Effects of Adsorbed Water upon Friction at Layered K_4Nb_6O_(17)·3H_2O Surfaces Studied with FFM

摘要

Frictional properties of two cleaved surfaces of K_4Nb_6O_(17)·3H_2O with different structures were compared with frictional force microscopy (FFM) at various relative humidity (RH) levels. Surface(I) is rough and facing the crystal water layer in the bulk. The other surface(II) is smooth and not hydrated in the bulk. In more humid (RH > 30%) conditions, lower friction was observed with the more easily hydrated surface I. When RH level was increased, frictional coefficients at the two surfaces both decreased. Adsorbed water is lubricating the surface. In drier conditions (RH < 30%), however, the RH dependence of the friction is reversed. Lower friction was observed at drier conditions at both surfaces and the difference between the two was small. The adsorbed water works as a glue at smaller surface coverage, while it acts as a lubricant at larger coverage.