**4.3. Effect of grain boundaries on tribological performance**

Many different surface properties of metals and alloys will influence tribological performance. These surface properties include surface energy, crystallographic orientation, grain boundaries, texturing of surface and crystal structure. Grain boundaries in the oxide layers can alter the underlying failure mechanisms of formed oxide scale, which affects their tribological performance during metal processing. There are various strained conditions along grain boundaries because many dislocations present to help accommodate the misfit or mismatch in adjacent orientations. These high energy regions at the surface could make sliding more difficult and increase the friction force of materials during metal forming.

Various mechanisms can be used to explain the role of grain boundaries in the tribolgoical properties of oxide scale during metal processing at high temperature. Our previous study [21] implies that grain boundary sliding contributes significantly to dissipation in oxide layers during hot rolling. If the oxidised grain boundary is under tension, both the metal and the oxide scale during thermal cycling tend to facilitate crack initiation [20]. A mechanism has been addressed for stress-aided grain boundary oxidation ahead of cracks. Oxygen embrittlement can therefore serve as the form of dynamic embrittlement or oxidation-induced grain boundary cracking during services at elevated temperatures [31]. In essence, the role of anisotropy needs to be investigated to clarify, which anisotropy (grain boundary energy or mobility) is dominant at which conditions. The local grain boundary planes can be dominated by the growing side of the boundary.

The friction characteristics with oxide scale also reveal a grain boundary effect—a profound dependence of friction on crystallographic direction and orientation and grain boundary characters. The variation of the coefficient of friction and rolling force in different thickness reductions during hot rolling associated with microtexture and grain boundary characters in magnetite/hematite scale formed on a microalloyed low carbon steel [17].

In summary, the role of grain boundary chemistry and structure on fundamental mechanisms and properties of oxide scale can help to accelerate the design optimisation of grain boundaries in oxidation or corrosion resistance.
