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Research on the rheological properties of magnetorheological grease(MRG) is of great significance for evaluating its properties, revealing the internal parameters, and guiding its application. The characterization methods of rheological properties usually include steady-state shear mode and oscillatory shear mode. However, the experiment of MRG rheological properties is mostly carried out in steady-state shear mode. To characterize the rheological properties of different base oil -based magnetorheological greases, the adaptability of the steady-state shear mode is discussed, and the feasibility of using the oscillatory shear mode is analyzed. The evaluation parameters of the MRG rheological properties are studied.

This study was conducted to investigate the influences of saponification time on the physical properties of mineral oil-based magnetorheological greases(MRGs). Three types of mineral oil-based MRG were prepared by applying different saponification time in this work. Then the physical properties, thermal stability and colloidal performance were tested. Besides, the micro-morphology was observed to analyze the microstructures of MRGs. The experimental results prove the formation of entanglement soap fibers micro-network. The oil separation on pressure and worked penetration of MRG decrease with the increase of saponification time, while the centrifugal stability becomes better. DSC-TG-DTG tests exhibit three endothermic peaks and reveal a double-stage thermal decomposition process. It also indicates that the MRG under 2 hours saponification has the smallest thermal decomposition and highest decomposition temperature, which demonstrates a good thermal stability.

FePt films with a thickness of 25 nm were deposited by magnetron sputtering onto the MgO(001) substrates at 400℃, and subsequently annealed at temperatures ( T a) between 500℃-800℃ for 5 hours. The structures and magnetic properties of FePt films were analyzed by X-ray diffraction, vibrating sample magnetometer, and in-field magnetic force microscope. The as-deposited films show a (001) epitaxy induced by the MgO(001) substrates, but a disordered soft magnetic A 1 phase. The ordering did not occur until T a was raised up to 600℃. At T a =600℃, the films changed into a mixture of soft A 1 phase and hard L 10 phase due to the partial A 1→ L 10 transformation, and a special soft-hard phase composite with magnetic anisotropy can be obtained. The in-plane magnetization is mainly contributed by the soft phase with a coercivity exceeding 10 kOe; whereas the out-of-plane magnetization is dominant by the hard phase with a coercivity of about 5 kOe. These results indicate the existence of strong coupling between the soft and hard phases, leading to the formation of exchange springs. After annealing at 700℃, the structure of FePt films became almost ordered. The easy magnetization axis turned completely to the out-of-plane direction and a large coercivity higher than 20 kOe was observed. The atomic force microscope images show that the films treated at 700℃ are composed of island-like grains. Additionally, the in-field magnetic force microscope images indicates that most of grains initially having multiple domains change into single domain by domain wall displacing and vanishing at low field. The nucleation is supposed to be the main mechanism of magnetization reversal of grains.