Effects of grain boundaries in oxide scale on tribological properties of nanoparticles lubrication

• Fe3O4 develops texture <001> parallel to the oxide growth.
• The deformed Fe3O4/α-Fe2O3 comprises a high proportion of low CSL boundaries.
• The decrease of coefficient of friction is due to changes in slip systems.
• Lubrication mechanism is proposed to improve tribological properties.

The characters of grain boundaries in oxide layers formed on substrates influence adhesion and friction behaviour, surface fracture and wear during high temperature steel processing. In this work, an electron backscattered diffraction (EBSD) analysis was conducted to investigate the role of surface grain boundary and orientation in magnetite (Fe3O4)/haematite (α-Fe2O3) scale during hot rolling, and further evaluate their effects on tribological properties of water-based nanoparticles lubrication. The results demonstrate that Fe3O4 (100) plane is strongly sensitive to the surface characteristics as the minimisation of surface energy. Coincident site lattice (CSL) boundaries in microstructure is in presence of Σ3 in the Fe3O4 and Σ13b in the Fe2O3 of the substrates subjected to a thickness reduction of 28% and cooling rate of 28 °C/s. This is due in great part to the changes in crystal slip systems. These low-Σ CSL boundaries in oxide scale offer obstacles to the propagation of cracks, where some of nanoparticles collected would be trapped at the interface and thereby may cause high wear rates. A lubrication mechanism is proposed to explain the grain boundary effect on nanoparticles lubrication, and further to determine the dependence of frictional behaviour on surface energy, crystallographic preferred orientation (microtexture) and crystal structure. These results provide an intriguing new insight into the application of water-based lubricant with graphite nanoparticles.