## 3 Results
It can be seen that the coefficient of friction of sample S-4 treated by laser nitriding with a scanning speed of 40 mm/s and frequency of 200 kHz at low temperature is similar to that of untreated NiTi alloy sample with a friction coefficient of 0.74. The reason for the results is that the thickness of the melted layer is very thin. The friction coefficient of sample S-2 treated by laser nitriding with a scanning speed of 80 mm/s and frequency of 100 kHz was lower than that of the untreated sample and increased gradually to around 0.74 at the fretting time of 100 s. The friction coefficient of sample S-3 treated by laser nitriding with 40 mm/s and 200 kHz at room temperature increased from 0.20 to 0.40 at the fretting time of 50 s and became stable with a friction coefficient of 0.4 after a fretting time of 50 s. As can be seen, the friction coefficient of NiTi alloy (S-3) treated with 40 mm/s and 200 kHz at room temperature is significantly lower compared to other samples. The lower friction coefficient  
Figure 5: Microhardness along the cross-section of NiTi alloy after laser nitriding. confirms that the laser nitriding with special parameters can increase the wear resistance of NiTi alloy. Figure 7 shows the optical microscope images of the worn scars on the surface of the NiTi alloy processed with different laser nitriding parameters, from which it can be found that the width of worn scars become smaller after laser treatment. The widths of worn scars of S-1, S-2, and S-4 were 464 \(\pm\) 6 \(\upmu\)m, 451 \(\pm\) 8 \(\upmu\)m, and 453 \(\pm\) 5 \(\upmu\)m respectively. However, the worn width of NiTi alloy (S-3) with laser nitriding of 40 mm/s and 200 kHz at room temperature has the narrowest width of 114 \(\pm\) 9 \(\upmu\)m compared with that of the untreated NiTi sample (464 \(\pm\) 6 \(\upmu\)m), which means that the worn width of NiTi decreased by 75% after laser nitriding treatment with the special process parameters. The enhanced surface microhardness and TiN layer were the main reason for the improvement of wear resistance. Figure 8 shows the SEM images of untreated and laser nitride NiTi samples (S-3: 40 mm/s and 200 kHz at room temperature). The particles with a small size can be observed in the worn area of untreated NiTi alloy. However, the worn area of treated NiTi alloy was very smooth and there were no worn particles due to the nitride layer. The movement of implant material-NiTi used as artificial joints would produce lots of  
Figure 6: The friction coefficient with a wear time of NiTi alloy with different laser nitriding parameters. Figure 7: The optical surface images of worn scars on the NiTi alloy with and without laser nitriding (**a**) untreated (sample S-1) (**b**). Laser nitriding with scanning speed of 80 mm/s and frequency of 100 kHz at room temperature (sample S-2) (**c**) laser nitriding with scanning speed of 40 mm/s, and frequency of 200 kHz at room temperature (sample S-3). (**d**) Laser nitriding with a scanning speed of 40 mm/s and a frequency of 200 kHz at low temperature (sample S-4). small-sized particles which can be found in the blood. The laser nitriding can be used to decrease the abrasion spallation and cracking so that the NiTi joints can be used for a longer time. It can be found that the wear mechanisms of NiTi alloy after laser nitriding is different from that of untreated specimens. The Al element and more O element were found in the area of the untreated NiTi alloy in worn scars (as shown in Table 3) when the EDX measurement (positions a1, a2, b1, and b2 marked in Fig. 8) was used to analyze the elemental composition, which is due to the Al\({}_{2}\)O\({}_{3}\) particles fall from the Al\({}_{2}\)O\({}_{3}\) ball during the wear testing. However, there was no Al element observed on the surface of nitriding NiTi alloy-worn scars, which is due to the fact that the nitriding layer can decrease the friction coefficient. Figure 8b also shows that the only minor wear occurred and there are no cracks in the worn scars of nitriding NiTi alloy.