Electrochemical boriding of molybdenum in molten borax

Abstract

In this study, molybdenum plates (99·5% purity) were subjected to electrochemical boriding in a molten borax electrolyte in order to synthesise molybdenum boride phases on exposed surfaces. Electrochemical boriding was carried out at temperatures of 900, 950 and 1000°C for a duration of 30–180 min at a current density of 0·5 A cm−2. Cross-sectional microscopic examination of the plates indicated formation of 20 to 50 μm thick boride layers, depending on process temperature and duration. These layers consisted of two distinct boride phases: Mo2B4·027 and Mo2B5. The boride layer cross-sectional hardness was in the range of 1900–3250 HV with 50 g load. The adhesion of the boride coating to the molybdenum substrate was determined by a Rockwell C indentation adhesion test machine. The boride layers produced due to the electrochemical boriding technique were thick, hard, dense, and homogeneous.