Tribology of viscoplastic magnetorheological fluids: The effect of the lubricant in a 4-ball test

Magnetorheological fluids and greases are intelligent materials capable of reversibly changing their rheology under a magnetic field. This study evaluated six formulations containing 25 vol.% carbonyl iron particles, HS grade (BASF, Germany), dispersed in: purified lanolin; a pre-formulated mineral oil–based semisolid matrix structured with polyethylene wax and added with 4 wt.% PTFE; three commercial greases (with molybdenum disulfide, or PTFE, or lithium soap); and a perfluoropolyether oil. Four-ball tests (ASTM D- 4172/D-2266) were performed with and without magnetic field application ( H ≈ 120 kA·m −1 ). The commercial greases exhibited catastrophic wear, whereas the semisolid matrices successfully completed the test. The formulation based on the mineral oil–polyethylene (PE) wax semisolid matrix with PTFE addition (field on) showed the lowest coefficient of friction (≈0.083 ± 0.004) and a wear scar diameter of 773 µm, attributed to the alignment of ferromagnetic particles and the formation of a protective film. The formulation with lanolin (field off) displayed the lowest surface roughness (Ra < 6 µm) and a wear scar of 796 µm, due to the high cohesiveness and adhesive nature of lanolin, which keeps the iron particles suspended and maintains the lubricating film. The tribological performance results from distinct mechanisms: lanolin iron grease favors film stability, whereas mineral oil-PE-PTFE iron grease promotes particle organization under magnetic field.