3.1.

Influence of Femtosecond Laser Fluence

Figure 2 represents the variation of surface roughness ($\mathrm{\Delta }Ra$) and mean diameter of the nanoparticles on the TC4 surface polished using different laser fluences (32.8, 74.4, and $109.4\mathrm{J}/{\mathrm{cm}}^2$). Triangular points on the solid line indicate the variation of surface roughness, and rectangular points on the dashed line represent the mean diameter of the nanoparticles on the TC4 surface. Points marked “A” and “B” and the ellipses on the lines correspond to insets (a) and (b), respectively. One of them depicts the variation of surface roughness ($\mathrm{\Delta }Ra$), which is shown in inset (a), the other depicts the size distribution of nanoparticles, which is shown in inset (b). The mean diameters of the nanoparticles on the TC4 surface were determined from the size distribution. All experimental data were normalized considering the inclinations and concave pits on the TC4 surface.

Fig. 2

Points on the solid lines depicting the variation of surface roughness on laser fluence. Points on the dashed line indicate the mean diameter of nanoparticles on the TC4 surface. Inset (a) shows the variation of surface roughness at point A and inset (b) presents the size distribution of nanoparticles at point B.

Figure 3 illustrates surface morphologies of the TC4 nanoparticles during ablation under different laser fluences with a single laser shot. There are dark and bright nanoparticles in the images, owing to the signals with different intensities and different angles in the SEM instrument. The nanoparticles on the TC4 surface are almost spherical.

Fig. 3

SEM images of TC4 surface subjected to single laser shot with laser fluences at (a) 109.4, (b) 74.4, and (c) $32.8\mathrm{J}/{\mathrm{cm}}^2$.

Figure 4 shows a comparison of the elemental composition on the TC4 surface. It can be seen that the chemical elements have little change after the TC4 surface was polished using the different laser fluences. The carbon on the sample surface could be because of surface contamination caused by organic material before the polishing process. On the other hand, oxygen was found on the sample surface after laser polishing, which could be generated from air during the polishing process. The rest of the composition on the sample surface remained unchanged.

Fig. 4

EDS spectra of the sample before and after polishing.

The experimental results clearly show that size of nanoparticles induced by the femtosecond laser depends on the laser fluence. The stronger the laser fluence utilized, the higher the variation of surface roughness is. Because small nanoparticles were effective in improving the surface roughness of the TC4 sample, a better surface was obtained with the smaller nanoparticles ablated using low laser fluence. So, it is obvious that $\mathrm{\Delta }Ra$ will decrease when the size of the nanoparticles induced by femtosecond laser polishing gets smaller. Moreover, the main chemical elements on the TC4 surface after femtosecond laser polishing remained almost unchangeable.

Our results show that laser polishing involves filling and fine-cutting. The depressions on the TC4 surface could be filled by the nanoparticles induced by the femtosecond laser, and the peaks on the TC4 surface could be cut away by femtosecond laser. Femtosecond polishing is different in approach but equally satisfactory in result with mechanical polishing, where the polishing mechanism is the resultant of filling adjoining depressions and fine cutting by the abrasive particles.¹⁰ Further, the statistical analysis results regarding nanoparticles further confirm the findings of many works, which state that nanoparticles induced by laser are large when laser fluence is strong.^12–14 The research in this part first combines the theory of conventional mechanical polishing with femtosecond-laser-ablated nanoparticles mechanism, and also it happens to hold the same view with femtosecond laser polishing. Moreover, we established the relationship between femtosecond laser fluence and TC4 surface roughness.

3.2.

Influence of Femtosecond Laser Overlapping Rate

Figure 5 shows the dependence of the surface roughness on the TC4 surface as function of the beam overlapping rate. Five overlapping rates at a laser fluence of $74.4\mathrm{J}/{\mathrm{cm}}^2$ were selected, and (I), (II), and (III) are the laser ablation regions with different trends in terms of the surface roughness on the TC4 surface after femtosecond laser polishing. The three images represent the characteristic morphology of regions (I), (II), and (III) respectively. The dotted line corresponding to $\mathrm{\Delta }Ra=0$ in the horizontal direction represents the original surface before laser polishing.

Fig. 5

Surface roughness as a function of beam overlapping rate (50%, 65%, 75%, 85%, and 90%). (I), (II), and (III) are the laser polishing regions with different trends in roughness variation. Insets reveal the surface morphologies resulting from using different overlapping rates.

In region (I), the greater the overlapping rate of the laser beam, the smaller is the size of the nanoparticles. The overlapping rate of the laser beam produces small nanoparticles because femtosecond laser fragmentation can produce smaller nanoparticles.^12,15,16 In region (II), particle aggregation is obvious when the laser beam overlapping rate increases, owing to the transition of mechanism of nanoparticle formation.^17–19 LM plays a leading role in this region, resulting in rising up of line and larger nanoparticles. In region (III), with the overlapping rate increasing, the reduction in surface roughness is obvious, due to agglomerate formation irradiated by femtosecond laser, and becomes even more apparently at high overlapping rate.²⁰ Thus, a fine surface can be attained by the certain overlapping rate and fluence of femtosecond laser, as reported in some studies on silicon.^21,22

The analysis above has sought to find out the inner relation under different overlapping rate of femtosecond laser polishing. In every region, there are obvious different mechanisms. The femtosecond laser ablated and fragmented on surface of TC4 to produce small nanoparticles in region (I). Mechanism of producing nanoparticles changes into LM in region (II). And there is the heat effect at a high overlapping rate beam shined in region (III). Therefore, the influence of femtosecond laser overlapping rate depends on the mechanism of femtosecond-laser-induced nanoparticles, and the interaction between femtosecond laser and TC4 surface is very complicated.

3.3.

Fine TC4 Surface after Femtosecond Laser Polishing

In Fig. 6, the surface roughness ($1.381\mu \mathrm{m}$) of TC4 after femtosecond laser polishing is 16.6% better than that of the original surface ($1.656\mu \mathrm{m}$). Moreover, the SEM image depicts the surface morphology, showing many nanoparticles. However, there is no microcrack observed on the polishing area. Moreover, in Table 1, the actual chemical compositions before and after femtosecond laser polishing are presented, which are consistent with the EDS spectra in Fig. 4.

Fig. 6

(a) Improved surface roughness ($1.381\mu \mathrm{m}$) of TC4 after femtosecond laser polishing. (b) Microstructure of TC4 surface after femtosecond laser polishing.

Table 1

Actual chemical compositions of TC4 surface polished by femtosecond laser.

Compared to nanosecond, millisecond, and continuous lasers, the surface polished by femtosecond laser not only has good surface roughness but also has much less defects, such as recast layer and microcracks. The microcracks generated by laser polishing mainly come from the thermal stress, and it is very challenge to eliminate the microcracks on TC4 surface polished by long-duration laser.^8,23 However, it is hard to generate microcracks and recast layer by femtosecond laser polishing, due to the weak thermal stress and residual stress on TC4 surface.²⁴

Although the mechanism of less surface defects observed on TC4 surface polished by femtosecond laser is still not quite clear, some relevant factors, such as laser duration, temporal shaping, and spatial shaping, have been discussed and analyzed.^25,26 Among them, laser duration is very important one. When TC4 surface is polished by femtosecond laser, the heat or other form of energy do not dissipate efficiently out of the beam spot because the laser pulse duration is much shorter than the thermal diffusivity time across the absorption depth, which is about ${10}^{-11}\mathrm{s}$.²⁷ In other words, there is no enough time to transmit energy via vibrations of crystal lattice during femtosecond laser polishing. The main mechanism of femtosecond laser processing is Coulomb explosion, although it sometime appears very weak and very limited melting under the condition of high laser fluence or high laser repetition frequency.^28–30

In a word, compared to other laser polishing with longer pulse durations, TC4 surface polished by femtosecond laser can obtain good surface without obvious microcrack and recast layer.