Low friction behavior of sintered hBN sliding in sodium chloride solution

The tribological behavior of pure sintered hexagonal boron nitride (p-hBN) and of sintered hexagonal boron nitride with CaO–B2O3, sintering additives (hBN–CaO) is investigated in distilled water and saline solution. The friction coefficient of the p-hBN sliding on itself in distilled water is 0.06, lower than that in air. However, a smaller amount of oxide was retained on the sliding surface in water than that in air and the pH of the test water increased with the production of ammonia due to the reaction between hBN and H2O. A transferred layer of the hBN debris observed on the sliding surface in water reveals the preferential plane (0 0 2) of the sliding layer of hBN. In a saline solution, the friction coefficient of p-hBN/p-hBN sliding increases to 0.1 or larger, while no change in friction coefficient is observed for hBN–CaO. EPMA and XRD analysis have revealed that sodium chloride is concentrated on the p-hBN sliding surface, but not on the hBN–CaO one. Sodium chloride adsorbs on the lubricative boron hydroxide or hydrate and slightly increases the friction of sliding hBN in aqueous solution. These results lead to the conclusion that the primary low friction sliding mechanism of hBN is a crystal lamella slip which promotes the lubricity of the tribo-products.