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

Abstract

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.