The morphology of the textures will be presented first to fully characterize the laser surface texture patterns' dimensions and properties. Results of the dry sliding friction test and the wear properties of the textured samples will be then presented, enabling comparisons to be made between smooth and textured samples through output variables.
3.1 Examination of laser textured samples
3.1.1 Morphology and characterization
Figure 2 shows the representative two-dimensional (2D) white light interferometry images and optical micrographs of both polished and textured surfaces. The polished surface is shown in Fig. 2a. Figure 2 (b-e) shows the square, parallel and perpendicular textures, respectively. According to the results found, the surface textures analyzed are arranged regularly and there are no defects on the surface.
Figure 3 (a-c) and Fig. 3 (d-f) show the 3D surface topographies and the sectional profile of the textures, respectively. As shown, the dimensions of the channels are 5 ± 0.2 μm width, 20 ± 0.3 μm pitch, and 1.5 ± 1 μm depth. The high changes in depth are because of the existence of carbide particles in the samples. This caused variations in the laser beam during the manufacturing operation.
Figure 4 (a, b) shows the 2D and 3D micrographs of a single micro-groove, respectively. The results observed the absence of re-solidified particles that are substantial enough. Additionally, the area surrounding the micro-groove relating to the zone that is affected by heat is narrow. The observation above is easily explainable using the fact that surfaces related to AISI 304 are vaporized directly by femtosecond laser especially during ablation. Since pulses of femtosecond laser have extremely short time scales, it is easy to see why consideration of the process of ablation as a solid-vapor transition is feasible [20].
Figure 5 displays the comparison of surface roughness for the smooth and textured surfaces. The Ra and Rz of the smooth surface were about 20 nm and 26 nm, respectively. As shown, surface roughness increased following the laser texturing process due to the existence of a textured surface. In surfaces with parallel and perpendicular patterns, the increase of these two values (Ra, Rz) was lesser than in the square textured one, whose Ra was 350% and Rz was 1.8 greater than those of the untextured surface.
3.1.2 Wettability analysis
Figure 6 shows the contact angles of the coolant droplets resting on untextured and textured surfaces where the hydrophilic nature of the initial polished untextured surface with a contact angle of 43.5° is noticed. The contact angle marginally decreases after laser treatment: 23.2° in the perpendicular textured surface, 22.8° in the parallel textured surface, and 22.2° in the square textured surface. This confirms the ability of laser surface texturing (LST) to improve the wettability of the surfaces. Results also indicate a greater decrease of contact angle in all textured surfaces. This because of the presence of the grooves which lead to an increase in the average surface roughness, as can be seen in Fig. 5.
The increase of surface roughness enables greater interaction of the sample with the drop. This causes a reduction in the contact angles by increasing the droplet diameter and reducing the height. In the meantime, there is a slight variation in contact angle values between parallel and perpendicular textured samples (Fig. 6) since the surface roughness values remain almost the same (Fig. 5).
3.1.3 Phase structure analysis and micro-hardness measurements
Apart from the topographic changes induced by the laser treatment, it is well known that its thermal effect can also alter the surface’s microstructure [20]. Because LST is a high-energy technique, it can induce phase transitions on the surface of the samples. Thus, XRD was performed on both smooth and textured samples to analyze these changes. Figure 7 presents a set of typical XRD patterns of the original and textured samples. It is known that the microstructure of the original surface is austenite and by taking a closer look at the XRD pattern, there is a confirmation that there is a presence of the austenite (γ) phase only. After conducting an LST, there is a lack of evidence supporting the presence of ferrite precipitates or phase from the profile gained from diffraction. These results confirm that LST does not affect the phase transformation of textured samples.
Figure 8a presents the optical images of the indentations close to the channels as well as the corresponding hardness values. Figure 8b shows the changes in hardness values as a function of the distance from the channels and as shown the hardness values close to the channels are lower compared with the hardness of the surface after LST. For square textures, the hardness is 490 HV at a distance of 15 μm from the channel edge, which is 30% less than its original hardness. The hardness decreases by a much lower amount in parallel and perpendicular textured surfaces at the same distance, about 26% and 20% less than the original sample.
3.2 Friction properties of the laser textured sample
The results of ball-on-disc tests showed a considerable decrease in the coefficient of friction (COF) for textured samples compared to the smooth specimen. Figure 9 depicts the relationship between the COF and the sliding time. As shown, a sudden increase in the COF was noticed in all test samples at the initial stage It gradually became stable after 10 seconds of sliding. This stable period was used to evaluate the COF of all test samples. Areas of severe deformation are produced by the action of entrapped wear debris at the sliding interface. Thus, minor fluctuations in the COF were indicated for all textured surfaces. Under the dry sliding condition, the smooth surface had a COF of 0.22 whereas that of the perpendicular and parallel textured specimens had COFs of 0.13 and 0.14, respectively. An even better result was obtained in the square textured surfaces, where the COF was found to be 0.07.
The smooth surface’s higher COF can be attributed to the higher material interactions at the microscopic level. Conversely, in textured surfaces, the reduction of the contact area improves the sliding performance and mitigates friction. The smaller contact area of the textured surfaces also accounts for their lower COF during all tribological tests. Usually, deformation mechanisms and adhesion that occur at microscopic levels are truly the predominant factors that influence dry friction. On one hand, deformation takes place because of the interlocking caused by micro asperities due to the applied load. On the other hand, adhesion is a result of the bonding that takes place between the surfaces that slide with each other. Therefore, an added level of force needs to be used when sliding occurs to make sure that the force components of both deformation and force are dealt with. Moreover, studies have shown that the additional force leads to the generation of wear particles that occur on the contact interface that usually act as an additional body that interacts with the surfaces sliding, which in that case causes the growth of friction even more. The force of deformation and adhesion are more pronounced in smooth surfaces because they have a higher rate of sliding and sticking due to the two forces occurring at the smallest level. However, the study found out that the reduction in contact length especially in the surfaces that are textured, the less the generation of frictional force by the relative motion that will occur.
Furthermore, the study found out that wear debris' entrapment that is done by the channels had a considerable reduction in COF. This is due to the further elimination of the abrasion of particles that were entrapped in between the surfaces that were sliding against each other. This was proved by the results where COF was reduced by 36% and 40% for perpendicular and parallel specimens, respectively. An even better COF reduction of 68% was observed in the square textured sample.
Figure 10 presents the EDS results of different spots: spot A between the grooves, spot B inside the groove, and spot C at the worn surfaces. Spot A contained a high amount of oxygen compared to the original surface, which means that oxidation had happened at the surface. This is beneficial as the oxidation can be used to minimize the contact between the cutting tool and the chip, which helps reduce the COF [21]. Also, the amount of Fe at spot C is lower than that at spot B, which indicates that the textures can trap wear debris. Conversely, on the smooth surface, the COF rapidly grows due to the lack of such a capability. The smaller contact area of the textured surfaces also reduces the resistance during the sliding process [9].
3.3 Wear properties of the laser textured samples
The calculated wear rates of the polished and textured samples are shown in Fig. 11. The graph showcases the optimal anti-wear properties of the square textured surface, whose wear rate is the lowest of all tested samples (the highest was that of the smooth surface). The wear rate values accord with the variations in average friction coefficients, demonstrating the wear resistance improvement that can be achieved by femtosecond LST. The textured samples exhibit a high wear resistance due to the combination of debris entrapment and lower contact surface. Thus, variation in the wear rates and average COF are similar.
Optical images of the worn surfaces of the polished and textured surfaces are shown in Fig. 12. As shown, wear scars were formed for all surfaces due to the mechanical rubbing between the ball and the samples. On the smooth surface, intense scratches and adhesions are visible on the wear surface (Fig. 12a). In the textured samples, the channels can be seen clearly on the wear sample and they are covered with wear debris (Fig. 12 (b-d)).
Figures (13-16) show the scratches and adhesion on the respective worn surfaces of the untextured, perpendicular, parallel, and square textured samples. Figure 13(a, b) shows the adhesion of the material to the untextured surface. Deep marks and adhesion are visible on the surface due to a combination of abrasive and adhesive wear [13,22–24]. For the untextured surface, it is difficult to reserve debris. Debris generated by friction can impact the untextured surface, thereby causing abrasive wear as confirmed by EDS analysis of point A in Fig. 13c.
However, Figs. (14-16) indicate that the worn areas are smoother with no evidence of furrowing lines on the patterns. The channels collect small wear particles, thereby preventing them from making furrows on the worn surface [11,21]. SEM images of worn areas and EDS analysis at point B of all textured samples revealed that wear debris fills most of the texture channels. EDS analyses also show that point A has a lower amount of W compared to point B.
Figure 17 (a-d) presents the cross-sectional profiles of different wear scars. These profiles are measured on untextured and textured surfaces using the Alicona microscope. The wear scar on the textured surfaces has a lower depth than those on the untextured sample. The square textured surface has the shallowest wear scar compared to other surfaces, demonstrating the improvement of wear resistance achieved by femtosecond laser applied textures.