Multifunctional Bi-Layered Tribofilm Generated on Steel Contact Interfaces under High-Temperature Melt Lubrication

The extreme state of high friction, severe wear, and oxidation invariably occur in mechanical contacts during high temperature steel processing. The application of lubricant to mitigate the aforementioned hindrances can enhance the process performance effectively. Melt lubricants are regarded as a highly promising class due to their good thermal stability and unique physical chemistry. The present study evaluates tribological responses of an alkaline metal borate on steel tribo-pair at 800 °C by ball-on-disk testing. It has been found that the borate melt significantly reduces the friction coefficient and the wear loss in accompany with providing excellent oxidation resistance. On the disk, the formation of a bilayered tribofilm dominates synergistic functionalities while the emergence of an ultrafine-grained layer considerably reinforces the interface integrity of the opposing ball. Cross-sectional examinations of the contact interfaces were carried out on both steel counterparts by FIB/STEM. STEM/HAADF-EDS reveal the formation of a boundary film featuring high concentration of B and significant depletion of O superimposed on a Na-rich film on the rubbing disk. On the opposing surface, a chemically complex film consisting of Na, Fe, O, amorphous C and [3]B which resides on nanograins of iron oxide is evidenced by STEM/EELS-EDS.