# Analysis of Coating Loss from Coated Stainless Steel Orthodontic Wire
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **TiN-Coated Wire** & **Au-Coated Wire** & **Rh-Coated Wire** & **PEN-Coated Wire** \\ \hline Ra (\(\mu\)m) & 0.061 & 0.028 & 0.074 & 0.532 \\ \hline SD & 0.007 & 0.004 & 0.002 & 0.014 \\ \hline \hline \end{tabular}  
Au, gold; PEN, polyethylene naphthalate; Ra, surface roughness; Rh, rhodium; TiN, titanium nitride.  
\end{table} 
Table 1: Surface roughness of coated wire. strength [27]. It would be difficult to make the TiN coating any thinner than that used in the present study, which had a thickness of 0.3 \(\upmu\)m. The relations between bend angle and the amount of metal ion elution from bend sites of coated wires immersed in HCI were analyzed by ICP-MS, and the results confirmed the SEM data and showed increased elution of metal ions derived from the substrate metal, SS, with increasing bend angle. These results suggest that with increasing bend angle, the coating cracked and detachment increased, exposing more of the substrate metal in the wire. On the other hand, the PEN coating suppressed metal ion elution even with bending at an angle of 120\({}^{\circ}\), suggesting no coating loss under these conditions. However, the amount of Mn ion elution from PEN-coated wire was significantly lower at a bend angle of 60\({}^{\circ}\) than at bend angles of 0\({}^{\circ}\) and 30\({}^{\circ}\). Only a trace amount of Mn ion elution was detected at all bend angles, suggesting that Mn elution did not occur at any bend angle examined. PEN has been shown to have good tensile strength and elastic modulus, and to undergo elongation at break [28]. Furthermore, the results of the present study suggest that PEN has sufficient durability for the range of bends used in orthodontic treatment. The null hypothesis (H02) was rejected, because bending of TiN-coated, Au-coated, and Rh-coated wires caused coating loss. As it was not possible to determine whether brushing affected coating loss on coated wires by stereomicroscopy, the surface properties of the wires were next examined by SEM. On SEM analysis, each type of wire showed black areas after brushing. EDS analysis was performed to determine whether there was coating loss and exposure of the substrate metal at these areas. TiN-coated, Au-coated, and PEN-coated wires did not appear to show significant changes before and after brushing. On the other hand, a change was observed in Rh-coated wires after brushing. In the Rh-coated wire, a peak of Rh was observed before brushing, which decreased after brushing and a new peak of Al was observed. It is possible that Al used in the pretreatment was exposed due to detachment of the Rh coating. However, although the manufacturer of the wire stated that they pretreat the Rh-coated wire to improve its coloration, precise data on the materials used were not available due to their policy of keeping this information confidential. In addition, Ti peaks were detected in the PEN coating both before and after brushing, which were due to the use of Ti oxide directly beneath the coating. As the PEN coating was as thin as 50 \(\upmu\)m, the Ti peak may have been detected even before brushing. Due to the thinness of the PEN coating and the detection of substrate metal both before and after brushing, it may be difficult to discuss coating loss of PEN-coated wire based on EDS analysis. However, new trace peaks of Fe were detected after brushing, suggesting that coating loss may have occurred in the PEN-coated wire. Taken together, these results suggest that brushing does not result in coating loss from TiN-coated and Au-coated wires, but that it may cause loss of the coating from Rh-coated and PEN-coated wires. In addition, coating loss may have occurred on the PEN-coated wire, but further analysis is needed. Therefore, the null hypothesis (H03) was rejected.