Silicon-based ceramics such as silicon nitride can be very effectively lubricated by water under certain conditions (1–3). After a wear-in period a condition of very low friction coefficient (<0.01) can be achieved for average contact pressures of 40 MPa and lower. Tomizawa & Fischer (1) proposed a chemomechanical polishing mechanism to reduce surface roughness enabling the onset of hydrodynamic lubrication by water. We suggest that there is an additional soft hydrated silica gel layer adhering to the rubbing surfaces that provides a deformable protective film to enable effective hydrodynamic lubrication of water and protect the surfaces from occasional asperity contacts. The existence of such films was indicated by the nanomechanical data performed on worn surfaces.

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