## 3 Results and Discussion
### Enhanced Subsurface Deformation of FS upon Indentation and Nanowear  
Not only has surface deformation of FS been reported in previous studies, but also the indentation-induced subsurface deformation of FS [40; 41]. To reveal the effect of H\({}_{2}\)O\({}_{2}\) treatment on the subsurface deformation of various FS surfaces following Vickers indentation, these Vickers indentations were annealed at sub-\(T_{g}\) temperature for 4 h, and re-imaged again using AFM. The densified volume (\(V_{d}\)), shear flow volume (\(V_{p}\)), and densification ratio (or volume recovery ratio, \(V_{R}\)) can be calculated as [40; 41]:  
\[V_{d}=\left(V_{i}^{-}-V_{a}^{-}\right)+\left(V_{a}^{+}-V_{i}^{+}\right) \tag{2}\]  
\[V_{p}=V_{i}^{-}-\left(V_{i}^{-}-V_{a}^{-}\right)+\left(V_{a}^{+}-V_{i}^{+} \right)=V_{i}^{-}-V_{d} \tag{3}\]  
\[V_{R}=\frac{\left(V_{i}^{-}-V_{a}^{-}\right)+\left(V_{a}^{+}-V_{i}^{+}\right) }{V_{i}^{-}} \tag{4}\]  
where the subscripts \(i\) and \(a\) indicate the initial volume and volume after annealing, and the superscripts \(-\) and \(+\) represent the indentation cavity volume and pile-up volume, with respect to the sample surface, respectively. It was found that as the H\({}_{2}\)O\({}_{2}\) concentration increases from 0% to 20%, the corresponding \(V_{d}\) of the FS surface following indentation increases from \(\sim\)0.8 \(\times\) 10\({}^{6}\) nm\({}^{3}\) to \(\sim\)2.5 \(\times\) 10\({}^{6}\) nm\({}^{3}\), the shear flow volume \(V_{p}\) of the FS surface decreases from \(\sim\)0.1 \(\times\) 10\({}^{6}\) nm\({}^{3}\) to \(\sim\)0.08 \(\times\) 10\({}^{6}\) nm\({}^{3}\) (Figure 5c), and the \(V_{R}\) of the FS surface increases from \(\sim\)88% to \(\sim\)96% (Figure 5d). The friction-induced subsurface densification can also occur as the normal load is applied along the tangential shear direction [42; 43]. Thus, following the same sub-\(T_{g}\) annealing treatments as used for the indentation marks, the nanowear marks on various FS surfaces were also annealed and the corresponding densified volume, shear flow volume, and volume recovery ratio were also calculated, as shown in Figure 6. It was found that as the H\({}_{2}\)O\({}_{2}\) concentration increases from 0% to 20%, the corresponding \(V_{d}\) of the FS surface following nanowear increases from \(\sim\)1 \(\times\) 10\({}^{6}\) nm\({}^{3}\) to \(\sim\)1.4 \(\times\) 10\({}^{6}\) nm\({}^{3}\), the \(V_{p}\) of the FS surface decreases from \(\sim\)0.25 \(\times\) 10\({}^{6}\) nm\({}^{3}\) to \(\sim\)0.08 \(\times\) 10\({}^{6}\) nm\({}^{3}\) (Figure 6c), and the \(V_{R}\) of the FS surface increases from \(\sim\)80% to \(\sim\)93% (Figure 6d). Figure 5: (**a**) AFM image and (**b**) cross\(-\)section line profiles of various FS surfaces after sub-\(T_{g}\) annealing treatment (red line). (**c**) Densified volume and shear flow volume of various FS surfaces during Vickers indentation tests. (**d**) Recovery ratio of various FS surfaces. Based on the results in Figures 5 and 6, it is clear that regardless of the nanoindentation and nanowear tests, both the \(V_{d}\) and \(V_{R}\) of the FS surface increase with H\({}_{2}\)O\({}_{2}\) concentration, while the \(V_{p}\) decreases with H\({}_{2}\)O\({}_{2}\) concentration, which indicates that the subsurface densification of the FS surface increases with H\({}_{2}\)O\({}_{2}\) concentration. After treatment with H\({}_{2}\)O\({}_{2}\), the FS surface becomes more hydrophilic (Figure 1), more adsorbed water on the FS surface can facilitate the surface contact damage caused by indentation, and the subsurface can undergo more stress since the penetration depth is deeper while the applied normal load remains the same (Figure 3); thus more subsurface densification of FS would be expected when treated with a higher concentration H\({}_{2}\)O\({}_{2}\) solution (Figure 5).