Effect of Surface Roughness on the Surface Texturing of 316 l Stainless Steel by Nanosecond Pulsed Laser

Sample coupons were prepared from stainless steel AISI316L (SS316L) supplied by Columbus Stainless Ltd. The size of the samples was 50 mm by 50 mm by 0.8 mm thick. Chemical composition of the SS316L was provided by supplier and this is summarised in the Table 1. The physicochemical properties of the SS316L [27‐29] can be observed in the Table 2.

Thermal diffusivity (\(\kappa\)) was calculated with Eq. 1 [26, 33, 40, 41].

Absorption coefficient (\(\alpha\)) was estimated using Eq. 2 [38].

where, \(\lambda\) is the wavelength (1064 ± 5 nm).

Surfaces of the sample coupons were polished with various methods before laser processing (pre-treatment). This was carried out to get surfaces with dissimilar average roughness (\({S}_{a}\)). The Sa values were measured using a Bruker ContourGT optical profiler. Two measurements were taken on five separate areas within each coupon. Assessment of the roughness was conducted with green light and at 27.5 × magnification. Silicon carbide paper of grit 80P was used to obtain a surface with a \({S}_{a}\) value of 360 ± 40 nm. A second surface was firstly polished with grit 80P abrasive paper and then with 400P silicon carbide paper. \({S}_{a}\) of the second surface was 180 ± 30 nm. Third surface had a \({S}_{a}\) of 100 ± 20 nm, which was achieved via a polishing process with subsequent abrasive papers that were grit 80P, grit 400P and grit 1200P. The next sample was polished with grit 80P, grit 400P and grit 1200P silicon carbide paper and, polycrystalline diamond paste of 3 μm as the last polishing stage. This treatment produced a S_a value of 24 ± 10 nm. The final surface had a \({S}_{a}\) value of 12 ± 8 nm that was generated through successive steps. First step was a grinding process with silicon carbide papers of grit 400P, grit 600P and grit 1200P. The next step was a sequential polishing via polycrystalline diamond pastes of 3 µm. The last step was mirror polishing employing a colloidal silica gel solution that was formed of 50% in volume of silica gel (0.04 µm of grain size) and 50% in volume of distilled water. All polishing consumables and items were provided by Struers. A cleaning process was conducted after each polishing treatment that consisted of first cleaning with commercial detergent and rinsing with fresh water, and then the surface was sprayed with isopropanol and, finally, a fast drying with a hot air dryer. Samples were named according to their \({S}_{a}\) and therefore, these were called as 360 nm, 180 nm, 100 nm, 24 nm and 12 nm as can be seen in Table 3. Note, \({S}_{a}\) was measured on ten areas with optical profilometer whose specifications will be detailed later.

The laser system used was formed of an infrared (IR) fibre laser, an optical system and a three axes automatic table. The IR fibre laser model was a SPI Laser (UK) G3 20 W nanosecond pulsed fibre laser. Optical system consisted of a manual 1064 nm beam expander (Linos (Qioptiq) 2-8x), a scanning galvanometer (Nutfield Extreme-15-YAG) controlled using SAMLight v3.05 software (SCAPS Gmbh), a focal length of 100 mm (Linos Ronar F-Theta focal lens) and four silver mirrors (Thorlab). The three axes computer-controlled table was provided by Aerotech Limited (UK). Figure 1 illustrates a schematic drawing of this equipment.

The laser has a wavelength (\(\lambda\)) 1064 ± 5 nm, and a near TEM₀₀ mode with beam quality factor \({M}^{2}\) = 2.1. The laser can work with several pulse lengths (\(\tau\)) varying from 9 to 200 ns and at various Pulse Repetition Frequencies (\(PRF\)) that range from 1 to 500 kHz. The laser has a maximum average power (\({P}_{m}\)) of 22 W and maximum pulse energy (\({E}_{pm}\)) of 880 µJ before the optical system. \({P}_{m}\) was measured after the beam expander with an Ophir laser power meter system, using a 30A-N-SH ROHS head and Nova II display software. The power measurements were taken for 60 s. Maximum pulse energy was estimated with the Eq. 3 [42, 43].

The beam expander was set to give a raw laser beam diameter (\({D}_{R}\)) of 5.7 mm. Four mirrors guide the laser beam to the scanning galvanometer mirror head fitted with a f-theta focussing lens. This device handled the displacement of the laser beam on the sample surface and the scan rate of the laser beam (\(SR\)) across the surface can vary from 0.01 mm/s to 20,000 mm/s. The lens had a focal length (\(FL\)) of 100 mm. The samples were placed on the three axes automatic table.

In this study, the laser is used in pulsed mode with \(\tau\) = 200 ns, \(PRF\) = 25 kHz and with straight line scanning. The laser beam was focused on the sample surface. Theoretical laser beam diameter (\({d}_{o\left(Theo\right)}\)) was around 51 µm, calculated using Eq. 4 [31, 44‐46] and the values of in Table 3 for the indicated parameters.

The focussed laser beam was scanned across the sample surfaces at a speed to produce single pulse shots. The distance between pulses (\({d}_{t}\)) was set to 200 µm to avoid any possible overlapping of the dimples. \(SR\) was 5000 mm/s using Eq. 5:

Pulse energy (\({E}_{p}\)) was varied from 4 µJ to 614 µJ in this work. \({E}_{p}\) were estimated using Eq. 6 [42] and the average power (\({P}_{ave}\)) measurements after the galvanometer mirror scanner.

Maximum \({E}_{p}\) is lower than \({E}_{pm}\) because the transmission of the system is 70%. This was measured by comparing an average power measurement of the laser beam after the focussing lens with the measurements after the beam expander outputs. The selected laser parameters for this study are summarised in Table 4.

The dimensions of the dimples were assessed using an Olympus BH2-UMA optical light microscope, equipped with DinoCapture 2.0 software. The topography and geometry of the dimples were evaluated with the Bruker ContourGT optical profiler. Assessment of the optical profilometry was conducted with green light and at 27.5 × magnification. The dimple shape features (e.g. width, diameter and depth) were calculated via the measuring of five dimples. Two measurements were carried out for each dimple and therefore, ten measurements for each experimental condition were conducted. It is noted that S_a measurements were not taken for the dimples, because this is not the object of this work.

The optical light microscopy pictures of the untextured (Fig. 2.a), d), g), j) and m)) and textured (Fig. 2.b-c), e–f), h-i), k-l) and n–o)) surfaces can be observed in Fig. 2. Laser impacts at ≥ 53 μJ produced textures on all samples. These show a circular shape with micro ripples in the centre and surrounded by crests. The type of textures was classified as dimples. Melting and evaporation of the material are usually the thermodynamic mechanisms that generate dimples on metals for laser irradiation with infrared laser pulses [47]. Material melting commonly creates the ablation mechanism of hydrodynamic expansion for nanosecond pulse lasers. This ablation mechanism is characterised by the displacement of molten material to edge of the laser impacted area. This displacement generates ripples and molten material massing on the edge of the dimples to create the crests [48‐51]. These structures remain after the laser pulse owing to the quick temperature changes of the laser shot area. The temperature of the metallic material is quickly increased during the laser pulse. After the laser pulse impact, the temperature of the laser impacted area of the alloy is rapidly reduced by heat diffusion into the material bulk. This rapid solidification retains the ripples and crest of the dimples [33, 52]. Two dissimilar types of dimples were found, according to \({S}_{a}\) and \({E}_{p}\). The first type of dimple (Fig. 2.b), e), h), k) and n)) are featured by smooth crests while the second dimple kind (Fig. 2.c), f), i), l) and o)) are characterised by an abrupt crest and molten material appearing outside of the laser exposed zone. This indicates that the ablation mechanisms of the textures were different for each type of dimple. The ablation mechanism of the first dimple kind is surface vaporisation that is featured by the conversion of liquid to gas in a liquid–gas interface that is usually named the Knudsen layer. Although the escape speed distribution of the gas material in the liquid layer initially is non-equilibrium, this distribution reaches equilibrium with time [47, 48, 53, 54]. The second type of dimple was produced by the ablation mechanism often called phase explosion, characterised by the creation and subsequent growth of a bubble that is formed of evaporated material surrounded by molten material. Gas material then presses on the molten material that when the pressure is sufficiently high, the bubble can explode. Explosion ejects the liquid material outside of the laser treated area [47, 48, 53‐58]. The first dimple type was furthermore found at lower \({E}_{p}\) than the second dimple kind as the phase explosion requires higher energy fluence to be produced than surface vaporisation for the nanosecond pulsed lasers [47].

Dimple kinds were found at dissimilar \({E}_{p}\) ranges for each sample. 360 nm (Fig. 2.a-c) and 180 nm (Fig. 2.d-f) samples have dimples of the first kind (Fig. 2.b and Fig. 2.e) at E_p ≤ 475 μJ while dimples produced by phase explosion (Fig. 2.b and Fig. 2.e) were created at E_p ≥ 554 μJ for these surfaces. In the case of the 100 nm surface (Fig. 2.g-i), the surface vaporisation dimple (Fig. 2.h) was generated at E_p ≤ 250 μJ and the second dimple kind (Fig. 2.i) was found at E_p ≥ 323 μJ. For the 24 nm (Fig. 2.j-l) and 12 nm (Fig. 2.m–o) samples, the laser shots at E^p ≤ 323 μJ produced dimples by mean of the surface vaporisation whilst phase explosion dimples were produced at E_p ≥ 402 μJ. This showed that the roughness of the original surface has an influence on the laser energy fluence absorbed by surface. The surface with \({S}_{a}\) ≥ 180 nm needed higher \({E}_{p}\) than surfaces with lower roughness because of the scattering effect of the high roughness. This effect enlarges the laser beam effective area, which reduces the energy fluence [36]. Note that the intrinsic laser parameter to determine one or other ablation mechanism is energy fluence. The phase explosion ablation mechanism is therefore produced at higher energy fluence than surface vaporisation. Phase explosion dimples were generated at lower \({E}_{p}\) for 120 nm samples than that for 24 nm and 12 nm samples as a result of the differences in the respective samples’ laser reflectivity. The reflectivity of metallic materials is inversely proportional to the surface roughness because the surface relief can cause the laser radiation to reflect at angles than the nominal incident angle of 90° [33]. The reflected laser radiation away from the normal can impact again within the laser shot area, increasing the laser radiation absorbed by the material. So, a certain roughness can be beneficial to laser absorption by metallic alloys [33]. These results indicate the existence of an optimal roughness surface for LST. An excessive roughness causes a reduction of the energy fluence while a smooth surface provokes a \({E}_{p}\) reduction. The assessments of the laser beam-material interaction factors were carried out to confirm this assertion, this will be discussed later. Dimples on the surface with a S_a of 360 nm (Fig. 2.b-c) and 180 nm (Fig. 2.e‐f) have an irregular circular shape due to the high surface roughness. The scattering effect of the roughness on the focussed laser beam causes a laser beam profile deformation that reduces the quality of the dimples [36].

The influences of the pulse energy, \({E}_{p}\), on the dimple width (Fig. 3.a-b) and average surface roughness, \({S}_{a}\), on the laser beam-material factors (Fig. 3.c-d), can be seen in Fig. 3. The width of the dimples (\(W\)), defined as the diameter of all the area processed by the laser pulse (ablation area and crest), was widened with increasing \({E}_{p}\), as can be in the graph of Fig. 3.a. This is due to the energy spatial distribution which is near Gaussian [45]. The dimples were wider on the surfaces with lower \({S}_{a}\) than on the higher \({S}_{a}\). This can indicate that the scattering effect of the roughness is stronger than the reflectivity effect of the smooth surfaces on the LST dimple creation. The increase of laser effective area by the roughness scattering effect decreases the energy fluence. This implies that the laser beam area that can melt the metallic material is reduced [33]. This in turns causes the reduction of the dimple width with the increase of the surface roughness.

The laser beam-material factors were calculated using Eqs. 7 and 8 [34, 45, 46, 59, 60] and the linear regression method in the graph data of Fig. 3.b.

where, \({d}_{o-m}\) is the experimentally determined focused laser beam diameter from the optical light microscopy data, \({E}_{th-m}\) is the experimental energy threshold for melting process and \({\varphi }_{th-m}\) is the fluence energy threshold for the melting process. The smooth surfaces typically showed lower \({\varphi }_{th-m}\) than higher roughness surfaces (Fig. 3.c). This indicates that the polished surface at low roughness has the most efficient laser absorption than the surface with high \({S}_{a}\). This is due to the reduction of the roughness effect on effective laser diameter enlargement [36]. The exception is the surface at 24 nm \({S}_{a}\) that has a higher \({\varphi }_{th-m}\) than that for the samples with \({S}_{a}\) of 100 nm and 180 nm. This \({\varphi }_{th-m}\) increase is due to the reflectivity effect that decreases the laser radiation absorbed by material [61]. This effect is attenuated by hydrodynamic expansion for the 12 nm samples. The low surface roughness can, moreover, result in the low resistance of the surface to the flow of molten material across the surface. This hydrodynamic expansion produces the movement of the liquid material outside of the laser impacted area [47]. This movement on the surface can be limited by surface relief that is proportional to \({S}_{a}\). These results also show that the roughness scattering effect on laser beam-material interactions is the predominant surface parameter at S_a ≥ 100 nm while the laser radiation reflection is the dominant factor for surfaces at S_a ≤ 24 nm. The melting fluence energy, \({\varphi }_{th-m}\), of the samples were slightly higher than the theoretical melting energy fluence (\({\varphi }_{Theo-m}\)) whose value is 2.625 J/cm², calculated using Table 2 data and Eq. 9 [26, 62, 63]. The slight difference with the samples at high roughness is due the roughness scattering effect [34].

\({d}_{o-m}\) of the samples (Fig. 3.c) increased as \({S}_{a}\) is reduced because it is the focused laser beam area that can melt the SS316L. The laser beam has an energy spatial distribution close to Gaussian. Maximum fluence is in the centre of the laser spot and reduces towards the edge of the spot, so the laser spot centre can melt the material while the laser beam edge can only heat the material [30]. The surface roughness enlarges the laser beam diameter by means of laser scattering [36]. This enlargement of the laser beam diameter decreases energy fluence and therefore, the melted surface area is reduced. Although the calculated \({d}_{o-m}\) of the samples varied, all diameters were only a 20% maximum deviation from \({d}_{o\left(Theo\right)}\) (51 µm), which is reasonable.

Dimples have different topographies according to the values of \({S}_{a}\) and \({E}_{p}\), as can be seen in Fig. 4 (3D pictures) and Fig. 5 (Profile images). Dimples mainly have a shape that consists of a depression surrounded by a crest. The depression is generated by the ablation of the material through the evaporation of the material and the movement and ejection of the molten material to the outside of the laser shot area [64, 65], as mentioned in 3.1. Surface Morphology. The dimple form has a spherical cap. These dimples are therefore classified as U-type [66‐69]. This geometry arises as the Gaussian peak of the laser beam possesses sufficient energy to ablate the metallic material [31, 69, 70]. Dimples with a different shape were found at E_p = 53 μJ for surfaces with 12 nm of S_a, (Fig. 4.n and Fig. 5.e). These dimples are formed of a peak in the centre of the laser impacted area that was surrounded by a valley that is, in-turn, surrounded by a crest. These dimples are classified according to their shape as “sombrero” type [31, 33, 71]. This kind of dimple is produced by the Marangoni effect that causes the movement of the liquid material to the edge and centre of the laser exposure point. This effect is caused by the thermal gradient differences in the liquid metal pool. The effect is found for the laser treatments where the melting is the dominant thermal process [31, 33]. This indicates that the 12 nm samples absorbed less laser radiation than other surfaces because melting needs less energy fluence than evaporation [47].

The crest of the dimples created by the ablation mechanism of phase explosion (Fig. 4.c, f, i, l and o) were wider and higher than the crest caused by surface vaporisation. Evaporated material pressure on the molten metal displaces the liquid material to edge of the dimple [10, 47, 65]. This increases the size of the crest. This pressure is considerable for the phase explosion owing to the confinement of the gas material into the bubble of the phase explosion [47, 72].

The quality of the dimple shape was better for the samples with smooth surface because the low relief (Fig. 5.a higher relief and Fig. 5.d low relief). Peaks and valleys of the non-textured surface cause the absorption of the laser radiation to be heterogeneous. This alters the Gaussian like shape of the laser beam as it is exposed on to the metallic surface [30]. The roughness scattering effect on the laser radiation can also modify the shape of the dimples [36].

The diameter of the dimples (\(d\)) is a texture feature that is defined as the diameter of the laser ablated area (laser exposure area without crest). Figure 6 shows the influence of \({E}_{p}\) on dimple diameter and \({S}_{a}\) on the laser beam-material interaction.

The diameter increased with \({E}_{p}\) (Fig. 6.a) as the laser beam has a Gaussian energy spatial distribution [9, 69]. \(d\) was higher on surface with low \({S}_{a}\) because of the laser scattering effect of the surface relief [36]. \({d}_{o-a}\) (focused laser beam diameter) and \({E}_{th-sa}\) (minimum laser energy to ablate the surface) were calculated using Eq. 10 [9, 34, 41, 56, 73] while \({\varphi }_{th-sa}\) (minimum laser energy fluence to ablate the surface) was estimated with Eq. 7 but with d_o-m and E_th-m replaced by d_o-a and E_th-sa.

The linear regression used to estimate \({\varphi }_{th-sa}\) and \({d}_{o-a}\) can be seen in Fig. 6.b. \({\varphi }_{th-sa}\) decreases with the reduction of the surface roughness as can be observed in Fig. 6.c. This seems to show that the laser scattering effect of the roughness is more predominant than the reflectivity effect for this roughness (from 12 to 360 nm). φ_th-sa of the samples (≈ 7 J/cm²) was lower than the theoretical ablation fluence threshold \({\varphi }_{th\left(Theo-e\right)}\) (11.168 J/cm²) that was calculated using Eq. 11 [26, 31, 58, 62, 74, 75]:

This is due to the explosive phase producing extra ablation material than the simple vaporising of the material. The explosion of the bubble provokes an increase of the amount of removed material owing to the ejection of the molten material, which is typical of the phase explosion [55]. \({\varphi }_{th-sa}\) was higher than \({\varphi }_{th-m}\) for all samples as the required energy to vaporise the material is larger than that for melting [30, 47].

\({d}_{o-a}\) increases with the decrease of \({S}_{a}\), as can be seen in Fig. 6.d. This is as a result of the reduction of laser effective area with rising \({S}_{a}\) [34]. This in turn causes the energy fluence to be higher for surfaces with low surface roughness and therefore, wider laser beam areas can be evaporated on the material.

\({d}_{o-a}\) of the samples (≈ 30 μm) are smaller than \({d}_{o(Theo)}\) (51 μm) because of the Gaussian like profile. Only the central areas of the laser beam can ablate the material, so, \({d}_{o-a}\) is lower than \({d}_{o-m}\) due to the higher energy fluence required for vaporising process than melting [30].

The influence of \({E}_{p}\) and \({S}_{a}\) on the depth of the dimples and the depth related laser-material interactions can be found in Fig. 7. All U-type dimples were deep while the sombrero-like dimples had raised central point (dimples on 12 nm surface at 53 μJ). These sombrero-like dimples were excluded from the analysis of \({S}_{a}\) and \({E}_{p}\) influence on dimple depth (\(De\)). An increase in \({E}_{p}\) deepens the dimples for all surface roughnesses (Fig. 7.a) as the rate of ablation is proportional to this laser parameter [64]. The samples with lower \({S}_{a}\) commonly have deeper dimples than the samples with high \({S}_{a}\). This is due to the reduction of the roughness scattering effect with decreasing \({S}_{a}\) [36]. The two exceptions were the dimples on \({S}_{a}\) = 360 nm samples that were deeper than that for samples with lower roughness, and \({S}_{a}\) = 12 nm sample dimples that were shallower than the \({S}_{a}\) = 24 nm sample dimples. In the case of the \({S}_{a}\) = 360 nm sample dimples, this can be due to the large relief of the original surface. On such surfaces, the difference of the height between the original surface peaks and valleys can reach 3 μm. This height can be of the same scale as the depth of the dimples for this surface. Thus, the depth of the dimples for the \({S}_{a}\) = 360 nm samples were greater than for those samples with low \({S}_{a}\). For the \({S}_{a}\) = 12 nm sample dimples. The explanation is the high reflectivity of the smooth surfaces [47].

Energy depth penetration (\(l\)) (the depth to which absorbed laser energy is transferred into the material) and pulse energy threshold (\({E}_{th-da}\)) (necessary minimum pulse energy to create a dimple with visible depth) were calculated using the graph of Fig. 7.b. (linear regressions) and Eq. 12 [26, 60, 64, 74].

Depth ablation energy fluence (\({\varphi }_{th-da}\)) were estimated with Eq. 8 but the \({d}_{o-m}\) and \({E}_{th-m}\) are replaced by \({d}_{o-a}\) and \({E}_{th-da}\). This was carried out to have an intrinsic and comparable value between samples. \({\varphi }_{th-da}\) of all samples (Fig. 7.c) were higher than all \({\varphi }_{th-sa}\) and \({\varphi }_{th-e}\), which can indicate the micro plasma generation at the bottom of the dimple. The evaporated material can interact with the laser radiation, which increases the temperature of the gas. If the material in gas state reaches a certain temperature, this can convert to plasma that can absorb, reflect, and scatter the laser radiation. This can diminish the laser energy absorbed by the material. The plasma effect on the laser radiation is often called the “plasma shielding effect” [48, 72, 76‐78]. It is noted that plasma can take from 10 ps to a few ns to form [77] and therefore, this can be produced during the laser pulse (200 ns pulse length). Plasma is only created in the centre of the dimple due to the high energy fluence that can be found in the centre of the laser spot [48]. \({\varphi }_{th-da}\) was increased with reducing \({S}_{a}\) until \({S}_{a}\) = 180 nm. This is as a result of the reduction in the roughness laser scattering effect that decreases the laser effective area [34]. This in turns enlarges the effective energy fluence. The temperature of the plasma is proportional to laser energy fluence and the plasma time generation is inversely proportional to this laser parameter. Plasma absorptivity is proportional to the increase in plasma temperature [72]. The smoother surface therefore produces a stronger plasma shielding effect than rougher surfaces up to \({S}_{a}\) = 24 nm. \({\varphi }_{th-da}\) of the \({S}_{a}\) = 24 nm and \({S}_{a}\) = 12 nm samples were lower than that for other samples due to the reflectivity effect. Part of the laser radiation is reflected by the metallic material, which decreases the laser energy absorbed by material [79]. This decreases the amount of the evaporated material that in turns reduces the density of the plasma. The plasma shielding effect is proportional to its density [48, 72, 76, 77]. Thus, the plasma shielding effect is weakened. So, the laser radiation absorbed by the material is increased as a result of the reduction of the plasma shielding effect. Note that the plasma shielding effect was absent in the case of the surface features (melt and ablation widths / diameters) because the plasma was only produced on bottom of the dimple. The \({S}_{a}\) = 24 nm (18 J/cm²) and \({S}_{a}\) = 12 nm (17 J/cm²) sample \({\varphi }_{th-da}\) were higher than \({\varphi }_{th-e}\) (11.168 J/cm²) as the plasma shielding effect still had influence on the laser radiation observed [72].

The influence of \({S}_{a}\) on energy depth penetration,\(l\), is similar to the case of \({\varphi }_{th-da}\) as can be seen in Fig. 7.d. This can indicate that the \(l\) is influenced by the same processes as\({\varphi }_{th-da}\). The value of this factor is lower than theoretical energy depth penetration \({l}_{Theo}\) (858 nm) that is calculated using Eq. 13 [31, 45, 57, 80, 81] and the data in Table 2.

This difference can be quantified through the geometrical factor (\(\zeta\)) that is estimated using the Eq. 14 [31, 45, 80]. It is noted that the influence of \(\alpha\) on \({l}_{Theo}\) is minimum for nanosecond pulsed laser texturing on metallic materials.

Figure 7e shows that the \({S}_{a}\) = 100 nm samples have the best surface to absorb the laser radiation in depth as this surface has \(\zeta\) nearest to one (≈ 0.7). An excessive roughness can produce a heterogeneous absorption of the laser in depth that decreases the effectiveness of this process [34] while an extremely smooth surface causes a reflectivity effect that diminishes the laser radiation absorbed by the material [61].