materials Article
The Synthesis of the Core/Shell Structured Diamond/Akageneite Hybrid Particles with Enhanced Polishing Performance Jing Lu 1,2, *, Yongchao Xu 1,2 , Dayu Zhang 1,2 and Xipeng Xu 1,2 1 2
*
Institute of Manufacturing Engineering, Huaqiao University, Xiamen 361021, China;
[email protected] (Y.X.);
[email protected] (D.Z.);
[email protected] (X.X.) MOE Engineering Research Center for Brittle Materials Machining, Huaqiao University, Xiamen 361021, China Correspondence:
[email protected]; Tel.: +86-592-6162359
Academic Editor: Jaroslaw Drelich Received: 16 February 2017; Accepted: 16 June 2017; Published: 20 June 2017
Abstract: In this study, the synthesis of the core/shell structured diamond/akageneite hybrid particles was performed through one-step isothermal hydrolyzing. The hybrid particle was characterized by X-ray diffraction, field emission scanning electron microscopy, and Fourier transform infrared spectra. The test results overall reveal that the akageneite coating, phase β-FeO(OH), was uniformly coated onto the diamond surface. The polishing performance of the pristine diamond and hybrid particles for the sapphire substrate was evaluated respectively. The experimental results show that the hybrid particles exhibited improved polishing quality and prolonged effective processing time of polishing pad compared with diamond particles without compromising the material remove rate and surface roughness. The improved polishing behavior might be attributed to the β-FeOOH coating, which is conducive to less abrasive shedding and reducing the scratch depth. Keywords: hybrid particle; isothermal hydrolyzing; sapphire; polishing performance
1. Introduction Sapphire, the most common substrate used in light emitting diodes (LEDs), exhibits excellent mechanical and optical properties and is widely used in a range of applications, such as optics, electronics, and temperature sensing, etc. [1–6]. In order to realize these applications based on sapphire, a smooth and planar surface without scratches and subsurface-damaged layers is required [7–10]. Currently, the main planarization machining to realize the precision requirement for sapphire substrate is still abrasive machining, especially at fine grain size. However, for the free abrasive polishing, such as chemical mechanical polishing (CMP), it is extremely difficult to polish the sapphire substrates to obtain high material remove rate (MRR) and perfect polished surface simultaneously. For the fixed abrasive polishing, the abrasive particles with agglomerate effect are difficult to be dispersed evenly in the binder when the size of the particle is small to a certain degree, which is difficult to obtain a high-quality surface [11,12]. So, the creation of nontraditional polishing tools has attracted considerable attention to process sapphire substrate. Based on the principle of sol-gel, a semi-fixed abrasive polishing pad with diamond abrasive, called SG polishing pad, is an ideal tool that satisfies the processing demands of various kinds of hard and brittle materials [13–15]. However, the abrasives easily fall off from the SG polishing tool during process, leading to the low efficiency and short life of the polishing tool. Moreover, the scratch defect and mechanical damage are easily induced because the diamond particles are irregular in shape and possess sharp edges, corners, and apexes with high hardness.
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Core/shell structures exhibit unique structure and special properties and thus can be used to efficiently change the mechanical properties of abrasives [16–19]. Akageneite features a tunnel structure and superior capacity in catalysts [20–22]. Akageneite also possesses high specific surface area and substantial –OH; as such, this mineral displays potential for promoting binding force with the matrix of polishing pad. Moreover, akageneite with low hardness value can reduce the mechanical damages caused by the sharp edges of diamond abrasives. In this study, we described a novel route for preparing core/shell structured diamond/akageneite hybrid particle via one-step isothermal hydrolyzing to reduce the abrasive shedding and mechanical damage during polishing. In contrast to forced hydrolysis and other severe preparations which are always under the temperature above 90 ◦ C [23], this isothermal hydrolyzing procedure under relatively low temperature (40 ◦ C) is very slow and soft [24,25]. The formation of akageneite is prone to occur on the surface of diamond rather than in the solution. To elucidate their structure and composition, the hybrid particles were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and Fourier transform infrared spectra (FTIR). Moreover, based on the SG polishing tool, the polishing performance of the pristine diamond and hybrid particles for sapphire substrate were evaluated respectively. 2. Experimental Section 2.1. Synthesis of Diamond/Akageneite Hybrid Particles The hybrid particles were synthesized by one-step isothermal hydrolyzing method. The iron (III) chloride hexahydrate (AR) was purchased from Xilong Chemical Co., Ltd. (Guangzhou, China). Diamond abrasives with similar shape and an average primary size of 3 µm were produced by Element Six Trading CO., Ltd. (Shanghai, China). Solid FeCl3 ·6H2 O was first dissolved in 400 mL of distilled water to produce 0.04 M solution under magnetic stirring. Then 0.5 g of diamond was added into the aqueous solution. After 5 min ultrasonic vibration, a yellow suspension with diamond abrasives homogeneously dispersed was obtained. The stable aqueous suspension was heated at 40 ◦ C and stirred in a thermostatic water bath for 48 h to ensure the isothermal hydrolyzing of FeCl3 . After that, the precipitate was further processed by centrifugal cleaning and drying. The mechanism for the formation of core-shell structural diamond/akageneite hybrid particle by the isothermal hydrolyzing of FeCl3 aqueous solution could be given as FeCl3 + 3H2 O → Fe(OH)3 + 3HCl
(1)
Fe(OH)3 → β-FeOOH + H2 O
(2)
The XRD profile of the hybrid particle was examined by an X-ray diffractometer (X’ Pert PRO, PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation (λ = 1.54055 Å) with an accelerating voltage of 40 kV and a current of 40 mA. The morphology and microstructure of the samples were observed by a FESEM (S-4800, Hitachi Limited, Tokyo, Japan), FTIR measurement of hybrid particle was conducted using a FT-IR spectrometer (Thermo Scientific Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA). 2.2. Polishing Setup and Process Control The epi-ready sapphire wafers were purchased commercially. Sapphire wafer-oriented (0 0 0 1) plane with two-inch diameter after grinding was used. The original surface roughness (Ra) of the wafer after lapping is about 1.8 nm. Pristine diamond and hybrid particles were used as abrasives in SG polishing pad, respectively. The flexible matrix of SG polishing pad was made from sodium alginate (AGS), and the fabrication of the SG polishing pad mainly included mixing, screeding, gelling, and drying. A rotary-type polishing machine (AUTOPOL-1200S, Kejing, Shenyang, China) was used in the sapphire-polishing experiments. The pressure between the sample clamp and the polishing
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pad was 5 kg. The speed of workpiece/pad rotation was 60/120 rpm and the treatment time was 3 h. Deionized water was applied as coolant. After processing, the wafers were cleaned immediately with Materials ethanol2017, and deionized water under sonication. The surface roughness (Ra) and morphology of 10, 673 3 of 8 processed substrates were measured by 3D optical interferometry profiler (New View™ 7300, Zygo, morphology processed substrates measured by 3D optical interferometry profiler (New Middlefield, OH,of USA). For the testing ofwere surface roughness polished by diamond abrasives, the deep View™ 7300, Zygo, Middlefield, OH, USA). For the testing of surface roughness polished by scratches were avoided. The MRR as a function of processing time is calculated by diamond abrasives, the deep scratches were avoided. The MRR as a function of processing time is calculated by 107 × ∆m
MRR =
=
ρ × 10 2.54×2 × ∆ π×t × 2.54 ×
×
(3)
(3)
where ∆m (mg) is the mass loss of sapphire wafer after polishing, t (min) is the processing time, ρ is 3 ). where Δm (mg) density is the mass lossg/cm of sapphire wafer after polishing, t (min) is the processing time, ρ is the sapphire wafer (3.98 the sapphire wafer density (3.98 g/cm3).
3. Results and Discussion
3. Results and Discussion
The XRD patterns of diamond/akageneite hybrid particle with different reaction durations are The XRD patterns of diamond/akageneite hybrid particle with different reaction durations are presented in Figure 1. The characteristic peaks at 2θ = 11.8◦ , 16.8◦ , 23.8◦ , 26.7◦ , 34.0◦ , 35.2◦ , 39.2◦ , presented in ◦Figure◦ 1. The characteristic peaks at 2θ = 11.8°, 16.8°, 23.8°, 26.7°, 34.0°, 35.2°, 39.2°, 46.4◦46.4°, , 52.0◦52.0°, , 55.955.9°, , 61.161.1° and 64.4◦ could be indexed as akageneite (PDF-34-1266). The characteristic and 64.4° could be indexed as akageneite (PDF-34-1266). The characteristic ◦ , 75.3◦ , and 91.5◦ could be indexed as diamond (PDF-06-0675). All of the visible peakspeaks at 2θat=2θ 43.9 = 43.9°, 75.3°, and 91.5° could be indexed as diamond (PDF-06-0675). All of the visible diffraction peaks match well with akageneiteand anddiamond, diamond, which were mixed diffraction peaks match well withthe thefeature featurepeaks peaks of of akageneite which were mixed with with the diamond and and akageneite phase. NoNo third phase was of the the diamond akageneite phase. third phase wasobserved. observed.The Thediffraction diffraction intensity intensity of the akageneite increased with its growth the reaction increased. akageneite increased with its growth as theasreaction timetime increased.
Figure 1. XRD patterns of diamond/akageneite hybrid particle with different reaction durations. Figure 1. XRD patterns of diamond/akageneite hybrid particle with different reaction durations.
Figure 2a shows the FESEM image of pristine diamond particles. It can be seen that the pristine Figure 2aparticles shows the of pristine particles. It canFigure be seen the pristine diamond are FESEM uniformimage in shape and the diamond particle surface is clean. 2b that presents the diamond particles are uniform in shape and the particle surface is clean. Figure 2b presents status of nucleation and growth of β-FeOOH on the diamond particle surface. β-FeOOH continues to the status of nucleation and growth diamond particle surface. continues grow with the reaction until of theβ-FeOOH formation on of the uniform spindle-shaped coatingβ-FeOOH on the diamond particle surface, as shown in Figure 2c. Figure of 2d uniform shows thespindle-shaped thermogravimetry-differential thermal to grow with the reaction until the formation coating on the diamond analysis (TGA-DTA) curves of the hybrid particle under 48 h of reaction. As shown in Figure 2d,thermal the particle surface, as shown in Figure 2c. Figure 2d shows the thermogravimetry-differential endothermic peak at about 700 °C corresponds to the 89.7% weight loss on thermogravimetry (TG) analysis (TGA-DTA) curves of the hybrid particle under 48 h of reaction. As shown in Figure 2d, curve, which is attributed to the of diamond. The weight loss ofloss diamond on TG curve ◦ C corresponds the endothermic peak at about 700oxidation to the 89.7% weight on thermogravimetry indicates that the weight percent of the coating is around 10%.
(TG) curve, which is attributed to the oxidation of diamond. The weight loss of diamond on TG curve indicates that the weight percent of the coating is around 10%.
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Figure 2. Morphologies of diamond/akageneite hybrid particles under different reaction durations: Figure 2. Morphologies of diamond/akageneite hybrid particles under different reaction durations: (a)Figure 0 h; (b) 24 h; (c) 48 h; and TGA-DTA curves (d) of the particles hybrid particle under 48 reaction h of reaction. of diamond/akageneite under different durations: (a) 0 h; (b)2.24Morphologies h; (c) 48 h; and (d) TGA-DTA curveshybrid of the hybrid particle under 48 h of reaction. (a) 0 h; (b) 24 h; (c) 48 h; and TGA-DTA curves (d) of the hybrid particle under 48 h of reaction.
The FTIR spectra of hybrid particles with different reaction durations are shown in Figure 3. The FTIR spectra with different reaction durations are shown in Figure The absorption peaks atof1087 and particles 699 cm−1 are attributed the –OH stretching and Fe–Oinvibrational The FTIR spectra ofhybrid hybrid particles with differenttoreaction durations are shown Figure 3. 3. −1 are attributed to the –OH stretching and Fe–O vibrational The absorption peaks at 1087 and 699 cm −1 modes of β-FeOOH [23,26,27], which benefit for the interface bonding. Compared with the FTIR The absorption peaks at 1087 and 699iscm are attributed to the –OH stretching and Fe–O vibrational modes ofofof β-FeOOH [23,26,27], which for interface bonding.Compared Compared with FTIR spectra pristine diamond particles, the absorption of β-FeOOH gradually increased with modes β-FeOOH [23,26,27], whichis isbenefit benefit for the thepeaks interface bonding. with thethe FTIR spectra of pristine diamond thethe absorption peaks of of β-FeOOH thespectra increasing reaction time.particles, of pristine diamond particles, absorption peaks β-FeOOHgradually graduallyincreased increasedwith withthe increasing reaction time. the increasing reaction time.
Figure 3. FTIR spectra of diamond/akageneite hybrid particles with different reaction durations. Figure FTIR spectraofofdiamond/akageneite diamond/akageneite hybrid durations. Figure 3. 3. FTIR spectra hybridparticles particleswith withdifferent differentreaction reaction durations.
Figure by diamond diamond and and hybrid hybrid Figure4 4exhibits exhibitsthe theroughness roughness ofof sapphire sapphire substrate substrate polished polished by abrasives at different processing time periods. In Figure 4, under the same processing conditions, the abrasives at different processing time periods. In Figure 4, under the same processing conditions, the roughness is 0.2 0.2 nm nm smaller smaller than thanthese these roughnessofofsapphire sapphiresubstrate substratesurface surfacepolished polished by by hybrid hybrid particles particles is
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Figure 4 exhibits the roughness of sapphire substrate polished by diamond and hybrid abrasives at different processing time periods. In Figure 4, under the same processing conditions, the5roughness Materials 2017, 10, 673 Materials 2017, 10, 673 of58of 8 of sapphire substrate surface polished by hybrid particles is 0.2 nm smaller than these polished by polished by diamond particles. Meanwhile, for diamond abrasives, thestandard standard deviation of Ra polished by diamond particles. Meanwhile, forthe theabrasives, diamond abrasives, the deviation of Ra diamond particles. Meanwhile, for the diamond the standard deviation of Ra increases when the processing time isis120 Then, theas Rathe increases thepolishing polishing process increases when the processing time 120 min. Then, the Ra increases asasthe process whenincreases the processing time is 120 min. Then, themin. Ra increases polishing process proceeds. For the proceeds. hybridabrasives, abrasives,the the mean mean value value and standard to to proceeds. ForFor thethe hybrid standard deviation deviationofofRaRacontinue continue hybrid abrasives, the mean value and standard deviation of Ra continue to decrease in 180 min of decrease in 180 min processingtime. time.So, So,aahigher higher and and more than that of of decrease in 180 min of of processing more uniform uniformsurface surfacequality quality than that processing time. abrasives So, a higher and more uniform surface quality than that of diamond abrasives can be diamond can be achieved by using the hybrid abrasives under the same processing diamond abrasives can be achieved by using the hybrid abrasives under the same processing achieved by using the hybrid abrasives under the same processing conditions. conditions. conditions. Diamond abrasives Diamond abrasives Hybrid abrasives
2.5
Surface roughness (nm)
Surface roughness (nm)
2.5
2.0
2.0
1.5
Hybrid abrasives
1.94
1.94
1.70
1.70
1.5
1.59
1.59 1.49 1.49
1.59
1.52
1.52 1.42
1.42
1.591.39 1.39
1.0
1.0 0.5
0.5
0
0
60 120 Processing time (min)
60
120
180
180
Processing time (min)and hybrid abrasives at different Figure 4. Roughness of sapphire substrate polished by diamond Figure 4. Roughness of sapphire substrate polished by diamond and hybrid abrasives at different processing time periods.
Figure 4. Roughness processing time periods.of sapphire substrate polished by diamond and hybrid abrasives at different
processing periods. Figure time 5 shows the binding condition of the diamond and hybrid particles with the matrix before polishing, and the shedding condition of diamond the diamond andhybrid hybrid particles particles from polishing Figure 5 shows the binding condition of the and withthethe matrix before Figure shows theSome binding condition the diamond hybrid particles of with the matrix pad after5 polishing. cracks could beofclearly observedand at the combination matrix and polishing, and the shedding condition of the diamond and hybrid particles from the polishing pad after before polishing, and before the shedding condition of the and hybrid particles fromwrapped the polishing diamond particles polishing (Figure 5a). By diamond contrast, hybrid particle was tightly in polishing. Some cracks could clearly observed at the combination ofcombination matrix and of diamond particles padthe after polishing. Some be cracks could beaddition, clearly observed matrix matrix before polishing (Figure 5b). In the numberatofthe hybrid particle residues in theand before polishing (Figure 5a). By contrast, hybrid particle was tightly wrapped in the matrix before diamond particles before polishing (Figure 5a). By contrast, hybrid particle was tightly wrapped matrix after polishing was significantly higher than that of diamond particles, as shown in Figure in polishing (Figure 5b). In addition, number ofthe hybrid residues the matrix polishing the 5c,d. matrix polishing (Figure 5b). Inthat addition, theparticle number of hybrid particle residues the Allbefore the results could bethe inferred presence of porous and in abundant –OHafter in in the akageneite coating are conductive to enhancing the binding force between the matrix andthe abrasives. after polishing was significantly higher than that diamond particles, as All shown in Figurecould was matrix significantly higher than that of diamond particles, asofshown in Figure 5c,d. results This feature is attributed to the tunnel structure and material properties of akageneite. The enhanced 5c,d. All the results could be inferred that the presence of porous and abundant –OH in the to be inferred that the presence of porous and abundant –OH in the akageneite coating are conductive binding force can reduce the amount of abrasive grains shedding from polishing film during akageneite coating are conductive enhancing the binding forceThis between theismatrix and abrasives. enhancing the binding force betweentothe matrix and abrasives. feature attributed to the tunnel as well as to thestructure effective processing time of the polishing pad. Moreover, the Thisprocessing, feature is attributed to prolong the tunnel and material properties of akageneite. The enhanced structure andofmaterial properties of by akageneite. The enhanced binding force can reducewhich the amount MRR sapphire wafer polished hybrid abrasives is similar to that of diamond abrasives, binding force can reduce the amount of abrasive grains shedding from polishing film during of abrasive grains0.28 shedding polishing filmalthough during processing, as well as to prolong the effective is around nm/min. from For hybrid abrasives, the coating with low hardness will weaken processing, as well as to prolong the effective processing time of the polishing pad. Moreover, the their material removal ability, a greater number of efficient particles in the polishing pad can make processing time of the polishing pad. Moreover, the MRR of sapphire wafer polished by hybrid abrasives MRR of sapphire wafer polished by hybrid abrasives is similar to that of diamond abrasives, which up for this defect to some extent during polishing. is similar to that of diamond abrasives, which is around 0.28 nm/min. For hybrid abrasives, although
is around 0.28 nm/min. For hybrid abrasives, although the coating with low hardness will weaken
the coating with low hardness will weakennumber their material removal ability, greater number efficient their material removal ability, a greater of efficient particles in thea polishing pad canof make particles inthis thedefect polishing padextent can make for this defect to some extent during polishing. up for to some duringup polishing.
Figure 5. Cont.
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Figure 5. Combination of diamond (a,c) and hybrid (b,d) abrasives with the matrix before (a,b) and
Figure 5. Combination of diamond (a,c) and hybrid (b,d) abrasives with the matrix before (a,b) and after (c,d) polishing. after (c,d) polishing.
On the other hand, the morphologies and profile curves of the sapphire substrate polished by diamond and hybrid were characterized by 3D optical Aspolished shown by On the other hand,abrasives the morphologies and profile curves ofinterferometry the sapphire profiler. substrate in Figure 6, due to the high hardness and sharp edges, the scratches caused by diamond particles diamond and hybrid abrasives were characterized by 3D optical interferometry profiler. Asare shown very deep. Bytocontrast, scratchesand caused by edges, hybrid the particles are very small. the in Figure 6, due the highthehardness sharp scratches caused byMeanwhile, diamond particles average value of PV (peak-to-valley) is 17.8 nm by diamond particles, and 7.5 nm by hybrid are very deep. By contrast, the scratches caused by hybrid particles are very small. Meanwhile, the particles. PV value which mainly indicates the depth of scratch is significantly decreased by using average value of PV (peak-to-valley) is 17.8 nm by diamond particles, and 7.5 nm by hybrid particles. hybrid particles. This phenomenon could be explained by the fact that the core/shell structured PV value mainly the depth of scratch is significantly decreased by using hybrid hybridwhich particles exhibitindicates a lower elastic modulus and hardness than the diamond particles. When particles. This the phenomenon be explained by of the that into the the core/shell hybrid considering deformationcould of the particle, the depth thefact particle substratestructured surface can be particles exhibit a lower elastic modulus and hardness than the diamond particles. When considering predicted as [16,28]
the deformation of the particle, the depth of the particle 1 into the substrate surface can be predicted 9F 2 3 as [16,28] (4) δ = 2 1 8 DEsw 9F2 3 δ= (4) 2 8DE vs2 sw 1 − vw2 1 1− E
δW
=
E
+
E
1 sw 1 − sv2s 1w− v2w = + 3 Esw Es Ew 2 " Esw δW = D − δ − δp = D − δ 1 + 32 # E Esw = D − δ − δp = D − δ 1+ sp Esp
(5)
(5) (6)
where δ is the deformation of the abrasive particle; F is the polishing pressure; D is the diameter of
(6)
where δ is the deformation of the abrasive particle; F is the polishing pressure; D is the diameter of the the particle; Esw is the Young’s modulus of the particle and wafer pair; δw is the the indentation depth particle; Esw is the Young’s modulus of the particle and wafer pair; δw is the the indentation depth of the particle into the substrate surface; δp is the indentation depth of the particle into the polishing of the particle into the substrate surface; δp is the indentation depth of the particle into the polishing pad; Esp is the Young’s modulus of the particle and pad pair; Es and vs are the Young’s modulus and pad;the EspPoisson’s is the Young’s modulus of the particle and padEpair; Eswand vs are the Young’s modulus ratio of the abrasive particle, respectively; w and v are the Young’s modulus and the and the Poisson’s ratioofofthe thewafer, abrasive particle, respectively; Ew and vw are the Young’s modulus and the Poisson’s ratio respectively. Poisson’sThe ratio of the wafer, respectively. indentation depth of the particle into the wafer is decreased with the increase of the deformation of the depth particle. these properties, core/shell structuredwith hybrid The indentation of Due the to particle into the wafer is decreased theparticles increasecan of the decrease of thethe contact stress and increase the contact area between abrasive andparticles wafer; thus, the deformation particle. Due to these properties, core/shell structured hybrid can decrease core/shellstress structured hybrid particles can decrease the scratch depth.and Thewafer; removal amount of the the contact and increase the contact area between abrasive thus, the core/shell next process is mainly determined by the depth of the deepest scratch. The achieved surface structured hybrid particles can decrease the scratch depth. The removal amount of the next process polished by hybrid particles with shallow and uniform scratches is critical to obtaining an is mainly determined by the depth of the deepest scratch. The achieved surface polished by hybrid ultra-smooth and defect-free sapphire substrate surface economically and efficiently. particles with shallow and uniform scratches is critical to obtaining an ultra-smooth and defect-free sapphire substrate surface economically and efficiently.
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Figure 6. Morphologies and profile curves of the sapphire substrate polished by diamond (a) and
Figure 6.hybrid Morphologies (b) abrasives.and profile curves of the sapphire substrate polished by diamond (a) and hybrid (b) abrasives. 4. Conclusions
4. Conclusions The core/shell structured diamond/akageneite hybrid particles were prepared via one-step isothermal hydrolyzing under relatively low temperature. The β-FeOOH coating is uniformly
Theformed core/shell structured particles were prepared via one-step on diamond particlediamond/akageneite surface, and the structurehybrid of coating is spindle-shaped. The hybrid isothermal hydrolyzing under relatively low temperature. The β-FeOOH coating uniformly formed particles can reduce the scratch depth without compromising the MRR and surfaceis roughness. Compared withsurface, pristine diamond, PV valueofofcoating sapphire is substrate surface after The polishing has particles on diamond particle and the the structure spindle-shaped. hybrid been decreased 10.3 nm by using hybrid particles. Moreover, the particles exhibited less abrasive can reduce the scratch depth without compromising the MRR and surface roughness. Compared shedding during the processing. The hybrid particle significantly improved the polishing with pristine diamond, PV value of sapphire sapphiresubstrate. substrate surface after polishing has coating been decreased performance of SGthe polishing pad for It can be attributed to the β-FeOOH 10.3 nm by using hybrid particles. Moreover, the particles exhibited less abrasive shedding which can improve the surface quality and prolong the effective processing time of polishing pad. during the processing. The hybrid particle significantly improved the polishing performance of SG polishing pad Acknowledgments: The authors appreciate financial support from National Natural Science Foundation of for sapphire substrate. It can be attributed to the β-FeOOH coating which can improve the surface China (Grant No. U1305241, 51475175), Promotion Program for Young and Middle-aged Teacher in Science, quality and prolong effective processing polishing pad. and Technologythe Research of Huaqiao Universitytime (Grantof No. ZQN-YX202). Author Contributions: Jing Lu designed the experiments; Yongchao Xu and Dayu Zhang performed the
Acknowledgments: The authors appreciate financial support from National Natural Science Foundation of experiments; Yongchao Xu analyzed the data; Xipeng Xu contributed materials and analysis tools; Jing Lu and China (Grant No. U1305241, 51475175), Promotion Program for Young and Middle-aged Teacher in Science, and Yongchao Xu wrote the paper. Technology Research of Huaqiao University (Grant No. ZQN-YX202). Conflicts of Interest: The authors declare no conflict of interest.
Author Contributions: Jing Lu designed the experiments; Yongchao Xu and Dayu Zhang performed the experiments; Yongchao Xu analyzed the data; Xipeng Xu contributed materials and analysis tools; Jing Lu References and Yongchao Xu wrote the paper. 1.
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