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A novel chemical mechanical polishing slurry for yttrium aluminum garnet crystal
Applied Surface Science 496 (2019) 143601
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A novel chemical mechanical polishing slurry for yttrium aluminum garnet crystal
T
⁎
Zili Zhang, Zhuji Jin, Jiang Guo , Xiaolong Han, Qing Mu, Xianglong Zhu Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Yttrium aluminum garnet Chemical mechanical polishing Surface roughness Material removal rate Reaction mechanism
Yttrium aluminum garnet (Y3Al5O12, YAG) crystal is widely used for laser applications. However, due to its high hard and brittle nature, high surface quality and high material removal rate (MRR) are difficult to be achieved. In this study, to solve the problem, a novel CMP slurry was developed, which contains 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water. After polishing, the surface roughness and MRR achieved 0.08 nm Ra and 34 nm/min respectively. The reaction mechanism during polishing was elucidated based on the results of grazing incidence small angle X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The results show that AlOOH and YOOH generated from hydration reaction react with Si–OH in Na2SiO3 aqueous solution to form soft andalusite and yttrium silicate, then MgO combines with andalusite to produce montmorillonite. All soft products are removed mechanically by ZrO2 abrasives, obtaining the ultrasmooth surface of YAG crystal.
1. Introduction With the development of laser technology [1–4], there has been increasing demand for some functional materials, such as yttrium aluminum garnet (Y3Al5O12, YAG) crystal with high thermal conductivity, superior ability of doping rare earth ions and fine optical property [5–7]. However, due to its high brittle nature, high surface quality is hard to be achieved, resulting in localized energy accumulation for laser scattering to deteriorate laser performances [8,9]. In addition, high hard nature and chemical stability lead to a low material removal rate (MRR), increasing the cost of processing. In recent decades, for minimizing the negative outcomes, different ultra-precision machining methods were utilized including mechanical polishing, chemical polishing and chemical mechanical polishing (CMP). Kim et al. [10,11] mechanically polished YAG crystal by hard abrasives, but surface was prone to dislocations, fine scratches and large damages after processing. Kostić et al. [7,12] chemically polished YAG crystal with high-concentration phosphoric acid at high temperature, after which the material removal was nonuniform and corrosion pits occurred. Moreover, this polishing method was harmful to the equipment and environment. Therefore, single mechanical or chemical polishing is disadvantageous to the processing of YAG crystal. In contrast to them, CMP can remove materials by soft abrasives and achieve high surface quality [13]. Furthermore, CMP slurry has an ⁎
important effect on the processing quality and cost of CMP. At present, traditional CMP slurries for YAG crystal are mainly colloidal silica. Li et al. [14] investigated the effects of polishing pad materials and pH of colloidal silica on the processing quality of YAG crystal, and found that a surface with 0.2 nm Ra in an area of 5 μm × 5 μm could be obtained with acid colloidal silica and chemcloth pad. In order to improve the surface quality, McKay [15] polished YAG crystal with mixed slurry of SiO2 suspension and NaOH, and a surface with 0.1 nm Ra in an area of 40 μm × 40 μm was achieved, but the MRR was only 0.3 nm/min. In addition, MRR declined during the reusing of colloidal silica for the consumption of Si–OH, affecting the utilization of colloidal silica [16,17]. Aiming at improving the performances of colloidal silica, some researchers [18–20] made efforts in the modification of colloidal silica, but the performance improvements were limited. Comprehensively, there are many problems in the CMP slurries for YAG crystal. Therefore, it is essential to develop a high-performance polishing slurry. During polishing with colloidal silica, Si–OH distributes on the surface of silica particles as reactant [16,21], so the chemical reaction and mechanical abrasion are bundled together. This interaction mode goes against the balance between chemical and mechanical effects and limits the performance. Thus, it is necessary to detach Si–OH from abrasive particles. In the Na2SiO3 aqueous solution, SiO32− mainly exists in the forms of HSiO43−, H2SiO42− and H3SiO4− due to hydrolysis reaction [22,23], resulting in the formation of Si–OH. In this study,
Corresponding author. E-mail address: [email protected] (J. Guo).
https://doi.org/10.1016/j.apsusc.2019.143601 Received 28 May 2019; Received in revised form 2 August 2019; Accepted 6 August 2019 Available online 06 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 496 (2019) 143601
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based on colloidal silica, the chemical effect of Si–OH was replaced by Na2SiO3·5H2O to detach it from mechanical effect. At the same time, the mechanical abrasion was realized by ZrO2 abrasives. In addition, on account of the result that adding MgO in colloidal silica can improve the surface quality and MRR during sapphire polishing [24], MgO was also considered in the components of polishing slurry. After optimizing the component concentrations by orthogonal experiments, a novel CMP slurry was developed for YAG crystal polishing, which contains 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water. According to the results of grazing incidence small angle X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS), reaction mechanism was elucidated.
Table 2 Components of different polishing slurries. Number
Solvent
ZrO2/wt%
Na2SiO3·5H2O/wt%
MgO/wt%
pH
1 2 3 4 5
Alcohol Deionized water Deionized water Deionized water Deionized water
8 8 8 8 8
0 0 0 5 5
0 0 0 0 0.3
– 3 13 13 13
3. Results and discussion 3.1. Performances of polishing slurries In order to compare the performances of the novel polishing slurry and colloidal silica, YAG crystals were polished with these two polishing slurries under the same processing parameters. Fig. 1 shows the results of surface roughness Ra and MRR. Surface roughness achieves 0.08 nm Ra in an area of 10 μm × 10 μm after polishing with the novel polishing slurry. Compared with 0.15 nm Ra obtained by colloidal silica, it reduces by 47%. Besides, MRR using the novel polishing slurry is 34 nm/min, which increases by 240% compared with 10 nm/min obtained by colloidal silica. Therefore, it is demonstrated that the novel polishing slurry has superior performances when polishing YAG crystal.
2. Experimental details 2.1. Materials and methods YAG crystal (111) was used as workpiece which was a cylinder with a diameter of 15 mm and a height of 2 mm. Three pieces of YAG crystals were adhered to a ZrO2 ceramic disc with paraffin. The purity of Na2SiO3·5H2O was 99.9%. The average diameter of MgO particles was 30 nm. The abrasives in the novel polishing slurry were monoclinic ZrO2 particles with an average diameter of 80 nm. The polishing experiments were performed on automatic grinding and polishing machine (UNIPOL1200S, Shenyang Kejing Automation Equipment Co., Ltd., China). The schematic of polishing machine is shown in Fig. S2. Commercial colloidal silica (COMPOL, FUJIMI Corporation) containing 10 wt% silica abrasives was used as a control of the novel CMP slurry. Table 1 shows the processing parameters of grinding and polishing. After polishing, YAG crystals were cleaned with deionized water and dried with an air syringe. For the purpose of investigating the effects of different components in the novel CMP slurry, different polishing slurries were prepared to polish YAG crystals. Table 2 lists the components of polishing slurries. NaOH was added into polishing slurry 3 to adjust pH to 13, which was consistent with polishing slurry 4.
3.2. Reaction mechanism 3.2.1. Effects of different components on polishing For the purpose of researching the effects of different components on polishing in the novel polishing slurry, MRR using different slurries shown in Table 2 was measured. As seen in Fig. S3, when alcohol is used as solvent of polishing slurry 1, MRR is extremely low and negligible. Because ZrO2 abrasives (Moh's hardness 7.5) are softer than YAG crystal (Moh's hardness 8.5), onefold mechanical action of ZrO2 abrasives can't achieve the material removal with polishing slurry 1. However, when deionized water is utilized as solvent of polishing slurry 2, MRR increases significantly. It is speculated that deionized water can react with the YAG crystal and a hydration layer is generated on the surface. Furthermore, the hardness of hydration layer is smaller than ZrO2 abrasives, so the hydration layer can be removed with ZrO2 abrasives. Besides, MRR is improved significantly with the successive addition of NaOH, Na2SiO3 and MgO. It indicates that NaOH, Na2SiO3 and MgO are beneficial to the chemical reaction and can promote material removal. Fig. S4 displays surface morphologies of YAG crystals after polishing for 120 min with different polishing slurries. It can be seen that surface quality becomes better and better with the successive addition of NaOH, Na2SiO3 and MgO, which is well consistent with MRR. Comprehensively, these phenomena indicate that deionized water, alkaline environment, Na2SiO3 and MgO all play important roles in the CMP of YAG crystal.
2.2. Characterization After grinding, YAG crystals were polished with different polishing slurries for 120 min, surface morphologies were observed by an optical microscope (MX-40, Olympus, Japan). Surface roughness Ra was measured by atomic force microscope (AFM, XE-200, Park system corporation, Korea). MRR was calculated by scratch method (The details are shown in supporting material). The depths of scratches were measured by surface profiler (NewView 5022, Zygo, USA). Before XRD and GIXRD detection, to obtain an initial flat surface, YAG crystals were polished with Al2O3 abrasives and then cleaned in alcohol by ultrasonic cleaner. Besides, YAG crystals were immersed into deionized water at 50 °C for 30 min to simulate the polishing environment before GIXRD measurement [25]. XRD and GIXRD patterns were measured in X-ray diffractometer (Empyrean, PANalytical B.V., Netherlands). XPS spectra of different elements were measured by X-ray photoelectron spectrometer (Escalab 250Xi, ThermoFisher, England) and calibrated by applying adventitious C 1s signal at 284.8 eV.
3.2.2. Hydration reaction In order to investigate the products of hydration reaction, GIXRD pattern of YAG crystal surface after being immersed in water and XRD pattern of initial YAG crystal surface were measured respectively. The patterns are shown in Fig. 2. There is only one characteristic peak of YAG at 52.7° in XRD pattern, which is attributed to the (111) orientation (JCPDS No. 72-1315) [26]. Instead, after immersing YAG crystal in deionized water, the characteristic peak of YAG crystal disappears. At the same time, the characteristic peaks of AlOOH at 36.3°, 38.0° and 43.4° and YOOH at 44.9° and 46.4° appear in GIXRD pattern (JCPDS No. 88-2111 and No. 74-2350) [27,28]. These changes of patterns prove that a hydration layer is generated on the surface of YAG crystal including AlOOH and YOOH during polishing. Many researchers have studied the interaction of water with solid surfaces [29,30]. Based on these researches and experiment results in
Table 1 Processing parameters. Process
Pad
Rotate speed of plate (r/min)
Flow rate (ml/min)
Pressure (MPa)
Grinding Polishing
3 M 673LAA10 IC1000
30 80
10 6
0.03 0.03
2
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Fig. 1. Surface morphologies and line profiles of YAG crystals after polishing with (a) colloidal silica and (b) the novel polishing slurry, and (c) contrasts of Ra and MRR after polishing with these two slurries.
Fig. 2. The XRD and GIXRD patterns of YAG crystal surface. 3
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Fig. 3. The hydration reaction process on YAG crystal surface.
Fig. 4. The XPS spectra of (a) Si 2p after polishing, (b) O 1s before polishing, (c) O 1s after polishing, (d) Al 2p before polishing, (e) Al 2p after polishing, (f) Y 3d before polishing and (g) Y 3d after polishing.
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as shown in Fig. 4. After analyzing the XPS spectra comprehensively, all changes of binding energies indicate that andalusite and yttrium silicate are generated on YAG crystal surface during polishing. Specific analyses are as following. Based on the Si 2p spectra in Fig. 4(a), after polishing, the Si 2p spectra of surface layer can be detected and resolved into two peaks: peaks at 102.51 eV and 101.40 eV, corresponding to andalusite and yttrium silicate respectively. Furthermore, as shown in Fig. 4(c), the O 1s peaks at 531.30 eV and 530.80 eV after polishing are well consistent with these two products. Besides, Al 2p peak at 74.27 eV assigned to andalusite appears in Fig. 4(e), and Y 3d peaks at 158.00 eV and 160.05 eV in Fig. 4(g) correspond to yttrium silicate. (The reference values of binding energies are listed in Table 3.) The hardness of andalusite (Moh's hardness 6.5–7) and yttrium silicate (Moh's hardness 5) are both smaller than that of ZrO2 abrasives, which is beneficial to material removal [31]. Reaction mechanism is proposed in Fig. 5. In the polishing slurry, Si–OH generated from hydrolysis reaction of SiO32− can be adsorbed on the surface of YAG crystal. Then Si–OH trends to combine with Al–OH and Y–OH of hydration layer, forming Si–O–Al and Si–O–Y by dehydration. The reaction equations are proposed as Eqs. (1) and (2).
Table 3 Binding energy of different elements (eV). Chemical state
Si 2p
Al 2p
O 1s
Y 3d
Mg 1s
YAG [32]
–
–
101.40
–
Andalusite [34] MgO [35] Montmorillonite [35]
102.51 – 102.95
74.27 – –
531.30 – –
157.43 159.44 158.00 160.05 – – –
–
Yttrium silicate [33]
530.47 531.94 530.80
– – 1303.90 1305.30
2AlOOH + SiO32 − → Al2SiO5 + 2OH−
(1)
2YOOH + SiO32 − → Y2SiO5 + 2OH−
(2)
3.2.4. The role of MgO Fig. 6 shows the Mg 1s and Si 2p spectra of YAG crystal surface after polishing with polishing slurry 5 (the novel polishing slurry). In Fig. 6(a), the peak of Mg 1s spectra is at the position of 1305.3 eV, which is higher than the binding energy of MgO in polishing slurry (1303.9 eV) and corresponds to Mg-montmorillonite. In addition, as seen in Fig. 6(b), the deconvolution of Si 2p spectra contains three overlapping peaks at 102.51 eV, 101.40 eV and 102.95 eV assigned to andalusite, yttrium silicate and montmorillonite respectively. (The reference values of binding energies are listed in Table 3.) Compared with the results in the Section 3.2.3, it is demonstrated that a new product Mg-montmorillonite is generated after adding MgO to the polishing slurry 4. Montmorillonite is a kind of aluminosilicate. At the same time, based on the XPS results, Mg 1s peak attributed to Mg-montmorillonite appears after polishing. It indicates that the components of montmorillonite include Mg, Si, Al and O. Therefore, the generation of montmorillonite results from the reaction of YAG crystal, Na2SiO3 and MgO in water. Harder [36] researched the synthesis of mineral and proposed that MgO could promote the conversion of silicate mineral salts to the mineral with three-layer structure. Furthermore, montmorillonite is a kind of mineral with three-layer structure as shown in Fig. S5 [37]. Therefore, the generation of montmorillonite is due to the secondary reaction of andalusite (the reaction product shown in Eq. (1)) and MgO. As a product of secondary reaction, the concentration of montmorillonite is low, corresponding to its small area of Si 2p peak shown in Fig. 6(b). Montmorillonite is softer than andalusite and can be removed
Fig. 5. The process of dehydration reaction.
this study, a hydration reaction mechanism is proposed. The generation of AlOOH and YOOH results from the formation of Al–OH and Y–OH on the YAG crystal surface. As shown in Fig. 3, due to the fracture of AleO bonds and YeO bonds, a lot of O vacancies and unsaturated metal atoms are generated on the surface of YAG crystal. The unsaturated metal atoms trend to adsorb H2O by forming AleOw bonds and YeOw bonds (Ow, O atoms in H2O). Meanwhile, the H2O dissociates into –OwH groups and H atoms. Then the H atoms combine with neighboring O atoms and form –OsH groups (Os, O atoms on YAG crystal surface). Consequently, two types of hydroxyl groups (–OwH and –OsH) are obtained, forming AlOOH and YOOH. Besides, H2O is more inclined to dissociate into –OwH groups in the alkali environment, promoting the hydration reaction [29]. Therefore, polishing slurry 3 has a better performance than polishing slurry 2.
3.2.3. The role of Na2SiO3 The XPS spectra of different elements on YAG crystal surface before and after polishing with polishing slurry 4 were detected respectively,
Fig. 6. The XPS spectra of YAG crystal surface after polishing with polishing slurry 5, (a) Mg 1s and (b) Si 2p. 5
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easily, so this conversion is beneficial to the improvement of MRR.
[8] Y.L. Fu, J. Li, Y. Liu, L. Liu, H. Zhao, Y.B. Pan, Influence of surface roughness on laser-induced damage of Nd:YAG transparent ceramics, Ceram. Int. 41 (2015) 12535–12542. [9] H. Kim, R.S. Hay, S.A. Mcdaniel, G. Cook, N.G. Usechak, A.M. Urbas, K.N. Shugart, H. Lee, A.H. Kadhim, D.P. Brown, Lasing of surface-polished polycrystalline Ho:YAG (yttrium aluminum garnet) fiber, Opt. Express 25 (2017) 6725–6731. [10] R. Daniel, Y. Hitomi, Nanometer-scale characteristics of polycrystalline YAG ceramic polishing, CIRP Ann. 67 (2018) 349–352. [11] H. Kim, R.S. Hay, S.A. McDaniel, G. Cook, N.G. Usechak, A.M. Urbas, H.D. Lee, R.G. Corns, K.N. Shugart, A.H. Kadhim, D.P. Brown, B. Griffin, Optical characterizations on surface-polished polycrystalline YAG fibers, Proc. SPIE 10192 (2017) 101920B-1–101920B-7. [12] P.Z. Yang, P.Z. Deng, Z.W. Yin, Y.L. Tian, The growth defects in czochralski-grown Yb:YAG crystal, J. Cryst. Growth 218 (2000) 87–92. [13] B.Z. Parshuram, K. Ashok, A.K. Sikder, Chemical mechanical planarization for microelectronics applications, Mater. Sci. Eng. R 45 (2004) 89–220. [14] J. Li, Y. Zhu, C.T. Chen, Chemical mechanical polishing of transparent Nd:YAG ceramics, Key Eng. Mater. 375-376 (2008) 278–282. [15] J. McKay, Chemical Mechanical polishing and Direct Bonding of YAG and Y2O3, Ph.D. Thesis University of California, 2016. [16] B.A. Natthaphon, Y. Yutaka, S. Koya, K. Panart, S. Keisuke, Study on effect of the surface variation of colloidal silica abrasive during chemical mechanical polishing of sapphire, Jpn. J. Appl. Phys. 56 (2017) 07KB01-1–07KB01-4. [17] X. Xing, L.Y. Wang, W.L. Liu, H.Q. Xie, Z.T. Song, L. Jiang, Q. Shao, J. Cheng, S.W. Xiong, H.T. Liu, Mechanism of sapphire chemical mechanical polishing by SiO2 particles, J. Shanghai Second Polytech. Univ. 30 (2013) 187–196. [18] T.T. Liu, L. Hong, Nd3+-doped colloidal SiO2 composite abrasives: synthesis and the effects on chemical mechanical polishing (CMP) performances of sapphire wafers, Appl. Surf. Sci. 413 (2017) 16–26. [19] S. Salleh, L. Sudin, A. Awang, Effects of non-spherical colloidal silica slurry on AlNiP hard disk substrate CMP application, Appl. Surf. Sci. 360 ( (2016) 59–68. [20] L. Xu, X. Zhang, C.X. Kang, R.R. Wang, C.L. Zou, Y. Zhou, G.S. Pan, Preparation of a novel catalyst (SoFeIII) and its catalytic performance towards the removal rate of sapphire substrate during CMP process, Tribol. Int. 120 (2018) 99–104. [21] Z.H. Liu, J. Gong, C. Xiao, P.F. Shi, S.H. Kim, L. Chen, L.M. Qian, Temperaturedependent mechanochemical wear of silicon in water: the role of Si–OH surfacial groups, Langmuir 35 (2019) 7735–7743. [22] N. Goudarzi, M.A. Chamjangali, A.H. Amin, M. Goodarzi, Effects of surfactant and polyelectrolyte on distribution of silicate species in alkaline aqueous tetraoctylammonium silicate solutions using 29Si NMR spectroscopy, Appl. Magn. Reson. 44 (2013) 1095–1103. [23] J.L. Bass, G.L. Turner, Anion distributions in sodium silicate solutions. Characterization by 29Si NMR and infrared spectroscopies and vapor phase osmometry, J. Phys. Chem. B 101 (1997) 10638–10644. [24] D. Yin, X.H. Niu, K. Zhang, J.C. Wang, Y.Q. Cui, Preparation of MgO doped colloidal SiO2 abrasive and their chemical mechanical polishing performance on c-, r- and aplane sapphire substrate, Ceram. Int. 44 (2018) 14631–14637. [25] V.H. Bulsara, Y. Ahn, S. Chandrasekar, T.N. Farris, Polishing and lapping temperatures, J. Tribol. 119 (1997) 163–170. [26] H.S. Yoder, M.L. Keith, Complete substitution of aluminum for silicon: the system 3MnO·Al2O3·3SiO2-3Y2O3·5Al2O3, Geol. Soc. Am. Bull. 61 (1950) 1516–1517. [27] A.N. Christensen, M.S. Lehmann, P. Convert, Deuteration of crystalline hydroxides. Hydrogen bonds of gamma-AlOO(H,D) and gamma-FeOO(H,D), Acta Chem. Scand. Ser. A 36 (1982) 303–308. [28] R.F. Klevtsova, P.V. Klevtsov, The crystal structure of YOOH, J. Struct. Chem. 5 (1965) 795–797. [29] M.A. Henderson, The interaction of water with solid surfaces: fundamental aspects revisited, Surf. Sci. Rep. 46 (2002) 1–308. [30] K.C. Hass, W.F. Schneider, A. Curioni, W. Andreoni, The chemistry of water on alumina surfaces: reaction dynamics from first principles, Science 282 (1998) 265–268. [31] Z.Q. Sun, M.S. Li, Y.C. Zhou, Recent progress on synthesis, multi-scale structure, and properties of Y–Si–O oxides, Int. Mater. Rev. 59 (2014) 357–383. [32] D.A. Pawlak, K. Woźniak, Z. Frukacz, T.L. Barr, D. Fiorentino, S. Seal, ESCA studies of yttrium aluminum garnets, J. Phys. Chem. B 103 (1999) 1454–1461. [33] J.J. Chambers, G.N. Parsons, Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon, J. Appl. Phys. 90 (2001) 918–933. [34] F.S. Ohuchi, S. Ghose, M.H. Engelhard, D.R. Baer, Chemical bonding and electronic structures of the Al2SiO5 polymorphs, andalusite, sillimanite and kyanite: X-ray photoelectron- and electron energy loss spectroscopy studies, Am. Mineral. 91 (2006) 740–746. [35] H. Seyama, M. Soma, X-ray photoelectron spectroscopic study of montmorillonite containing exchangeable divalent cations, J. Chem. Soc. Faraday Trans. 80 (1) (1984) 237–248. [36] H. Harder, Clay mineral formation under lateritic weathering conditions, Clay Miner. 12 (1977) 281–288. [37] S. Cadars, R. Guégan, M.N. Garaga, X. Bourrat, L.L. Forestier, F. Fayon, V.H. Tan, T. Allier, Z. Nour, D. Massiot, New insights into the molecular structures, compositions, and cation distributions in synthetic and natural montmorillonite clays, Chem. Mater. 24 (2012) 4376–4389.
4. Conclusions In this study, a novel polishing slurry for YAG crystal consisting of 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water was developed. High MRR with low surface roughness was achieved. The reaction mechanism was studied by GIXRD and XPS to verify the chemical composition of reaction layer. The following conclusions can be drawn: (1) Using the novel polishing slurry, the surface roughness and MRR achieve 0.08 nm Ra and 34 nm/min respectively. (2) AlOOH and YOOH generated from hydration reaction react with SiOH in Na2SiO3 aqueous solution to form soft andalusite and yttrium silicate, then MgO combines with andalusite to produce montmorillonite. All soft products are removed mechanically by ZrO2 abrasives, obtaining the ultra-smooth surface of YAG crystal. As a multifunctional polishing slurry, during polishing with colloidal silica, Si–OH can react with various materials, which is also existed in the novel polishing slurry. It is promising to apply the polishing slurry to other materials such as sapphire, silicon wafer and diamond. The polishing experiments with polishing slurry containing Na2SiO3·5H2O and ZrO2 abrasives will be conducted to study the applicability to other materials. The optimized component concentrations for different materials will also be investigated to achieve better performances. Declaration of competing interest The authors declare no conflict of interest. Acknowledgements The authors would like to appreciate the financial support from National Key Research and Development Program of China (Grant No. 2016YFB1102205), Science Fund for Creative Research Groups of NSFC (No. 51621064) and National Science Foundation of China (Grant No. 51775084). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143601. References [1] H.S. Dogan, S. Tekgul, B. Akdogan, M.S. Keskin, A. Sahin, Use of the holmium:YAG laser for ureterolithotripsy in children, BJU Int. 94 (2015) 131–133. [2] G.D. Gautam, A.K. Pandey, Pulsed Nd:YAG laser beam drilling: a review, Opt. Laser Technol. 100 (2018) 183–215. [3] P.Z.Z.J. Liu, X.J. Xu, X.L. Wang, Y.X. Ma, Coherent beam combining of high powerfiber lasers: progress and prospect, Sci. China Technol. Sci. 56 (2013) 1597–1606. [4] S.J. Qin, W.J. Li, Micromachining of complex channel systems in 3D quartz substrates using Q-switched Nd:YAG laser, Appl. Phys. A Mater. Sci. Process. 74 (2002) 773–777. [5] R.L. Aggarwal, D.J. Ripin, J.R. Ochoa, T.Y. Fan, Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2 and KY(WO4)2 laser crystals in the 80–300 K temperature range, J. Appl. Phys. 98 (2005) 103514-1–103514-14. [6] J. Hostaša, V. Nečina, T. Uhlířová, V. Biasini, Effect of rare earth ions doping on the thermal properties of YAG transparent ceramics, J. Eur. Ceram. Soc. 39 (2019) 53–58. [7] S. Kostić, Z.Ž. Lazarević, V. Radojević, A. Milutinović, M. Romčević, N.Ž. Romčević, A. Valčić, Study of structural and optical properties of YAG and Nd:YAG single crystals, Mater. Res. Bull. 63 (2015) 80–87.
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A novel chemical mechanical polishing slurry for yttrium aluminum garnet crystal
T
⁎
Zili Zhang, Zhuji Jin, Jiang Guo , Xiaolong Han, Qing Mu, Xianglong Zhu Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Yttrium aluminum garnet Chemical mechanical polishing Surface roughness Material removal rate Reaction mechanism
Yttrium aluminum garnet (Y3Al5O12, YAG) crystal is widely used for laser applications. However, due to its high hard and brittle nature, high surface quality and high material removal rate (MRR) are difficult to be achieved. In this study, to solve the problem, a novel CMP slurry was developed, which contains 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water. After polishing, the surface roughness and MRR achieved 0.08 nm Ra and 34 nm/min respectively. The reaction mechanism during polishing was elucidated based on the results of grazing incidence small angle X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The results show that AlOOH and YOOH generated from hydration reaction react with Si–OH in Na2SiO3 aqueous solution to form soft andalusite and yttrium silicate, then MgO combines with andalusite to produce montmorillonite. All soft products are removed mechanically by ZrO2 abrasives, obtaining the ultrasmooth surface of YAG crystal.
1. Introduction With the development of laser technology [1–4], there has been increasing demand for some functional materials, such as yttrium aluminum garnet (Y3Al5O12, YAG) crystal with high thermal conductivity, superior ability of doping rare earth ions and fine optical property [5–7]. However, due to its high brittle nature, high surface quality is hard to be achieved, resulting in localized energy accumulation for laser scattering to deteriorate laser performances [8,9]. In addition, high hard nature and chemical stability lead to a low material removal rate (MRR), increasing the cost of processing. In recent decades, for minimizing the negative outcomes, different ultra-precision machining methods were utilized including mechanical polishing, chemical polishing and chemical mechanical polishing (CMP). Kim et al. [10,11] mechanically polished YAG crystal by hard abrasives, but surface was prone to dislocations, fine scratches and large damages after processing. Kostić et al. [7,12] chemically polished YAG crystal with high-concentration phosphoric acid at high temperature, after which the material removal was nonuniform and corrosion pits occurred. Moreover, this polishing method was harmful to the equipment and environment. Therefore, single mechanical or chemical polishing is disadvantageous to the processing of YAG crystal. In contrast to them, CMP can remove materials by soft abrasives and achieve high surface quality [13]. Furthermore, CMP slurry has an ⁎
important effect on the processing quality and cost of CMP. At present, traditional CMP slurries for YAG crystal are mainly colloidal silica. Li et al. [14] investigated the effects of polishing pad materials and pH of colloidal silica on the processing quality of YAG crystal, and found that a surface with 0.2 nm Ra in an area of 5 μm × 5 μm could be obtained with acid colloidal silica and chemcloth pad. In order to improve the surface quality, McKay [15] polished YAG crystal with mixed slurry of SiO2 suspension and NaOH, and a surface with 0.1 nm Ra in an area of 40 μm × 40 μm was achieved, but the MRR was only 0.3 nm/min. In addition, MRR declined during the reusing of colloidal silica for the consumption of Si–OH, affecting the utilization of colloidal silica [16,17]. Aiming at improving the performances of colloidal silica, some researchers [18–20] made efforts in the modification of colloidal silica, but the performance improvements were limited. Comprehensively, there are many problems in the CMP slurries for YAG crystal. Therefore, it is essential to develop a high-performance polishing slurry. During polishing with colloidal silica, Si–OH distributes on the surface of silica particles as reactant [16,21], so the chemical reaction and mechanical abrasion are bundled together. This interaction mode goes against the balance between chemical and mechanical effects and limits the performance. Thus, it is necessary to detach Si–OH from abrasive particles. In the Na2SiO3 aqueous solution, SiO32− mainly exists in the forms of HSiO43−, H2SiO42− and H3SiO4− due to hydrolysis reaction [22,23], resulting in the formation of Si–OH. In this study,
Corresponding author. E-mail address: [email protected] (J. Guo).
https://doi.org/10.1016/j.apsusc.2019.143601 Received 28 May 2019; Received in revised form 2 August 2019; Accepted 6 August 2019 Available online 06 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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based on colloidal silica, the chemical effect of Si–OH was replaced by Na2SiO3·5H2O to detach it from mechanical effect. At the same time, the mechanical abrasion was realized by ZrO2 abrasives. In addition, on account of the result that adding MgO in colloidal silica can improve the surface quality and MRR during sapphire polishing [24], MgO was also considered in the components of polishing slurry. After optimizing the component concentrations by orthogonal experiments, a novel CMP slurry was developed for YAG crystal polishing, which contains 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water. According to the results of grazing incidence small angle X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS), reaction mechanism was elucidated.
Table 2 Components of different polishing slurries. Number
Solvent
ZrO2/wt%
Na2SiO3·5H2O/wt%
MgO/wt%
pH
1 2 3 4 5
Alcohol Deionized water Deionized water Deionized water Deionized water
8 8 8 8 8
0 0 0 5 5
0 0 0 0 0.3
– 3 13 13 13
3. Results and discussion 3.1. Performances of polishing slurries In order to compare the performances of the novel polishing slurry and colloidal silica, YAG crystals were polished with these two polishing slurries under the same processing parameters. Fig. 1 shows the results of surface roughness Ra and MRR. Surface roughness achieves 0.08 nm Ra in an area of 10 μm × 10 μm after polishing with the novel polishing slurry. Compared with 0.15 nm Ra obtained by colloidal silica, it reduces by 47%. Besides, MRR using the novel polishing slurry is 34 nm/min, which increases by 240% compared with 10 nm/min obtained by colloidal silica. Therefore, it is demonstrated that the novel polishing slurry has superior performances when polishing YAG crystal.
2. Experimental details 2.1. Materials and methods YAG crystal (111) was used as workpiece which was a cylinder with a diameter of 15 mm and a height of 2 mm. Three pieces of YAG crystals were adhered to a ZrO2 ceramic disc with paraffin. The purity of Na2SiO3·5H2O was 99.9%. The average diameter of MgO particles was 30 nm. The abrasives in the novel polishing slurry were monoclinic ZrO2 particles with an average diameter of 80 nm. The polishing experiments were performed on automatic grinding and polishing machine (UNIPOL1200S, Shenyang Kejing Automation Equipment Co., Ltd., China). The schematic of polishing machine is shown in Fig. S2. Commercial colloidal silica (COMPOL, FUJIMI Corporation) containing 10 wt% silica abrasives was used as a control of the novel CMP slurry. Table 1 shows the processing parameters of grinding and polishing. After polishing, YAG crystals were cleaned with deionized water and dried with an air syringe. For the purpose of investigating the effects of different components in the novel CMP slurry, different polishing slurries were prepared to polish YAG crystals. Table 2 lists the components of polishing slurries. NaOH was added into polishing slurry 3 to adjust pH to 13, which was consistent with polishing slurry 4.
3.2. Reaction mechanism 3.2.1. Effects of different components on polishing For the purpose of researching the effects of different components on polishing in the novel polishing slurry, MRR using different slurries shown in Table 2 was measured. As seen in Fig. S3, when alcohol is used as solvent of polishing slurry 1, MRR is extremely low and negligible. Because ZrO2 abrasives (Moh's hardness 7.5) are softer than YAG crystal (Moh's hardness 8.5), onefold mechanical action of ZrO2 abrasives can't achieve the material removal with polishing slurry 1. However, when deionized water is utilized as solvent of polishing slurry 2, MRR increases significantly. It is speculated that deionized water can react with the YAG crystal and a hydration layer is generated on the surface. Furthermore, the hardness of hydration layer is smaller than ZrO2 abrasives, so the hydration layer can be removed with ZrO2 abrasives. Besides, MRR is improved significantly with the successive addition of NaOH, Na2SiO3 and MgO. It indicates that NaOH, Na2SiO3 and MgO are beneficial to the chemical reaction and can promote material removal. Fig. S4 displays surface morphologies of YAG crystals after polishing for 120 min with different polishing slurries. It can be seen that surface quality becomes better and better with the successive addition of NaOH, Na2SiO3 and MgO, which is well consistent with MRR. Comprehensively, these phenomena indicate that deionized water, alkaline environment, Na2SiO3 and MgO all play important roles in the CMP of YAG crystal.
2.2. Characterization After grinding, YAG crystals were polished with different polishing slurries for 120 min, surface morphologies were observed by an optical microscope (MX-40, Olympus, Japan). Surface roughness Ra was measured by atomic force microscope (AFM, XE-200, Park system corporation, Korea). MRR was calculated by scratch method (The details are shown in supporting material). The depths of scratches were measured by surface profiler (NewView 5022, Zygo, USA). Before XRD and GIXRD detection, to obtain an initial flat surface, YAG crystals were polished with Al2O3 abrasives and then cleaned in alcohol by ultrasonic cleaner. Besides, YAG crystals were immersed into deionized water at 50 °C for 30 min to simulate the polishing environment before GIXRD measurement [25]. XRD and GIXRD patterns were measured in X-ray diffractometer (Empyrean, PANalytical B.V., Netherlands). XPS spectra of different elements were measured by X-ray photoelectron spectrometer (Escalab 250Xi, ThermoFisher, England) and calibrated by applying adventitious C 1s signal at 284.8 eV.
3.2.2. Hydration reaction In order to investigate the products of hydration reaction, GIXRD pattern of YAG crystal surface after being immersed in water and XRD pattern of initial YAG crystal surface were measured respectively. The patterns are shown in Fig. 2. There is only one characteristic peak of YAG at 52.7° in XRD pattern, which is attributed to the (111) orientation (JCPDS No. 72-1315) [26]. Instead, after immersing YAG crystal in deionized water, the characteristic peak of YAG crystal disappears. At the same time, the characteristic peaks of AlOOH at 36.3°, 38.0° and 43.4° and YOOH at 44.9° and 46.4° appear in GIXRD pattern (JCPDS No. 88-2111 and No. 74-2350) [27,28]. These changes of patterns prove that a hydration layer is generated on the surface of YAG crystal including AlOOH and YOOH during polishing. Many researchers have studied the interaction of water with solid surfaces [29,30]. Based on these researches and experiment results in
Table 1 Processing parameters. Process
Pad
Rotate speed of plate (r/min)
Flow rate (ml/min)
Pressure (MPa)
Grinding Polishing
3 M 673LAA10 IC1000
30 80
10 6
0.03 0.03
2
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Fig. 1. Surface morphologies and line profiles of YAG crystals after polishing with (a) colloidal silica and (b) the novel polishing slurry, and (c) contrasts of Ra and MRR after polishing with these two slurries.
Fig. 2. The XRD and GIXRD patterns of YAG crystal surface. 3
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Fig. 3. The hydration reaction process on YAG crystal surface.
Fig. 4. The XPS spectra of (a) Si 2p after polishing, (b) O 1s before polishing, (c) O 1s after polishing, (d) Al 2p before polishing, (e) Al 2p after polishing, (f) Y 3d before polishing and (g) Y 3d after polishing.
4
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as shown in Fig. 4. After analyzing the XPS spectra comprehensively, all changes of binding energies indicate that andalusite and yttrium silicate are generated on YAG crystal surface during polishing. Specific analyses are as following. Based on the Si 2p spectra in Fig. 4(a), after polishing, the Si 2p spectra of surface layer can be detected and resolved into two peaks: peaks at 102.51 eV and 101.40 eV, corresponding to andalusite and yttrium silicate respectively. Furthermore, as shown in Fig. 4(c), the O 1s peaks at 531.30 eV and 530.80 eV after polishing are well consistent with these two products. Besides, Al 2p peak at 74.27 eV assigned to andalusite appears in Fig. 4(e), and Y 3d peaks at 158.00 eV and 160.05 eV in Fig. 4(g) correspond to yttrium silicate. (The reference values of binding energies are listed in Table 3.) The hardness of andalusite (Moh's hardness 6.5–7) and yttrium silicate (Moh's hardness 5) are both smaller than that of ZrO2 abrasives, which is beneficial to material removal [31]. Reaction mechanism is proposed in Fig. 5. In the polishing slurry, Si–OH generated from hydrolysis reaction of SiO32− can be adsorbed on the surface of YAG crystal. Then Si–OH trends to combine with Al–OH and Y–OH of hydration layer, forming Si–O–Al and Si–O–Y by dehydration. The reaction equations are proposed as Eqs. (1) and (2).
Table 3 Binding energy of different elements (eV). Chemical state
Si 2p
Al 2p
O 1s
Y 3d
Mg 1s
YAG [32]
–
–
101.40
–
Andalusite [34] MgO [35] Montmorillonite [35]
102.51 – 102.95
74.27 – –
531.30 – –
157.43 159.44 158.00 160.05 – – –
–
Yttrium silicate [33]
530.47 531.94 530.80
– – 1303.90 1305.30
2AlOOH + SiO32 − → Al2SiO5 + 2OH−
(1)
2YOOH + SiO32 − → Y2SiO5 + 2OH−
(2)
3.2.4. The role of MgO Fig. 6 shows the Mg 1s and Si 2p spectra of YAG crystal surface after polishing with polishing slurry 5 (the novel polishing slurry). In Fig. 6(a), the peak of Mg 1s spectra is at the position of 1305.3 eV, which is higher than the binding energy of MgO in polishing slurry (1303.9 eV) and corresponds to Mg-montmorillonite. In addition, as seen in Fig. 6(b), the deconvolution of Si 2p spectra contains three overlapping peaks at 102.51 eV, 101.40 eV and 102.95 eV assigned to andalusite, yttrium silicate and montmorillonite respectively. (The reference values of binding energies are listed in Table 3.) Compared with the results in the Section 3.2.3, it is demonstrated that a new product Mg-montmorillonite is generated after adding MgO to the polishing slurry 4. Montmorillonite is a kind of aluminosilicate. At the same time, based on the XPS results, Mg 1s peak attributed to Mg-montmorillonite appears after polishing. It indicates that the components of montmorillonite include Mg, Si, Al and O. Therefore, the generation of montmorillonite results from the reaction of YAG crystal, Na2SiO3 and MgO in water. Harder [36] researched the synthesis of mineral and proposed that MgO could promote the conversion of silicate mineral salts to the mineral with three-layer structure. Furthermore, montmorillonite is a kind of mineral with three-layer structure as shown in Fig. S5 [37]. Therefore, the generation of montmorillonite is due to the secondary reaction of andalusite (the reaction product shown in Eq. (1)) and MgO. As a product of secondary reaction, the concentration of montmorillonite is low, corresponding to its small area of Si 2p peak shown in Fig. 6(b). Montmorillonite is softer than andalusite and can be removed
Fig. 5. The process of dehydration reaction.
this study, a hydration reaction mechanism is proposed. The generation of AlOOH and YOOH results from the formation of Al–OH and Y–OH on the YAG crystal surface. As shown in Fig. 3, due to the fracture of AleO bonds and YeO bonds, a lot of O vacancies and unsaturated metal atoms are generated on the surface of YAG crystal. The unsaturated metal atoms trend to adsorb H2O by forming AleOw bonds and YeOw bonds (Ow, O atoms in H2O). Meanwhile, the H2O dissociates into –OwH groups and H atoms. Then the H atoms combine with neighboring O atoms and form –OsH groups (Os, O atoms on YAG crystal surface). Consequently, two types of hydroxyl groups (–OwH and –OsH) are obtained, forming AlOOH and YOOH. Besides, H2O is more inclined to dissociate into –OwH groups in the alkali environment, promoting the hydration reaction [29]. Therefore, polishing slurry 3 has a better performance than polishing slurry 2.
3.2.3. The role of Na2SiO3 The XPS spectra of different elements on YAG crystal surface before and after polishing with polishing slurry 4 were detected respectively,
Fig. 6. The XPS spectra of YAG crystal surface after polishing with polishing slurry 5, (a) Mg 1s and (b) Si 2p. 5
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easily, so this conversion is beneficial to the improvement of MRR.
[8] Y.L. Fu, J. Li, Y. Liu, L. Liu, H. Zhao, Y.B. Pan, Influence of surface roughness on laser-induced damage of Nd:YAG transparent ceramics, Ceram. Int. 41 (2015) 12535–12542. [9] H. Kim, R.S. Hay, S.A. Mcdaniel, G. Cook, N.G. Usechak, A.M. Urbas, K.N. Shugart, H. Lee, A.H. Kadhim, D.P. Brown, Lasing of surface-polished polycrystalline Ho:YAG (yttrium aluminum garnet) fiber, Opt. Express 25 (2017) 6725–6731. [10] R. Daniel, Y. Hitomi, Nanometer-scale characteristics of polycrystalline YAG ceramic polishing, CIRP Ann. 67 (2018) 349–352. [11] H. Kim, R.S. Hay, S.A. McDaniel, G. Cook, N.G. Usechak, A.M. Urbas, H.D. Lee, R.G. Corns, K.N. Shugart, A.H. Kadhim, D.P. Brown, B. Griffin, Optical characterizations on surface-polished polycrystalline YAG fibers, Proc. SPIE 10192 (2017) 101920B-1–101920B-7. [12] P.Z. Yang, P.Z. Deng, Z.W. Yin, Y.L. Tian, The growth defects in czochralski-grown Yb:YAG crystal, J. Cryst. Growth 218 (2000) 87–92. [13] B.Z. Parshuram, K. Ashok, A.K. Sikder, Chemical mechanical planarization for microelectronics applications, Mater. Sci. Eng. R 45 (2004) 89–220. [14] J. Li, Y. Zhu, C.T. Chen, Chemical mechanical polishing of transparent Nd:YAG ceramics, Key Eng. Mater. 375-376 (2008) 278–282. [15] J. McKay, Chemical Mechanical polishing and Direct Bonding of YAG and Y2O3, Ph.D. Thesis University of California, 2016. [16] B.A. Natthaphon, Y. Yutaka, S. Koya, K. Panart, S. Keisuke, Study on effect of the surface variation of colloidal silica abrasive during chemical mechanical polishing of sapphire, Jpn. J. Appl. Phys. 56 (2017) 07KB01-1–07KB01-4. [17] X. Xing, L.Y. Wang, W.L. Liu, H.Q. Xie, Z.T. Song, L. Jiang, Q. Shao, J. Cheng, S.W. Xiong, H.T. Liu, Mechanism of sapphire chemical mechanical polishing by SiO2 particles, J. Shanghai Second Polytech. Univ. 30 (2013) 187–196. [18] T.T. Liu, L. Hong, Nd3+-doped colloidal SiO2 composite abrasives: synthesis and the effects on chemical mechanical polishing (CMP) performances of sapphire wafers, Appl. Surf. Sci. 413 (2017) 16–26. [19] S. Salleh, L. Sudin, A. Awang, Effects of non-spherical colloidal silica slurry on AlNiP hard disk substrate CMP application, Appl. Surf. Sci. 360 ( (2016) 59–68. [20] L. Xu, X. Zhang, C.X. Kang, R.R. Wang, C.L. Zou, Y. Zhou, G.S. Pan, Preparation of a novel catalyst (SoFeIII) and its catalytic performance towards the removal rate of sapphire substrate during CMP process, Tribol. Int. 120 (2018) 99–104. [21] Z.H. Liu, J. Gong, C. Xiao, P.F. Shi, S.H. Kim, L. Chen, L.M. Qian, Temperaturedependent mechanochemical wear of silicon in water: the role of Si–OH surfacial groups, Langmuir 35 (2019) 7735–7743. [22] N. Goudarzi, M.A. Chamjangali, A.H. Amin, M. Goodarzi, Effects of surfactant and polyelectrolyte on distribution of silicate species in alkaline aqueous tetraoctylammonium silicate solutions using 29Si NMR spectroscopy, Appl. Magn. Reson. 44 (2013) 1095–1103. [23] J.L. Bass, G.L. Turner, Anion distributions in sodium silicate solutions. Characterization by 29Si NMR and infrared spectroscopies and vapor phase osmometry, J. Phys. Chem. B 101 (1997) 10638–10644. [24] D. Yin, X.H. Niu, K. Zhang, J.C. Wang, Y.Q. Cui, Preparation of MgO doped colloidal SiO2 abrasive and their chemical mechanical polishing performance on c-, r- and aplane sapphire substrate, Ceram. Int. 44 (2018) 14631–14637. [25] V.H. Bulsara, Y. Ahn, S. Chandrasekar, T.N. Farris, Polishing and lapping temperatures, J. Tribol. 119 (1997) 163–170. [26] H.S. Yoder, M.L. Keith, Complete substitution of aluminum for silicon: the system 3MnO·Al2O3·3SiO2-3Y2O3·5Al2O3, Geol. Soc. Am. Bull. 61 (1950) 1516–1517. [27] A.N. Christensen, M.S. Lehmann, P. Convert, Deuteration of crystalline hydroxides. Hydrogen bonds of gamma-AlOO(H,D) and gamma-FeOO(H,D), Acta Chem. Scand. Ser. A 36 (1982) 303–308. [28] R.F. Klevtsova, P.V. Klevtsov, The crystal structure of YOOH, J. Struct. Chem. 5 (1965) 795–797. [29] M.A. Henderson, The interaction of water with solid surfaces: fundamental aspects revisited, Surf. Sci. Rep. 46 (2002) 1–308. [30] K.C. Hass, W.F. Schneider, A. Curioni, W. Andreoni, The chemistry of water on alumina surfaces: reaction dynamics from first principles, Science 282 (1998) 265–268. [31] Z.Q. Sun, M.S. Li, Y.C. Zhou, Recent progress on synthesis, multi-scale structure, and properties of Y–Si–O oxides, Int. Mater. Rev. 59 (2014) 357–383. [32] D.A. Pawlak, K. Woźniak, Z. Frukacz, T.L. Barr, D. Fiorentino, S. Seal, ESCA studies of yttrium aluminum garnets, J. Phys. Chem. B 103 (1999) 1454–1461. [33] J.J. Chambers, G.N. Parsons, Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon, J. Appl. Phys. 90 (2001) 918–933. [34] F.S. Ohuchi, S. Ghose, M.H. Engelhard, D.R. Baer, Chemical bonding and electronic structures of the Al2SiO5 polymorphs, andalusite, sillimanite and kyanite: X-ray photoelectron- and electron energy loss spectroscopy studies, Am. Mineral. 91 (2006) 740–746. [35] H. Seyama, M. Soma, X-ray photoelectron spectroscopic study of montmorillonite containing exchangeable divalent cations, J. Chem. Soc. Faraday Trans. 80 (1) (1984) 237–248. [36] H. Harder, Clay mineral formation under lateritic weathering conditions, Clay Miner. 12 (1977) 281–288. [37] S. Cadars, R. Guégan, M.N. Garaga, X. Bourrat, L.L. Forestier, F. Fayon, V.H. Tan, T. Allier, Z. Nour, D. Massiot, New insights into the molecular structures, compositions, and cation distributions in synthetic and natural montmorillonite clays, Chem. Mater. 24 (2012) 4376–4389.
4. Conclusions In this study, a novel polishing slurry for YAG crystal consisting of 8 wt% ZrO2 abrasives, 5 wt% Na2SiO3·5H2O, 0.3 wt% MgO and deionized water was developed. High MRR with low surface roughness was achieved. The reaction mechanism was studied by GIXRD and XPS to verify the chemical composition of reaction layer. The following conclusions can be drawn: (1) Using the novel polishing slurry, the surface roughness and MRR achieve 0.08 nm Ra and 34 nm/min respectively. (2) AlOOH and YOOH generated from hydration reaction react with SiOH in Na2SiO3 aqueous solution to form soft andalusite and yttrium silicate, then MgO combines with andalusite to produce montmorillonite. All soft products are removed mechanically by ZrO2 abrasives, obtaining the ultra-smooth surface of YAG crystal. As a multifunctional polishing slurry, during polishing with colloidal silica, Si–OH can react with various materials, which is also existed in the novel polishing slurry. It is promising to apply the polishing slurry to other materials such as sapphire, silicon wafer and diamond. The polishing experiments with polishing slurry containing Na2SiO3·5H2O and ZrO2 abrasives will be conducted to study the applicability to other materials. The optimized component concentrations for different materials will also be investigated to achieve better performances. Declaration of competing interest The authors declare no conflict of interest. Acknowledgements The authors would like to appreciate the financial support from National Key Research and Development Program of China (Grant No. 2016YFB1102205), Science Fund for Creative Research Groups of NSFC (No. 51621064) and National Science Foundation of China (Grant No. 51775084). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143601. References [1] H.S. Dogan, S. Tekgul, B. Akdogan, M.S. Keskin, A. Sahin, Use of the holmium:YAG laser for ureterolithotripsy in children, BJU Int. 94 (2015) 131–133. [2] G.D. Gautam, A.K. Pandey, Pulsed Nd:YAG laser beam drilling: a review, Opt. Laser Technol. 100 (2018) 183–215. [3] P.Z.Z.J. Liu, X.J. Xu, X.L. Wang, Y.X. Ma, Coherent beam combining of high powerfiber lasers: progress and prospect, Sci. China Technol. Sci. 56 (2013) 1597–1606. [4] S.J. Qin, W.J. Li, Micromachining of complex channel systems in 3D quartz substrates using Q-switched Nd:YAG laser, Appl. Phys. A Mater. Sci. Process. 74 (2002) 773–777. [5] R.L. Aggarwal, D.J. Ripin, J.R. Ochoa, T.Y. Fan, Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2 and KY(WO4)2 laser crystals in the 80–300 K temperature range, J. Appl. Phys. 98 (2005) 103514-1–103514-14. [6] J. Hostaša, V. Nečina, T. Uhlířová, V. Biasini, Effect of rare earth ions doping on the thermal properties of YAG transparent ceramics, J. Eur. Ceram. Soc. 39 (2019) 53–58. [7] S. Kostić, Z.Ž. Lazarević, V. Radojević, A. Milutinović, M. Romčević, N.Ž. Romčević, A. Valčić, Study of structural and optical properties of YAG and Nd:YAG single crystals, Mater. Res. Bull. 63 (2015) 80–87.
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