## 3 Results and Discussions
### Tribological Properties
Comparing the measured wear depth value of treated samples and the untreated sample, when the boriding temperature was 950 \({}^{\circ}\)C, the wear track of the treated sample was relatively deeper, but lower than that of the untreated sample. This is attributed to the increased surface hardness enhancing wear resistance. The borided layer was very thin, the binding force at this processing temperature was low, and microparticles would be generated to increase wear loss. This consideration is supported by the fact that linear scratches were observed in the observation result of wear scars. When the boring temperature increased to 1150 \({}^{\circ}\)C, the worn surface of the treated Ti had no large-scale deformation and abrasion powder, and the wear marks were very narrow. It is estimated that the formation of the borided layer reduces the tendency of the adhesive to wear, resulting in very shallow and smooth wear marks. Samples have not been peeled or peeled off. The wear depth clearly decreased with increasing thickness of the hardened layers. The wear resistance of the treated samples was significantly improved due to the hard and adhesive layers formed during the boring process (Figure 7). A hardened layer was achieved after boriding, containing a surface layer with ultra-high hardness with a slightly softer layer beneath. Such a surface layer is beneficial, especially for wear-resistant materials, as it improves the adhesion and wear resistance of the soft titanium substrate. The softer TiB layer beneath the surface layer acts as damping and prevents delamination during wear, especially under heavy force. For the conditions studied in this research, the maximum boring temperature of 1150 \({}^{\circ}\)C resulted in minimum wear width and depth values of 400 \(\upmu\)m and 3.02 \(\upmu\)m, respectively. As shown in Table 7, the calculated wear depth values exceeded the measured ones. This suggests that the abrasion of the ball surface occurred in addition to the abrasion of the hardened layer, resulting in a transition from mechanical wear to abrasive wear. When the boring temperature was 1050 \({}^{\circ}\)C, the worn surface of the treated sample showed very limited deformation and smearing effects. As a result, the wear depth decreased significantly, which may be related to the sufficiently hardened layer being formed. Referring to Figures 7 and 10, it can be clearly seen that the wear depth was shallower when the hardened layer was thicker. This extraordinary improvement in wear resistance of the treated sample was attributed to the hard and adhesive subsurface layer formed during boriding (Figure 7). As pointed out above, a surface layer with ultra-high hardness was achieved on the outermost layer, \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Boriding**} & \multicolumn{3}{c}{**Wear Depth/\(\upmu\)m**} \\ \cline{2-4}
**Temperature/\({}^{\circ}\)C** & **Wear Width/\(\upmu\)m** & **Measured** & **Calculated** \\ \hline untreated & 1240 & 58 & 82 \\
950 & 1192 & 51 & 76 \\
1050 & 641 & 3.42 & 22 \\
1150 & 400 & 3.02 & 8.41 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Wear width and depth values of samples prepared using different boriding temperatures measured after wear testing (calculated wear depths are also given for comparison). with the substrate having gradually decreasing hardness. The hard surface layer increased both adhesive and abrasive wear resistance of the soft Ti substrate. The layer just below the outermost layer with relatively low hardness may have a good damping effect which prevents delamination during rubbing, especially under heavy loading conditions. Although not clearly observed in the micrographs, it is well-known that oxidation readily occurs due to friction heating during wear testing because of the high affinity of Ti for oxygen. Therefore, we assume oxidative wear occurred in the samples at all bording temperatures. The hardened layer can prevent the physical reaction of friction, thus reducing oxidation wear.