Load Induced Microstructure Evolution and Friction in an Organic Monolayer Self-assembled on a Silicon Substrate

In sliding of organic self-assembled monolayer against a probe the friction force is generally found to vary linearly with normal load. Here, lateral force microscopy is used to track the physical changes at the interface brought about when an octadecyltrichlorosilane monolayer, self-assembled on a silicon wafer, is slid against a Si₃N₄ tip in the 0–30 nN load range. Regarding a morphologically heterogeneous monolayer domain to be made up of tiles of characteristic friction forces, each tile is in a unique physical state; the variation of area fraction (in a scan area) of each tile is tracked as a function of normal load. The area averaged friction force at a load is obtained by summing the fractional forces of constituent friction tiles. The friction force obtained thus, is found to vary linearly with normal tip load. It is observed that this force is dominated by the low-friction crystalline tiles at low loads and by the high friction more amorphous tiles at high loads. This suggests that for a self- assembled monolayer the load governance of friction as implied by the Amontons Law may be attributed to the physical changes that are brought about at the interface by changing the normal load.