## 3 Results and Discussion
### Nanoscratch Testing  
The accent in the implementation of the nanoscratch surface testing of the nanocomposite specimens fell on an accurate calculation of the coefficient of friction taken by the linear part of the curve depicting the friction as correlation between the enclosed by the indenter lateral and normal forces in function of experimental time. Another main objective was the consideration of the amount of pile-up along the scratch groove that can be dependent on the type or combination of carbon nanofillers used as reinforcing material in the composite structure [31]. In general, the formation of side and rear pile-up of material is predetermined by extrinsic parameters as the radius and shape of a Berkovich indenter [32] without ignoring the impact of the interface features in the nanocomposite structure. The amount of surface deformation during nanoscratching depends on the intrinsic mechanical and physical parameters of the material and, as in our case, on the thermoplastic nature of the polymer die [33]. The essential mechanical data obtained from nanoscratch testing, encompassing COF values, magnitude of lateral pile-up, and average surface roughness, can be seen in Table 4.  
_In-situ_ SPM imaging was used to illustrate the effect of GNPs and MWCNTs in the nanocomposite architecture on the height of material pile-up as a nanomechanical surface peculiarity affecting the study of the samples' friction behavior (Figure 4a,b). Apparently, the presence of carbon nanotubes in the HDPE matrices of the nanocomposites leads to a lower height of pile-up that can be due to the better elastic and wear resistance properties of these two compositions, as already demonstrated by the nanoindentation analysis. Most significant scratch resilience at a 1000 \(\upmu\)N constant load function was ascertained as regards the bifiller composite sample, whose lowest degree of side pile-up means shallowest residual penetration depth (see the 3D plots of scratch grooves in Figure 4c,d). The blue lines denote the loop of the indenter notch measuring the vertical distance of percolation, which was found to be 30 nm shorter with respect to the bifiller hybrid sample in contrast to the neat HDPE. For, the distinguishable elastic behavior of that material could be assigned to the hybrid nanocomposite structure of well-dispersed and interacting carbon nanofillers. On the other hand, the highest coefficient of friction has been calculated with respect to the monofiller nanocomposite containing 4 wt% graphene nanoplatelets--0.401. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Composite Sample** & **COF (F\({}_{\text{x}}\)/F\({}_{\text{z}}\))** & **Lateral Pile-Up** & **Average Surface** \\  & & **(nm)** & **Roughness (nm)** \\ \hline HDPE & 0.332 & 243.9 & 17.5 \\ \hline
4 wt\%GNP/HDPE & 0.401 & 221.6 & 24.1 \\ \hline
4 wt\%MWCNT/HDPE & 0.343 & 133.1 & 29.6 \\ \hline
2 wt\%GNP2 wt\%MWCNT/HDPE & 0.344 & 73.7 & 18.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Coefficient of friction (COF), height of lateral pile-up, and average surface roughness of the composite specimens estimated from nanoscratch mechanical data. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **H/E at 100 \(\upmu\)N** & **H/E at 1000 \(\upmu\)N** & **H/E at 10,000 \(\upmu\)N** \\  & **Peak Load** & **Peak Load** & **Peak Load** \\ \hline HDPE & 0.037 & 0.034 & 0.033 \\ \hline
4 wt\%GNP/HDPE & 0.038 & 0.033 & 0.033 \\ \hline
4 wt\%MWCNT/HDPE & 0.040 & 0.038 & 0.037 \\ \hline
2 wt\%GNP2 wt\%MWCNT/HDPE & 0.040 & 0.034 & 0.032 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Hardness-vs.-elasticity ratio of the polymer compositions as a function of nanoindentation peak load. This value could signify good scratch resistance as it means a significant stress rendered against the motion of the indenter on the sample surface [29], whereas the mean COFs of the other two nanocomposite materials are due to the registering of lower lateral force.