## 3 Results
Moreover, from sample S-3, processed at room temperature with 40 mm/s and 200 kHz in Figure 2, we can see the periodic corrugation, evidencing the Marangoni-induced convection flow. This flow induces convective mixing and hence, an additional mechanism responsible for the transport of TiN from the surface into the volume of the sample at a depth comparable to the depth of the molten pool. From Table 1, it can be found that the highest back temperature (about 240 \({}^{\circ}\)C) of NiTi alloy during the laser nitriding process is the NiTi alloy (S-3) treated by 40 mm/s and 200 kHz at room temperature, which means that the more energy deposition at room temperature is an effective method to induce deeper nitride layer for NiTi alloy. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Positions** & **O** & **C** & **N** & **Ti** & **Ni** & **Ba** & **Ta** \\ \hline a1 & 4.67 & 43.44 & 1.79 & 20.52 & 22.43 & 6.25 & 0.90 \\ a2 & – & 7.70 & – & 35.21 & 44.60 & 10.70 & 1.78 \\ a3 & – & 7.77 & – & 35.83 & 43.87 & 10.78 & 1.75 \\ \hline b1 & – & 4.81 & – & 34.98 & 43.90 & 10.93 & 1.78 \\ b2 & – & 7.31 & – & 36.17 & 43.65 & 11.12 & 1.74 \\ b3 & – & 7.46 & – & 35.44 & 44.41 & 10.92 & 1.77 \\ \hline c1 & – & 13.29 & 16.25 & 33.47 & 25.69 & 10.32 & 0.98 \\ c2 & – & 6.01 & 8.55 & 38.38 & 33.72 & 11.95 & 1.38 \\ c3 & – & 7.35 & – & 35.98 & 43.94 & 10.92 & 1.81 \\ c4 & – & 5.00 & 14.43 & 40.23 & 26.80 & 12.42 & 1.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The EDX elemental analysis along the cross-section of the NiTi alloy after laser nitriding (marked in Figure 4, At%). Figure 4: The cross-section SEM images of NiTi alloy after laser nitriding: (**a**) laser nitriding with a scanning speed of 80 mm/s and frequency of 100 kHz at room temperature (sample S-2), (**b**) laser nitriding with a scanning speed of 40 mm/s, and frequency of 200 kHz at low temperature (sample S-4). (**c**) laser nitriding with a scanning speed of 40 mm/s and a frequency of 200 kHz at room temperature (sample S-3). The microhardness along the cross-section of the NiTi sample was measured as shown in Figure 5. The hardness of the base sample was less than 350 HV when the load force is 100 g during hardness measurement. The hardness results show that the gradient of hardness can be found along the cross-section of NiTi alloy after the laser nitriding. The top layer has the highest hardness for different laser parameters. It can be observed that the highest hardness of NiTi alloy (S-3) treated with a scanning speed of 40 mm/s and a frequency of 200 kHz at room temperature is 569 \(\pm\) 25 HV, which is much higher than that of the base material. The higher hardness of the S-3 sample is attributed to the distribution of the TiN layer with a larger thickness. The laser nitriding is a heat-treatment process that can induce the diffusion of nitrogen into the subsurface of NiTi alloy to enhance its surface mechanical property. From Table 1, it can be found that laser nitriding with a scanning speed of 40 mm/s and a frequency of 200 kHz at room temperature can induce higher temperature in the treated area and generate a thicker and harder TiN layer. Normally, the hardness can be used as the criteria for judging the wear and abrasion resistance of alloys and the higher hardness can induce better wear resistance. Since the body implants made of NiTi alloy have to be replaced after their service life due to their wear and abrasion, it is important to enhance and improve their surface wear and abrasion resistance. The wear property of NiTi is measured before and after laser nitriding treatment to study the effect of laser nitriding on the wear resistance of NiTi alloy. The coefficient of fiction of NiTi alloy can be used to represent its wear property during wear testing. Figure 6 presents the relationship between the coefficient of friction and wear time on the NiTi alloy with different laser nitriding treatment.