The boron oxide-boric acid system: Nanoscale mechanical and wear properties

The film that forms spontaneously when boron oxide (B2O3) is exposed to hum id air is a solid lubricant. This film is usually assumed to be boric acid (H3BO3), the stable bulk phase. We describe the nanometer-scale surface mor phology, mechanical properties, and tribological properties of these films and compare them with crystals precipitated from saturated solutions of bor ic acid. Scanning force microscopy (SFM) and low-load indentation were the primary experimental tools. Mechanical properties and their Variation with depth are reported. In all cases, the surfaces were covered with a layer th at has different mechanical properties than the underlying bulk. The films formed on boron oxide showed no evidence of crystalline structure. A thin s urface layer was rapidly removed, followed by slower wear of the underlying film. The thickness of this initial layer was sensitive to sample preparat ion conditions, including humidity. Friction on the worn surface was lower than on the as-formed surface in all cases. In contrast, the SFM tip was un able to cause any wear to the surface film on the precipitated crystals. In dentation pop-in features were common for precipitated crystals but did not occur on the films formed on boron oxide. The surface structures were more complex than assumed in models put forth previously to explain the mechani sm of lubricity in the boron oxide-boric acid-water system.