# Wear Effect on the Contact between a Metallic Pin and a Rotating Polymeric Specim
## 5 Conclusions  
The wear behavior between a Ti6 Al4V pin and a UHMWPE rotating sheet was investigated through morphological observations of the surfaces in contact, highlighting the detachment of material particles and the beneficial effects of lubrication in reducing debris production.  
A method for direct measurements of volume loss performed by optical microscopy observations was proposed. Even though the absolute error increases with the test time, for the duration usually considered, this procedure is characterized by a greater degree of concordance between the value of the measured quantity and the real one with respect to that intrinsic to weight measurements. Furthermore, volume measurements allow for avoiding any possibility of error in weighing the polymeric specimens, which could have absorbed humidity during the wear tests.  
The behavior of the debris volume according to the sliding length is characterized by an initial transient stage until a stationary linear stage is reached. Our investigations confirmed the positive effect of lubrication in simulated synovial fluid, which significantly reduces polymer wear compared to dry conditions. Moreover, the morphology of the wear track was characterized through evaluating the rates of material that, in part, is detached in the form of flow or debris, leaving a wear track on the polymer surface, and in part contributes to the formation of plastic pile-ups.  
Figure 10: Debris volume vs. slight length in dry conditions and in SSF: (**a**) experimental points of \(V_{\text{debris}}\) vs. L deduced from weight measurements [20], (**b**) Comparison between curves deduced from weight measurements (blue ribbons) and curves obtained with volume measurements (red lines).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lubricating Condition** & \multicolumn{2}{c}{**(a) Weight Measurements [20]**} & \multicolumn{2}{c}{**(b) Volume Measurements**} \\ \hline  & **(dV/dL)s** & \(W_{s}\) & **(dV/dL)s** & \(W_{s}\) \\  & **(mm\({}^{3}\)/m)** & **(mm\({}^{3}\)/Nm)** & **(mm\({}^{3}\)/m)** & **(mm\({}^{3}\)/Nm)** \\ \hline Dry & 0.00138 & \(4.6\times 10^{-5}\) (\(6.2\cdot 10^{-5}\)) & 0.00155 & \(5.2\times 10^{-5}\) \\ SSF & 0.00078 & \(2.6\times 10^{-5}\) (\(3.6\cdot 10^{-5}\)) & 0.00092 & \(3.1\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Specific wear rates obtained with weight (a) and volume measurements (b).  
As for the metallic pin, the volumetric measurements showed the formation of a mass of debris lower than the accuracy of the weight scale, but it could be potentially dangerous to the prosthetic joints in the long run and deserves to be further explored.  
Conceptualization, A.V. and A.S.; methodology, A.V., G.E., F.G., C.S. and A.S.; validation, A.V., G.E., F.G., C.S. and A.S.; formal analysis, A.V., G.E., F.G., C.S. and A.S.; investigation, A.V., G.E., F.G., C.S. and A.S.; resources, A.V.; data curation, A.V., G.E., F.G., C.S. and A.S.; writing--original draft preparation, F.G. and A.S.; writing--review and editing, A.V., G.E., F.G., C.S. and A.S.; visualization, A.V., G.E., F.G., C.S. and A.S.; supervision A.V. and A.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.