**5. Conclusions**

In conclusion, according to the results obtained by studies on particle coating that was then incorporated in MR materials, the coating layer was not only able to protect the magnetic particles from damaging factors such as oxidation, but can also contribute to the enhancement of magnetorheological properties of the material such as the improvement of the sedimentation stability of MRF, increment in wear and friction resistance, as well as reinforcement of the rheological properties of MR materials. It is also emphasized that the coated particles can contribute to more than one advantage that can be o ffered.

It was clear that almost all MR materials that were impregnated with coated magnetic particles have lower magnetic saturation compared to that of uncoated particles. This is unavoidable because most coatings used are made up from non-magnetic materials such as polymers. However, there were some coated particles that exhibited only a slight drop in magnetic properties, where the materials that were grafted onto the particles were made up from magnetizable materials such as multi-walled carbon nanotubes (MWCNT) [109]. However, when this MWCNT-coated CIP was re-coated with COOH–MWCNT, the magnetic saturation of the particles decreased by about 25%. Therefore, it was ascertained that the thicker the coating layer grafted onto the magnetic particles, the lower the magnetization of the particles, regardless of the type of the coating material.

On the other hand, ascertaining the variety of coating methods that were employed in this MR field may also contribute to the di fferent coating functionalities and properties that can influence the characteristic of the MR materials, with various types of polymers that can be used as the coating layer. Table 3 shows the aforementioned coating methods along with their respective responding variables that are crucial in MR studies in terms of storage modulus, loss factor/loss modulus, MR e ffect, oxidation stability, sedimentation stability and magnetic saturation as well as other notable outcomes. Some characterizations are unable to be presented into numerical data due to di fferent methods

employed by respective authors. Therefore, we only indicated the improvement of the particle coated MR materials characteristics using symbols of ↑ (increase) and ↓ (decrease) when comparing the materials with the uncoated ones. From Table 3, it can be deduced that ATRP is the most commonly used coating method employed in particle coating for MR materials. This might be due to controllable polymerization of ATRP with a wider variety of functionalities that the method could o ffer, as well as the fact that most MR materials employed in this method have a much lower decrease in magnetic saturation compared to other methods listed in the table below.

Although there are some unavoidable disadvantages when utilizing particle coating in MR materials, such as the lower magnetic saturation, there are still a lot of opportunities for enhancement that can be developed in the near future.



**Author Contributions:** Conceptualization, S.K.M.J., N.A.N., U. and S.A.M.; methodology, S.K.M.J.; formal analysis, S.K.M.J.; investigation, S.K.M.J.; resources, N.A.N. and S.A.M.; data curation, S.K.M.J.; writing—original draft preparation, S.K.M.J.; writing—review and editing, N.A.N., U., S.A.A.A., S.A.M. and N.N.; visualization, S.K.M.J.; supervision, N.A.N. and S.A.M.; project administration, S.A.M.; funding acquisition, N.A.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Universiti Teknologi Malaysia—Collaborative Research Grant UTM-CRG, gran<sup>t</sup> number 4B461 and Fundamental Research Grant Scheme FRGS, gran<sup>t</sup> number 5F001.

**Acknowledgments:** The authors acknowledge the partial financial support provided by Universitas Sebelas Maret for Hibah Kolaborasi Internasional 2021.

**Conflicts of Interest:** The authors declare no conflict of interest.
