Laser-assisted surface modification of TRIP steels: chemical and microstructural evolution, residual stresses, and micromechanical properties

ORIGINAL RESEARCH ARTICLE Laser-Assisted Surface Modification of TRIP Steels: Chemical and Microstructural Evolution, Residual Stresses, and Micromechanical Properties G. RIU-PERDRIX, S. SLAWIK, O. GAVALDA-DIAZ, J.A. TRAVIESO-RODRIGUEZ, G. FARGAS, ANTONIO MATEO, F. MU ¨CKLICH, and J.J. ROA The use of new lightweight design strategies is implemented in the automotive industry, for applications where high friction forces are present and lubrication systems need to be implemented. Micro-topography modification through laser texturing is an advantageous manufacturing technique that is currently being used by the industry. Combining both ideas, this manuscript aims to analyze the effect of surface modification laser-texturing process on a TRansformation-Induced Plasticity steel. Residual stresses, superficial chemistry, microstructure, and mechanical properties at the submicrometric length scale are investigated. Results show that the intensity and the number of pulses strongly modify the pattern geometry. Also, the pattern distance affects the residual stress state, changing from tensile to compressive stresses for distances higher than 20 lm. Chemical and microstructural changes at the vicinity of the laser may be associated to the fast cooling of the sublimated material around the pattern induced during the laser-assisted surface modification thermal process. Furthermore, the hardness remains stable, and no changes are evident in the bulk material. However, a hardness reduction of around 95 pct in the austenitic value is evident in the pile-up zone around the laser pattern due to (1) the microporosity and defects confined inside the pile-up region and (2) due to the residual chemical composition created during the solidification process, which is rich in iron, chromium, nickel, and manganese and poor in oxygen. https://doi.org/10.1007/s11663-024-03330-9 The Author(s) 2024 I. INTRODUCTION Given the actual regulations developed to reduce the carbon dioxide emission and the financial penalties that bring along, fuel economy and the reduction of the emissions have become key factors for the automotive industry. Both factors are clearly related to produce lightweight vehicles. [1] But beside lightweight, good formability, high mechanical strength, and capacity for energy absorption are the main requirements of the automotive industry, which also requires applicability at high production rates and low cost. In this sense, the metastable austenitic stainless steels, and in particular TRansformation-Induced Plasticity (TRIP) steels, are the perfect option concerning crash behavior properties, due to their high energy absorption potential. These steels present an austenitic (face-centered cubic—fcc) phase which is not thermodynamically stable at room temperature. Then, when plastically deformed, austenite (c) experiences martensitic phase transformation that can produce two different types of martensite: ewhich has a hexagonal close packed (hcp) structure and a¢with a body-centered cubic (bcc) structure. Typically, the phase transformation can be defined through two different reactions, directly or indirectly. [2] In the first case, cfia¢, [3,4] while for the second case, where a¢is generated passing through an intermediate state, the emartensite, which transforms almost immediately in a¢as follows: cfiefia¢. [5,6] The difference in packing density between cand a¢provides a volume expansion of ~3 pct, [7] inducing an internal compressive residual stress that enhances the mechanical properties in terms of hardness and fatigue behavior. G. RIU-PERDRIX, G. FARGAS, A. MATEO, and J.J. ROA are with the Center for Structural Integrity, Reliability and Micromechanics of Materials (CIEFMA), Department of Materials Science and Engineering/Barcelona Research Center in Multiscale Science and Engineering, Escola d’Enginyeria de Barcelona Est, Universitat Polite `cnica de Catalunya, 08019 Barcelona, Spain. Contact e-mail: [email protected] S. SLAWIK and F. MU ¨CKLICH are with the Functional Materials, Department of Materials Science and Engineering, Saarland University, 66123 Saarbruecken, Germany. O. GAVALDA-DIAZ is with the Department of Materials, Centre of Advanced Structural Ceramics, Imperial College London, London SW7 2AZ, UK. J.A. TRAVIESORODRIGUEZ is with the Mechanical Engineering Department, Escola d’Enginyeria de Barcelona Est, Universitat Polite `cnica de Catalunya, 08019 Barcelona, Spain. Manuscript submitted March 9, 2022; accepted October 5, 2024. METALLURGICAL AND MATERIALS TRANSACTIONS B Under service conditions (e.g., fatigue, wear, etc.), the failure of these steels is usually initiated at the surface. Therefore, surface modification has become a key factor to take into consideration for increasing their lifetime. This can be done through surface modifications such as nanotexturing, in order to enhance the mechanical and tribological response under different complex stress fields. In this regard, several techniques (i.e., grinding, shot peening, laser patterning, among others [8–11] )have been used to microstructurally change the superficial properties and, as a consequence, the residual stress state. [12,13] Currently, shot peening is the most employed technique to enhance the fatigue resistance. The compressive residual stresses induced by shot peening increase the amount of martensitic phase at the surface, reaching values up to 20 to 30 pct. [14] Despite this achievement, this technique has limitations, like inaccuracy of the reproducibility, induced defects (i.e., folds, micro-, and submicrometric cracks, etc.), among others. For overcoming these problems, during the last decade, the laser surface texturing (LST) technique has been widely studied to superficially induce micro-texturing. LST is considered a technique that precisely controls the energy of laser beams to create patterns at different length scales on steel surfaces. [15] In particular, the parameters that influence the final properties include surface chemistry (e.g., chemical composition), topography (e.g., roughness), surface energy, charge, and potential. It is worth to mention that large amount of work on laser surface texturing has been carried out mainly to modify seals, piston rings, and thrust bearings in order to enhance the tribological properties. [16–22] However, scarce information on this subject is present in the literature related to TRIP steels. In this sense, during the last years, Rezayat et al. [15,23–27] investigated the effect of laser surface texturing on linear patterns conducted on TRIP steels. Changes in microstructure, mechanical properties, grain characteristics, and the main mechanical properties at the static state have been reported. These diverse studies present in the literature contribute to the advancement of precise º and reliable surface roughness measurements particularly by laser confocal microscopy as well as to better understand the chemical, microstructural, and mechanical superficial effect induced by a nanosecond (ns-) laser on TRIP steels at the edges of the patterns. While some preliminary studies have shown promising results in selecting the employed texturing parameters to control the surface roughness from nanoup to micrometric length scale on linear patterns, [28,29] more research is needed to demonstrate practical applications of ns-laser surface texturing for different patterning geometries for realworld industrial applications on TRIP steels. Within this context, the main goal behind this research is to understand the interaction laser/material—in terms of residual stresses, chemical composition, microstructure, and mechanical properties—on superficially modified TRIP steels by using laser-assisted modification. In order to achieve this aim, it is necessary to optimize the laser working parameters: intensity, frequency, number of pulses, and distance between patterns. Afterward, the reference and the superficially modified samples will be investigated from a chemically, microstructurally, and mechanically point-view to get a deeper understanding of the laser-texturing interaction on TRIP steels. II. MATERIALS AND METHODS A. Material The material was a commercial AISI 301LN stainless steel (corresponding to European standard EN 1.4318) provided by Outokumpu (Finland). It was supplied as 1.5-mm-thick sheets with a duplex microstructure (around 92 pct of the c-phase, while the rest corresponds to a¢-phase, in agreement with the data presented in Reference 22). Its chemical composition is summarized in Table I. Prior to surface modification, the specimens were sequentially polished until a mirror-like surface, by using the following diamond suspensions: 30, 6, and 3lm. Finally, they were chemo-mechanically polished by using a neutral suspension of alumina (20 to 45 nm of average particle size). Subsequently, in order to remove the work hardening layer, as well as the a¢-phase induced during the polishing process, samples were electro-chemically polished at room temperature by using a Buehler PoliMat 2 (Mikron) at a constant voltage of around 11.6 V for 15 seconds. B. Surface Modification Surface modification was carried out using a nanosecond pulsed laser (Spectra Physics, One Explore 346-120) with Nd:YLF solid-state medium and 346 nm of wavelength. The working conditions were optimized by using a Design of Experiments (DoE), taking into consideration the following parameters: frequency (Hz), pulse intensity (A), number of pulses, and spacing between patterns. More information related with the DoE is available in Reference 30. C. Microstructural Characterization The laser pattern generated was observed with a confocal laser scanning microscope (LSCM, LEXT OL31000) and in more detail with a Field Emission Scanning Electron Microscopy (FESEM, Carl Zeiss Neon 40), in order to characterize the induced topography. The superficial residual stresses (r) induced during the LST process was measured by using the x-ray diffraction (XRD) technique. The equipment used was PANalytical Empyrean, which has been operated with Cr tube (40 kV and 40 mA). A polycapillary optics (x-ray lens) with cross aperture 1 91 mm and V-filter METALLURGICAL AND MATERIALS TRANSACTIONS B (primary side) and a parallel plate collimator with Xe proportional detector (secondary side) were employed. The 220 peak found at 2h=126 deg was used for the stress calculations, at 8 Chi angles from 0 to + 70 deg. Stress analysis was performed by using the PANalytical X’Pert Stress plus software using the sin 2 wand by using X-ray elastic constants of S1=1.50 TPa 1 and 1/2 S2 = 6.50 TPa 1 . Damage induced as well as the microstructural changes due to the laser patterning were investigated by focused ion beam (FIB) milling of cross section and scanning electron microscopy (SEM) inspection. It was done by using a dual-beam workstation, Zeiss Neon 40 with a Ga+ ion source at a voltage of 30 kV. The final polishing process of the cross section was performed at a current of 500 pA and 30 kV of acceleration voltage. D. Chemical Characterization Electron probe x-ray microanalysis (EPMA), using wavelength-dispersive spectrometry (WDS), was conducted in a small area, where several laser patterns were included using a JEOL JXA-8230 microprobe. The x-ray maps were recorded in high-resolution mode, using a channel width of 1 eV. Electron currents were selected to ensure that counting rates were less than 10 4 counts/s. More information about the protocol followed to conduct this analysis can be found elsewhere. [31,32] X-ray photoelectron spectroscopy (XPS) experiments were performed in two different samples: the reference and the superficially modified one for comparison purposes. PHI 5500 Multitechnique System (from Physical Electronics) with a monochromatic X-ray source (AlKaline of 1486.6 eV energy and 350 W) was placed perpendicular to the analyzer axis and calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 eV. The analyzed area was a circle of 0.8 mm in diameter, and the selected resolution for the spectra was 187.5 eV of Pass Energy and 0.1 eV/step for the spectra of the different elements. All measurements were made in an ultra-high vacuum (UHV) chamber at a pressure between 5 910 9 and 5 910 8 Torr. To study the morphology and element distribution around ablated patterns, electron transparent samples for transmission electron microscopy (TEM) were prepared using a FIB lift-out technique (Helios NanoLab SEM, FEI). These samples where characterized at 200 kV using TEM bright-field (BF) imaging, scanning transmission electron microscopy (STEM), and energy-dispersive x-ray spectroscopy (EDS) with a TEM/STEM at 200 kV (JEM-2100F, Jeol) equipped with an EDS detector (X-Max detector STEM-EDS, Oxford Instruments). E. Mechanical Characterization at the Submicrometric Length Scale Mechanical properties at the micrometric length scale, mainly hardness (H) and elastic modulus (E), were determined by means of nanoindentation for both the non-modified (reference material) and the modified TRIP steel. These micromechanical tests were performed with a Nanoindenter XP (MTS) unit, equipped with a Berkovich diamond tip indenter and a continuous stiffness measurement mode (CSM), allowing a dynamic determination of the mechanical properties during the indentation process by the Oliver and Pharr method [33,34] and subsequently applying the Ulm et al. method [35–38] (more information about this methodology is available in Appendix A1). A total of 400 imprints (20 by 20) were performed under displacement control mode at 200 nm of maximum displacement into surface. The distance between imprints was kept constant at 10 lm, in order to avoid any overlapping effect. Strain rate was held constant at 0.05 s 1 and the indenter shape was carefully calibrated for true indentation depth as small as 25 nm by indenting fused silica standard of known Young’s modulus of 72 GPa. [33] III. RESULTS AND DISCUSSION A. DoE Analysis—Optimization of the Nanosecond Pulsed Laser Design of Experiments (DoE) is defined as a planned approach for determining cause and effect relationships. [39] This method consists of (1) reducing the number of needed experiments, (2) verifying all factors that affect an experiment, and (3) defining a strategy to obtain reliable solutions after a set of experiments. This methodology has been used, following a full factorial approach, to optimize the laser parameters looking for the lowest surface roughness as well as the highest tensile residual stresses. A schematic representation of the different steps taken into consideration is summarized in Figure 1. The critical parameters to take into consideration for this analysis are as follows: laser beam intensity, frequency, and number of pulses. After that, the three different roughness parameters (Ø t ,Ø wp , and h, defined as the total diameter which includes the pattern as well as the ablation pile-up, the patterning inner diameter without taking into account the ablation pile-up, and the maximum depth reached during the laser patterning process, respectively) were measured for each condition by LSCM technique, as shown in Figures 2(a) and (b). As it is evident, hdoes not depend of the different laser parameters investigated here. On one hand, both Ø t and Ø wp linearly increase with the laser intensity and the number of pulses. On the other hand, the frequency is not a key factor as the different morphological parameters investigated remain constant. According to the different measurements by LSCM conducted on the different investigated patterns, the optimal parameters are as follows: frequency of 1000 Hz (being this the minimum frequency employed in this study), a laser Table I. Chemical Composition of AISI 301LN Element Cr Ni Mn Si N Mo C Fe Wt. Pct 17.6 6.5 1.13 0.42 0.17 0.04 0.02 bal. METALLURGICAL AND MATERIALS TRANSACTIONS B intensity of 4 A, and a constant number of pulses of 20 spots at the same region (as shown in Figure 2(b), the number of pulses does not modify the h). In this particular study, 20 was chosen because it was the minimum number of pulses, which was able to produce perfect patterns with the minimum pile-up around the desired pattern. Once the laser parameters related to beam intensity, frequency, and number of pulses were optimized, the distance between the laser patterns was also evaluated. In order to optimize this distance, four different (x,y) values were investigated (20/20, 50/50, 100/100, and 150/ 150 lm) (see Figure 2(c)) in order to see the effect of the laser spot on the residual stresses, and therefore be able to choose the distance which produces less compressive residual stresses. In this sense, the residual stresses were measured by means of XRD by using the sin 2 Wmethod in the r 11 and r 22 directions (see Figure 2(c)) of those four distances between spots. The resulting data are summarized in Table II. Finally, by using these optimized laser parameters, the desired patterns investigated present the following dimensions: about 26.02 ± 0.98 lm in diameter with pile-up (Ø t as depicted in Figure 2) and about 32.2 ±4.8 lm in depth. Fig. 1—Schematic diagram of the parameter optimization steps to produce the final laser-textured specimens. Fig. 2—(a) Schematic representation of the morphological parameters employed to optimize the main laser parameters through DoE analysis. (b) Variation of the depth and/or width as a function of the laser intensity, frequency, and number of pulses and (c) optical image showing the different laser patterns using the optimized laser conditions at different distances (20, 50, 100, and 150 lm). The r 11 (longitudinal) and r 22 (transversal) overlapped to the optical image denote the direction measurement of the residual stresses by using the sin 2 Wmethod. METALLURGICAL AND MATERIALS TRANSACTIONS B It is well known that a thermal process, such as laser, will induce tensile residual stresses, while a mechanical process (i.e., shot peening, grinding, etc.) will induce compressive residual stresses, as found in Reference 40. In view of the results obtained in terms of residual stresses in both directions (r 11 and r 22 ) as a function of the distance between each pattern, that distance was chosen to be 20 lm due to two different reasons: (1) for smaller distances between patterns the amount of ablated material around the pattern is higher and the interaction laser/material is more evident, and (2) the residual stresses are tensile and scarce information is available on TRIP steels with this kind of stresses. Both reasons will lead to better understanding of the laser/material interaction on TRIP steels. With all this information, a homogeneous laser patterning was conducted at 1000 Hz, 4 A, 20 pulses, and the distance between patterns was kept constant and equal to 20 lm (from now labeled as SM 20). B. Microstructural Characterization The surface modification process induces chemical, microstructural, and mechanical changes. In this regard, a heterogeneous layer is created during the laser ablation patterning process, as depicted in straw tones in the LSCM micrograph (Figure 3(a)). When the distance between the different spots is small, this effect provides more thermal impact and, as a consequence, a change in physical/chemical and mechanical properties of the surface. Also, the roughness is increased. Figure 3(b) shows the SEM general micrograph of the laser-textured surface with a constant distance of ~20 lm. Furthermore, as it is evident in Figure 3(c), the surface seems to be partially melted. The affected molten material (spatter) has been ejected from the laser pattern due to the high energy and long pulse duration and appears deposited at the vicinity of the spot forming a of pile-up, as depicted in Figure 3(d). This ablated material is heterogeneously re-deposited pulse by pulse along the laser-texturing process. This process can be assimilated to a local thermal shock, including a continuous process of melting and solidification. Also, between each ablated layer a thermal-affected zone co-exists, generating some defects (i.e., cracks, folds, chemical and microstructural heterogeneities, etc.) along the pile-up (see white arrow in Figure 3(e)). C. Chemical Characterization Aiming to extract more detailed information around the laser pattern and in between, the surface chemistry of both investigated samples was observed by punctual analysis thorough EPMA technique. Five different points, heterogeneously distributed, were measured to atomically (at. pct) quantify the main constitutive elements. The results are summarized in Table III.In addition, compositional maps for a small area of the TRIP laser-textured surface were obtained (Figure 4). Three different zones with a heterogeneous chemical composition distribution could be distinguished (see marks in Figure 4labeled as 1, 2, and 3, corresponding to the different regions of the base material, deposited layer after the laser-patterning process, and the pile-up zone, respectively) from the different composition maps. Regions 1 and 3 presented similar chemical composition of the main constitutive alloy elements, while in region 2 the at. pct content of Fe, Cr, and Ni was lower than in the other investigated regions. On the other hand, the O chemical composition map (Figure 4) in between the different pattern’s present oxygen heterogeneously distribution, highlights the possibility to generate a thin oxide layer. On the other hand, around the laser pattern (pile-up region), lower content of oxygen is present, highlighting the possibility to create an intermetallic material due to the fast-thermal cooling process inducing just after the ablation process generated by the nanosecond laser. This fact remarks that the surface layer generated in the region between the different laser-texturing patters (region 1 and 2 in Figure 4) was an oxide layer. The chemical analysis presented in Figure 4and summarized in Table III does not present significant differences between the reference and the surface-modified specimens. This fact manifests that the induced layer is so thin that this technique is not suitable to precisely determine their chemical composition. Thus, for the thin layer generated during the laser-texturing process, the data obtained by means of this technique only could provide a qualitatively study. This may be attributed to the fact that electron/sample interaction is too deep and mainly confined in the bulk material of the region of study, being impossible to accurately determine the chemical composition of the superficial layer induced during the laser-texturing process and correctly identify and quantify the chemical composition of the surface layer generated during the local thermal ablation process induced during the laser-texturing process. To get a deeper knowledge related to the chemical composition of the thin layer induced during the laser-texturing process, an analysis at the surface level (first nm) by means of XPS technique was done. Figures 5(a) and 5(b) exhibit general XPS spectra for the reference and laser-textured specimens, respectively. As can be seen for the laser-textured specimen (Figure 5(b)), the XPS spectrum highlights the presence of Zn (not present in the reference specimen—Figure 5(a)) as well as the increase of the Ni content for the laser-textured specimen as summarized in Table IV. Comparing these data with those determined by Table II. Summary of the Residual Stresses Determined in r 11 and r 22 Directions for the Different Laser Distances Investigated (20, 50, 100, and 150 lm) Distance (lm) r 11 (MPa) r 22 (MPa) Reference 15.7 ±18.9 24.8 ±15.6 20 45.5 ±15.8 67.4 ±12.1 50 71.5 ±5.5 20.0 ±16.1 100 59.7 ±18.4 57.1 ±15.8 150 177.5 ±15.7 113.5 ±19.4 The laser beam intensity, frequency, and number of pulses were held constant and equals to 4 A, 1000 Hz, and 20 pulses, respectively. METALLURGICAL AND MATERIALS TRANSACTIONS B SEM-EDS and reported for the reference material in Table I, Zn does not appear in the chemical composition of the commercial TRIP steel, so it is likely to be below the detection limit, in the form of traces in the reference, of the SEM-EDS technique. The presence of Zn in the laser-textured specimen can be due because perhaps laser ablation volatilizes and re-deposits this element on the surface, generating a new intermetallic-based Zn compound with the other elements. Figures 5(c) through (h) summarize the XPS spectra of the induced layer for each majority element for the reference (for comparison purposes) and laser-textured (induced during the fast-thermal cooling process) specimens, left and right spectra, respectively. The reference XPS spectra for the Ni element are very noisy (see Figure 5(h)), highlighting that the content present on the reference sample is at the detection limit sensitivity of the XPS equipment. In this sense, it is not possible to deconvolute this peak and be able to quantify the content of Ni present on the reference sample. Furthermore, the Si-spectra for the reference and laser-textured specimen present a different crystallographic state (see Figure 5(f)), which presented a doublet peak distribution for the laser-textured specimen (right) and a monomodal peak distribution for the reference sample (left). This phenomenon may be related to the fact that silica (SiO 2 ) presents an allotropic phase transformation at high temperature (being the most common phase; quartz, cristobalite and tridymite [41] ) and each one of them presents a low and high atomic order phase (a-and b-, respectively [1] ). So, the peaks presented in the XPS spectra may be related to aand b-Si. Furthermore, XPS analysis gave qualitative information on the chemical composition of the thin layer generated during laser-texturing process. In an attempt to get more detail knowledge of the ablation pile-up, as well as on the layer induced during the laser-patterning process, and on the bimodal SiO 2 peak distribution, a TEM lamella was extracted by FIB from the region of interest. It was directly extracted from the white dash line presented in Figure 6(a). A TEM observation of the pile-up ablated zone is presented in Figure 6(b). The TEM analysis in Figure 6(c) shows the presence of some crystalline and amorphous regions. Furthermore, this TEM micrograph displayed a complex microstructure, like micrograins, pores, and holes among others. The defects observed though TEM inspection and confined in the ablation pile-up region agree with those reported in Reference 26. This phenomenon highlights that the solidification process presents two different regions: one phase of low and other of high atomic order, in agreement with the data obtained by XPS and presented in Figure 5(f) (right side). The EDS maps of the TEM micrograph presented in Figure 6(c) show a homogeneous distribution of the main elements within the TRIP steel. From this TEM analysis, it is concluded that the ablated and also the layer induced during the laser-texturing process present a homogeneous chemical composition. In this sense, further chemical assays by using Fig. 3—Microstructural analysis of the TRIP laser surface specimen. (a) LSCM micrograph showing the different heterogeneities induced during the ablation process; (b) SEM micrographs showing the homogeneous array of patterns; (c) SEM magnification micrograph of a pattern showing the pile-up as well as the redeposition effect induced during the ablation process at the vicinity of the pattern; (d) tilted-SEM micrograph in order to highlight the pile-up effect induced during the ablation process; and (e) SEM micrograph showing the defects present in the pile-up region. METALLURGICAL AND MATERIALS TRANSACTIONS B atomistic length scale techniques, like atom probe tomography may be done in order to quantify and determine the thickness of this intermetallic layer. D. Mechanical Characterization Attempting to observe the energy/material interaction in terms of mechanical properties (mainly hardness, H), nanoindentation technique was used to evaluate the hardness for the reference and laser-textured specimens and, in particular, to determine the intrinsic hardness for the straw layer from Figure 3(a). In order to measure the mechanical properties of the surface and be able to correlate the microstructure and the mechanical properties, prior and after the laser-texturing process, for each constitutive phase (c,a¢as well as the c/a¢-interphase), 400 imprints were conducted at 2 mN of maximum applied load. Prior using the methodology explained in Appendix A1 it is necessary to determine the right bin size. This parameter leads to qualitatively distinguish how many phases are present in the investigated material, as shown in Figure 7(a). For a constant bin size of around 1 GPa, only one peak is clearly visible, while three different peaks are discernible Table III. Chemical Analysis (Wt Pct) by EPMA Test for Two Different Samples Sample Fe Cr Ni Mn O Reference 73.72 ±0.28 17.82 ±0.12 6.54 ±0.11 1.20 ±0.02 0.09 ±0.01 SM 20 73.73 ±0.14 17.82 ±0.06 6.38 ±0.08 1.17 ±0.01 0.08 ±0.01 Fig. 4—EPMA analysis performed on a TRIP laser-textured specimen. (a) Quality SEM micrograph of the analyzed region, (b) iron—Fe, (c) chromium—Cr, (d) nickel—Ni, (e) manganese—Mn, and (f) oxygen—O. METALLURGICAL AND MATERIALS TRANSACTIONS B reducing this parameter up to 0.25 GPa. Figure 7(b) exhibits the hardness histogram overlapped to the Gaussian simulation curves for each constitutive phase. Three mean peaks were observed for the TRIP steel. The highest and the lowest value peaks, centered at 7.67 ± 0.75 and 5.74 ±0.74 GPa were attributed to the a¢and c phases, respectively, while the intermediate peak centered at 6.27 ±0.87 GPa corresponded to the c/ a¢-interphase. The hardness for the layer generated during the texturing process was determined following the same protocol methodology presented in Appendix A1 and also explained in Figure 7for the reference sample. In this sense, from the data obtained through the statistical method, the value of intrinsic hardness for the thin layer induced during the laser-texturing process was deconvoluted by implementing established thin film models. The use of thin film models is expected to yield reasonable values of hardness and the lateral stiffness is considered low in comparison to the normal stiffness, due to the large opening angle of the Berkovich indenter tip. In this study, the models of Korsunsky et al. [42] and Puchi-Cabrera et al. [43] were used to extract the intrinsic hardness for the base Zn interface. They are described by Eqs. [1] and [2], respectively, as follows: HC¼HSþHfHs 1þkb2½1 where H C (laser-texturing layer + TRIP bulk material), H f (laser-texturing layer), and H S (TRIP bulk material) are the composite, film, and substrate x 104 5 10 15 20 25 30 35 40 45 50 CPS 900 600 300 0 Binding Energy (eV) (Si) (N) (O) (C) (Cr) (Mn) (Fe) (Ni) (Zn) (C) (N) (O) (Si) (Cr) (Mn) (Fe) (Ni) General spectra Oxygen Iron (a) (b) (c) (d) (e) Chromium Silicon Nitrogen Nickel (g)(f) (h) Fig. 5—XPS spectra. (a) Reference sample, (b) laser-textured specimen with a constant patterning distance of 20 lm, (c) O-spectra, (d) Fe-spectra, (e) Cr-spectra, (f) Si-spectra, (g) N-spectra, and (h) Ni-spectra. The left and right spectra for (c) through (h) correspond to the XPS spectra for the reference and laser-textured specimen. METALLURGICAL AND MATERIALS TRANSACTIONS B hardness, respectively, while bis the relative indentation depth and kis a constant related to the film thickness. HC¼HSþHfHS ðÞeðkbnÞ;½2 where kand nrepresent material parameters that characterize the change in hardness as the indenter passes from the laser-textured film to the substrate. As a result, an intrinsic hardness of the laser-textured coating was calculated and summarized in Table V. The lower values obtained, compared to those of those of the reference sample, showed a softening in terms of hardness when it is measured in the ablation pile-up. This hardness reduction may be related to two different effects that take place at the same time: (1) from a chemical point of view, the ablation pile-up is a region rich of intermetallic elements and (2) from a microstructural point of view, the presence of submicrometric porosity as presented in the TEM micrograph (Figure 6(b)) and also reported in Reference 26 produces a dissipation of the plastic stress field induced during the indentation process and as a direct consequence a reduction of their hardness. This fact reveals that the superficial chemical composition of the ablation pile-up has changed and does not correspond to the generation of an oxide. In this sense, these findings fit with the data presented in Section III–C. IV. CONCLUSIONS The results shown in this contribution have demonstrated that the laser-assisted surface modification of TRIP steels superficially changes the microstructure, the chemical composition of the ablated material observed around the patterns in forms of pile-up, and the mechanical properties at the surface level. The following conclusions can be drawn: 1. The DoE analysis highlights that the laser intensity and the number of pulses change the final shape of the pattern, in terms of diameter and depth. FurTable IV. Chemical Analysis (At. Pct) Determined by XPS Test for the Reference and Laser-Textured Specimens Sample O Fe Cr Si N Ni Reference Etching 46.254 6.339 3.387 2.193 0.911 DL SM 20 Etching 47.553 9.287 3.278 2.628 2.096 0.318 Prior the chemical composition analysis, the surface of both analyzed specimens was etched with argon in order to reduce the contamination and be able to increase the metallic peaks intensity. DL means detection limit—it means the spectra are noisy and cannot be deconvoluted to correctly determine the Ni content present on the reference sample. Fig. 6—(a) SEM micrograph of the region of study. The white dash line shows the region, where TEM lamellae was extracted, (b) TEM micrograph in bright-field (BF) mode showing the microstructure in the pile-up ablated zone, and (c) EELS chemical composition maps for the main constitutive elements present in TRIP steels. METALLURGICAL AND MATERIALS TRANSACTIONS B