Abrasion and chemical–mechanical polishing between steel and a sputtered boron carbide coating

When a steel ball undergoes dry sliding against a boron carbide-coated steel coupon, both the steel ball and the boron carbide are polished, even though the boron carbide is several times harder than the steel. We have measured the abrasion rate of the steel under loads ranging from 5 to 1100 g. The average abrasion rate at and above 100 g of load shows a simple power law dependence on the number of cycles, but a deviation from this simple dependence is observed at lower loads. The loss of abrasiveness of the boron carbide correlates with a reduction of the RMS angle on the boron carbide surface, which eventually becomes nearly atomically smooth over regions as large as 1 μm×1 μm. Our results suggest that under the right conditions, a boron carbide coating could be designed to polish a counterpart gear or bearing surface with little risk that the counterpart would be substantially worn and with little concern for how the load varied during a break-in period. We suggest that polishing of the boron carbide surface by steel is a chemical process, while the abrasion of the steel by boron carbide is a mechanical process.