Microstructure and tribological performance of Co-B intermetallic compounds under water environment

Cobalt-boride (Co-B) alloys, as promising wear resistant materials, can form different intermetallic compounds. In this work, supercooling treatment is applied to control the microstructures and compounds in the alloys, and the related mechanical and tribological properties have been investigated systematically. The results suggest that besides FCC-Co and HCP-Co, it forms Co₃B and minor Co₂B in the as-cast samples, Co₃B, Co₂B and Co₂₃B₆ under different supercooling degree. Tribological experiments of these alloys under distilled water environment demonstrate that the Co-B alloy containing Co₂₃B₆ has the best tribological performance, followed by Co₂B and Co₃B. The coefficient of friction and wear rate of alloy containing Co₂₃B₆ are about 0.2 and 1.6 × 10⁻⁶ mm³/Nm, respectively, which are reduced by 60 % and one magnitude lower compared to the as-cast sample containing Co₂B and Co₃B. The promising tribological result of Co₂₃B₆ is attributed to the formation of tribolayer, which is rich in Co(OH)₂, silicate, SiO₂ and possibly H₃BO₃. This work provides guideline for tuning phases of Co-B alloys when using as rubbing parts in moisture environment.