Ultralow friction behavior of borided steel surfaces after flash annealing

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Abstract
In this letter, we describe the ultralow friction mechanism of borided steel surfaces subjected to a short‐duration, or ‘‘flash,’’ annealing procedure. In this procedure, a borided steel surface is exposed to high temperature (600 to 800 °C) for a short time (3 to 5 min) and then cooled to room temperature in open air. During the high‐temperature exposure, boron atoms within the borided layer diffuse to the surface and react spontaneously with oxygen in air. The reaction product is a thin boron oxide film. During cooling, the boron oxide reacts spontaneously with moisture in the surrounding air to form a thin boric acid film. The sliding friction coefficient of a Si3N4 ball against this flash‐annealed surface is about 0.06, but is 0.5 and higher against the unborided or borided‐only surfaces. Mechanistically, we propose that the ultralow friction behavior of the borided and flash‐annealed surface is due mainly to the layered‐crystal structure of the boric acid film that forms on the sliding surface.

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