**4. The development trend of nanofluids**

The successful application of nanofluids in MQL technique using vegetable oils becomes a big step toward sustainable manufacturing. This new trend is well suited to encounter the issue of environmental change. MQL machining under nanofluid condition significantly develops the cutting processes, which generate large amount of heat such as hard turning, hard milling, grinding, and so on. The machining capability and achieved surface integrity of hard cutting operations are much improved and equivalent to those of grinding, but the productivity is higher. They can replace or supplement to some of grinding processes. The progression of hard machining will reach further achievements.

Like the idea coming from composite materials, the use of hybrid nanofluids (two or more different types of nanoparticles suspended in fluids) recently gains much attention due to better performance when compared to nanofluids [63–65], but the ratio of mixing and the types of nanoparticles will be continually studied and proven. On the one hand, the MQCL technique, the novel development of the MQL technique will be the future of modern machining processes. MQCL technique has overcome the cooling effect, the main drawback of MQL method [66, 67]. MQCL technique with nanofluids or hybrid nanofluids definitely brings out novel solutions and develops hard machining and grinding to the new level in the near future. On the other hand, the development of ice jet machining is one of the greatest achievements in the field of modern/advanced machining. It is a nondestructive, nonabrasive, residue-free, and environmentally friendly way of machining [68].
