7.4 Nanomachining 189 move through a sea of molecules. Eyring's theory of liquid flow is men- tioned here since it explains why the flow of a liquid approximates the flow of metal passed a tool in chip formation. In this case microcracks (voids) move through a sea of crystalline solid. 7.3 SIZE EFFECTS IN MICROMACHINING It is appropriate at this point to mention that an alternative explanation for the increase in hardness that occurs when the indentation size is reduced in metals has recently been introduced 27­38 . This is based on the fact that there is an increase in the strain gradient with reduction in indentation size. This has been extended by Dinesh et al. 39 to explain the size effect in machining. In the Dinesh et al. 39 analysis the size effect in hardness is related to that in cutting by assuming the von Mises criterion is applicable. Based on the experiments of Merchant, it is evident that this is not applica- ble in steady-state chip formation.