Self-replenishing solid lubricant films on boron carbide

Abstract

This study introduces a simple annealing procedure that leads to the formation of a self-replenishing solid lubricant film on bulk boron carbide (B₄ C) and its coatings. The procedure uses short duration annealing at 800°C for 15 min. Subsequently, the specimens are cooled to room temperature and tested in a pin on disc machine. During annealing, B₄ C reacts with oxygen in air and forms a glasslike boron oxide (B₂ O₃) layer on the exposed surface. Eventually, this layer reacts spontaneously with moisture in the surrounding air, thus forming a thin boric acid (H₃ BO₃) film. The friction coefficients of 440C steel and zirconia pins against this film are 0·04–0·06. Electron microscopy and Raman spectroscopy were used to elucidate the formation and self-lubricating mechanisms of this solid boundary film on sliding B₄ C surfaces.