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HIGHLIGHTSFive magnetorheological fluids are tested for MR response, and wear and friction A standard four-ball method is used for wear testingThe bimodal (with a magnetite ferrofluid as base liquid) fluid displays the best MR performance both before and after testing Steel scratches are observed in fluids containing the iron particles in base oil, the effect being more important in absence of anti-wear additiveThe bimodal fluid produces a milder wear, at the price of a larger friction coefficient ABSTRACTIn this work the friction and wear properties of five magnetorheological (MR) fluids with varying compositions are investigated. Considering that many of the proposed applications for these fluids involve lubricated contact between mobile metal-metal or polymer-metal parts, the relationship between MR response and wear behavior appears of fundamental importance. One of the fluids (MR#1) contains only the iron microparticles and the base oil; the second and third ones (MR#2 and MR#3) contain an anti-wear additive as well. The fourth one (MR#4) is a well know commercial MR fluid. Finally, MR#5 is stabilized by dispersing the iron particles in a magnetite ferrofluid. The MR response of the latter fluid is better (higher yield stress and postyield viscosity) than that of the others. More important, it remains (and even improves) after the wear test: the pressure applied in the four-ball apparatus produces a compaction of the magnetite layer around the iron microparticles. Additionally, the friction coefficient is larger, which seems paradoxical in principle, but can be explained by considering the stability of MR#5 in comparison to the other four MRs, which appear to undergo partial phase separation during the test. In fact, electron and optical microscope observations confirm a milder wear effect of MR#5, with almost complete absence of scars from the steel test spheres and homogeneous and shallow 3 grooves on them. Comparatively, MR#2, MR#3 and, particularly, MR#1 produce a much more significant wear.

The flow behavior of a magnetorheological (MR) fluid, consisting of iron particles dispersed in a ferrofluid carrier (‘MRFF’) in a commercial monotube MR shock absorber is studied. The magnetorheological properties of the MRFF suspensions are compared with those of a conventional oil-based MR fluid (‘MRF’). The mechanical behavior of the MR damper, filled with the MRFF or alternatively with the MRF, is characterized by means of different oscillatory force–displacement and force–velocity tests. The MR shock absorber has an internal electromagnet that generates a controlled magnetic field in the channels through which the MR suspensions flow under operation conditions. The results obtained indicate that the damper filled with MRFF shows a reliable and reproducible behavior. In particular, the response of the shock absorber can be controlled to a large extent by adjusting the electromagnetic current, showing a response that is independent of the mechanical and magnetic history of the MRFF. The non-linear hysteresis model proposed for predicting the damping force provides good agreement with the experimental results when the MRFF is employed. The improved response of the damper loaded with ferrofluid-based MRFF (instead of the conventional MRF) is explained considering the physical properties and the internal structure of the suspension.