A mechanism to achieve superlow friction in water lubrication is still in the debate because friction is accompanied by complex mechanochemical processes at sliding interfaces. Here, we experimentally demonstrate that superlow friction of silicon-based ceramics in water is achievable because of a self-formed double tribolayer. The outermost tribolayer can be removed from the surface, but the second tribolayer is strongly bound to the surface. Molecular dynamics simulations revealed that the outermost tribolayer was colloidal silica and the second tribolayer was a hydrophilic hydrate. The outermost colloidal silica layer acted as a lubricant and reduced friction. Retention of the water and colloidal silica layer at the sliding interface due to the hydrophilicity of the hydrate layer led to high load-carrying capacity. The reduction of friction by the first tribolayer and the increased load-carrying capacity associated with the second tribolayer played collaborative roles that made superlow friction achievable in water.
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