Formation and self-lubricating mechanisms of boric acid on borided steel surfaces

Abstract

In this study, we investigated the formation and self-lubricating mechanisms of thin boron oxide and boric acid films on the surfaces of borided steel substrates. Specifically, we developed an annealing procedure which resulted in the formation of a glasslike boron oxide layer on borided steel surfaces. Annealing was performed at 750 degrees C for 5 min. During cooling, the boron oxide layer reacted with moisture in the surrounding atmosphere to form a thin boric acid film. The measured friction coefficients of sapphire and 440C steel balls against the unborided and borided sides ranged from 0.4 to 0.6. However, after short-duration annealing at 750 degrees C, the friction coefficients of borided steel surfaces were about 0.05 at steady state. Crystal chemical knowledge and Raman spectroscopy evidence were used to determine the formation and self-lubricating mechanisms of the thin boron oxide and boric acid films on borided surfaces.