# Influence of the Drive Plate with Different Surface Textures on the Property of Carbonyl Iron-Based Magnetorheological Fluid

Authors: He Lu; Lifeng Wang; Lili Dai; Hongxing Ding; Qiuxiang Zhang; Xinhua Liu
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

Aiming to study the effect of drive plate with different surface topographies on the wear property of magnetorheological fluid (MRF), some specific experiments are carried out and analyzed in this paper. Firstly, experiment materials and test methods for MRF are elaborated, and four different surface topography drive plates are designed and manufactured. MRF test-bed is built up to test the rheological properties of MRF. Moreover, the surface morphology of particles is taken using a scanning electron microscope (SEM). Finally, experimental results show that there are remarkable influences on tribological properties of carbonyl iron-based MRF with different drive plates. Therefore, the drive plate with specific surface topography can meet the needs of different MRF transmission systems.


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## Body

## 1. Introduction

As a kind of smart material that is sensitive to magnetic field, magnetorheological fluid (MRF) is a solid-liquid two-phase system formed by micron or submicron magnetic particles and additives disperse in a nonmagnetic carrier liquid [1, 2]. MRF changes into a solid-like structure from a liquid-like structure when exposed to an external magnetic field. When the external magnetic field is removed, the behavior of MRF reverts to the previous state within a few milliseconds. Such phenomenon is called magnetorheological effect [3, 4]. As magnetorheological effect is rapid, continuous, and reversible, MRF is widely applied in biological medicine [5], automobile industry [6], polishing technology [7, 8], aerospace [9], mechanical engineering, and other fields [10, 11].In practical engineering applications, carbonyl iron-based magnetorheological fluid with different physical parameters has different application properties, which has received extensive research by scholars. The corrosion process of carbonyl iron particles on magnetorheological behavior was investigated by Plachy et al., indicating that carbonyl iron particles were oxidized at fierce thermal oxidation at 500°C in the air and mild chemical oxidation in 0.05 M HCl [12]. Sedlacik and Pavlinek performed an experimental investigation to elucidate the effect of partial substitution on the overall MR performance as well as sedimentation stability; the sedimentation test showed positive role of dimorphic composition of dispersed phase on the sedimentation stability [13]. A systematical study of the overall influence of carbon allotrope additives on performance, stability, and redispersibility of magnetorheological fluids was carried out by Cvek et al., showing that carbon nanotubes had a better effect on stability and redispersibility of MRF [14]. In addition, the effect of acid additives on the stability and rheological properties of a suspension of carbonyl iron (CI) microparticles dispersed in silicone oil was studied by Ashtiani and Hashemabadi. Experimental results showed that, by increasing carbon chain length of acids, yield stress and stability increased up to 22 times (at H = 362 kA/m) and 7 times, respectively [15]. The relationship between shear stress and volume fraction and shear rate of MRFs was studied by Sun et al., and it was found that the volume fraction had great effect on the yield stress of MRF [16].Particle morphology also has great influence on the properties of magnetorheological fluids. The characterization, magnetic, and rheological properties of plate-like carbonyl iron particle (CIP) in comparison with conventional spherical CIP were studied by Shilan et al., and they indicated that the plate-like CIP obtained higher saturation magnetization (about 8%) than that of the spherical particles [17]. Moreover, a nonspherically shaped iron particle-based magnetorheological (MR) fluid, particularly flake-shaped, is synthesized to evaluate the performance of an MR brake by Patel et al., and it was shown that flake-shaped particle-based MR fluid with 70% weight fraction of iron particles exhibits 17% higher breaking torque at relatively low magnetic field strength compared to spherically shaped MR fluid with 72% particle weight fraction [18]. Effect of nanocelluloses on the magnetoresponsive behavior and stability of MR fluids was studied by Wang et al., and they indicated that both CNC and CNF can stabilize MR fluids and improve their sensitivity to alterations of magnetic field strengths [19]. The effect of nanodiamond on the MRF was studied by Zhao et al., and they demonstrated that the physical properties and external working conditions of the nanodiamond could have a higher impact on MRF, which was of high significance to the preparation of MRF with excellent performance [20]. The iron nanoparticles and commercial carbonyl iron microsized particles were used in the dispersing phase to prepare MR fluids; magnetorheological effect and sedimentation stability were measured for comparison by Zhu et al., and they indicated that the iron nanoparticles-based MR fluids present a slightly lower MR effect but much better sedimentation stability with respect to the MR fluids with carbonyl iron microsized particles [21].Furthermore, external working condition is also one of the key factors affecting the properties of MRF. The temperature effect on performance of compressible magnetorheological fluid suspension systems was studied by McKee et al. and it was found that the shear yield stress of the magnetorheological fluid remained unchanged within the testing range, while both the plastic viscosity, using the Bingham plastic model, and the bulk modulus of the magnetorheological fluid decreased as the temperature of the fluid increased [22]. The microscopic characteristics of a magnetorheological fluid (MRF) in a magnetic field was studied by Wang et al., and they indicated that the chain structure of the same MRF becomes more apparent as the magnetic field strength increases and, in the same external magnetic field, the chain structure also becomes more apparent with an increase in the particle volume fraction [23].Bearing the above observations in mind, the research of MRF is greatly in the component parameters of MRF (magnetic particles, carrier fluid, and additives) and external conditions, such as magnetic field and temperature. However, few studies focus on surface texture of the drive plate on the property of MRF. In this paper, MRF test-bed is used to study the effects of different surface topography on properties of carbonyl iron-based magnetorheological fluid. Sedimentation stability, zero-field viscosity, maximum transmittable torque, and shear yield stress of the samples have been studied and analyzed. Test results show that there are remarkable influences on tribological properties of carbonyl iron-based MRF with different surface texture.The rest of this paper is organized as follows: experimental methods and preparation of the MRF are elaborated in Section2. Results and discussion based on drive plate with different surface texture on MRF experiments are discussed in Section 3. Our conclusions and future work are summarized in Section 4.


## 2. Experiment

### 2.1. Preparation of MRF

Carbonyl iron particles and synthesized base oil are used as magnetic particles and carrier fluid for MRF, respectively. Carbonyl iron powders consisted of polydisperse spherical particles with diameter ranging between 1 and 2μm. Due to its excellent high-temperature and low-temperature performance and wide range of working temperatures, PAO 6 (Poly-Alpha-Olefins 6) is chosen as carrier fluid. Firstly, the synthesized base oil is heated from normal temperature to 60°C by a digital magnetic agitator. Secondly, dispersant, thickener, and activator are added to the synthesized base oil with certain proportion successively. Thirdly, the mixture is magnetically stirred for about 2 hours until it is well blended and the speed is kept at 300 rpm; then we get compound liquid. Fourthly, a certain percentage of carbonyl iron powder is added to the compound liquid gradually. Fifthly, stirring is performed and mechanical stirring speed is kept at 1800 rpm for about 8 hours. Sixthly, the suspension liquid is stirred at 25°C for about 1 hour. MRF of 30% mass fraction is prepared by the above method. The preparation process of MRF is shown in Figure 1. Then we get the prepared carbonyl iron-based magnetorheological fluid sample that is marked as MRF-0.Figure 1
Preparation process of MRF.


### 2.2. Experimental Methods for Properties of MRF

As shown in Figure2, there are three working modes of MRF device: flow mode, shear mode, and squeeze mode. MRF transmission device is based on the shear mode. According to the model of Newton fluid and Bingham fluid, the relation between total shear stress τ and shear strain rate γ˙ is given by [24](1)τ=τyBsgnγ˙+ηγ˙,τ>τyB,(2)γ˙=0,τ≤τyB,where τyB is the yield stress in response to the applied magnetic field intensity B and η is the constant plastic viscosity, which is the apparent viscosity.Figure 2
Working modes of MRF device.Before the magnetic particles are not magnetized to saturation,τyB increases with the increasing magnetic field intensity B and it is given by [24](3)τyB=αBn,where B is the applied magnetic flux density and α and n are constant parameters that approximate the relationship between the magnetic field intensity and the yield stress for MRF, which is determined by the properties of MRF.In this paper, a disc rotary shear test device is used to test the transferable torque and its working principle is shown in Figure3, and the resistant torque can be written as [24](4)T=2π∫r1r2τr2dr.Figure 3
Working principle of MRF shear.The angular velocities of the rotating disks areω1 and ω2, and h is the thickness of the MR fluid gap; then(5)γ˙=rω1−ω2h.Substituting equations (1)–(5), the resistant torque is given by(6)T=∫r1r22πr2dr=2πτy3r23−r13+πηΔω2hr24−r14.It can be seen from the above equation that the resistant torque of MRF transmission device is mainly composed of two parts,TB=2πτy/3r23−r13 and Tη=πηΔω/2hr24−r14. TB is produced by shearing chain columnar structure of MRF, and Tη is formed by the viscidity of MRF that is related to physical property of MRF.In order to observe the dynamic evolution process of magnetorheological fluid with the applied magnetic field, MRF microstructure observation device is built up (Figure4). As shown in Figure 5, magnetic particles are randomly distributed in carrier liquid without an applied magnetic field. The magnetic particles attract each other and line up into chains in the direction of an applied magnetic field and the chains are wider and longer with the intensity of magnetic field increasing.Figure 4
Diagram of MRF microstructure observation device.Figure 5
Microstructure of magnetorheological fluid under different magnetic intensity.
(a)(b)(c)A testing device is designed and its integral structure is shown in Figure6. In order to study the effects of different surface topography on properties of carbonyl iron-based magnetorheological fluid, drive plate with smooth surface, ring groove, radial groove, and pitted surface are designed, where the depth of the grooves and pitted surface is 2 mm, as shown in Figure 7. MRF test-bed is designed and constructed, as shown in Figure 8. Firstly, four samples of MRF with the same composition are prepared as a reference sample before the experiment. Sedimentation stability, zero-field viscosity, and shear yield stress are tested, respectively. Secondly, the sample of MRF is injected into the MRF working gap of the testing device. Thirdly, loads of magnetic powder brake are kept at 20 N/m, regulating DC electrical source and delivering up to 3.5 A of continuous output current, running the system. Fourthly, the maximum load is recorded once an hour, which can be transferred by the test device when the systems stabilize. Fifthly, the system is stopped when the load remains constant, and samples are taken to test its dispersion stability, zero-field viscosity, and shear yield stress. Sixthly, drive plate is changed with four different surface topographies shown in Figure 2, and steps 2 through 5 are repeated.Figure 6
Explosive view of testing device.Figure 7
Drive plate with different surface topography.Figure 8
MRF test-bed.


## 2.1. Preparation of MRF

Carbonyl iron particles and synthesized base oil are used as magnetic particles and carrier fluid for MRF, respectively. Carbonyl iron powders consisted of polydisperse spherical particles with diameter ranging between 1 and 2μm. Due to its excellent high-temperature and low-temperature performance and wide range of working temperatures, PAO 6 (Poly-Alpha-Olefins 6) is chosen as carrier fluid. Firstly, the synthesized base oil is heated from normal temperature to 60°C by a digital magnetic agitator. Secondly, dispersant, thickener, and activator are added to the synthesized base oil with certain proportion successively. Thirdly, the mixture is magnetically stirred for about 2 hours until it is well blended and the speed is kept at 300 rpm; then we get compound liquid. Fourthly, a certain percentage of carbonyl iron powder is added to the compound liquid gradually. Fifthly, stirring is performed and mechanical stirring speed is kept at 1800 rpm for about 8 hours. Sixthly, the suspension liquid is stirred at 25°C for about 1 hour. MRF of 30% mass fraction is prepared by the above method. The preparation process of MRF is shown in Figure 1. Then we get the prepared carbonyl iron-based magnetorheological fluid sample that is marked as MRF-0.Figure 1
Preparation process of MRF.


## 2.2. Experimental Methods for Properties of MRF

As shown in Figure2, there are three working modes of MRF device: flow mode, shear mode, and squeeze mode. MRF transmission device is based on the shear mode. According to the model of Newton fluid and Bingham fluid, the relation between total shear stress τ and shear strain rate γ˙ is given by [24](1)τ=τyBsgnγ˙+ηγ˙,τ>τyB,(2)γ˙=0,τ≤τyB,where τyB is the yield stress in response to the applied magnetic field intensity B and η is the constant plastic viscosity, which is the apparent viscosity.Figure 2
Working modes of MRF device.Before the magnetic particles are not magnetized to saturation,τyB increases with the increasing magnetic field intensity B and it is given by [24](3)τyB=αBn,where B is the applied magnetic flux density and α and n are constant parameters that approximate the relationship between the magnetic field intensity and the yield stress for MRF, which is determined by the properties of MRF.In this paper, a disc rotary shear test device is used to test the transferable torque and its working principle is shown in Figure3, and the resistant torque can be written as [24](4)T=2π∫r1r2τr2dr.Figure 3
Working principle of MRF shear.The angular velocities of the rotating disks areω1 and ω2, and h is the thickness of the MR fluid gap; then(5)γ˙=rω1−ω2h.Substituting equations (1)–(5), the resistant torque is given by(6)T=∫r1r22πr2dr=2πτy3r23−r13+πηΔω2hr24−r14.It can be seen from the above equation that the resistant torque of MRF transmission device is mainly composed of two parts,TB=2πτy/3r23−r13 and Tη=πηΔω/2hr24−r14. TB is produced by shearing chain columnar structure of MRF, and Tη is formed by the viscidity of MRF that is related to physical property of MRF.In order to observe the dynamic evolution process of magnetorheological fluid with the applied magnetic field, MRF microstructure observation device is built up (Figure4). As shown in Figure 5, magnetic particles are randomly distributed in carrier liquid without an applied magnetic field. The magnetic particles attract each other and line up into chains in the direction of an applied magnetic field and the chains are wider and longer with the intensity of magnetic field increasing.Figure 4
Diagram of MRF microstructure observation device.Figure 5
Microstructure of magnetorheological fluid under different magnetic intensity.
(a)(b)(c)A testing device is designed and its integral structure is shown in Figure6. In order to study the effects of different surface topography on properties of carbonyl iron-based magnetorheological fluid, drive plate with smooth surface, ring groove, radial groove, and pitted surface are designed, where the depth of the grooves and pitted surface is 2 mm, as shown in Figure 7. MRF test-bed is designed and constructed, as shown in Figure 8. Firstly, four samples of MRF with the same composition are prepared as a reference sample before the experiment. Sedimentation stability, zero-field viscosity, and shear yield stress are tested, respectively. Secondly, the sample of MRF is injected into the MRF working gap of the testing device. Thirdly, loads of magnetic powder brake are kept at 20 N/m, regulating DC electrical source and delivering up to 3.5 A of continuous output current, running the system. Fourthly, the maximum load is recorded once an hour, which can be transferred by the test device when the systems stabilize. Fifthly, the system is stopped when the load remains constant, and samples are taken to test its dispersion stability, zero-field viscosity, and shear yield stress. Sixthly, drive plate is changed with four different surface topographies shown in Figure 2, and steps 2 through 5 are repeated.Figure 6
Explosive view of testing device.Figure 7
Drive plate with different surface topography.Figure 8
MRF test-bed.


## 3. Results and Discussion

MRF samples’ index and the corresponding test drive plate in the wear experiments are given in Table1.Table 1
Index of MRF samples and the corresponding test drive plate.
IndexExperimental projectsSurface morphology of test drive plateMRF-0——MRF-1Sedimentation stabilityRing groove surfaceMRF-2Pitted surfaceMRF-3Smooth surfaceMRF-4Radial groove surfaceMRF-1Zero-field viscosityRing groove surfaceMRF-2Pitted surfaceMRF-3Smooth surfaceMRF-4Radial groove surfaceMRF-1Maximum transmittable torqueRing groove surfaceMRF-2Pitted surfaceMRF-3Smooth surfaceMRF-4Radial groove surfaceMRF-1Shear yield stressRing groove surfaceMRF-2Pitted surfaceMRF-3Smooth surfaceMRF-4Radial groove surface


### 3.1. SEM Images of Magnetic Particles Morphology

The surface morphology of magnetic particles is observed by scanning electron microscopy (SEM). Carbonyl iron particles are essentially spheroids, as shown in Figure9(a). The modified particles with floccule surface can improve the suspension stability of MRF shown in Figure 9(b). Figures 9(c) and 9(d) show that particles are worn after the wear test due to the interactions between the particles [25, 26].Figure 9
(a) SEM images of magnetic carbonyl iron particles. (b) Modified particles of MRF before experimental test. (c) Multiple particles of MRF after experimental test. (d) Particles of MRF after experimental test.
(a)(b)(c)(d)


### 3.2. Sedimentation Test

The sedimentation stability of the samples is investigated by visual observation method and can be expressed by sedimentation ratioV [27]:(7)V=hH×100%.In this formula,h is the height of the particle-rich phase relative to H, which is the total MR fluid height.The sedimentation stability curve is shown in Figure10. It is illustrated that the sedimentation stability of all the samples decreases with time, and at last it remains stable. The sedimentation ratios of samples that experience test are lower than MRF-0, which indicates that the surface of modified particles is damaged by the interactions force between particles during the experiment. In addition, MRF-4 has the lowest sedimentation ratio of 73.8%, and it is followed by MRF-2, 74.5%, MRF-3, 76.1%, and MRF-1, 78.7%, respectively.Figure 10
Sedimentation ratio of MRF before and after experiments.


### 3.3. Zero-Field Viscosity

The experiments of zero-field viscosity are carried out at room temperature, 25°C, and the rheometer (version: Anton Paar MCR 302) is used in the test of zero-field viscosity. From Figure11, the zero-field viscosity of all the samples decreases with the increment of shear rate and zero-field viscosity of MRF-0 is lower than that of the others. In addition, MRF-4 has the highest zero-field viscosity. Therefore, MRF subjected to wear experiment has higher zero-field viscosity.Figure 11
Zero-field viscosity of the samples.


### 3.4. Shear Yield Stress

The change of maximum transmittable torques of MRF samples with four different drive plates with working time is shown in Figure12. The maximum transmittable torques of the samples are different. From the maximum to minimum are drive plates with radial groove, pitted surface, smooth surface, and ring groove. The corresponding maximum transmittable torques are 27.7 Nm, 25 Nm, 24.4 Nm, and 22.3 Nm. The rates of decline are 29%, 30.6%, 30.3%, and 31.2%, respectively. Drive plate with ring groove has the minimum transmit of torque; the reason may be the increase of the working gap of MR fluid as well as the same direction of ring groove and the shear direction of MR fluid, which reduces transmittable torque of MRF.Figure 12
Relationship between maximum transmittable torque of MRF and working time.The relation curves of field current and shear yield stress are obtained in Figure13. In the working gap between the drive plate and brake plate, the magnetic flux density in x-axis direction with different field current is shown in Figure 14. As shown in Figure 13, the shear yield stress of MRF becomes lower after the wear experiment. The curves of MRF-1 and MRF-3 are almost the same, and the shear yield stress reaches 39.7 kPa and 39.8 kPa, respectively, when the field current is 4 A. However, MRF-4 has minimum shear yield stress, which is 31.8 kPa, when the field current reaches 4 A. Through the above analysis, radial groove and pitted surface drive plates not only improve the transmittable torque of MRF but also increase the wear of MRF.Figure 13
Shear yield stress curves before and after wear experiments.Figure 14
Magnetic flux density curves with different field current and position coordinate of coil.


### 3.5. Discussion

There are remarkable influences on tribological properties of carbonyl iron-based MRF with different surface topography drive plates. Compared with smooth surface plate, drive plates with radial groove surface and pitted surface can improve transmittable torques of MRF, but drive plate with ring groove surface cannot. However, radial groove surface plate and pitted surface plate can also exacerbate the wear of MRF, which will reduce shear yield stress of MRF. In addition, with intensification of the wear of MRF, there results an increase in worn particles, which leads to reduction of zero-field viscosity of MRF. Zero-field viscosity of MRF subjected to wear experiment is as follows in decreasing order: drive plate with radial groove surface, drive plate with pitted surface, drive plate with smooth surface, and drive plate with ring groove surface. Moreover, the sedimentation stability of MRF worsens due to the damage of additives after wear experiments. Therefore, the wear property of MRF needs to be considered in the choice of surface texture for drive plate applied to MRF transmission system.


## 3.1. SEM Images of Magnetic Particles Morphology

The surface morphology of magnetic particles is observed by scanning electron microscopy (SEM). Carbonyl iron particles are essentially spheroids, as shown in Figure9(a). The modified particles with floccule surface can improve the suspension stability of MRF shown in Figure 9(b). Figures 9(c) and 9(d) show that particles are worn after the wear test due to the interactions between the particles [25, 26].Figure 9
(a) SEM images of magnetic carbonyl iron particles. (b) Modified particles of MRF before experimental test. (c) Multiple particles of MRF after experimental test. (d) Particles of MRF after experimental test.
(a)(b)(c)(d)


## 3.2. Sedimentation Test

The sedimentation stability of the samples is investigated by visual observation method and can be expressed by sedimentation ratioV [27]:(7)V=hH×100%.In this formula,h is the height of the particle-rich phase relative to H, which is the total MR fluid height.The sedimentation stability curve is shown in Figure10. It is illustrated that the sedimentation stability of all the samples decreases with time, and at last it remains stable. The sedimentation ratios of samples that experience test are lower than MRF-0, which indicates that the surface of modified particles is damaged by the interactions force between particles during the experiment. In addition, MRF-4 has the lowest sedimentation ratio of 73.8%, and it is followed by MRF-2, 74.5%, MRF-3, 76.1%, and MRF-1, 78.7%, respectively.Figure 10
Sedimentation ratio of MRF before and after experiments.


## 3.3. Zero-Field Viscosity

The experiments of zero-field viscosity are carried out at room temperature, 25°C, and the rheometer (version: Anton Paar MCR 302) is used in the test of zero-field viscosity. From Figure11, the zero-field viscosity of all the samples decreases with the increment of shear rate and zero-field viscosity of MRF-0 is lower than that of the others. In addition, MRF-4 has the highest zero-field viscosity. Therefore, MRF subjected to wear experiment has higher zero-field viscosity.Figure 11
Zero-field viscosity of the samples.


## 3.4. Shear Yield Stress

The change of maximum transmittable torques of MRF samples with four different drive plates with working time is shown in Figure12. The maximum transmittable torques of the samples are different. From the maximum to minimum are drive plates with radial groove, pitted surface, smooth surface, and ring groove. The corresponding maximum transmittable torques are 27.7 Nm, 25 Nm, 24.4 Nm, and 22.3 Nm. The rates of decline are 29%, 30.6%, 30.3%, and 31.2%, respectively. Drive plate with ring groove has the minimum transmit of torque; the reason may be the increase of the working gap of MR fluid as well as the same direction of ring groove and the shear direction of MR fluid, which reduces transmittable torque of MRF.Figure 12
Relationship between maximum transmittable torque of MRF and working time.The relation curves of field current and shear yield stress are obtained in Figure13. In the working gap between the drive plate and brake plate, the magnetic flux density in x-axis direction with different field current is shown in Figure 14. As shown in Figure 13, the shear yield stress of MRF becomes lower after the wear experiment. The curves of MRF-1 and MRF-3 are almost the same, and the shear yield stress reaches 39.7 kPa and 39.8 kPa, respectively, when the field current is 4 A. However, MRF-4 has minimum shear yield stress, which is 31.8 kPa, when the field current reaches 4 A. Through the above analysis, radial groove and pitted surface drive plates not only improve the transmittable torque of MRF but also increase the wear of MRF.Figure 13
Shear yield stress curves before and after wear experiments.Figure 14
Magnetic flux density curves with different field current and position coordinate of coil.


## 3.5. Discussion

There are remarkable influences on tribological properties of carbonyl iron-based MRF with different surface topography drive plates. Compared with smooth surface plate, drive plates with radial groove surface and pitted surface can improve transmittable torques of MRF, but drive plate with ring groove surface cannot. However, radial groove surface plate and pitted surface plate can also exacerbate the wear of MRF, which will reduce shear yield stress of MRF. In addition, with intensification of the wear of MRF, there results an increase in worn particles, which leads to reduction of zero-field viscosity of MRF. Zero-field viscosity of MRF subjected to wear experiment is as follows in decreasing order: drive plate with radial groove surface, drive plate with pitted surface, drive plate with smooth surface, and drive plate with ring groove surface. Moreover, the sedimentation stability of MRF worsens due to the damage of additives after wear experiments. Therefore, the wear property of MRF needs to be considered in the choice of surface texture for drive plate applied to MRF transmission system.


## 4. Conclusions and Future Work

A manufacturing process method for MRF was described in detail, drive plates with four different surface topographies were designed, and an experiment table of MRF transmission test-bed was designed and constructed to study the wear property of carbonyl iron-based MRF.The future work will focus on other factors on the properties of MRF, such as improving control algorithm of MRF transmission system to optimal property of carbonyl iron-based magnetorheological fluid, and the study of the mechanism of friction and wear of MRF is also our future work.



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