2.6 Sustainable cutting fluidsSustainable cutting fluids are one of the main development areas for the lubricants market due to restrictions and individual pressures and environmental concerns, which is opening multiple research lines. On the one hand, the market is surveying the use of biodegradable vegetable or synthetic oils as the substitute of petroleum products 36. The fast upsurge in prices of petroleum products, the more reliance on offshore sources, the decreasing rate of production and the decreasing rate of finding new reserves are some of the reasons to eliminate mineral based oils 24.

On the other hand, there is a tendency to remove or reduce the use of dangerous substances in the cutting fluids formulation. For example, the removal of sodium nitrite and aromatic compounds, the use of more biodegradable additives and an efficient use of biocides in water-based fluids 37.There is a wide range of base fluids that can be used as an alternative to mineral oil based like poly-alkylene glycols, vegetable oil, poly-alphaolefins, dibasic acid esters, polyol esters 38, and polymer-based fluids 39.             2.7 NanofluidsBesides these methods, “Nanofluids” a new class of cutting fluids, introduced by Choi 41 can be prepared by suspending metallic, non-metallic, ceramics or carbon nanoparticles in a conventional cutting fluid, resulting in good stability, rheological properties, tremendous improved thermal conductivity and negligible influence on pressure drop 40. These novel fluids indicated improved heat transfer properties such as higher thermal conductivity, long-standing stability, and uniformity along with the negligible obstruction in flow channels due to very small sizes and large specific areas of the nanoparticles.

Large number of reports in heat transfer improvement shows the appreciable growth and its importance for developing technology in the field of nanofluids. Table 1, bellows shows examples of nanoparticles used commonly for such applications.