“…Numerous mechanisms, including rolling/ball-bearing effect [9,18,45,46], protective film/tribofilm [11,14,19,22,[47][48][49][50][51], mending effect [10,52], polishing effect [18,46], synergistic effect [53,54], and thirdbody effect [28,45,[55][56][57], have been proposed to explain the lubrication enhancement of the nanolubricants. The rolling effect means that NPs can act as ball bearings between the rubbing surfaces in case that sliding or ploughing occurs [45].…”