## 3 Results  
When the laser beam irradiated the surface of NiTi samples, the surface temperature would rise very fast. The highest temperature of the NiTi sample on its backside was measured during the laser nitriding. It can be found that the backside temperature of S-3 during laser nitriding was the highest (about 240 \({}^{\circ}\)C). The temperatures of other NiTi alloy of S-2 and S-4 were about 90 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, which was much lower than that of S-3. Figure 2 shows the SEM images and energy dispersive X-ray (EDX) mapping results of a single laser scanning line on the surface of NiTi targets with different laser parameters and temperatures. From the SEM images, it can be found that no cracks are appearing on the surface of NiTi alloy after laser nitriding treatment. Because of the Gaussian profile of the laser beam, the melt track can be observed in the center of the laser scanning area. From the EDX mapping, the elements of Ti and N (bright area) were mainly concentrated in the central part of the laser spot in the scanning lines, which is because of the remelting in the special area. Due to the laser remelting, the roughness of the treated region is higher than that of the untreated surface. Figure 3 shows the roughness of NiTi alloy after laser nitriding with different laser parameters. The Ra and Rz of untreated NiTi alloy were 0.13 \(\pm\) 0.03 \(\upmu\)m and 0.83 \(\pm\) 0.22 \(\upmu\)m respectively. The Ra of S-2, S-3, and S-4 are 1.05 \(\pm\) 0.15 \(\upmu\)m, 0.39 \(\pm\) 0.04 \(\upmu\)m, and 0.84 \(\pm\) 0.22 \(\upmu\)m respectively. The Rz of S-2, S-3, and S-4 are 6.81 \(\pm\) 0.44 \(\upmu\)m, 3.98 \(\pm\) 0.27 \(\upmu\)m, and 5.87 \(\pm\) 0.73 \(\upmu\)m respectively. The roughness of S-3 was higher than as-received NiTi alloy, but it was lower than that of other samples with different laser nitriding parameters. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Scanning** & \multirow{2}{*}{**Frequency**} & **Nitrogen** & **Process** & **Highest** \\  & **Speed** & & **Gas Pressure** & **Temperature** & **Temperature** \\ \hline S-1 & – & – & – & 20 \({}^{\circ}\)C & 20 \({}^{\circ}\)C \\ S-2 & 80 mm/s & 100 KHz & 2 Mbar & 20 \({}^{\circ}\)C & 90 \({}^{\circ}\)C \\ S-3 & 40 mm/s & 200 KHz & 2 Mbar & 20 \({}^{\circ}\)C & 240 \({}^{\circ}\)C \\ S-4 & 40 mm/s & 200 KHz & 2 Mbar & \(-\)190 \({}^{\circ}\)C & 120 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The laser parameter for different NiTi samples. Figure 1: The schematic diagram of the laser nitriding process on NiTi alloy. Figure 4 shows the SEM images along the cross-section of NiTi targets after laser nitriding with different parameters. The thickness of the melted zone of the laser nitriding in the top layer of NiTi alloy was different when it was treated with different laser parameters. Both melted layers of S-2 and S-4 were less than 10 \(\upmu\)m. The thickness of the laser nitride layer of S-3 was about 65 \(\upmu\)m due to its slow scanning speed and higher laser repetition. The elemental analysis was measured with energy-dispersive X-ray spectroscopy and the positions were marked in Figure 4. Table 2 shows the element resulting in different positions of the NiTi alloy. It can be noted that the sample of S-3 that was treated with  
Figure 3: The surface roughness of NiTi alloy before and after laser nitriding. Figure 2: The SEM images and EDX mapping results on the surface of NiTi alloy after single laser scanning (red scale bar: 40 \(\upmu\)m and yellow: 40 \(\upmu\)m). Laser processing parameters (scanning velocity, laser repetition rate, temperature) were given in the first row of the table. RT stays for the room temperature, LT-for process with liquid nitrogen temperature. 40 mm/s and 200 kHz at room temperature could induce a deeper layer with the TiN. The reason is that the surface temperature is a very important factor for the formation of the nitride layer during the laser nitriding.