# A New Plasma Surface Alloying to Improve the Wear Resistance of the Metallic Card Clothing
## 3 Results and Discussion
### Nanoindentation Tests
The friction coefficients of the chromizing coating decreased first and consequently increased as the sliding speed increased. When the sliding speed was 4 m/min, the chromizing coating presented the lowest friction coefficient. When the sliding speed was 4 m/min, the friction coefficient curve slope of the metallic card clothing changed. Therefore, as the sliding speed increased, the friction coefficient of the metallic card clothing gradually increased. The significantly lower friction coefficients of the chromizing coating demonstrated that the alloy layers were effective in the tribological properties improvement [22]. The wear profiles of the metallic card clothing before and after plasma surface alloying under various sliding speeds are presented in Figure 8. Figure 8d presents the wear volume variation curve of the samples along with sliding speeds. The wear volume increased along with the sliding speeds. It's remarkable that the chromizing coating wear slowed down, when the sliding speed exceeded 4 m/min. The specific wear rates of the metallic card clothing before and after plasma surface alloying at different sliding speeds are presented in Table 4. It was apparent that the specific wear rate of the chromizing coating with low fluctuation was significantly lower compared to metallic card clothing under various sliding speeds. This indicated the excellent wear resistance of the coating. The lower wear rate of the chromizing coating was attributed to the high H/E and H\({}^{3}\)/E ratios. Figure 7: Friction coefficient vs. sliding time for untreated and treated samples under various sliding speeds: (**a**) 2 m/min, 420 g; (**b**) 4 m/min, 420 g; (**c**) 6 m/min, 420 g; and (**d**) variation tendency of friction coefficient. As the sliding speed increased, the wear mechanisms of the steel, the aluminum alloys and other aluminum foundation composite materials would also change [23]. Figure 9 presents the wear track morphology of the chromizing coating at various sliding speeds. When the sliding speed was 2 m/min, only slight scratches existed on the wear track surface, while the alloy layer mainly presented adhesion wear. When the sliding speed increased to 4 m/min, the fast friction produced a thermal effect. As the sliding velocity increased, the friction pair interface shear stress increased [24]. Under the shear stress action, the wear debris as well as a high amount of scratches appeared on the wear track surface, while the wear mechanism was converted to abrasive wear. As the sliding speed continued to increase, the shear pressure became high and the coating was flaked. The friction mechanism was mainly abrasive wear. A large amount of scratches could be observed in Figure 9c. Moreover, the adhesion film could be observed near the edge of the wear track. The good bonding strength between the deposition layer and the diffusion layer prevented delamination of the deposited layer during friction. The deformed deposition layer acted as a soft film lubricant, subsequently reducing both the friction coefficient and specific wear rate. When the sliding speed was 2 m/min, the wear mechanism of the chromizing coating was abrasive wear. Moreover, the wear mechanism gradually changed from adhesive wear to abrasive wear as the sliding velocity increased (4-6 m/min). It could be observed from Figure 8d that \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Load (g)**} & \multirow{2}{*}{**Sliding Speed (m/min)**} & \multicolumn{2}{c}{**Specific Wear Rate (10\({}^{-4}\)-mm\({}^{3}\)-N\({}^{-1}\)-m\({}^{-1}\))**} \\ \cline{3-4}  & & **Metallic Card Clothing** & **Chromizing Coating** \\ \hline \multirow{3}{*}{420} & 2 & 16.38 & 2.91 \\  & 4 & 9.06 & 3.30 \\ \cline{1-1}  & 6 & 6.26 & 2.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specific wear rate under various sliding speeds for the metallic card clothing before and after plasma surface alloying.