, there is maximum amplitude, which diminishes without returning to its original maximum value.
By matching with the observations in the LIBS analysis, there is coincidence between the decrease in amplitude in relation to the investment of the intensity of the selected lines, that suggests the majority ablation of the paint layer.
Another method of visualizing possible changes in the acoustic pulse is to study the frequencies contained in the measured interval by applying the Faster Fourier Transform (FFT) to the sample. In order to obtain the clearest result, the magnitude of the complex numbers corresponding to the frequencies is to be determined. The result of this procedure can be seen in Figure 6, where the results of the first five shots are superimposed on one another. At first glance, additional frequency bands between shots cannot be distinguished, but after the second shot, an overall decrease is observed.
Considering the band of 5000 Hz the most representative, Figure 7 shows the progression of its relative intensity for the fired shots. A notable difference can be seen in the intensity between the first two shots and the rest, whose value diminishes drastically after the third pulse and maintains essentially stable until the process is completed. This behavior is also, similar to what is observed in the intensity progression of the characteristic line in the LIBS case.
This similar behavior makes the study of PILA suitable for use as an alternative qualitative monitoring method, enabling the distinction between where the ablation ends for one layer and where it begins for the next layer.
Figure 5. Peak-to-peak voltage tendency of the most significant oscillation.
Figure 6. FFT magnitude of the first five shots in the sample.
Figure 7. Magnitude tendency of the 5000 Hz band.
3.2. Analysis of Surface Morphology
Figure 8(a) shows an image of the surface of the study samples before the treatment magnified 100×, where one can see that the surface is uniform along the entire surface, this does not have imperfections such as craters or visible layers. Figure 8(b) illustrates a reconstruction of the same surface, generated by the OCT device, which was imaged in an area of 3 × 3 cm. Therefore, it corroborates the uniformity of the surface shown in Figure 8(a).
Figure 9 shows images after the treatment obtained by magnification at 100× of the surface by optical microscopy (top pictures), and OCT reconstructions where laser
Figure 8. Images of the surface: (a) magnified by optical microscope at 100×, (b) and OCT tomography.
(a) (b) (c)
Figure 9. Images of the surface after: (a) one shot, (b) two shots, (c) three shots.
shots occurred (bottom pictures). As we can see that in Figure 9(a) the crater begins to form, in Figure 9(b) the paint has been completely removed in the incidence zone of the laser, and in Figure 9(c), the laser finally begins to interact with the substrate. This visual observation are in agreement with the results obtained in both the LIBS and the PILA monitoring techniques, i.e. the optical microscope and the OCT imaging indicate that ablation of the material from the substrate begins with the third shot.
3.3. Discussion of the Results
Monitoring the depth of the elemental composition by LIBS has been reported previously for LIBS systems with monopulse excitation  . The experiments conducted in this study confirm that in the case of multipulse excitation, LIBS monitoring is still possible, despite the greater structural complexity of the laser pulse and consequently, and despite the greater complexity of the ablation process and acoustic wave formation.
On the other hand, the use of PILA demonstrates a potential ad depth-profiling low cost and simple method. The information obtained in this PILA study herein was divided into two parts: the amplitude study of the first oscillation of the pulse and the magnitude of the Fourier-transformed sampled time interval. Both methods demonstrated they can be used for real-time monitoring of ablation process.
This is clearly observed when proving that, in the case of the amplitude of the most significant acoustic oscillation, a greater amplitude of said oscillation was detected, a finding which we associate with the ablation of the paint finish material. This phenomenon occurred at the same time and with the same behavior as observed in LIBS, where a predominant presence of emission derived from the paint finish is detected during the first two pulses, but afterward diminishes and does not return to its previous intensity.
For the purpose of corroborating the behavior of the PILA monitoring using another criteria, the total sampled interval was calculated using Fourier transform, which showed that before the characteristic lines are inverted, a greater general magnitude of the frequency bands exists, which we associate with the paint finish ablation. The subsequent behavior is characterized by the decrease of the bands until they reach a relatively constant magnitude.
The aim of this work was to demonstrate the potential of PILA technique for monitoring the laser ablation of paint layers by using PILA technique.
In order to monitor the process by using the signal intensity, the most significant oscillation was detected in the acoustic signal, which diminished when ablating the substrate. In addition, the results suggest that the behavior of the FFT magnitude is similar to the one observed for the amplitude.
Using said analysis, a greater intensity can be observed in the frequency band associated with the paint, if compared to the intensity of the bands associated with the substrate.
In the LIBS analysis, the information about the presence of characteristic emission lines for each material, confirming that the PILA technique holds potential for qualitative monitoring of the ablation process of paint, was produced by using multi-pulse laser excitation. In resume, no differences in behavior of PILA results between single and multi-pulse regimes were observed despite the complexity of acoustic signal for the last regime.
The authors thank CONACYT for its financial support. This project was co-financed by the SIP-IPN 20150573 project.