The effect of graphite content on the rheological performance of magnetorheological grease

Abstract

Magnetorheological grease (MRG) is classified as a smart material because its properties can be changed by applying a magnetic field. MRG is made up of magnetic carbonyl iron particles (CIPs) dispersed in a grease medium. The utilization of grease with high viscosity as a medium in MRG has benefit in preventing the settlement of CIPs from occurring. However, it limits the increment of yield stress in the on-state condition, thus reducing application performance during operation. Therefore, the introduction of graphite as an additive was investigated in this study to improve the rheological properties of MRG including apparent viscosity, shear stress and viscoelastic properties. Apart from that, the performance of MRG in term of yield stress was determined as well as the increment of yield stress from off-state and on-state condition were also evaluated to see the effect of different graphite contents on the properties of MRG. MRGs with graphite weight percentages ranging from 5, 10, and 15 wt.% were developed through conventional mixing method, namely MRG5, MRG10, and MRG15. The properties of all fabricated samples were then compared to those of a reference MRG sample. The microstructure of MRG and MRG15 was characterized using an environmental scanning electron microscope (ESEM). The rheological properties of all samples, including apparent viscosity and shear stress were examined using a shear rheometer in the rotational mode with shear rate ranging from 0.01 to 100s-1. While the viscoelastic properties in term of storage and loss modulus of all samples were carried out through shear rheometer under oscillatory mode with varied strains range from 0.001 to 10% at fixed frequency of 1Hz for strain sweep and frequency range from 0.1 to 80 Hz at fixed 0.01% strain for frequency sweep. The results demonstrated a uniform distribution of CIPs in MRG and CIPs with graphite in MRG15 under ESEM analysis at the off-state condition. Based on rheological testing, addition of graphite displayed a slight increment in the apparent viscosity of MRG5, MRG10, and MRG15, and a significant improvement in the yield stress. The highest yield stress achieved in this study is 61.778 kPa with increment of yield stress of 52.645 kPa from 0A to 3A. An expansion of the linear viscoelastic region from 0.01% to 0.1% was also observed for the MRG10 and MRG15 samples, credited to the domination of the elastic properties on the sample. Furthermore, all samples displayed a strong solid-like (elastic) behavior due to high value of storage modulus, G’ acquired compared to loss modulus, G’’ at all frequency ranges. These obtained results were confirmed based on ESEM under on-state condition, which described the contribution of graphite to constructing a more stable chain structure in the MRG. In conclusion, the findings highlight the influence of the addition of graphite on improving the rheological properties of MRG. Hence, the addition of graphite in MRG is a great potential to be applied in many applications such as in brake, damper and clutch.