Suspension Rheology and Magnetorheological Finishing

Feb 19, 2017 - Youngwook P. Seo , Sangsok Han , Junsok Choi , Atsushi Takahara , Hyoung Jin Choi , Yongsok Seo. Advanced Materials 2018 48, 1704769 ...
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Suspension Rheology and Magnetorheological Finishing Characteristics of Biopolymer-Coated Carbonyliron Particles Jung-Won Lee,† Kwang-Pyo Hong,† Seung Hyuk Kwon,‡ Hyoung Jin Choi,*,‡ and Myeong-Woo Cho*,† †

Department of Mechanical Engineering and ‡Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea ABSTRACT: Magnetorheological (MR) finishing has been adopted as a newly ultraprecision finishing method for microparticles, an optical system, and an aspherical lens, mainly by MR fluids consisting of magnetic carbonyliron (CI) particles and a carrier fluid such as silicone oil or diwater. However, CI particles are related to corrosion problems because of their oxidation, leading to unpredictable polishing results and a short lifetime of the MR fluids. To resolve this problem, we coated CI particles with a biopolymer of xanthan gum (XG) for better MR polishing and measured their MR properties using a rotational rheometer under applied magnetic fields of different strengths. Experiments were carried out to examine the material removal depth and the surface roughness for BK7 (borosilicate) glass by changing the experimental parameters. While the material-removal depth obtained using raw CI particles was deeper than that for the XG-coated CI particles, the surface roughness obtained using the XG-coated CI particles after MR polishing was lower than that for pure CI particles. These results confirm that XG-coated CI particles can be applied for the MR polishing of a BK7 glass well.

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INTRODUCTION Magnetorheological (MR) fluids, which are mainly composed of microscale magnetic particles with almost no magnetic hysteresis and a nonmagnetic medium, are some of the most advanced smart and intelligent materials because of not only their interesting physical phenomena but also their crucial engineering applications.1−4 Among their various industrial applications, including shock absorbers, brakes, and haptic devices, MR finishing is an advanced and smart polishing method using MR fluids.5,6 This technology can minimize the surface damage of a workpiece surface. Thereby, it can be applied to various materials with different shapes.7 The MR fluids employing MR polishing consist of carbonyliron (CI) particles, water, and stabilizers.8,9 However, because CI particles, which are the major component of MR fluids, are composed of more than 95 wt % iron, corrosion due to oxidation can occur when they are exposed to air or water for a long time. This leads to uncertain polishing results and can reduce the lifetime of MR fluids.10 In addition, CI particles can cause sedimentation problems because of the big difference in the density between deionized (DI) water and CI particles.11,12 To resolve this drawback, rheological studies on CI particles and the composition of MR fluids have been actively performed. With coating technologies that have been introduced to resolve sedimentation problems by reducing the density difference between the fluid and metallic particles, coated CI particles with polymers or inorganics can be an alternative method for handling sedimentation, improving dispersion stability, and resolving corrosion problems.13−15 © XXXX American Chemical Society

Note that a tensiometric method was recently introduced to study both the surface free energy of the magnetic dispersed particles and their dispersion stability.16 The MR properties of a bilayer coating with a polymer and a multiwalled carbon nanotube were also investigated, revealing that sedimentation due to the density difference was improved and stable.11 Recently, for the antioxidation of magnetic CI particles in an MR fluid, silica was adopted as a coating material for CI. The pH of the silica-coated CI particles in an HCl solution did not change,17 demonstrating that the coating inhibited corrosion. A silica coating method was introduced to enhance the rheological properties,18 by comparing the pH and viscosity of coated and uncoated CI particles having different compositions. Therefore, a study involving polymer- and/or composite-coated CI particles was conducted in an effort to improve the stability of dispersion, corrosion, and sedimentation. Such developments regarding MR fluids can have a positive effect on their performance in MR polishing. Thereby, MR fluids together with polymer- or composite-material-coated magnetic particles have been used to polish materials. Zirconia was adopted to coat CI particles, and the viscoelastic behaviors of the coated CI particles were investigated. Glass and ceramics were polished by the coated CI particles,19 revealing that zirconia-coated CI particles can be used for the MR polishing of Received: Revised: Accepted: Published: A

September 29, 2016 February 18, 2017 February 19, 2017 February 19, 2017 DOI: 10.1021/acs.iecr.6b03790 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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respectively. In addition, the saturation magnetic values of two magnetic particles evaluated by a vibrating sample magnetometer (DMS 1660) in their powder forms were 201 and 151 emu/g, respectively, as shown in Figure 2 without magnetic hysteresis.21 The lessened magnetic saturation value of the XGcoated CI particles was caused by the nonmagnetic XG coating onto the surface of CI particles.

optical materials. Therefore, the CI coating technique can be considered to be a greatly helpful technique for MR polishing by using MR fluids. In this study, xanthan gum (XG), a hydrophilic biopolymer, was coated onto CI particles, and their rheological and MR properties were measured after an MR fluid was fabricated as a continuation of our previous work on its MR polishing application.8 The MR fluid was then used to test the depth of material removal and the roughness profile of the surface in an actual MR polishing process for BK7 glass, which is widely used for the fabrication of aspheric lenses because of its good optical properties. Polishing spots were examined using a threedimensional (3D) noncontact surface profiler (NewView 7300, Zygo, USA).

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EXPERIMENTAL SECTION Materials. The CI particles purchased from ISP, USA (particle density = 7.63 g/cm3), were used as magnetic-core materials. Semiflexible biocompatible XG (Sigma Aldrich), which is generally obtained from exopolysaccharide upon fermentation of Xanthomonas campestris and composed of Dglucopyranosyl-, D-mannopyranosyl-, and D-glucopyranosylglucuronic acid, was used as a coating material. XG has a high solution viscosity even at low concentration owing to its unique chemical structure in water, with a relatively strong mechanical strength of the chain under a high shear rate.20 The 3D net structure of the molecule gives stability to the suspension and emulsion. However, this structure is strongly influenced by the temperature: the molecules have a regular structure and a low viscosity at 25−40 °C, but at slightly higher temperatures (40− 60 °C), they have an irregular structure and a high viscosity. Figure 1 exhibits a synthesis scheme of the XG-coated CI

Figure 2. Magnetic hysteresis curves of both pure CI and XG-coated CI particles.

On the other hand, aqueous media of both MR fluids with and without coating were slightly different from each other for various tests such that, for XG-coated MR fluids, all were in distilled water, while for pure CI MR fluids, all were with additives of 0.3 wt % sodium carbonate (Na2CO3) and 2 wt % glycerin except the oxidation test with distilled water. Figure 3 demonstrates scanning electron microscopy (SEM) images of both particles. The bumpy CI surface was clearly observed as a result of coating onto XG particles. MR Characterization. To investigate the MR behavior of MR fluids, we examined the rheological properties of the fluids

Figure 1. Schematic diagram of the CI coating process with XG.

particles. First, the CI particles were rinsed using acetone to eliminate impurities at the CI surface, while XG was dissolved in water by stirring at a rotational speed of 400 rpm for 30 min. The rinsed CI particles and prepared XG solution were mixed for 30 min at a 600 rpm speed. Then ethanol was added to the mixture, and because ethanol is a nonsolvent of XG, it effectively generated a coating layer of XG on the surface of the CI particles, with hydrogen bonds between the CI particles and XG. The coated particles were washed with methyl alcohol several times and kept in a vacuum oven for 1 day. The particle density of both pure CI and XG-coated CI was measured using a gas pycnometer, and the values were 7.63 and 5.80 g/cm3,

Figure 3. SEM images of (a) pure CI and (b) XG-coated CI particles. B

DOI: 10.1021/acs.iecr.6b03790 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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field was observed to be higher than those of MR fluids with pure CI. In addition, it can also be noted that the efficiency of the coating on the MR characteristics has been widely reported for both MR fluids22 and MR elastomers23 in terms of the MR efficiency. Figure 4b represents the shear viscosity curves at shear rate sweep tests deduced from Figure 4a for both MR fluids with pure CI and XG-coated CI particles at the same weight ratio as the magnetic particles. Nonmagnetic XG particles caused the shear viscosity to decrease under the same value of magnetic field strength. The shear viscosity of both MR fluids decreased as the shear rate increased, indicating a severe shear-thinning effect.24 Note that, during the MR polishing process, MR fluids were used as a polishing pad. Therefore, it was necessary to observe the shear viscosity of the MR fluids under the same magnetic field intensities. Further shear viscosity tests of the MR fluids containing pure and coated CI particles were performed to investigate the influence of the XG coating and different particle fractions. Figure 5 shows that the shear viscosity of the

using a controlled stress rotational rheometer (MCR 300; Paar Physica, Graz, Germany) equipped with a magnetic-fieldgenerating device (MRD 180; Paar Physica, Graz, Germany). A PP 20 (parallel-plate) measuring system (diameter = 20 mm; gap size = 0.8 mm) was used over a shear rate range of 0.1−200 s−1. The MR fluids had a 30 wt % particle concentration for both CI and XG-coated CI particles. Figure 4 demonstrates the

Figure 5. Shear viscosity curve of both pure CI and XG-coated CIbased MR fluids at different compositions under the same magnetic field strength.

Figure 4. Flow curves of both pure CI and XG-coated CI particles based on MR fluids at various magnetic field strengths.

60 wt % XG-coated CI-based MR fluid was similar to that of the pristine CI-based MR fluid with 30 wt % particle concentration. As for the reference of the shear viscosity depending the dispersed particle concentration, the shear viscosity of the 50 wt % MR fluid with XG-coated CI particles, which was lower than that of the 60 wt % MR fluid with XGcoated CI particles, is also given. Thus, the optimal composition was determined according to the variation of the viscosity for other compositions. MR Polishing System. In the existing commercially available systems, aqueous MR fluids, which are used in the polishing system of QED Technologies, consist of CI particles, nonmagnetic abrasives, DI water, and stabilizers. In the polishing method of QED Technologies, a nonmagnetized MR fluid is supplied to a converging gap between a rotating wheel and the workpiece surface. When the nonmagnetic MR fluid is moved to the gap on the wheel, the section of the gap is magnetized by an electromagnet. The magnetized MR fluid performs material removal and then is brought to the outside of the field, where it is removed from the rotating wheel. It is again

steady shear test results of the two MR fluids at four different magnetic fields. For each step in the test, the duration of the measuring point was fixed from the initial stage 30 s in a lowshear-rate region to the final 0.5 s in a high-shear-rate region on a log−log scale. These duration times are considered to be enough to ensure their equilibrium condition of each measuring point regarding the response time of the magnetic particles dispersed under magnetic and flow fields. As shown in Figure 4a, even without a magnetic field strength, all MR fluids deviated from a Newtonian fluid behavior because of the high weight percentage of the magnetic particles. However, when an external magnetic field strength was applied, the shear stress of all MR fluids experienced a huge increase with increasing magnetic field strength. Also, the shear stress of MR fluids (open symbols) with XG-coated CI particles was slightly lower than that of the MR fluids (closed symbols) with pure CI. This was attributed to the nonmagnetic XG coating on the spherical CI surfaces, which prohibited direct contact of the magnetic particles. However, the change of the MR characteristics of MR fluids with XG-coated CI particles under an applied magnetic C

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Figure 6. Schematic diagram of the MR polishing system used.

transferred to the container. Therefore, the magnetic field is concentrated in the polishing area, that is, the top-dead-center position on the rotating wheel. A pole piece is manufactured to polish the workpiece using the concentrated magnetic field. To reuse and resupply the MR fluid, the system is designed such that the magnetic field cannot be generated to the pole piece. Unlike the existing system, our custom-made MR polishing system uses an MR fluid that contains only CI particles, DI water, and stabilizers of dispersion, as shown in Figure 6.25 The circulation of abrasives (slurry type) is possible because it is supplied by a developed abrasives supply system. The shear viscosity can be kept constant, and the fluid rotates onto a magnetized wheel surface. The MR fluid does not need a specific collection system, and the input amount of the abrasive can be controlled. Therefore, an additional polishing efficiency and an additional cooling effect can be obtained. The MR fluid is supplied to the rim of the magnetized wheel, and the abrasive is supplied to the stiffened MR fluid layer. When a magnetic field is applied to the rotating wheel, the rim of the wheel surface is magnetized. The required amount of MR fluid is provided on the rim through nozzle A1, as shown in Figure 6. The MR fluid becomes stiffened by the applied magnetic field, while the abrasives are introduced onto the stiffened MR fluid layer through nozzle B. The stiffened MR fluid layer behaves as a polishing pad. Nozzle A2 can make the stiffened MR fluid layer thickness constant and provide an additional new MR fluid. Nozzle B is used to remove the used abrasives for recycling. Because the MR fluid and abrasive supply systems are separated, easy recycling of the abrasives is possible and requires a relatively small amount of MR fluid compared with that for existing systems. Additionally, the flow rate of the abrasives can be easily controlled for better polishing. MR fluid scriber C is used to skim off the abrasive slurries remaining on the outer site of the used MR fluid. Nozzles A1 and A2 can always supply a novel MR fluid. As shown in Figure 7, the MR polishing machine is presented. A rotatable wheel with electromagnets and motion stages are installed on the table. The rotatable is worked by a servomotor, and the wheel is controlled within a range of 0−1000 rpm. The whole workpiece is fixed on a rotational motion stage by using a vacuum. A laser sensor is additionally installed to precisely compensate for tilting errors on the table.

Figure 7. Experimental setup for MR polishing experiments.

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RESULTS AND DISCUSSION Sedimentation Test. While MR fluids have many advantages, such as a fast response time, high yield strength, large working temperature range, and good stability against impurities, they have a sedimentation problem because of the density difference between the particles and the continuous phase. To fabricate an applicable MR fluid, the dispersion stability of the fluid should be guaranteed. When no additive is added to an uncoated CI particle-based MR fluid, a sedimentation problem can arise. Fang et al.14 and Liu et D

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Table 1. Experimental Conditions for the MR Polishing Test no. 1 2 3 4 5

electric current (A)

magnetic field Intensity (A/m)

polishing time (s)

gap size (mm)

1.0

4700

60

0.8

wheel speed (rpm) 100 200 300 400 500

Figure 8. Sedimentation test of uncoated and coated CI particles ( in minutes).

Figure 11. Measured removed depth after 1 min of MR polishing.

Figure 9. Sedimentation test of uncoated and coated CI particles (in seconds) from a centrifugation method.

Figure 12. Variation of material removal according to uncoated and coated CI particles. Figure 10. pH variations of uncoated and coated CI particles according to time.

coated and uncoated CI particles), the settled height of the MR fluids was measured for unit time. The sedimentation ratio was calculated according to the settled height versus the MR fluid height for unit time. As shown in Figure 8, the sedimentation process of the uncoated MR fluid was finished after 15 min. In contrast, the sedimentation of the coated MR fluid continued at length. This method thus requires a long time.

al.17 conducted studies to solve the sedimentation problem by changing the density difference. Here, the sedimentation ratio with two different MR fluids was investigated using a test tube method. After a test tube was filled with the MR fluids (XGE

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Figure 13. Measured normal force of uncoated and coated CI particles with applied magnetic fields.

Figure 14. Surface roughness variations according to the MR polishing time.

Table 2. Experimental Conditions for the MR Polishing Test no. 1 2 3 4

wheel speed (rpm)

electric current (A)

magnetic field Intensity (A/m)

polishing time (min)

gap size (mm)

300

1

4700

5 10 15 20

0.8

containing both XG-coated and uncoated CI particles were put in DI water, compared to all other tests in this study. Then, the pH was measured until unchangeable time by a pH meter in intervals of a few minutes. First, the pH probe was soaked in a standard solution. Then, the neutral pH was controlled at 7. Finally, the pH of the MR fluids was measured. The pH decreased over time. The rate of change of the pH of the MR fluid with pure CI was larger than that of the MR fluid with XG-coated CI, as shown in Figure 10. The MR fluid with XGcoated CI remained alkaline after a few hours. Note that, during the MR polishing of glass materials, the pH should remain alkaline. Therefore, this fluid with coated XG can be applied for the MR polishing process and offers enhanced antioxidation characteristics. Material-Removal Depth at a Polishing Spot. The experiments were carried out to evaluate the material-removal depth. The composition of the MR fluid and abrasive slurry was considered to be very important in this polishing experiment because it has a decisive effect on the material-removal depth and the surface roughness of the glass material. In particular, the abrasive slurries can be used selectively according to the material. The compositions of the applied MR fluid for the experiments are 30 wt % pure CI particles in 67.7 wt % DI water with 0.3 wt % Na2CO3 and 2 wt % glycerin and 60 wt % XG-coated CI particles in 40 wt % DI water, respectively. In addition, a nanoceria slurry was used as the abrasive. The wheel speed, magnetic field strength, polishing time, and gap

Therefore, a centrifuge was utilized to reduce the time. The centrifugation method relies on centrifugal force. This method can separate particles with high and low densities. The test was performed for 1 min in 10 s intervals at 300 rpm. As shown in Figure 9, the resulting sedimentation of the uncoated MR fluid was clear. It was finished in 60 s. On the other hand, in the case of the coated MR fluid, 20% of the MR fluid was settled after 60 s. The stability of the fluid was improved owing to the hydrophilic property of the coated macromolecular chains of XG. Therefore, the XG-coated CI MR fluid is more stable than the uncoated MR fluid, mainly because of the density difference between 7.63 and 5.8 g/cm3 for pure CI and XG-coated CI particles, respectively. Oxidation Test. The corrosion problem due to oxidation of the magnetic particles induces unpredictable polishing results or surface damage. To compare the corrosion of the coated MR fluid with that of the uncoated MR fluid, the pH values of both MR fluids were measured using a pH meter. MR fluids F

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Figure 15. Measured surface roughness after MR polishing.

under a magnetic field strength of 170 kA/m. When the magnetic field was applied, the magnetic particles in the MR fluid were oriented in the direction of the magnetic field. Therefore, positive normal forces are generated, thus pushing the upper plates. The normal force was measured with a sensor built into the air bearing system, and it could be recorded from 0.01 to 50 N. For each step in the normal-force measuring test, the duration of the measuring point was set every 5 s for 60 s. The gap size from plate to plate was changed from 0.6 to 1.0 mm. Figure 13a shows the normal force of the MR fluids with both CI particles with a gap of 0.8 mm. Figure 13a also shows the same result as the shear stress and viscosity results, meaning that the chain structure of XG-coated CI particles becomes soft under a magnetic field. This causes XG to serve as a protection layer on the CI particles. The measured normal forces were maintained during the polishing time (60 s in this study). With regard to the polishing time, the magnetized MR fluids can be used for MR polishing. Note that normal force is the applied force that influences material removal and surface roughness along with shear force in the MR polishing process. Also, normal force is the factor affecting the surface-quality improvement. In order to improve the surface roughness and

decisively affect the results of MR polishing. To evaluate the material-removal depth, an MR fluid and an abrasive slurry material were used in this experiment. The experimental conditions for MR polishing are listed in Table 1. The experiment was performed three times under identical conditions, and the average results were obtained. The results were obtained by using the noncontact 3D profiler (Nano system, NS-E1000, and Zygo, NV-7300) and showed that the material-removal depth increased with the rotational wheel speed. The depth of material removal decreased at a rotational wheel speed of 400 rpm, demonstrating that the higher rotation of the wheel could break the chain structures of the MR fluid (Figure 11). Furthermore, the irregular supply of the abrasive slurry caused an erratic material-removal depth because the contact area between the abrasive particles and the surface was formed irregularly. The material-removal depth of the coated MR fluid was lower than that of the uncoated MR fluid, as shown in Figure 12. On the other hand, the changed shear viscosity, applied pressure, and normal force of the MR fluid can induce the yield stress to remove the material. The normal force was measured by a parallel-plate geometry measuring system of the rotational rheometer with a diameter of 20 mm G

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nm for the uncoated CI particles and P-v = 23.562 nm for the XG-coated CI particles. Hereafter, with the optimum conditions from both polishing experiments and rheological analyses of MR fluids, ultraprecision polishing with an MR fluid may find uses in various industrial fields.

minimize the surface defects, the polishing process should be carried out at a relatively low pressure. In this study, MR fluids based on XG-coated CI particles, which produce low pressure compared to the two types of normal force, have a better effect on the improvement of the surface roughness. Figure 13b also presents the measured normal force according to the size of the gap between the MR fluid and the surface of the workpiece. As shown in Figure 13b, the highest normal force was observed at a gap size of 0.7 mm. However, in this study, a 0.8 mm gap was selected. This difference results from the abrasive slurry thickness due to the separating supply system. Surface Roughness. Additional experiments were performed to evaluate the surface roughness of the workpiece. The experimental conditions for these assessments are listed in Table 2. An experiment was performed three times under identical conditions, and the average results were obtained. Figure 14 shows that the measured surface roughness improved as the polishing time increased. In particular, the variations of the surface roughness improved considerably up to 10 min. After 10 min, a few changes were observed. Surface roughness values of Ra = 1.036 nm (uncoated CI particles) and 0.966 nm (XG-coated CI particles) were obtained when both uncoated and coated MR fluids were applied for 20 min. The peak-tovalley (P-v) value was investigated to analyze the surface topography in the measured area using a 3D noncontact surface profiler with an accuracy of 0.001 nm range. The P-v value exhibited a trend similar to that of the Ra value. After 20 min, the P-v value was 26.562 nm for the MR fluid with pure CI and 23.562 nm for the MR fluid with XG-coated CI. Furthermore, Figure 15 shows that the surface integrity was improved because of the low pressure on the surface of the workpiece and the decreased number of scratches. Additionally, similar polishing results can be obtained with two MR fluids. The results of this study thus demonstrate that the coated MR fluid can be effectively applied for the MR polishing process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hyoung Jin Choi: 0000-0001-6915-4882 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.J.C. and M.-W.C. were partially supported by the Ministry of Trade, Industry & Energy, Korea (Grant 10047791) and the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, Korea (Grant NRF2015R1A2A2A01005811), respectively.

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CONCLUSION Once CI particles were coated with XG, the rheological properties of MR fluids containing both coated and uncoated CI particles were examined. The shear viscosity of the XGcoated CI MR fluid was lower than that of the uncoated MR fluid at similar compositions. At 60 wt % XG-coated CI particles, the viscosity of the coated MR fluid was similar to that of the uncoated CI particles and was feasible for MR polishing. To be correlated with the rheological properties, polishing experiments were performed using the fabricated MR fluid, in which BK7 glass was employed as the workpiece, and a nanoceria slurry was used to compare the results obtained using the coated CI particles with those obtained using the uncoated CI particles. The best performance of the removal depth was obtained at a wheel rotation of 300 rpm for both of the MR fluids. However, the coated MR fluid had a noticeably lower material-removal depth because of deceleration of the magnetic properties and the soft-chain structures. On the other hand, the low pressure on the workpiece due to the soft-chain structures can decrease the scratches on the workpiece. The polishing results show that the surface roughness values obtained using the XG-coated CI particles (Ra = 0.966 nm) and the uncoated CI particles (Ra = 1.036 nm) were similar, demonstrating that the coated MR fluid can be applied for MR polishing. The P-v value was further investigated to analyze the surface topography in the measured area, exhibiting a trend similar to that of the Ra value. After 20 min, the polishing results showed P-v = 26.562 H

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