Preliminary investigation of the effect of environmental humidity on the nanoscale mechanical removal of KDP crystal

In this paper, the effect of environmental humidity on the mechanical removal of potassium dihydrogen phosphate (KDP) crystal was preliminarily investigated using an environment control atomic force microscope at the nanoscale. The results reveal that the pre-wear storage time in humid environments has nearly no impact on the removal of KDP crystal with ∼50 nm wear depth whereas the post-wear storage time imposes remarkable influence. Moreover, the material removal volume gradually increases from 29 × 105 to 127 × 105 nm3 when the relative humidity (RH) increases from 0 to 60%. Such significant increment is mainly attributed to the synergism of mechanical force and deliquescence. More specifically, mechanical force-enhanced deliquescence is achieved by the acceleration of the deliquescence rate in the plastic deformation zone probably due to the high concentrations of catalytically active sites for water molecules adsorption; deliquescence-enhanced mechanical force is achieved by the formation of a thin deliquescence layer with much reduced mechanical strength and the resultant promotion of the material removal. With the increase of RH, more water molecules will be adsorbed on the KDP crystal surface, leading to the continuous mutual promotion between mechanical force and deliquescence, and thereby the material removal volume greatly increases in a high RH environment.