3,32 The ceramic fracture is a process of crack propagation and growth, in which the cracks inevitably produce interaction with the interphases. In previous reports for the Al2 O3 /YAG binary eutectic,6,24 the straight indentation crack normally propagates directly across the Al2 O3 and YAG phases, leading to material fracture. As shown in the arrows in Fig. 6(b), for the MLS-processed Al2 O3 /YAG composite, the crack bridging and deflection at interface are clearly observed, indicating more fracture energy can be absorbed. The crack is finally stopped by the fine phase interface shown by red arrow in Fig. 7, and therefore the fracture toughness is effectively increased. The eutectic ceramic presents typical transgranular cracking propagation. Moreover, the considerable linear dislocations formed
Fig. 7. The TEM showing the dislocation distribution in the Al2 O3 /YAG eutectic phases.
in the Al2 O3 phase as compared with the YAG phase can also contribute to the toughness improvement,9 as shown in Fig. 7. 4. Conclusions Rapidly solidified Al2 O3 /YAG nanoeutectic ceramic rods with large size (diameter >6 mm) and nearly full density are produced by a modified laser surface processing method. The laser two-side surface remelting and high temperature preheating effectively enhance the sample size, avoid crack generation and achieve continuous growth between two solidified layers. The composite presents a homogeneous three-dimensional network with the irregular eutectic structure consisting of micro/nano structured Al2 O3 and YAG phases with a defined ¯ YAG orientation relationship of [1 0 1¯ 0] Al2 O3 //[1 0 1] and (0 0 0 1)Al2 O3 //(1 2 1) YAG. The improvement of fracture
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toughness is mainly ascribed to the microstructure refinement by rapid solidification, crack bridging and deflection induced at phase interfaces as well as the high density dislocations produced in the YAG phases. Acknowledgements This work was supported by the National Natural Science Foundation of China (51272211, 51002122), Provincial Natural Science Foundation of Shaanxi Province, the China Postdoctoral Science Foundation (2012M51028), the Hong Kong Scholars Program Fund (XJ2011030), Research Grants Council of the Hong Kong Special Administrative Region (HKU 717011E), and the 111 Project (B08040). References 1. Waku Y, Sakum T. Dislocation mechanism of deformation and strength of Al2 O3 -YAG single crystal composites at high temperatures above 1500 ◦ C. J Eur Ceram Soc 2000;20:1453–8. 2. Orera VM, Pe˜na JI, Oliete PB, Merino RI, Larrea A. Growth of eutectic ceramic structures by directional solidification methods. J Cryst Growth 2013;360:99–104. 3. Zhang J, Su HJ, Song K, Liu L, Fu HZ. Microstructure, growth mechanism and mechanical property of Al2 O3 -based eutectic ceramic in-situ composites. J Eur Ceram Soc 2011;31(7):1191–8. 4. Waku Y, Nakagaw N, Wakamoto T, Ohtsubo H, Shimizu K, Kohtoku Y. A ductile ceramic eutectic composite with high strength at 1873 K. Nature 1997;389:49–52. 5. Hirano K. Application of eutectic composites to gas turbine system and fundamental fracture properties up to 1700 ◦ C. J Eur Ceram Soc 2005;25(8):1191–9. 6. Llorca J, Orera VM. Directionally solidified eutectic ceramic oxides. Prog Mater Sci 2006;51:711–809. 7. Pastor JY, Martín A, Molina-Aldareguía JM, Llorca J, Oliete PB, Larrea A, et al. Superplastic deformation of directionally solidified nanofibrillar Al2 O3 -Y3 Al5 O12 -ZrO2 eutectics. J Eur Ceram Soc 2013;33(13/14):2579–86. 8. Oliete PB, Pe˜na JI, Larrea A, Orera VM, Llorca J, Pastor JY, et al. Ultra-high-strength nanofibrillar Al2 O3 -YAG-YSZ eutectics. Adv Mater 2007;19:2313–8. 9. Mazerolles L, Perriere L, Lartigue-Korinek S, Parlier M. Creep behavior and related structural defects in Al2 O3 -Ln2 O3 (ZrO2 ) directionally solidified eutectics (Ln = Gd, Er, Y). J Eur Ceram Soc 2011;31: 1219–25. 10. Waku Y, Nakagawa N, Ohtsubo H, Mitani A, Shimizu K. Fracture and deformation behaviour of melt growth composites at very high temperatures. J Mater Sci 2001;36:1585–94. 11. Waku Y, Sakata S, Mitani A, Shimizu K, Ohtsuka K, Hasebe M. Microstructure and high-temperature strength of Al2 O3 /Er3 Al5 O12 /ZrO2 ternary melt growth composite. J Mater Sci 2005;40:711–7. 12. Pastor JY, LLorca J, Martín A, Pe˜na JI, Oliete PB. Fracture toughness and strength of Al2 O3 -Y3 Al5 O12 and Al2 O3 -Y3 Al5 O12 -ZrO2 directionally solidified eutectic oxides up to 1900 K. J Eur Ceram Soc 2008;28(12):2345–51. 13. Perrière L, Valle R, Mazerolles L, Parlier M. Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites. J Eur Ceram Soc 2008;28(12):2301–8.
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