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This review paper highlights the tribological behaviour and performance of smart fluids. Two main kinds of smart fluids, that respond to a suitable external stimulus are investigated -electrorheological and magnetorheological fluid. The excitation of an external active field significantly increases or decreases the fluidity, depending on the fluid type. First, the tribological behaviour of both types of smart fluids without the influence of an external field and under the external active field, respectively, are reviewed. Subsequently, the influence of external field activation on the lubricating layer is described. Finally, a summary of the most notable findings of investigated smart fluids performance and behaviour is provided. Possible future applications are then considered for smart lubricants technology and engineering devices.

Real-time external alteration of the internal properties of lubricants is highly desirable in all mechanical systems. However, fabricating a suitable and effective smart lubricant is a long-lasting experimental process. In this study, the film thickness and frictional response of ionic liquid-lubricated non-conformal contacts to an electric field excitation under elastohydrodynamic conditions were examined. Film thickness was evaluated using a "ball-on-disc" optical tribometer with an electric circuit. Friction tests were carried on a mini traction machine (MTM) tribometer with a "ball-on-disc" rotation module and an electric circuit for contact area excitation. The results demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation at the evaluated film thicknesses. The results of evaluated film thicknesses demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation. Therefore, the ionic liquids could be a new basis for the smart lubrication of mechanical components. Moreover, the proposed experimental approach can be used to identify electrosensitive fluids. Friction 8(5): 982-994 (2020) 983 |www.Springer.com/journal/40544 | Friction http://friction.tsinghuajournals.comFriction 8(5): 982-994 (2020) | https://mc03.manuscriptcentral.com/frictionFriction 8(5): 982-994 (2020)

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Metal matrix nanocomposites represent a relatively new class of material, which is still being extensively investigated. Most of the studies, however, are devoted to aluminium- or magnesium-based nanocomposites. A limited number of studies focus on zinc alloy base nanocomposites, with fewer still concentrating on zinc alloy base micro-nanocomposites. In addition, most of the tribological studies investigate adhesive or abrasive wear resistance, whereas studies of erosive wear resistance lag well behind. It was previously shown that the presence of nanoparticles in ZA-27 alloy-based nanocomposites led to a slight increase in erosive wear resistance. Upon discovering that, the aim became to produce micro-nanocomposites that would retain the positive effect of nanoparticles, while further elevating performance, by combining microparticles with nanoparticles. The ZA-27 alloy-based micro-nanocomposites were reinforced with 3 wt. % Al2O3 microparticles (particle size approx. 36 μm) and with four different amounts (0.3, 0.5, 0.7 and 1 wt. %) of Al2O3 nanoparticles (particle size 20–30 nm). Tested materials were produced by the compocasting process, with mechanical alloying pre-processing. Solid particle erosive wear testing, with particle impact angle of 90°, showed that all micro-nanocomposites had significantly increased wear resistance in comparison to the reference material.