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Development of a novel chemical mechanical polishing slurry and its polishing mechanisms on a nickel alloy
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Development of a novel chemical mechanical polishing slurry and its polishing mechanisms on a nickel alloy ⁎
Zhenyu Zhang , Longxing Liao, Xinze Wang, Wenxiang Xie, Dongming Guo Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, 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: Ni alloy CMP Environment friendly XPS Infrared spectroscopy
Conventional chemical mechanical polishing (CMP) slurries of pure nickel (Ni) and its alloys usually consist of toxic and corrosive acids, which is dangerous and contaminative to the operators and environment. It is a big challenge to develop a novel environment friendly CMP slurry for Ni alloys. In this study, a novel environment friendly CMP slurry was developed, containing of silica, hydrogen peroxide (H2O2), malic acid and deionized water. The surface roughness Ra, and peak-to-valley (PV) values are 0.44, and 4.49 nm, respectively with an area of 71 × 53 μm2. To the best of our knowledge, surface roughness in this work is the lowest for pure Ni and its alloys at a scan area of 71 × 53 μm2. The CMP mechanisms are elucidated by electrochemical, X-ray photoelectron spectroscopy, and infrared measurements. Firstly, H2O2 dominated the oxidation process in CMP, forming oxides of nickel (Ni), chromium (Cr), and molybdenum (Mo) on the surface of Ni alloy. Then, the Ni oxides were dissolved by hydrogen (H) ions. The oxides of Cr and Mo were stable in malic acid. Chelating formulas are proposed between malic acid and Ni ions. Finally, the passivated film was removed by the polishing pad.
1. Introduction
[8,14,20,21], potassium hydroxide (KOH) [14], hydrofluoric acid (HF) [10,19–23], and nitric acid (HNO3) [10,19–23]. These toxic, corrosive and explosive acids and alkalis are dangerous to the operators and contaminative to the environment. Therefore, it is a great challenge to develop a novel environment friendly CMP slurry for Ni alloys. It is reported that the surface roughness Ra is 0.08 nm at a scan area 3 × 3 μm2, measured by atomic force microscopy (AFM) after float polishing on a Ni plated sample [12]. This is the lowest surface roughness reported on Ni and Ni alloys after polishing. The polishing slurry in float polishing is pure water and silica (SiO2) powder, and the polishing tool is the resin pad. Even for the lowest surface roughness, there are still small scratches on the polished surface [12], due to the absent of chemical function. The polishing time is very long in float polishing. To remove the scratches and reduce the polishing time, CMP is necessary to be employed for Ni and its alloys. Moreover, an asperous surface could change to a flat surface by reducing the scan area from 10 × 10 μm2 to 1 × 1 μm2, which was measured by an AFM [24]. In this regard, measurements of AFM at small scan areas are not appropriate for industrial measurements. It is reported that the surface roughness Ra is 1.28 nm at a scan area of 70 × 53 μm2 measured by a Zygo profilometer, corresponding to Ra of 0.365 nm at a scan area of 5 × 5 μm2 tested by AFM [14]. At present, the lowest surface roughness Ra on a Ni alloy is 0.509 nm at a scan area of 70 × 53 μm2 [14]. Hence,
Cosmochemical, geochemical and geophysical studies provide evidence that the core of Earth contains iron (Fe) with substantial amounts for 5 to 15% of nickel (Ni) [1]. Pure Ni and Ni alloys exhibit extraordinary mechanical and physical properties [2–6], high temperature strength, high corrosion resistance [7], high hardness [8], low friction coefficient, good wear resistance, nonmagnetic behavior, high electrocatalytic activity [9], and good biocompatibility [10]. Due to the superior mechanical, physical and chemical properties, Ni and Ni alloys are widely used in aircraft [7], medical [10], energy [11], optical [12], aerospace, automotive, electronics [13] and chemical industries [8]. Nevertheless, high-performance components demand the surface roughness Ra at angstrom level, as well as damage-free on the surface and subsurface to satisfy the stringent requirements of high performance devices [12,14]. Chemical mechanical polishing (CMP) has become the primary global planarization method [14,15], and is widely used in semiconductor fabrication and microelectronics industries [16]. In traditional CMP slurries for Ni alloys, free abrasives of alumina (Al2O3) are usually employed [16–18], resulting in thermal asperities defects by the embedment of alumina [17]. In addition, conventional CMP slurries for Ni and Ni alloys normally include benzotriazole (BTA) [14], perchloric acid (HClO4) [19], hydrochloric acid (HCl) ⁎
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144670 Received 7 September 2019; Received in revised form 20 October 2019; Accepted 11 November 2019 Available online 14 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhenyu Zhang, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144670
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it is difficult to achieve the surface roughness Ra less than 0.509 nm at a scan area of 70 × 53 μm2 on a polished surface of Ni alloys. In this study, a novel CMP slurry was developed for a Ni alloy. The CMP mechanisms were explored by electrochemical, X-ray photoelectron spectroscopy (XPS) and infrared (IR) measurements. Chelating and reaction formulas are suggested during CMP, in terms of the experimental measurements. 2. Experimental details Hastelloy® C-2000® alloy (UNS N06200) was selected as the CMP specimens. It had a nominal composition of 59 balance Ni, 23 chromium (Cr), 16 molybdenum (Mo), 2 max. cobalt (Co), 1.6 copper (Cu), 3 max. iron (Fe), 0.5 max. manganese (Mg), 0.5 max. aluminum (Al), 0.08 max. silicon, and 0.01 max. wt% carbon (C). Three blocks of Ni alloy were adhered uniformly at the periphery of an Al alloy carrier. Each block had a length of 10 mm, a width of 10 mm and a thickness of 1 mm. Mechanical lapping, polishing and CMP were performed on a precision polisher (YJ-Y380, Shenyang Yanjia Technology Co., Ltd., China). Firstly, the mechanical lapping was conducted by waterproof sandpapers of silicon carbide (SiC) with mesh sizes at a sequence of 600, 1000, 2000, and 3000. The lapping solution and tool were deionized water and a cast iron plate, respectively. The lapping time was 4 min. Mechanical polishing was carried out by a polyurethane pad. The slurry of mechanical polishing consisted of 6 wt% α-Al2O3 with a diameter of 5 μm and deionized water. The CMP slurry contained hydrogen peroxide (H2O2), SiO2, malic acid, and deionized water. It had a pH value of 3.5. The concentration of H2O2 was 30 wt%, and the colloidal SiO2 was 30 wt% with diameters varying from 10 to 35 nm. During mechanical polishing and CMP, the rotation speeds of the polishing pad and workpiece were 60 rpm. A nubuck pad was used in CMP to replace the polyurethane pad employed in mechanical polishing. The polishing pressure was 45 and 20 kPa in CMP and mechanical polishing, respectively, corresponding to their polishing time of 18 and 6 min. After CMP, the polished surface was flushed and dried by deionized water and compressed air, respectively. Surface morphology was characterized by an optical microscope (MX 40, Olympus, Japan). Surface roughness was measured by a precision non-contact surface profilometer (NewView 5022, Zygo, USA). Electrochemical measurements were performed by an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, Ametek, USA). They were carried out by a reference electrode of silver (Ag)/Ag chloride (AgCl), and an auxiliary electrode of platinum (Pt). Scanning range was from −0.25 to 1 V, and the scanning speed was 1 mV/s. Prior to electrochemical measurements, the Ni alloy blocks were lapped by sandpapers with mesh sizes in a sequence of 600, 1000, 2000, and 3000. Then, the lapped blocks were ultrasonically cleaned for 10 min, and dried by compressed air. For the measurement of polarization curves, the blocks were immersed in slurry for 30 min. XPS spectra were measured by an X-ray photoelectron spectrometer (Axis Ultra DLD, Shimadzu, Japan). They were calibrated by C 1s peak at 284.8 eV. IR spectra were measured by a Fourier transformation IR spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA). Colloidal SiO2 was characterized by a transmission electron microscope (TEM) (Tecnai F20, FEI, USA) at an acceleration voltage of 200 kV.
Fig. 1. TEM image of colloidal SiO2 nanospheres.
(c) and (d), while Fig. 2(b) has no these defects. Accordingly, pH 3.5 is an optimal value for developed CMP slurry. Fig. 3 shows the surface topography on the polished surfaces of Ni alloy generated by CMP slurries with different H2O2 concentrations. The developed CMP slurries include 12 wt% SiO2, malic acid and deionized water, and the pH value is 3.5. When the H2O2 is zero, i.e. without H2O2, scratches and obvious etched pits are present on the polished surface in Fig. 3(a). After increasing H2O2 to 4.5 wt%, scratches and etched pits are removed basically, and only small etched pits are left in Fig. 3(b). When the H2O2 is 9 wt%, the polished surface looks like a flat mirror, without scratches and etched pits. With increasing the concentration of H2O2 to 13.5 wt%, obvious etched pits appear again. In a consequence, the optimal concentration of H2O2 is 9 wt%. Furthermore, from Fig. 3, it is confirmed that H2O2 plays an important role during CMP on the Ni alloy. Fig. 4 draws the dynamic potential polarization curves as a function of current density of Ni alloy blocks after immersion in slurries with different pH values and H2O2 concentrations. In Fig. 4(a), the corrosive current Icorr is 7.51, 1.71 and 1.36 μA for pH values at 3.5, 7 and 10, respectively, corresponding to the corrosive voltage Ecorr of 453, 207 and 142 mV. With increasing the pH values, Icorr and Ecorr decrease monotonically. This indicates that the more pH value, the more difficult to corrode. When the pH value is 3.5, the Ecorr is remarkably higher than those of 7 and 10, meaning a balance state between mechanical and chemical functions during CMP. This results in the best surface quality, which is in good agreement with the surface morphology in Fig. 2(b). When the pH value is 10, the Ecorr is the smallest, signifying the thinnest corrosive film. This leads to the worst surface quality, which is consistent with the surface morphology in Fig. 2(d). In Fig. 4(b), when the H2O2 concentration is 0, 4.5, 9 and 13.5 wt%, the Icorr is 0.134, 1.67, 7.51 and 12.93 μA, respectively, corresponding to the Ecorr of −186, 391, 453 and 480 mV. With increasing the concentration of H2O2 from 0 to 13.5, Icorr increases two orders magnitude, indicating the high oxidation ability of H2O2 on the Ni alloy. However, when the concentration of H2O2 increases from 4.5 to 13.5, the Ecorr increases tenderly, meaning the ability for corrosion is similar between each other. When the H2O2 is absent, Icorr and Ecorr are extremely low, indicating the chemical function is very weak. In this condition, the mechanical function dominates the polishing process, resulting in the evident scratches and etched pits in Fig. 3(a). When the H2O2 concentration is 9 wt%, a balance is achieved between mechanical and chemical functions, leading to a good surface quality in Fig. 3(c). When the H2O2 concentration reaches 13.5 wt%, the chemical function
3. Results and discussion Fig. 1 illustrates the TEM image of colloidal SiO2 nanospheres. The diameters of SiO2 vary from 10 to 35 nm, which is beneficial to remove the passivated film generated during CMP. Fig. 2 depicts the surface morphology polished by developed CMP slurries with different pH values. The developed CMP slurries consist of 12 wt% SiO2 and 9 wt% H2O2 and deionized water. PH values from 3 to 7 was adjusted by malic acid, and 10 was modulated by KOH for a comparison. Small scratches and etched pits are observed in Fig. 2(a), 2
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Fig. 2. Optical images on the polished surfaces of Ni alloy induced by developed CMP slurries with pH values at (a) 3, (b) 3.5, (c) 7 and (d) 10.
Fig. 3. Optical images on the polished surfaces of Ni alloy generated by developed CMP slurries with H2O2 concentration at (a) 0, (b) 4.5, (c) 9 and (d) 13.5 wt%.
pits after CMP by the optimal slurry. Surface roughness Ra, rms and PV values are 0.44, 0.553 and 4.489 nm (Fig. 5(d)), respectively at a scan area of 71 × 53 μm2 on the polished surface in Fig. 5(c) by the optimal CMP slurry. It is lower than previous reports, in which surface roughness Ra and PV values are 0.509 and 4.95 nm at a scan area of 70 × 53 μm2 [14]. To the best of our knowledge, the surface roughness achieved in this study is the lowest after CMP for pure Ni and Ni alloys at a scan area of 70 × 53 μm2. In the developed CMP slurry, it consists of SiO2, H2O2, malic acid and deionized water. SiO2 widely exists in
dominates, making the etched pits appear. According to the electrochemical measurements and CMP experiments, the optimal CMP slurry contains 12 wt% SiO2, 9 wt% H2O2 and deionized water, and the pH value is 3.5 adjusted by malic acid. After mechanical lapping in Fig. 5(a), the surface is full of dense scratches, indicating the mechanical removal by fixed abrasives. In Fig. 5(b), most scratches are removed, and only a few shallow scratches are left after mechanical polishing. There is no embedment of free abrasives. The surface looks like a flat mirror in Fig. 5(c), without scratches and etched 3
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Fig. 4. Dynamic potential polarization curves as a function of current density of Ni alloy blocks after immersion in slurries with (a) different pH values and (b) H2O2 concentrations.
(Fig. 6(d)). Fig. 7 illustrates the XPS spectra of Cr element on the four kinds of different surfaces. The peaks at 574 and 577.1 eV are Cr 2p3/2 and Cr2O3 2p3/2 respectively, corresponding to Cr 2p1/2 and Cr2O3 2p1/2 at 583.2 and 586.9 eV [25]. Chemical states of Cr element are approximately invariable after immersion in H2O2 (Fig. 7(b)), malic acid (Fig. 7(c)) and CMP slurry (Fig. 7(d)). Cr2O3 dominates on the surface, which is different from the pure Ni in Fig. 6. This is attributed to the oxidation of Cr element on the surfaces. Fig. 8 depicts the XPS spectra of Mo element on the four kinds of different surfaces. Three peaks at 227.8, 229.1 and 232.4 eV belong to Mo 3d5/2, MoO2 3d5/2, and MoO3 3d5/2 respectively. This agrees well with peaks at 230.9, 232.2 and 235.5 eV for Mo 3d3/2, MoO2 3d3/2 and MoO3 3d3/2, respectively [25]. Similar to Ni and Cr elements, the chemical states of Mo element have no variation after immersion in H2O2 (Fig. 8(b)), malic acid (Fig. 8(c)) and CMP slurry (Fig. 8(d)). To identify the mechanical function in CMP, XPS spectra of Ni, Cr and Mo elements after CMP are diagrammed in Fig. 9. After CMP, Ni oxides (Fig. 9(a)) were removed compared with those in Fig. 6(a). Most
nature, occupying 12 wt% of the crust of the earth. It is the main composition of rocks. SiO2 has crystalline and amorphous phases in nature, such as crystalline quartz and amorphous kieselguhr·H2O2 is used for the disinfection on environment, food and medical wound. It could decompose into water and oxygen, without the residue of metallic ions, which is different from the oxidizer of KIO3 [16]. Malic acid was firstly separated from apple juice, which is widely used for the additives of food. It is a nutritious material for the human being. After CMP, the Ni alloys were cleaned and dried by deionized water and compressed air, respectively, without the organic solvents, such as acetone and alcohol. From the above analysis, the developed CMP slurry and processes are environment friendly. XPS spectra of Ni element on four kinds of different surfaces are diagrammed in Fig. 6. There are three peaks at 852.8, 853.9 and 856.0 eV, corresponding to Ni 2p3/2 [25], NiO 2p3/2 [26], and Ni2O3 2p3/2 [25]. This is in good agreement with three peaks at 870.1, 871.4 and 873.5 eV for Ni 2p1/2, NiO 2p1/2, Ni2O3 2p1/2 [25], respectively. From Fig. 6, chemical states of Ni element keep basically constant after immersion in H2O2 (Fig. 6(b)), malic acid (Fig. 6(c)) and CMP slurry
Fig. 5. Optical images on the machined surfaces of Ni alloy by (a) mechanical lapping, (b) mechanical polishing, (c) CMP, and (d) surface roughness after CMP. 4
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Fig. 6. XPS spectra of Ni element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
Fig. 7. XPS spectra of Cr element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
on the surface of Ni alloy. Without the mechanical function, the chemical reaction would be terminated due to the passivation induced by CMP slurry. Fig. 10 illustrates the atomic percentage of Ni, Cr, Mo and their oxides on the surfaces of pristine Ni alloy and after immersion in three
oxides of Mo (Fig. 9(b)) were got rid of after CMP in comparison with those in Fig. 7(a). The left Cr2O3 might be formed by oxidation in air after CMP. Similar to Cr element, oxides of Mo in Fig. 9(c) are greatly reduced compared with those in Fig. 8(a), especially for the MoO3 3d3/2 peak at 235.5 eV. Mechanical function is obvious to remove the oxides 5
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Fig. 8. XPS spectra of Mo element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
the stability of oxides of Cr and Mo. In optimal CMP slurry, the oxides of Ni slightly reduce compared with those in malic acid, which is the synergistic effect between H2O2 and malic acid. In comparison between H2O2 and malic acid, dissolution function of malic acid dominates in CMP slurry for the oxides of Ni. As the oxides of Cr and Mo are stable in malic acid, H2O2 leads the chemical process in the CMP slurry. After
kinds of slurries and CMP machining. The atomic percentages of oxides for all the Ni (Fig. 10(a)), Cr (Fig. 10(b)) and Mo (Fig. 10(c)) elements increase after immersion in H2O2, indicating the oxidizing function. After immersion in malic acid, the oxides of Ni decrease (Fig. 10(a)), meaning the dissolving of oxides of Ni. However, the oxides of Cr (Fig. 10(b)) and Mo (Fig. 10(c)) keep constant in malic acid, signifying
Fig. 9. XPS spectra of (a) Ni, (b) Cr, and (c) Mo elements on the surface after CMP by the optimal CMP slurry. 6
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Fig. 10. Atomic percentage of pure (a) Ni, (b) Cr, (c) Mo and their oxides on the surfaces of pristine Ni alloy and after immersion in H2O2, malic acid, CMP slurries and CMP machining.
CeO reduces from 1381 to 1360 cm−1, and COOH varies from 902 to 835 cm−1 (Fig. 11) [27]. Chelating formulas are proposed in Fig. 12 for malic acid and Ni2+ ions and Fig. 13 for malic acid and Ni3+ ions, according to the molecular structure of malic acid [32,33], XPS and IR experimental results. Schematic diagrams of the CMP mechanism are diagrammed in Fig. 14 for the developed optimal CMP slurry. According to the XPS and IR results in Figs. 6–10, four steps are suggested for the CMP mechanism. Firstly, H2O2 dominates for the oxidation process during CMP (Fig. 14(a)). The formulas of oxidation by H2O2 are presented [34,35],
CMP machining, all the oxides of Ni, Cr and Mo reduce greatly, indicating the oxides formed are removed by mechanical function during CMP process. Pristine Ni alloy was immersed in malic acid for 24 h, and then it was picked out. Malic acid crystals were obtained after evaporation of aqueous solution. The IR spectra of malic acid crystals are drawn in Fig. 11 prior to and after immersion by the Ni alloy. The molecular formula of malic acid is COOHCH2CHOHCOOH [27,28], which is a dicarboxylic acid with a secondary hydroxyl [29]. Malic acid exists in many fruits, including apples, grapes, peaches, apricots, blackberries, blueberries and plums [29], which is widely used in food and cosmetic industries [30,31]. From the XPS measurements in Figs. 6 and 10, malic acid can dissolve oxides of Ni, forming the Ni2+ and Ni3+ ions in solution. Owing to the reactive carboxyl and hydroxyl groups, malic acid could chelate with Ni2+ and Ni3+ ions, resulting in the shifts of IR spectra. After chelating with Ni ions, stretching vibration of eOH shifts from 3415 to 3389 cm−1, C]O decreases from 1638 to 1613 cm−1,
H2O2 + Ni → NiO + H2O
(1)
3H2O2 + 2Ni → Ni2O3 + 3H2O
(2)
3H2O2 + Cr → CrO3 + 3H2O
(3)
2H2O2 + Mo → MoO2 + 2H2O
(4)
3H2O2 + Mo → MoO3 + 3H2O
(5)
+
H ions are derived mainly from the malic acid, and partially from the H2O2. The oxides of Cr and Mo are stable in malic acid, while the oxides of Ni were dissolved by malic acid (Fig. 10(a)). The formulas of dissolution for oxides of Ni are described [36], NiO + 2H+ → Ni2++H2O
(6)
Ni2O3 + 6H+ → 2Ni3++3H2O
(7)
The oxides of Ni are dissolved mainly by malic acid (Fig. 14(b)). After the release of Ni2+ and Ni3+ ions to the CMP slurry, they were chelated by malic acid (Fig. 14(d)). This prohibited the release of toxic Ni ions to the environment by malic acid, which is very significant to avoid the pollution to the environment. The oxides of Ni, Cr and Mo produced in CMP are removed by mechanical action (Fig. 14(c)). The passivated film formed by oxides at plateau on the surface was removed by mechanical action, resulting in the exposure of Ni alloy. In
Fig. 11. IR spectra of malic acid prior to and after immersion by the Ni alloy. 7
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Fig. 12. Chelating formulas between malic acid and Ni2+ ions.
Fig. 14. Schematic diagrams of the CMP mechanism for the developed optimal CMP slurry.
developed for Ni alloy, consisting of H2O2, malic acid, SiO2 and deionized water. According to the electrochemical and optical measurements, the optimal CMP slurry was obtained. After CMP, angstrom level surfaces were achieved at a scan area of 71 × 53 μm2. The CMP mechanism was elucidated by XPS and IR measurements. Firstly, H2O2 oxidized the Ni alloy, forming oxides of Ni, Cr and Mo on the surface. Malic acid dissolved the oxides of Ni, while the oxides of Cr and Mo kept constant. After the release of Ni2+ and Ni3+ ions to the CMP slurry, malic acid chelated with them, avoiding the release of Ni ions to the environment. The produced oxides in CMP were removed by mechanical action. Thus, the CMP process is environment friendly.
Fig. 13. Chelating formulas between malic acid and Ni3+ ions.
comparison, the passivated film at basin on the surface protected the Ni alloy, terminating the chemical reaction. With this method, the plateau on the surface of Ni alloy was flattened by CMP, achieving the angstrom level surfaces. The developed CMP slurry and process are both environment friendly.
Declaration of Competing Interest The authors declare no competing interests. Acknowledgements
4. Conclusions
The authors acknowledge the financial support from the National
In summary, a novel environment friendly CMP slurry was 8
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Key R&D Program of China (2018YFA0703400), Excellent Young Scientists Fund of NSFC (51422502), Science Fund for Creative Research Groups of NSFC (51621064), Program for Creative Talents in University of Liaoning Province (LR2016006), Distinguished Young Scholars for Science and Technology of Dalian City (2016RJ05), the Xinghai Science Funds for Distinguished Young Scholars and Thousand Youth Talents at Dalian University of Technology, Science Fund of State Key Laboratory of Tribology, Tsinghua University (SKLTKF17B19), Science Fund of State Key Laboratory of Metastable Materials Science and Technology, Yanshan University (201813), and the Collaborative Innovation Center of Major Machine Manufacturing in Liaoning.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Development of a novel chemical mechanical polishing slurry and its polishing mechanisms on a nickel alloy ⁎
Zhenyu Zhang , Longxing Liao, Xinze Wang, Wenxiang Xie, Dongming Guo Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, 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: Ni alloy CMP Environment friendly XPS Infrared spectroscopy
Conventional chemical mechanical polishing (CMP) slurries of pure nickel (Ni) and its alloys usually consist of toxic and corrosive acids, which is dangerous and contaminative to the operators and environment. It is a big challenge to develop a novel environment friendly CMP slurry for Ni alloys. In this study, a novel environment friendly CMP slurry was developed, containing of silica, hydrogen peroxide (H2O2), malic acid and deionized water. The surface roughness Ra, and peak-to-valley (PV) values are 0.44, and 4.49 nm, respectively with an area of 71 × 53 μm2. To the best of our knowledge, surface roughness in this work is the lowest for pure Ni and its alloys at a scan area of 71 × 53 μm2. The CMP mechanisms are elucidated by electrochemical, X-ray photoelectron spectroscopy, and infrared measurements. Firstly, H2O2 dominated the oxidation process in CMP, forming oxides of nickel (Ni), chromium (Cr), and molybdenum (Mo) on the surface of Ni alloy. Then, the Ni oxides were dissolved by hydrogen (H) ions. The oxides of Cr and Mo were stable in malic acid. Chelating formulas are proposed between malic acid and Ni ions. Finally, the passivated film was removed by the polishing pad.
1. Introduction
[8,14,20,21], potassium hydroxide (KOH) [14], hydrofluoric acid (HF) [10,19–23], and nitric acid (HNO3) [10,19–23]. These toxic, corrosive and explosive acids and alkalis are dangerous to the operators and contaminative to the environment. Therefore, it is a great challenge to develop a novel environment friendly CMP slurry for Ni alloys. It is reported that the surface roughness Ra is 0.08 nm at a scan area 3 × 3 μm2, measured by atomic force microscopy (AFM) after float polishing on a Ni plated sample [12]. This is the lowest surface roughness reported on Ni and Ni alloys after polishing. The polishing slurry in float polishing is pure water and silica (SiO2) powder, and the polishing tool is the resin pad. Even for the lowest surface roughness, there are still small scratches on the polished surface [12], due to the absent of chemical function. The polishing time is very long in float polishing. To remove the scratches and reduce the polishing time, CMP is necessary to be employed for Ni and its alloys. Moreover, an asperous surface could change to a flat surface by reducing the scan area from 10 × 10 μm2 to 1 × 1 μm2, which was measured by an AFM [24]. In this regard, measurements of AFM at small scan areas are not appropriate for industrial measurements. It is reported that the surface roughness Ra is 1.28 nm at a scan area of 70 × 53 μm2 measured by a Zygo profilometer, corresponding to Ra of 0.365 nm at a scan area of 5 × 5 μm2 tested by AFM [14]. At present, the lowest surface roughness Ra on a Ni alloy is 0.509 nm at a scan area of 70 × 53 μm2 [14]. Hence,
Cosmochemical, geochemical and geophysical studies provide evidence that the core of Earth contains iron (Fe) with substantial amounts for 5 to 15% of nickel (Ni) [1]. Pure Ni and Ni alloys exhibit extraordinary mechanical and physical properties [2–6], high temperature strength, high corrosion resistance [7], high hardness [8], low friction coefficient, good wear resistance, nonmagnetic behavior, high electrocatalytic activity [9], and good biocompatibility [10]. Due to the superior mechanical, physical and chemical properties, Ni and Ni alloys are widely used in aircraft [7], medical [10], energy [11], optical [12], aerospace, automotive, electronics [13] and chemical industries [8]. Nevertheless, high-performance components demand the surface roughness Ra at angstrom level, as well as damage-free on the surface and subsurface to satisfy the stringent requirements of high performance devices [12,14]. Chemical mechanical polishing (CMP) has become the primary global planarization method [14,15], and is widely used in semiconductor fabrication and microelectronics industries [16]. In traditional CMP slurries for Ni alloys, free abrasives of alumina (Al2O3) are usually employed [16–18], resulting in thermal asperities defects by the embedment of alumina [17]. In addition, conventional CMP slurries for Ni and Ni alloys normally include benzotriazole (BTA) [14], perchloric acid (HClO4) [19], hydrochloric acid (HCl) ⁎
Corresponding author. E-mail address: [email protected] (Z. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144670 Received 7 September 2019; Received in revised form 20 October 2019; Accepted 11 November 2019 Available online 14 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhenyu Zhang, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144670
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it is difficult to achieve the surface roughness Ra less than 0.509 nm at a scan area of 70 × 53 μm2 on a polished surface of Ni alloys. In this study, a novel CMP slurry was developed for a Ni alloy. The CMP mechanisms were explored by electrochemical, X-ray photoelectron spectroscopy (XPS) and infrared (IR) measurements. Chelating and reaction formulas are suggested during CMP, in terms of the experimental measurements. 2. Experimental details Hastelloy® C-2000® alloy (UNS N06200) was selected as the CMP specimens. It had a nominal composition of 59 balance Ni, 23 chromium (Cr), 16 molybdenum (Mo), 2 max. cobalt (Co), 1.6 copper (Cu), 3 max. iron (Fe), 0.5 max. manganese (Mg), 0.5 max. aluminum (Al), 0.08 max. silicon, and 0.01 max. wt% carbon (C). Three blocks of Ni alloy were adhered uniformly at the periphery of an Al alloy carrier. Each block had a length of 10 mm, a width of 10 mm and a thickness of 1 mm. Mechanical lapping, polishing and CMP were performed on a precision polisher (YJ-Y380, Shenyang Yanjia Technology Co., Ltd., China). Firstly, the mechanical lapping was conducted by waterproof sandpapers of silicon carbide (SiC) with mesh sizes at a sequence of 600, 1000, 2000, and 3000. The lapping solution and tool were deionized water and a cast iron plate, respectively. The lapping time was 4 min. Mechanical polishing was carried out by a polyurethane pad. The slurry of mechanical polishing consisted of 6 wt% α-Al2O3 with a diameter of 5 μm and deionized water. The CMP slurry contained hydrogen peroxide (H2O2), SiO2, malic acid, and deionized water. It had a pH value of 3.5. The concentration of H2O2 was 30 wt%, and the colloidal SiO2 was 30 wt% with diameters varying from 10 to 35 nm. During mechanical polishing and CMP, the rotation speeds of the polishing pad and workpiece were 60 rpm. A nubuck pad was used in CMP to replace the polyurethane pad employed in mechanical polishing. The polishing pressure was 45 and 20 kPa in CMP and mechanical polishing, respectively, corresponding to their polishing time of 18 and 6 min. After CMP, the polished surface was flushed and dried by deionized water and compressed air, respectively. Surface morphology was characterized by an optical microscope (MX 40, Olympus, Japan). Surface roughness was measured by a precision non-contact surface profilometer (NewView 5022, Zygo, USA). Electrochemical measurements were performed by an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, Ametek, USA). They were carried out by a reference electrode of silver (Ag)/Ag chloride (AgCl), and an auxiliary electrode of platinum (Pt). Scanning range was from −0.25 to 1 V, and the scanning speed was 1 mV/s. Prior to electrochemical measurements, the Ni alloy blocks were lapped by sandpapers with mesh sizes in a sequence of 600, 1000, 2000, and 3000. Then, the lapped blocks were ultrasonically cleaned for 10 min, and dried by compressed air. For the measurement of polarization curves, the blocks were immersed in slurry for 30 min. XPS spectra were measured by an X-ray photoelectron spectrometer (Axis Ultra DLD, Shimadzu, Japan). They were calibrated by C 1s peak at 284.8 eV. IR spectra were measured by a Fourier transformation IR spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA). Colloidal SiO2 was characterized by a transmission electron microscope (TEM) (Tecnai F20, FEI, USA) at an acceleration voltage of 200 kV.
Fig. 1. TEM image of colloidal SiO2 nanospheres.
(c) and (d), while Fig. 2(b) has no these defects. Accordingly, pH 3.5 is an optimal value for developed CMP slurry. Fig. 3 shows the surface topography on the polished surfaces of Ni alloy generated by CMP slurries with different H2O2 concentrations. The developed CMP slurries include 12 wt% SiO2, malic acid and deionized water, and the pH value is 3.5. When the H2O2 is zero, i.e. without H2O2, scratches and obvious etched pits are present on the polished surface in Fig. 3(a). After increasing H2O2 to 4.5 wt%, scratches and etched pits are removed basically, and only small etched pits are left in Fig. 3(b). When the H2O2 is 9 wt%, the polished surface looks like a flat mirror, without scratches and etched pits. With increasing the concentration of H2O2 to 13.5 wt%, obvious etched pits appear again. In a consequence, the optimal concentration of H2O2 is 9 wt%. Furthermore, from Fig. 3, it is confirmed that H2O2 plays an important role during CMP on the Ni alloy. Fig. 4 draws the dynamic potential polarization curves as a function of current density of Ni alloy blocks after immersion in slurries with different pH values and H2O2 concentrations. In Fig. 4(a), the corrosive current Icorr is 7.51, 1.71 and 1.36 μA for pH values at 3.5, 7 and 10, respectively, corresponding to the corrosive voltage Ecorr of 453, 207 and 142 mV. With increasing the pH values, Icorr and Ecorr decrease monotonically. This indicates that the more pH value, the more difficult to corrode. When the pH value is 3.5, the Ecorr is remarkably higher than those of 7 and 10, meaning a balance state between mechanical and chemical functions during CMP. This results in the best surface quality, which is in good agreement with the surface morphology in Fig. 2(b). When the pH value is 10, the Ecorr is the smallest, signifying the thinnest corrosive film. This leads to the worst surface quality, which is consistent with the surface morphology in Fig. 2(d). In Fig. 4(b), when the H2O2 concentration is 0, 4.5, 9 and 13.5 wt%, the Icorr is 0.134, 1.67, 7.51 and 12.93 μA, respectively, corresponding to the Ecorr of −186, 391, 453 and 480 mV. With increasing the concentration of H2O2 from 0 to 13.5, Icorr increases two orders magnitude, indicating the high oxidation ability of H2O2 on the Ni alloy. However, when the concentration of H2O2 increases from 4.5 to 13.5, the Ecorr increases tenderly, meaning the ability for corrosion is similar between each other. When the H2O2 is absent, Icorr and Ecorr are extremely low, indicating the chemical function is very weak. In this condition, the mechanical function dominates the polishing process, resulting in the evident scratches and etched pits in Fig. 3(a). When the H2O2 concentration is 9 wt%, a balance is achieved between mechanical and chemical functions, leading to a good surface quality in Fig. 3(c). When the H2O2 concentration reaches 13.5 wt%, the chemical function
3. Results and discussion Fig. 1 illustrates the TEM image of colloidal SiO2 nanospheres. The diameters of SiO2 vary from 10 to 35 nm, which is beneficial to remove the passivated film generated during CMP. Fig. 2 depicts the surface morphology polished by developed CMP slurries with different pH values. The developed CMP slurries consist of 12 wt% SiO2 and 9 wt% H2O2 and deionized water. PH values from 3 to 7 was adjusted by malic acid, and 10 was modulated by KOH for a comparison. Small scratches and etched pits are observed in Fig. 2(a), 2
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Fig. 2. Optical images on the polished surfaces of Ni alloy induced by developed CMP slurries with pH values at (a) 3, (b) 3.5, (c) 7 and (d) 10.
Fig. 3. Optical images on the polished surfaces of Ni alloy generated by developed CMP slurries with H2O2 concentration at (a) 0, (b) 4.5, (c) 9 and (d) 13.5 wt%.
pits after CMP by the optimal slurry. Surface roughness Ra, rms and PV values are 0.44, 0.553 and 4.489 nm (Fig. 5(d)), respectively at a scan area of 71 × 53 μm2 on the polished surface in Fig. 5(c) by the optimal CMP slurry. It is lower than previous reports, in which surface roughness Ra and PV values are 0.509 and 4.95 nm at a scan area of 70 × 53 μm2 [14]. To the best of our knowledge, the surface roughness achieved in this study is the lowest after CMP for pure Ni and Ni alloys at a scan area of 70 × 53 μm2. In the developed CMP slurry, it consists of SiO2, H2O2, malic acid and deionized water. SiO2 widely exists in
dominates, making the etched pits appear. According to the electrochemical measurements and CMP experiments, the optimal CMP slurry contains 12 wt% SiO2, 9 wt% H2O2 and deionized water, and the pH value is 3.5 adjusted by malic acid. After mechanical lapping in Fig. 5(a), the surface is full of dense scratches, indicating the mechanical removal by fixed abrasives. In Fig. 5(b), most scratches are removed, and only a few shallow scratches are left after mechanical polishing. There is no embedment of free abrasives. The surface looks like a flat mirror in Fig. 5(c), without scratches and etched 3
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Fig. 4. Dynamic potential polarization curves as a function of current density of Ni alloy blocks after immersion in slurries with (a) different pH values and (b) H2O2 concentrations.
(Fig. 6(d)). Fig. 7 illustrates the XPS spectra of Cr element on the four kinds of different surfaces. The peaks at 574 and 577.1 eV are Cr 2p3/2 and Cr2O3 2p3/2 respectively, corresponding to Cr 2p1/2 and Cr2O3 2p1/2 at 583.2 and 586.9 eV [25]. Chemical states of Cr element are approximately invariable after immersion in H2O2 (Fig. 7(b)), malic acid (Fig. 7(c)) and CMP slurry (Fig. 7(d)). Cr2O3 dominates on the surface, which is different from the pure Ni in Fig. 6. This is attributed to the oxidation of Cr element on the surfaces. Fig. 8 depicts the XPS spectra of Mo element on the four kinds of different surfaces. Three peaks at 227.8, 229.1 and 232.4 eV belong to Mo 3d5/2, MoO2 3d5/2, and MoO3 3d5/2 respectively. This agrees well with peaks at 230.9, 232.2 and 235.5 eV for Mo 3d3/2, MoO2 3d3/2 and MoO3 3d3/2, respectively [25]. Similar to Ni and Cr elements, the chemical states of Mo element have no variation after immersion in H2O2 (Fig. 8(b)), malic acid (Fig. 8(c)) and CMP slurry (Fig. 8(d)). To identify the mechanical function in CMP, XPS spectra of Ni, Cr and Mo elements after CMP are diagrammed in Fig. 9. After CMP, Ni oxides (Fig. 9(a)) were removed compared with those in Fig. 6(a). Most
nature, occupying 12 wt% of the crust of the earth. It is the main composition of rocks. SiO2 has crystalline and amorphous phases in nature, such as crystalline quartz and amorphous kieselguhr·H2O2 is used for the disinfection on environment, food and medical wound. It could decompose into water and oxygen, without the residue of metallic ions, which is different from the oxidizer of KIO3 [16]. Malic acid was firstly separated from apple juice, which is widely used for the additives of food. It is a nutritious material for the human being. After CMP, the Ni alloys were cleaned and dried by deionized water and compressed air, respectively, without the organic solvents, such as acetone and alcohol. From the above analysis, the developed CMP slurry and processes are environment friendly. XPS spectra of Ni element on four kinds of different surfaces are diagrammed in Fig. 6. There are three peaks at 852.8, 853.9 and 856.0 eV, corresponding to Ni 2p3/2 [25], NiO 2p3/2 [26], and Ni2O3 2p3/2 [25]. This is in good agreement with three peaks at 870.1, 871.4 and 873.5 eV for Ni 2p1/2, NiO 2p1/2, Ni2O3 2p1/2 [25], respectively. From Fig. 6, chemical states of Ni element keep basically constant after immersion in H2O2 (Fig. 6(b)), malic acid (Fig. 6(c)) and CMP slurry
Fig. 5. Optical images on the machined surfaces of Ni alloy by (a) mechanical lapping, (b) mechanical polishing, (c) CMP, and (d) surface roughness after CMP. 4
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Fig. 6. XPS spectra of Ni element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
Fig. 7. XPS spectra of Cr element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
on the surface of Ni alloy. Without the mechanical function, the chemical reaction would be terminated due to the passivation induced by CMP slurry. Fig. 10 illustrates the atomic percentage of Ni, Cr, Mo and their oxides on the surfaces of pristine Ni alloy and after immersion in three
oxides of Mo (Fig. 9(b)) were got rid of after CMP in comparison with those in Fig. 7(a). The left Cr2O3 might be formed by oxidation in air after CMP. Similar to Cr element, oxides of Mo in Fig. 9(c) are greatly reduced compared with those in Fig. 8(a), especially for the MoO3 3d3/2 peak at 235.5 eV. Mechanical function is obvious to remove the oxides 5
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Fig. 8. XPS spectra of Mo element on the surfaces of (a) as-received Ni alloy, and after immersion in (b) H2O2, (c) malic acid, and (d) the optimal CMP slurry.
the stability of oxides of Cr and Mo. In optimal CMP slurry, the oxides of Ni slightly reduce compared with those in malic acid, which is the synergistic effect between H2O2 and malic acid. In comparison between H2O2 and malic acid, dissolution function of malic acid dominates in CMP slurry for the oxides of Ni. As the oxides of Cr and Mo are stable in malic acid, H2O2 leads the chemical process in the CMP slurry. After
kinds of slurries and CMP machining. The atomic percentages of oxides for all the Ni (Fig. 10(a)), Cr (Fig. 10(b)) and Mo (Fig. 10(c)) elements increase after immersion in H2O2, indicating the oxidizing function. After immersion in malic acid, the oxides of Ni decrease (Fig. 10(a)), meaning the dissolving of oxides of Ni. However, the oxides of Cr (Fig. 10(b)) and Mo (Fig. 10(c)) keep constant in malic acid, signifying
Fig. 9. XPS spectra of (a) Ni, (b) Cr, and (c) Mo elements on the surface after CMP by the optimal CMP slurry. 6
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Fig. 10. Atomic percentage of pure (a) Ni, (b) Cr, (c) Mo and their oxides on the surfaces of pristine Ni alloy and after immersion in H2O2, malic acid, CMP slurries and CMP machining.
CeO reduces from 1381 to 1360 cm−1, and COOH varies from 902 to 835 cm−1 (Fig. 11) [27]. Chelating formulas are proposed in Fig. 12 for malic acid and Ni2+ ions and Fig. 13 for malic acid and Ni3+ ions, according to the molecular structure of malic acid [32,33], XPS and IR experimental results. Schematic diagrams of the CMP mechanism are diagrammed in Fig. 14 for the developed optimal CMP slurry. According to the XPS and IR results in Figs. 6–10, four steps are suggested for the CMP mechanism. Firstly, H2O2 dominates for the oxidation process during CMP (Fig. 14(a)). The formulas of oxidation by H2O2 are presented [34,35],
CMP machining, all the oxides of Ni, Cr and Mo reduce greatly, indicating the oxides formed are removed by mechanical function during CMP process. Pristine Ni alloy was immersed in malic acid for 24 h, and then it was picked out. Malic acid crystals were obtained after evaporation of aqueous solution. The IR spectra of malic acid crystals are drawn in Fig. 11 prior to and after immersion by the Ni alloy. The molecular formula of malic acid is COOHCH2CHOHCOOH [27,28], which is a dicarboxylic acid with a secondary hydroxyl [29]. Malic acid exists in many fruits, including apples, grapes, peaches, apricots, blackberries, blueberries and plums [29], which is widely used in food and cosmetic industries [30,31]. From the XPS measurements in Figs. 6 and 10, malic acid can dissolve oxides of Ni, forming the Ni2+ and Ni3+ ions in solution. Owing to the reactive carboxyl and hydroxyl groups, malic acid could chelate with Ni2+ and Ni3+ ions, resulting in the shifts of IR spectra. After chelating with Ni ions, stretching vibration of eOH shifts from 3415 to 3389 cm−1, C]O decreases from 1638 to 1613 cm−1,
H2O2 + Ni → NiO + H2O
(1)
3H2O2 + 2Ni → Ni2O3 + 3H2O
(2)
3H2O2 + Cr → CrO3 + 3H2O
(3)
2H2O2 + Mo → MoO2 + 2H2O
(4)
3H2O2 + Mo → MoO3 + 3H2O
(5)
+
H ions are derived mainly from the malic acid, and partially from the H2O2. The oxides of Cr and Mo are stable in malic acid, while the oxides of Ni were dissolved by malic acid (Fig. 10(a)). The formulas of dissolution for oxides of Ni are described [36], NiO + 2H+ → Ni2++H2O
(6)
Ni2O3 + 6H+ → 2Ni3++3H2O
(7)
The oxides of Ni are dissolved mainly by malic acid (Fig. 14(b)). After the release of Ni2+ and Ni3+ ions to the CMP slurry, they were chelated by malic acid (Fig. 14(d)). This prohibited the release of toxic Ni ions to the environment by malic acid, which is very significant to avoid the pollution to the environment. The oxides of Ni, Cr and Mo produced in CMP are removed by mechanical action (Fig. 14(c)). The passivated film formed by oxides at plateau on the surface was removed by mechanical action, resulting in the exposure of Ni alloy. In
Fig. 11. IR spectra of malic acid prior to and after immersion by the Ni alloy. 7
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Fig. 12. Chelating formulas between malic acid and Ni2+ ions.
Fig. 14. Schematic diagrams of the CMP mechanism for the developed optimal CMP slurry.
developed for Ni alloy, consisting of H2O2, malic acid, SiO2 and deionized water. According to the electrochemical and optical measurements, the optimal CMP slurry was obtained. After CMP, angstrom level surfaces were achieved at a scan area of 71 × 53 μm2. The CMP mechanism was elucidated by XPS and IR measurements. Firstly, H2O2 oxidized the Ni alloy, forming oxides of Ni, Cr and Mo on the surface. Malic acid dissolved the oxides of Ni, while the oxides of Cr and Mo kept constant. After the release of Ni2+ and Ni3+ ions to the CMP slurry, malic acid chelated with them, avoiding the release of Ni ions to the environment. The produced oxides in CMP were removed by mechanical action. Thus, the CMP process is environment friendly.
Fig. 13. Chelating formulas between malic acid and Ni3+ ions.
comparison, the passivated film at basin on the surface protected the Ni alloy, terminating the chemical reaction. With this method, the plateau on the surface of Ni alloy was flattened by CMP, achieving the angstrom level surfaces. The developed CMP slurry and process are both environment friendly.
Declaration of Competing Interest The authors declare no competing interests. Acknowledgements
4. Conclusions
The authors acknowledge the financial support from the National
In summary, a novel environment friendly CMP slurry was 8
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Key R&D Program of China (2018YFA0703400), Excellent Young Scientists Fund of NSFC (51422502), Science Fund for Creative Research Groups of NSFC (51621064), Program for Creative Talents in University of Liaoning Province (LR2016006), Distinguished Young Scholars for Science and Technology of Dalian City (2016RJ05), the Xinghai Science Funds for Distinguished Young Scholars and Thousand Youth Talents at Dalian University of Technology, Science Fund of State Key Laboratory of Tribology, Tsinghua University (SKLTKF17B19), Science Fund of State Key Laboratory of Metastable Materials Science and Technology, Yanshan University (201813), and the Collaborative Innovation Center of Major Machine Manufacturing in Liaoning.
[16]
[17] [18] [19]
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