## 3 Results and Discussion
### Tribocrrosion Tests  
The tribocorrosion tests (combined corrosion and wear tests) were performed according to the experimental procedure described in Section 2.5. The variation in the coefficient of friction (COF) and open circuit potential (OCP) of substrates and Nb-implanted samples is shown in Figure 5. In general, the friction coefficient will vary with the degree of surface damage and wear. Starting load at after 5 min, the COF of the substrate increases rapidly and enters the stable wear stage with an average friction coefficient of 0.395, while that of the Nb-implanted sample remains around 0.15 at the initial stage of wear due to the reduced surface roughness of the sample caused by ion implantation. However, it increases sharply and becomes similar to that of the substrate after about 2 min, which may be due to wear of the implanted layer and the exposure of the substrate as wear deepens. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **E\({}_{\text{corr}}\) (V)** & **I\({}_{\text{corr}}\) (\(\mu\)A/cm\({}^{2}\))** & **R\({}_{\text{p}}\) (\(k\)C/cm\({}^{2}\))** \\ \hline Substrate & \(-\)0.349 & 0.194 & 340 \\ Nb implanted & \(-\)0.199 & 0.051 & 1110 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Corrosion parameters calculated from the potentiodynamic polarization curves. Figure 4: The potentiodynamic polarization curves of the untreated and treated samples in Ringer’s solution. In general, the variation in the OCP is affected by friction and dielectric corrosion [28]. It can be observed in Figure 5b that the OCP of both samples reaches a steady state after 5 min of immersion in Ringer's and before the tribological process, indicating the formation of a stable passivation film. The OCP of samples mutates and moves towards a lower potential due to local damage to the passivation film of the worn surface after a tribological load. The exposed inner metal is more prone to corrosion due to the wear behavior. In addition, the agitation in the friction process promotes the corrosion reaction, which explains the mutation of the OCP. After unloading the load, the OCP of both samples increases rapidly and gradually becomes stable because the metal at the abrasion mark being exposed to the solution promotes the formation of a new passive layer. The presence of defects at the wear marks decreases the OCP to a lower level than it was before the load. It can be observed that the potential of the implanted sample becomes stable faster and is higher than that of the substrate, which indicates that Nb can still improve the corrosion behavior of the substrate after the wear behavior. The OCP of the implanted samples is higher than that of the substrate before, during, and after the wear process, indicating that the implantation of Nb ions can reduce the corrosion sensitivity and improve corrosion resistance. The morphology and element composition of the wear scar were tested and shown in Figure 6. A certain number of grooves were seen on the wear scar because the wear debris are repeatedly rolled and embedded into the substrate surface. This is a typical feature of abrasive wear. Many cracks and pitting pits were observed on the surface of the wear scar, indicating that corrosion wear occurred. Where cracks and pitting occur, there is a high corrosion tendency [29]. The elemental composition in Figure 6c shows that the surface of the wear scar is mainly composed of elements Ti, O and Cl. When the oxide film ruptures during tribocorrosion, Cl ions diffuse easily along the defect and thus form soluble chlorine salts. In addition, the corrosion effect of [PERSON]'s solution accelerates the initiation and propagation of cracks and finally leads to the shedding of wear debris, thus forming pitting pits, following Equation (1):  
\[nMe(O^{2-},2H^{+})+xCl-\to McCl_{x}+nH_{2}O \tag{1}\]  
where \(Me(O^{2-},2H^{+})\) represents the metal oxide film in solution and \(McCl_{x}\) is the soluble chlorine salt. In addition, the heat generated in the wear process will promote the oxidation reaction.