Effect of the laser scanned pattern over the surface morphology

Another process parameter taken into account is the texture induced by the laser ablation on the treated surface. Besides the treatment carried out making the laser beam scan the surface along parallel lines from an extremity to the other, even a texture realized by means of perpendicular crossed trajectories is studied. Although a single transition along every trajectory is performed, there are of course several points, in correspondence of the crosses, which receive the pulse twice and therefore absorb a double amount of energy.

The influence of the employment of every ablated pattern over the surface morphology is evaluated by means of the same methods followed in the previously exposed characterization.

In Fig. 4.13 the measured values of surface roughness induced by different energy density processes carried out with the intent of producing a sort of regular grid all over the treated surfaced are plotted. As in the unidirectional case, the hatch distance is varied between the same three values. The letter "C" in the graph legend stands for "crossed", it agrees with the nomenclature exposed in Chapter 3 and it is reported to promote the understanding of the figure when comparing it with Fig. 4.9.

Fig. 4.13 Measured value of surface roughness obtained with crossed pattern treatment S_avs Energy density ED for different hatch distances H [149]

Comparing Fig. 4.13 with the results of roughness for the laser scanning unidirectional direction (Fig. 4.9), it is possible to notice how the ablation with a grid pattern induces a surface morphology characterized by significantly higher values of Sathan in the previously

discussed case. The increase of roughness appears very strong especially for high values of hatch distance and energy density, while for the H=25 µm set the values are consistent with what was found for the unidirectional pattern. The dependence of the surface skewness from the energy density varying the hatch distance for the pattern realized with crossed trajectories is showed in Fig. 4.14.

Fig. 4.14 Measured value of surface skewness S_skvs Energy density ED for different hatch distances H,for the crossed pattern case [149]

In general, comparing Fig. 4.14 with Fig. 4.10, it is possible to state that the surfaces characterized by a grid pattern induced by the laser ablation present an increase of the S_sk value when the hatch distance is equal to 25 and 50 µm and the energy density is lower than 2 J/mm²with respect to the ones pre-treated unidirectionally. The set with H=100 µm of Fig.

4.14 present values of S_sk slightly higher than the corresponding ones of the unidirectional surfaces, with the exception of the highest ED values, in correspondence of which S_sk is very low.

In order to justify the differences achievable by means of a variation of the laser scanning direction, even for the crossed pattern case some images showing the morphology maps among the corresponding SEM images are provided in Figs. from 4.15 to 4.17.

Fig. 4.15 refers to a surface ablated with the lowest ED value considered in this work, with different values of H. It is possible to observe how when H is equal to 25 and 50 µm the interaction between grooves induced on adjacent lines is very high and the ablation coverage

4.2 Surface characterization results 105

Fig. 4.15 Morphology maps and SEM images of surfaces ablated using a crossed pattern strategy with ED=0.17 J/mm²and a) H=25 µm, b) H=50 µm and c) H=100 µm [149]

of the surface is complete. Instead, in the H=100 µm case a wide portion of the surface remains untreated because of the poor interaction between adjacent craters.

Moving to consider the images corresponding to the ED=1.71 J/mm²case (Fig. 4.16), the first and the second one (H=25 µm and H=50 µm, respectively) do not exhibit any particular shape in terms of peaks and valleys, because of both the fact that the width of every groove is significantly higher than the hatch distance and the stacking of the debris ejected during the

Fig. 4.16 Morphology maps and SEM images of surfaces ablated using a crossed pattern strategy with ED=1.71 J/mm²and a) H=25 µm, b) H=50 µm and c) H=100 µm [149]

treatment due to the material ablation. When H becomes significantly higher than the spot diameter (H=100 µm), there is not any trace of overlay and the surface appears organized in a sort of grid generated by the presence of high peaks, in the untreated zones where the ablated material is stacked, and deep valleys caused by the overlap of two perpendicular laser beam trajectories.

Finally considering the high ED treated samples presented in Fig. 4.17 , it is worth noting the high accumulation of the solidified melted material expelled during the treatment due to

4.2 Surface characterization results 107

Fig. 4.17 Morphology maps and SEM images of surfaces ablated using a crossed pattern strategy with ED=3.81 J/mm²and a) H=25 µm, b) H=50 µm and c) H=100 µm [149]

the high amount of energy provided to the surface. As a direct consequence, although the laser scan is performed along two perpendicular directions, the final shape of the surface is essentially the result of the ablation carried out along the last followed direction. This is especially pronounced for the H=100 µm case, while when the hatch distance value is lower the grooves appear partially hidden by the debris. Additionally, the high ED value employed produces a surface characterized by remarkably deep valleys, which justifies both the high

surface roughness S_aand the very low surface skewness S_sk found for the H=100 µm set when ED is higher than 3 J/mm².