Anti-oxidation mechanism and interfacial chemistry of BN@CaCO3-SiO2 microcapsule-added sodium borate melt on the sliding steel surfaces at elevated temperatures

Abstract This study reports the chemistry underpinning the superior anti-oxidation and lubrication of the BN@CaCO3-SiO2 microcapsule-added sodium borate melt at 930 °C by surface-interface tailoring. Under static oxidation at 930 °C, sodium borate reacts to the microcapsule’s shell that generates the sodium-calcium borosilicate melt and releases the h-BN nanosheets. Interfacial characterizations reveal a negligible corrosion attack and superior anti-oxidation of the sodium-calcium borosilicate compared to sodium borate (by ~ 86%) which is due to the weak mobility of sodium in the melt network. Under sliding conditions at 930 °C, friction modifies profoundly the interfacial reactivity of the sodium-calcium borosilicate melt toward the oxide surfaces. An intermixing effect and the friction-induced heat accelerates the high-temperature reactions between the melt and the spinel oxides on the sliding surfaces that result in the formation of the plate-like non-layered M-type CaCrxFe12-xO19 hexaferrite. A synergy between the h-BN nanosheets, the grain-refinement of the sliding surfaces, and the intercalation of the M-type hexaferrite at the sliding interfaces contribute to a superior friction reduction (by ~ 70%) compared to sodium borate melt under boundary conditions with high contact pressures. These findings highlight a potential strategy for achieving excellent lubricity, superior anti-oxidation, and reduced corrosion from the melt lubricants at high temperatures.