Application of Hybrid Nanofluids in MQL Assisted Machining Operations: Exploring Synergies and Establishing Guidelines

Minimum quantity lubrication (MQL) is considered a promising lubricating technique for machining processes that can reduce the environmental impact of conventional flooding lubrication systems and improve operators' safety. The addition of nanoparticles has been proved to increase the lubricating performance of MQL systems, and recently, the use of hybrid nanofluids is getting the attention of many researchers. The combination of different nanoparticles can provide a synergic effect leading to MQL systems with a high improvement in terms of friction and wear reduction, cooling, good stability, wettability, etc. However, the benefits of nanoparticles combination and the potential synergies that can be obtained have not been studied in detail. Current hybrid nanofluids formulation is proposed by a trial an error procedure, without a clear methodology to follow. In this review, recent research on hybrid nanofluids in MQL for machining operations is examined, and five main effects/benefits and three synergies of applying hybrid nanofluids are identified. A comparison between the main nanoparticles used in MQL is provided, showing the nanoparticles performance in terms of friction and wear reduction, cooling, mending, polishing and potential toxicity. This research work can be of interest to practitioners that want to use hybrid nanofluids in MQL systems, and they are looking for a comprehensive review in the field with practical guidelines about optimal formulation and preparation.