Tribological behavior of electroless Ni–B–W coating at room and elevated temperatures

Abstract

Sodium borohydride reduced electroless Ni–B coatings possess high hardness, wear resistance, and low coefficient of friction. They are found to be suitable candidates for wear reduction of mechanical components. In a quest to achieve enhanced tribological behavior and high thermal stability, the present work reports the inclusion of W to Ni–B coatings. Electroless method is employed for Ni–B–W coating deposition on AISI 1040 steel specimens. Post deposition, the coatings are heat treated at 350 ℃, 400 ℃, and 450 ℃. Deposit characterization is carried out using energy-dispersive X-ray analysis, X-ray diffraction, and scanning electron microscopy. Inclusion of W leads to an increase in microhardness and thermal stability of Ni–B coatings. The tribological behavior of as-deposited and heat-treated Ni–B–W coatings are investigated at room and elevated temperatures (100 ℃, 300 ℃, and 500 ℃). Heat-treated coatings show lower wear rate at room temperature compared to as-deposited ones but the coefficient of friction increases. Tribological test results at elevated temperatures suggest an improvement in the wear resistance and coefficient of friction at 300 ℃ and 500 ℃ in comparison with 100 ℃. Phase transformation study post wear test indicate microstructural changes in the coating due to the in situ heat treatment at high temperature. The tribological behavior of the coatings at 100 ℃ and 300 ℃ is mainly governed by the loose wear debris and formation of debris patches, respectively. Whereas at 500 ℃, formation of protective tribo-oxide patches is also observed.