1. Introduction
Laser surface texturing (LST) is a widely used method worldwide for surface functionalization [
1,
2,
3,
4], being used in various fields: medical implants [
5,
6,
7,
8,
9,
10,
11], wettability tuning [
12], optical properties [
13,
14], hybrid joining [
15,
16,
17], increasing adhesion [
18], or cutting tools [
19,
20,
21,
22]. The materials that are the object of microtexturing are varied, from dentin and enamel materials, to polymers, ceramics, ferrous and non-ferrous metallic materials, and finally composite materials, in various domains such as engineering, medicine, bioengineering, etc. The most widespread LST patterns used are dimples [
4,
5,
12,
17], lines (parallel, crosshatch) [
4,
5,
8,
9,
10,
12,
20,
22], square [
6,
12], conical [
19,
22], rhombic [
12,
22], ripple texture (riblet texture inspired from the sharkskin, U-shapes (waviness texture) [
8,
18], and ring (bulge, smooth staked) [
11]. The possibility of using the laser equipment to obtain texturing patterns with different geometries of high-quality surfaces to improve the materials’ performances in a single step of a technological process represents a special advantage. That is why ultrashort laser ablation is an ideal solution in creating structures with known dimensions and perfectly repeatable, on a nanoscale and beyond. Approaching this technique is a challenge, but at the same time it offers real solutions in terms of material removal.
Zhao Y. et al. [
23] present Si and Cu samples laser ablated with the following characteristics of an X-ray laser: energy 50 µJ, wavelength 46.9 nm, and focused by means of a toroidal mirror at grazing incidence. Two hundred shots were applied to each of the samples, with a power density of ~2 × 10
7 W/cm
2. The results obtained and the related conclusion that the laser fluence is directly influenced by ablation, is consistent with the results obtained in relation with this research and presented in a previous paper of Moldovan E. et al. [
24].
In value, the fluence is different, depending on the material. For example, for polymers it is between 0.1 to 1 J/cm
2, while for inorganic insulators it is between 0.5 and 2.0 J/cm
2 [
2]. Related to the present study, Moldovan E. et al. [
24] showed a decrease in fluence values from 8.49 J/cm
2 to 2.55 J/cm
2, in the frequency range of 30–100 kHz. The materials have a different behavior under the action of the laser. For example, the absorption of energy from a laser beam strongly influences the ablation mechanism. Thus, polymers absorb in a non-linear manner the laser energy compared to metals [
2].
The literature provides information on the possibility of using nanosecond, picosecond, and femtosecond laser equipment for texturing. Pou-Álvarez P. et al. [
25] studied the difference between the three variants, applying pulse lengths of 20 ns (in present study 9 ns pulse duration), 800 ps, and 266 fs on AZ31B magnesium alloy. In the nanosecond and picosecond lasers, the wavelength was 1064 nm (as in the present study) and 520 nm for the femtosecond one. The power was 6.4 W, 6.2 W, and 5.9 W (in the present study 20 W). The spot size in the nanosecond case was 163 µm (compared to 100 µm in the current study). All parameters can be analyzed comparatively in the papers [
24,
25,
26]. SEM analysis shows uniform Mg matrix with dispersed Al-Mn-Fe particles. It is important to consider what happens due to the reduction of pulse duration. Groove profiles differ from nanosecond to femtosecond, as does the position for the recast layer. The molten material is drastically reduced with the reduction of the pulse length, the same as recast material. In the case of recast material in nanosecond situation, this is present as a uniform and thicker layer. By reducing the pulse length, an increase in width and depth of grooves is obtained.
The ablation was also studied by Cui H. et al. [
27] on copper samples, using a laser with a grazing incidence angle of 7° (in the present study 3°), wavelength 46.9 nm, energy 50 µJ and 100 µJ with a pulse width of 1.7 ns. The pulse numbers were 1 to 6. By increasing the energy, the evaporation on the surface increased. At 100 µJ the average fluence were calculated at 250 mJ/cm
2. The explanation of the mechanism was as follows: the laser energy was absorbed by the surface and after melting/evaporation the material was resolidified into nanoparticles. The placement of the sample is very important, because the vertical positioning allows, due to gravity, the nano particles to fall. The interesting results obtained encourage further research into the phenomenon.
Microtexturing also finds its place in the field of hip implants, and the couple bearing between the acetabular cup and the femoral head is very important. The combination of materials can be varied, from metal–metal to metal–ceramic and to metal–polymer. The appropriate choice of the combination of materials translates into the reduction of unwanted effects, such as aseptic loosening, tissue constraints in the body, etc. Using the finite element simulation model presented by Jamari J. et al. [
28] for the combination of UHMWPE as material for acetabular cup and SS316L, CoCrMo and Ti6Al4V as metallic materials for femoral head, the contact pressure can be established for other combinations of materials.
Delgado-Ruiz R. A. et al. [
5] studied zirconium dioxide for dental implants microtextured with a femtosecond laser device. They applied 120 fs pulses at 795 wavelengths, with a repetition rate of 1 kHz, a pulse energy that can reach a maximum of 1.1 mJ. As a result of surface modification, all roughness parameters significantlyincrease. Concerning the surface morphology, the pores have a conical section and the grooves a pyramidal one. The elemental analysis reveals the diminishing presence of carbon and aluminum and a larger proportion of oxygen and zirconium. The SEM analysis of surfaces showed the presence of crests and valleys and microcracks areas.
To improve osteoblastic bond of human stem cells, Cunha A. et al. [
6] investigated laser surface texturing of Ti-6Al-4V alloy. The laser wavelength was 1030 nm and pulse duration 500 fs. The surface texture consisted in nanoripples, nanopillars, and microcolumns. SEM analysis shows the differences between the three types of texturing and is supplemented by X-ray photoelectron spectroscopy (XPS). Nanoripples and microcolumns induce cell stretching. Nanopillars tend to increase the formation of filopodia. After seeding and cell growth, the first two microstructures appear to be suitable for use.
In dentistry and orthodontics, it is necessary to ensure adhesion properties. Maria De la Cruz Lorenzo et al. [
7] studied the microstructuring of the dentin and enamel surfaces with ultrashort pulsed laser microstructuring (wavelength: 795 nm, pulse duration 120 fs and repetition rate 1 kHz). SEM observations of the processed and failure surfaces reveal that the bonding strength is similar to other techniques and in some cases even increased.
For biomedical materials, including magnesium and titanium, laser microprocessing is beneficial. Thus, for Hu G. et al. [
8], the study material was hot-extruded Mg–6Gd–0.6Ca alloy bars and Ti6Al4V. SEM images were performed on the samples after cell seeding on laser-melted and laser-melted and LIPSS technique. The wavelength of 1064 nm, the pulse width of 800 fs, and the repetition rate of 400 kHz were considered for the laser. The cell spreading was anisotropic on the laser melted and LIPSS surfaces, observing many focal adhesions (filopodia). The ossification is accelerated due to good adhesion.
Implantation of stem cells cultured in bio-resorbable polymeric scaffolds requires microstructuring technologies, useful in regenerative medicine. Poly-L-Lactide (PLLA) was tested by Rocio Ortiz et al. [
9], applying grooves on the samples surfaces with a picosecond pulse laser, UV wavelength 355 nm applying an energy of 0.9 μJ at a frequency of 100 kHz, and 5 μm of pulse distance. As a result, the SEM images reveal a depression caused by material removal and protrusions of the recast material at the grooves’ ends and edges. The protrusion is the consequence of the first pulse effect. The conclusion is that grooves influence cell orientation and grooves’ edges promote cell adherence and provide guidance. The authors recommend ways to functionalize biomaterials.
Laser microprocessing using high repetition rate of a femtosecond lasers was used by Schille J. [
12] for stainless steel, aluminum, and copper. SEM analysis on stainless steel, for example, reveals the hydrophobic behavior of high volumes of microcones, with contact angles of up to 150°. The reflectivity in this situation is less than 5%. The decreasing of volumes causes the abatement of hydrophobic behavior. Thermal conductivity and highly repetitive laser pulses influence the morphology of the surface materials. Around the irradiated area, a high temperature determined a higher ablation rate.
Calcium fluorite as transparent material was microstructured with a femto laser device by Rupasov A.E. et al. [
13]. Nanogratings were produced at the wavelength of 515 nm and at a wavelength of 1030 nm. The doubling of the period of the nanostructures from 200–250 nm to 400–450 nm can be related with the appearance of a crater on the surface of the material showed by SEM images and the redistribution of energy in the volume.
Hybrid joints metal-polymers were of interest for Van der Straeten K. et al. [
15] using X5CrNi18-10 (1.4301) stainless steel and glass fiber reinforced PA6 composite polymer. The pulse duration was about 6 ps at a wavelength of 1030 nm. SEM images revealed cone-like protrusions (CLP) which show dots and holes partially connected with bridges. The structure seems to have no predominant orientation direction. The surface roughness can be growth by increasing pulse energy and fluency.
One condition of the joint improvement is the modification of the surface roughness. Nguyen, A.T.T. et al. [
16] studied metal-composite joints using adhesive, the surface being microstructured with outward/inward dimples or outward/inward grooves. It was revealed that surface irregularities ensure improved mechanical interlocking, the resin flowing through the relief being observed in SEM characterization.
Surface microtexturing has been broadly applied for improving the tribological properties of cutting tools, such as improving the friction behavior and anti-wear. The types of fabricated patterns on the cutting tool surfaces were most often micro-grooves, micro-holes, micro-stripe grooves, and micro-pools. Su Y. et al. [
20] studied the influence of micro-grooved polycrystalline diamond tools manufactured with a fiber laser (scanning speed: 2 mm/s, pulse repetition rate: 20 kHz, average output power: 12 W) on Ti6Al4V, which is the workpiece material. Three cutting speeds were investigated: 16.485 m/min, 56.52 m/min, and 125.6 m/min. For the first speed the results show that the low cutting speed generates low cutting temperature and causes slight adhesion. The results for the other two speeds show excellent anti-adhesive effects compared with those of the untextured PCD tools, even without lubricants. Low tool-chip contact length can be obtained by the micro-grooved PCD tools which also determines the improvement of the friction behavior.
Nickel-based alloys have special properties, such as high corrosion resistance and high temperature resistance, but their big problem is the difficult machining. Among the materials with suitable cutting possibilities is polycrystalline cubic boron nitride (CBN), although it encounters difficulties at high cutting speeds. Sugihara T. et al. [
21] chose Inconel 718 as the workpiece material and cubic boron nitride (CBN) as cutting tools, which are among the hardest materials. Grooves (grooves parallel and orthogonal to the main cutting edge, microgrooves orthogonal to the main cutting edge and placed 30 µm away from the main cutting edge) with a depth of 5 µm and width and separation of 20 µm were fabricated on the flank face of the CBN cutting tool with a femtosecond laser (wavelength 800 nm, pulse width 150 fs, cyclic frequency 1 kHz, pulse energy 300 µJ). The purpose was to stabilize the adhesion layer on the flank face and prevent it from flaking. A cutting tool with micro grooves orthogonal to the cutting edge and set back from the cutting edge significantly reduced the amount of tool retreat. This proves that microgrooving is a helpful method in machining special alloys.
The research in this article is a response to the challenge demanded by industry, namely microstructuring using nanosecond pulsed industrial laser source. Experimental research has generally shown better results in picoseconds and femtoseconds laser, but the encouraging results in the case of nanoseconds, and especially the applications for industrial and economic reasons, determined our choice in the study of the latest technical solution. The goal is to perform the morphological and elemental analysis of the laser surface texture patterns on AISI 430 stainless steel, continuing the studies completed in [
24,
26] for geometry, wettability, and roughness characterization.
2. Materials and Methods
AISI 430 ferritic stainless steel was supplied by Acerinox (ACX 500, Madrid, Spain) in the form of a large sheet, from which the samples were cut to size 80 × 25 × 0.5 mm. The material selected for the present research has a low content of carbon and nitrogen (improving the weldability, toughness, and ductility) and a good resistance in corrosive environment and exposure. Ferritic stainless steel has an elongation at room temperature (20 °C) higher than 20%, a nominal yield strength 0.2% offset greater than 260 MPa, and tensile strength between 450 MPa and 600 MPa. With low carbon and chromium between 16.00–18.00% content (
Table 1), the material has a body-centered cubic (BCC) crystalline structure. After welding, the 430 ferritic stainless steel can propound corrosion at the intergranular level or/and an oxidation layer. The oxidation layer (chromium oxide) can behave as a protective layer.
The equipment used to achieve the microtexturing patterns on the surface of the ferritic stainless steel was a nanosecond pulsed laser TruMark 5020 (Trumpf Laser und Systemtechnik GmbH, Ditzingen, Germany). The laser beam is generated by an active medium Nd: Fiber Diode-pumped, with average power of 20 W, beam quality M
2~2, 1064 nm wavelength, and 100 µm spot (at 254 mm focal distance). The 3.8 kW pulsed peak power at 20 W constant mean power ensures the highest pulse energy (>400 µJ) when frequency varies from 30 kHz to 50 kHz. The equipment has an integrated computer-aided design software (AutoCAD, version 23.0) that offers the possibility to acquire a wide variety of geometrical shapes’ design and a facile transfer between working stations. The maximum marking area is 180 × 180 mm at 254 mm focal distance and a pulse frequency from 5 kHz to 1000 kHz (9 ns pulse duration). Because of the specular reflection (surface finish of the ferritic stainless steel is bright annealed), the laser beam axis was settled at 3° angle to prevent possible damage of the laser optical fiber. Throughout the experiments, the constant parameters were: spot diameter 100 µm, power density 2.55 × 10
5 W/cm
2, impulses per point 1, track width of the spot 0.5 mm, overlap of the spot 99% (there is a correlation between the speed and frequency: if the speed increases/decreases the frequency increases/decreases too, to not affect the overlapping of the laser beam spot), a pulsed power of 30.5 kW at 19 kHz, and hatch distance center-to-center 0.25 mm (1st design-octagonal shape-design type A [
23,
24]), 0.5 mm (2nd design-two ellipses at 90° angle-type B [
23,
24]), and 2 mm (3rd design-crater array–type C [
23,
24]). The variable parameters of LST are frequency, speed, and number of repetitions. Repeatability was chosen randomly, starting from five passes, then every five repetitions up to a maximum of 15 repetitions. The maximum number of repetitions was chosen after observing the intensity of the splashes and burrs. Increasing the amount of expulsed material will increase the influenced thermal area and amount of recast material.
The laser density profile was investigated prior the laser processing to ensure that proper laser absorption into material will be obtained. The intensity distribution profile (
Figure 1a) of the laser radiation on the surface plane can be described as Gaussian, which provides an image on the future groove’s shape. During the laser beam-surface interactions, a high intensity plasma surrounded by an electron charged field is formed which leads to material melting-recasting phenomena and even to delamination of the surface layer.
Figure 1b shows an almost perfect outcome for the laser beam: perfect beam and real beam are almost identical.
The time cycle refers to a single sequence of repetition, decreasing with increasing frequency (
Figure 2). The time cycle is an important parameter, as it can indicate the time required to perform micro-texturing. The importance comes from the influence on the capacity and volume of production that can be achieved, thus being able to influence the automation of the LST process.
The surface morphology of the samples was investigated by scanning electron microscopy (SEM FEI Quanta 200 3D Dual beam, equipped with energy-dispersive X-ray spectroscopy analysis unit-X flash Bruker, Billerica, MA, USA). The working distance is set-up at 15 mm in low vacuum mode, with a spot size 5, high voltage (20 kV), and detector LFD (Large Field Detector). Scanning electron microscopy and energy-dispersive X-ray spectroscopy provide non-destructive, rapid, qualitative, and quantitative analysis.
The microscopic analysis for sectional images and measurements (
Figure 3) was performed with an optical microscope high-quality phase contrast Leica (Leica Microsystems, Heerbrugg, Switzerland, Ltd., model DMIL M LED). AutoCAD software was used to measure the ablated and recast areas.
When LST was applied on the surface of the AISI 430, a minimum of 3 mm distance was maintained from the start edge in the speed direction of each sample to mitigate the characteristic predisposition for a thermally influenced area of the ferritic stainless steel.
4. Conclusions
The SEM images (top view and cross section) offer information about the phenomena that occur during LST. The crater array (design type C) and octagonal (design type A) patterns present a smaller area of splashes compared to the ellipse pattern (design type B). The same phenomenon of reduction appears in the case of the recast material for ellipses and dimple/hole/crater array pattern. For pattern design type B, this is due to the 99% overlapping of the laser spot. In the case of the crater array (where there is no overlapping), the recast material is almost non-existent.
In two out of the three patterns (design type B and design type C) the presence of the oxygen element is indicated, which is beneficial for ferritic stainless steel in creating the passive layer (an oxide layer, formed from chromium and oxygen and displaying an inert reaction to the environment). The appearance of oxygen in the case of design patterns B and C, which can lead to the appearance of the passive layer (protective layer), signifies an optimal direction for the use of the patterns in tribological applications. The lack of the oxygen element in the case of pattern design type A lays out the possibility of the pattern to be used in hybrid joining.
In the area of the recast material, the EDX analysis indicates almost a double value for the carbon element (weight %) for octagonal pattern. The results, for ellipse and crater array patterns, are very appropriate (iron, chromium, and the carbon elements). At the bottom of the groove, the pattern that offers different results for elemental analysis is the octagonal design. In the thermally affected area, the measured values of the elemental analysis are appropriate for all the patterns. The cross-section images of EDX analysis offer the answer that nothing is changed regarding microtextured area.
Morphological analysis provides valuable information about the microrelief of laser textured surfaces and clues for mechanical interlocking in the case of hybrid joints. Of the three analyzed models, the highest level of recast material is observed for design type A, being more recommended in application for the preparation of the metal surface before hybrid welding.
Future studies will focus on corrosion testing, XPS, or AES characterization and FEM simulation of the microtextured specimens.