Study on the tribological and rheological properties of magnetorheological fluids based on different base oils

Base oil has great influence on the tribological and rheological properties of magnetorheological fluid. In this paper, four types of magnetorheological fluid are prepared respectively by silicone oil, mineral oil, synthetic oil (PAO) and castor oil, and their tribological and rheological properties are investigated. Firstly, the viscosity of the magnetorheological fluid is measured by a viscometer. Then the friction coefficient and wear scar diameter of the magnetorheological fluid is measured by a four ball friction testing machine. Next, the sedimentation rate of the magnetorheological fluids is calculated by the observation method. Finally, the shear yield stress of the magnetorheological fluid is measured by a rheometer. By analyzing the experimental data, it is concluded that the magnetorheological fluid prepared by white mineral oil and castor oil has excellent wear resistance. The magnetorheological fluid prepared by castor oil has better sedimentation stability and higher shear yield stress. Consequently, the magnetorheological fluid prepared by castor oil has better comprehensive properties.


Introduction
Magnetorheological fluid (MRF) is a suspension consisting of three parts: base oil, magnetic particle and additives.In the absence of an external magnetic field, MRF has good fluidity.In the presence of a magnetic field, the MRF can be transformed continuously and reversibly from liquid to solid in milliseconds.The MRF will exhibit solid-like mechanical property and can be actively controlled timely with minimal energy consumption.This intelligent fluid with solid-liquid conversion properties is now applied to dampers [1,2], clutches [3,4], brakes [5,6], polishing devices [7,8].The friction performance of MRF directly affects the overall stability and reliability of magnetorheological devices, which has become a major factor restricting the further development and application of MRF [9,10].In the working process of MRF, since the base oil occupies the highest volume fraction in MRF, the characteristics of base oil directly affect the friction performance of MRF [11].Therefore, it is of great significance to study the friction behavior of MRFs prepared by different types of base oils to reduce friction, lower wear and prolong the service life of the devices.
At present, researchers mainly focus on the magnetic particles and additives in the composition of MRF to improve the friction and wear characteristics of MRF.Kumar [12] found through experiments that the carbonyl iron powder (CIP), which is composed of graphene oxide (GO) and iron oxide (Fe 2 O 3 ) nano-composite particles, can effectively improve the friction properties of MRF.Zhang [13] used dispersion polymerization to coat polystyrene on CIP to improve the friction property of MRF.Upadhyay [14] prepared a new kind of MRF with flake-shaped iron particle, which can reduce friction.Li [15] obtained the formula of MRF with excellent frictional properties through experimental research on six different types and different mass fractions of lubricating additives.Lv [16] proposed a new method to improve the tribological properties of MRF using nonpolar oil-soluble graphene, and the results showed that MRF get the best anti-wear effect when graphene concentration in the base oil is 2 mg/ml.Thakur [17] prepared a new kind of MRF with graphite flake, and the results showed that the graphite flake can be used as lubricants to reduce wear.
In the above studies on friction and wear of MRF, the tribological property of MRF is mainly improved by searching for different magnetic particle and additive, but the influence of base oil on the comprehensive performance of MRF is neglected.In this paper, four kinds of MRFs are prepared by silicone oil [18], mineral oil [19], synthetic oil (PAO) [20] and castor oil [21].Subsequently, the friction coefficient of MRF is measured by the four ball friction testing machine, and the wear scar diameter of the steel ball is measured by an optical microscope.Finally, the friction coefficient, sedimentation stability, zero-field viscosity and shear yield stress characteristics of MRF are considered comprehensively to provide a basis for the selection of base oil types for MRF.

Materials
Four different types of oils are selected as the base oil of MRF, which are silicone oil with viscosity of 100 cSt produced by Dow Corning (USA), 100# white mineral oil produced by Toma (China), PAO16 fully synthetic oil produced by Huameite (China), and castor oil produced by Tongliao (China), the specific parameters are shown in table 1.The magnetic particle of MRF is CIP with an average particle size of 3.5 μm produced by Tianyi (China).The additives include the anti-wear agent MoS 2 produced by Yuandong (China), the silane coupling agent KH-560 produced by Chuangshi (China), and thixotropic agent SiO 2 produced by Wacker (Germany).In this experiment, the CIP is 60 wt%, base oil is 36% wt%, HK-560 is 1 wt%, SiO 2 is 1 wt%, and MoS 2 is 2 wt% [15].As the mass fraction of CIP increases, the mass fraction of base oil decreases.The oil film on the surface of the CIP will become thin, and the frictional properties of the MRF will be poor.However, increasing the mass fraction of CIP can effectively enhance the shear yield stress of MRF.The silane coupling agent HK-560 and thixotropic agent SiO 2 can improve the sedimentation stability of MRF.MoS 2 , as a wear-resistant agent, can reduce the friction between CIPs.
Figure 1 shows the process preparation of MRFs.The CIP, base oil and additives are weighed by a balance and poured into a beaker.The MRF is stirred with a high-speed stirrer so that the CIP and additives are uniformly distributed in the base oil.In order to investigate the effect of different base oil on the tribological and rheological properties of MRF, the design of scheme is shown in table 2.
Due to the obvious density difference between CIP and base oil, it will cause the CIP with higher density to settle easily in the base oil.The zero-field viscosity of the MRF has a great influence on the sedimentation stability of the MRF.The viscosity of base oil and MRF is measured by the NDJ-1 rotational viscometer (China) in the water bath at 25 °C, as shown in figures 2, 3(a) shows the sedimentation diagram of four MRFs after 7 days.When the total volume of MRFs is a and the volume of the precipitated layer is b, the sedimentation rate is the ratio of b to a, as shown in figure 3(b).The sedimentation rate x(t) of the MRFs can be derived from equation (1).

Friction experiment
The friction experiment is conducted using a SGW-10A four ball friction testing machine (China), as shown in figure 4    the three fixed steel balls are measured using optical microscope, and the average value is taken as the experimental wear scar diameter.

Magnetorheological experiment
The magnetorheological experiment is conducted using the Discovery HR-20 rheometer (USA), as shown in figure 5(a).There is the 1 mm gap between the rotor plate and the bottom plate for deposit the MRF.The magnetic field generated by the electromagnetic coil is uniformly passed through the MRF, as shown in figure 5(b).The rheometer is used to obtain the relationship between the shear yield stress and the magnetic induction strength of the MRF.The experimental conditions are as follows: temperature of 25 °C, shear rate of 100 s −1 , and magnetic induction strength of 0-0.9 T.

Effect of different base oils on the friction and wear properties of MRFs
Friction and wear experiments are conducted using MRFs prepared by four different types of base oils.Figure 6 shows friction coefficient of four kinds of MRFs.The friction coefficient is measured every 10 s, and the average friction coefficient is the average one of all the data.It can be seen from the error bars in figure 6 that the friction coefficients of MRFs-1 fluctuate greatly, while those of MRFs-   coefficients.The MRF prepared by silicone oil exhibits the highest average friction coefficient, while MRFs prepared by white mineral oil exhibits the lowest average friction coefficient.Figure 7 shows the average wear scar diameters on the fixed steel balls in the friction experiment of MRFs prepared by four kinds of base oils.Three fixed balls in the four ball friction testing machine have wear diameters, so the average value of the wear spot diameters of the three fixed steel balls is the average wear spot diameter.The wear scar diameters of MRFs-1, MRFs-2, MRFs-3, and MRFs-4 are 2.05, 0.61, 0.67, and 0.64 mm, respectively.According to the data in figures 6 and 7, the average friction coefficients and wear scar diameters of the four kinds of MRFs are summarized in table 3. The results show that the MRFs prepared by white mineral oil and castor oil have smaller average friction coefficients and wear spot diameters.In contrast, MRF prepared by silicone oil have the largest average coefficient of friction and wear spot diameter.Therefore, white mineral oil and castor oil have excellent wear resistance as base oil for MRF.

Effect of different base oils on the sedimentation rate of MRFs
In order to understand the effect of different base oils on the sedimentation stability of MRFs, the sedimentation rates of four MRFs are observed.Figure 8 shows the 7-day sedimentation rates of MRFs prepared by four base oils.The sedimentation mainly occurred within the first 3 days after preparation, and the sedimentation rates gradually stabilized after 7 days.The 7-day sedimentation rates of MRFs-1, MRFs-2, MRFs-3, and MRFs-4 are 97.2%,98.8%, 94.4%, and 99.8%, respectively.The sedimentation rates of these four MRFs are higher than sedimentation rate (90%) of commercial MRF (Lord-132DG) [22].Since viscosity has a great influence on sedimentation stability, the viscosity of four MRFs are measured by the viscometer.Because of viscositytemperature characteristics, the viscosity will decrease with the increase of temperature.Therefore, when testing the viscosity of four MRFs, the temperature is controlled at 25 °C by using the water bath.The zero-field viscosities of MRFs-1, MRFs-2, MRFs-3, and MRFs-4 are shown in table 4. By comparing the viscosity of silicone oil in table 1, the viscosity of MRF is positively correlated with viscosity of base oil.The different viscosity of the four base oils is the main factor which makes the viscosity of the four MRFs different.The higher zero-field viscosity prevents the sedimentation of CIP, while lower viscosity accelerates the sedimentation of CIP.Since the MRF prepared by PAO has low zero-field viscosity, its sedimentation stability is poor.While the MRF prepared from castor oil has higher zero-field viscosity, so its sedimentation stability is better.

Effect of different base oils on the shear yield stress of MRFs
Figure 9 shows the shear yield stress curves of the four MRFs at 0-0.9 T magnetic induction strength.In the 0-0.7 T stage, the shear yield stresses of the four MRFs increased rapidly with the increase of the magnetic induction intensity.After the magnetic induction strength reaches 0.7 T, the shear yield stresses of the MRFs no longer increase.The main reason is that when the magnetic induction intensity reaches 0.7 T, the CIP reach magnetization saturation resulting in the shear yield stress of the MRF no longer increases.When the magnetic induction intensity is 0.7 T, the shear yield stresses of MRFs-1, MRFs-2, MRFs-3, and MRFs-4 are 15.2, 14.8, 13.8, and 17.4 kPa, respectively.The shear yield stress of the MRF prepared by castor oil is the largest, and the shear yield stress of the MRF prepared by PAO is relatively small.spots of MRFs prepared from silicone oil, white mineral oil, PAO and castor oil were observed to be 2.05, 0.61, 0.67 and 0.64 mm, respectively.The MRFs prepared by white mineral oil and castor oil have excellent friction and wear properties.When the magnetic induction intensity is 0.7 T, the shear yield stresses of MRFs prepared by silicone oil, white mineral oil, PAO, and castor oil are 15.2, 14.8, 13.8, and 17.4 kPa, respectively.The MRF prepared by castor oil have higher shear yield stress.In conclusion, the MRF prepared by castor oil exhibit both excellent tribological and rheological properties.
(a).The schematic diagram of the four ball friction testing machine is shown in figure 4(b).The four ball friction testing machine consists of three fixed steel balls on the base and one rotating steel ball clamped by a chuck.The MRF is placed in the gap between four steel balls.The steel balls are made of GCr15 steel with the hardness of 58-62 HRC and the diameter of 12.7 mm.The experimental conditions are as follows: rotation speed of 1200 rpm, load of 100 N, and duration of 30 min.To obtain the accurate variation of the MRFs friction coefficient, the test time is set to 30 min.At the end of the friction experiment, the diameters of the wear scars on

Figure 3 .
Figure 3. Sedimentation stability of different MRFs; (a) Sedimentation stability of four MRFs after 7days.(b) Schematic diagram of sedimentation rate.

Figure 4 .
Figure 4. Four ball friction testing machine; (a) Photograph of the testing machine.(b) Schematic diagram of four ball friction.

Table 1 .
Performance parameters of the base oils.
Figure 9. Shear yield stress of four MRFs.