Μm Depth Research Articles

Improved corrosion resistance of 42CrMo4 steel by reconstructing surface integrity using ultrasonic surface rolling process

The ultrasonic surface rolling process (USRP) was adopted to improve the corrosion resistance of 42CrMo4 steel by reconstructing the surface integrity in terms of surface roughness, microhardness, residual stress, and microstructures. The corrosion behaviors of 42CrMo4 steel were studied by salt spray corrosion, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results indicated that the surface integrity of 42CrMo4 steel was restructured after USRP with the surface roughness reduced to 0.052 µm, the surface microhardness improved to 305 HV0.1, the residual compressive stress achieved up to − 274 MPa at 0.6 mm depth, and the average grain diameter minimized to 2.02 µm in the zone of 5–150 µm depth. Owing to the modified surface roughness, hardened layer, and induced residual compressive stress, the corrosion resistance of 42CrMo4 steel was enhanced by USRP with the annual corrosion rate of 2.972 mm/a reduced about 5.2 %, the corrosion current density of 2.748 μA·cm−2 decreased up to 45.2 %, and the charge transfer resistance of 1809 Ω·cm2 increased up to 19.6 %.

Micro ECDM scanning process with feedback control of flexible contact force

Quartz glass has been widely used in frontier fields of MEMS (micro electro-mechanical systems) and sensors due to its excellent chemical and mechanical properties. Although ECDM (electrochemical discharge machining) is a non-traditional material removal process suitable for machining electrically non-conductive materials of high temperature resistance and hard brittleness, the adopted feeding strategy of tool electrode directly affects the machining results, especially in a micro ECDM scanning process. The existing feed methods still cannot maintain a suitable machining gap with a minimum given contact force during the micro ECDM scanning process, causing inconsistent gap state and the damage to machining stability and geometric accuracy of micro structures. In this research, a novel spindle combining a flexure hinge and a high-resolution micro-force sensor is designed, achieving the function of real-time servo control of a given micro contact force. Using the micro tool electrodes (<Φ200 μm) fabricated by the combined process of reverse-copying micro EDM and TF-WEDG (tangential feed-wire electrode discharge grinding), the circular structures are successfully machined on quartz glass with the optimized parameters of open-circuit voltage, scanning speed, and number of scanning. To guarantee the consistent depth along a scanning layer, a feed strategy of quantified depth with servo control of flexible micro force (SCFMF-QDFS) is proposed, and micro 3D structures of S-shape and square are machined with controllable accuracy of ±2 μm and ±4 μm in depth and width.

Nanoscratch-healing of single and bilayer thin films of hindered urea bond-based self-healable copolymers

In this study, the effects of the surface properties of single and bilayer thin films of hindered urea bond (HUB)-based self-healable copolymers on their scratch-wound formation and self-healing were investigated. Initially, three prepolymers with bulky amine groups, PtB (control), PFtB (hydrophobic), and PMtB (hydrophilic), were synthesised by free-radical polymerisation. Subsequently, they were used to fabricate self-healing copolymers by crosslinking with HDI. The three crosslinked polymers (PtB-H, PFtB-H, and PMtB-H) showed contact angles of 80°, 93°, and 73°, respectively. Moreover, all the single-layer samples showed complete scratch healing within 2 h at ambient temperature, with PFtB-H and PMtB-H showing an initial elastic recovery immediately after scratch formation, unlike PtB-H. The formation of bilayer samples by stepwise coating was confirmed by X-ray photoelectron spectroscopy (XPS). Owing to HUB networks formed on their surface similar to the single-layer, bilayer coatings showed excellent scratch-healing performance at room temperature for nano-scratches with 40–50 nm depth, as indicated by AFM. PMtB-H@PtB-H showed a better scratch-healing performance than PFtB-H@PtB-H, and the scratch-healing ability of PMtB-H@PtB-H was improved by moisture. Their nano-scale self-healing ability was verified by micro-scale scratch tests; scratches with 4–12 µm depth healed completely with ∼100% SHE% at 60 °C.

Measurement of ablation impulse and direction with oblique laser-pulse irradiation

In this study, a dual-axis impulse stand, having an elastic pivot with a precision of ±0.07 μNs, was newly developed, and the impulse induced by a laser pulse irradiation with a fluence of 10 J/cm2 was measured for incidence angles of 0°, 30°, 45°, and 60°. In all cases, the impulse vector was initially directed perpendicularly to the surface of the target. However, as the number of the laser pulse irradiations reached 200, only when the incidence angle was 60°, did the impulse direction shift to the laser-beam direction by approximately 10°. This impulse performance was consistent with the ablation crater morphology consisting in a periodic structure with 100 μm in width and 20 μm in depth, which was observed, through a laser microscope, only at this incidence angle.

Effect of Characteristic Parameters and Distribution of Friction Pair Surface Texture on Lubrication Properties

Based on the basic mechanism and bionics principle that texture affects the dynamic pressure effect of lubricating medium, a V-shaped texture that converges along the sliding direction is designed. Through numerical simulation, the optimal geometric parameters and distribution of the V-shaped and textures are obtained. A textured surface with various texture features is prepared using a nanosecond ultraviolet laser with bearing steel as substrate. Tribological experiments with friction and wear tester are performed to investigate the effect of characteristic parameters and distribution of surface texture on the lubrication performance and the lubrication properties are compared and analyzed with that of circular texture. Hence, this investigation provides a research direction to improve the lubrication performance between frictional pairs under fluid lubrication condition to reduce the frictional wear of mechanical systems. The results show that under the conditions of optimal parameters, due to the effect of convergence and extrusion on the flow of lubrication medium, the V-shape texture is better than the circular texture in improving the lubrication performance. The optimal characteristic parameters of V-shape texture are: 60° for the angle between the two wings, 0.53 for shape parameter, 25.9% for area ratio, 13 μm depth, 60% texture area coverage ratio, and the inlet of flow field of the texture distribution position.

Quantified trend of photosynthetic rate along the depth of microalgae biofilm

Compared with suspended microalgae cultivation, attached microalgae cultivation for wastewater treatment has advantages of low biomass recovery costs and high robustness. As a heterogeneous system, the variation of photosynthetic capacity along biofilm depth lacks quantitative conclusions. The distribution curve of oxygen concentration along the depth of attached microalgae biofilm (f(x)) was detected by dissolved oxygen (DO) microelectrode, and a quantified model was built based on mass conservation and Fick's law. It revealed that the net photosynthetic rate at a certain depth (x) in the biofilm showed a linear relationship with the second derivatives of the distribution curve of oxygen concentration (f″(x)). In addition, the declining trend of photosynthetic rate along attached microalgae biofilm was relatively slow compared with the suspended system. The photosynthetic rate at 150–200 μm depth of algae biofilm was only 3.60 %–17.86 % of that at the surface layer. Moreover, the light saturation points of the attached microalgae got lower along the depth of biofilm. Compared to 400 lx light intensity, the net photosynthetic rate of microalgae biofilm at the depths of 100–150 μm and 150–200 μm increased by 389 % and 956 % under 5000 lx, respectively, showing the high photosynthesis potential with increasing light.

Protease-Activatable Porphyrin Molecular Beacon for Osteoarthritis Management.

Despite the substantial burden posed by osteoarthritis (OA) globally, difficult challenges remain in achieving early OA diagnosis and adopting effective disease-modifying treatments. In this study, we use a biomolecular approach to address these limitations by creating an inherently theranostic molecular beacon whose imaging and therapeutic capabilities are activated by early pathological changes in OA. This platform comprised (1) a peptide linker substrate for metalloproteinase-13 (MMP-13), a pathological protease upregulated in OA, which was conjugated to (2) a porphyrin moiety with inherent multimodal imaging, photodynamic therapy, and drug delivery capabilities, and (3) a quencher that silences the porphyrin's endogenous fluorescence and photoreactivity when the beacon is intact. In diseased OA tissue with upregulated MMP-13 expression, this porphyrin molecular beacon (PPMMP13B) was expected to undergo sequence-specific cleavage, yielding porphyrin fragments with restored fluorescence and photoreactivity that could, respectively, be used as a readout of MMP-13 activity within the joint for early OA imaging and disease-targeted photodynamic therapy. This study focused on the synthesis and characterization of PPMMP13B, followed by a proof-of-concept evaluation of its OA imaging and drug delivery potential. In solution, PPMMP13B demonstrated 90% photoactivity quenching in its intact form and robust MMP-13 activation, yielding a 13-fold increase in fluorescence post-cleavage. In vitro, PPMMP13B was readily uptaken and activated in an MMP-13 cell expression-dependent manner in primary OA synoviocytes without exuding significant cytotoxicity. This translated into effective intra-articular cartilage (to a 50 μm depth) and synovial uptake and activation of PPMMP13B in a destabilization of the medial meniscus OA mouse model, yielding strong fluorescence contrast (7-fold higher signal than background) at the diseased joint site. These results provide the foundation for further exploration of porphyrin molecular beacons for image-guided OA disease stratification, effective articular delivery of disease-modify agents, and OA photodynamic therapy.

Effective fluence and dose at skin depth of daylight and lamp sources for PpIX-based photodynamic therapy.

Skin-based photodynamic therapy (PDT) is used for the clinical treatment of actinic keratosis (AKs) and other skin lesions with continued expansion into the standard of care. Due to the spectral dependency of photosensitizer activation and skin optical fluence, there is a need for more accurate methods to estimate the delivered dose at depth from different PDT light sources and treatment regimens. Develop radiometric methods for calculating photosensitizer-effective fluence and dose at depth and determine differences between red-lamp, blue-lamp, and daylight-based PDT treatments. Radiometric measurements of FDA-approved PDT lamp sources, outdoor daylight, and indoor daylight were performed for clinically relevant AK treatments. The protoporphyrin IX (PpIX) equivalent irradiance, fluence, and dose for each light source were calculated from the PpIX absorption spectrum and a 7-layer skin fluence model. The effective fluence and dose at depth was estimated by combining the spectral attenuation predicted at each wavelength and depth with the source fluence at each wavelength. The red-lamp source had the highest illuminance (112,000lumen/m2), but lowest PpIX-effective irradiance (9.6W/m2), and highest effective fluence at depth (10.8W/m2 at 500µm). In contrast, the blue light source had the lowest illuminance (2300lumen/m2), but highest PpIX effective irradiance (37.0W/m2), and ultimately the lowest effective fluence at depth (0.18W/cm2 at 500µm). The daylight source had values of (outdoor | indoor) illuminance of (49,200 | 37,800 lumen/m2), effective irradiance of (19.2 | 10.7W/m2), and effective fluence of (1.50 | 1.08W/m2 at 500µm). The effective fluence and dose at depth facilitated the comparison of treatment regimens, for example, calculating an equivalent dose for a 2 hr indoor daylight treatment and a 10min red-light treatment for the 300-1000μm depth range. The consideration of PpIX-effective fluence at varying depths is necessary to provide adequate comparisons of the delivered dose from PDT light sources. Methods for calculating radiometric fluence and delivered dose at depth were introduced, with open source MATLAB code, to help overcome the limitations of commonly used photometric and irradiance-based reporting.

Effect of Cr/Ti/Al Elements on High Temperature Oxidation Behavior of a Ni-Based Superalloy with Thermal Exposure

High-temperature oxidation of a Ni-based superalloy was analyzed with samples taken from gas turbine blades, where the samples were heat-treated and thermally exposed. The effect of Cr/Ti/Al elements in the alloy on high temperature oxidation was investigated using an optical microscope, SEM/EDS, and TEM. A high-Cr/high-Ti oxide layer was formed on the blade surface under the heat-treated state considered to be the initial stage of high-temperature oxidation. In addition, a PFZ (γ’ precipitate free zone) accompanied by Cr carbide of Cr23C6 and high Cr-Co phase as a kind of TCP precipitation was formed under the surface layer. Pits of several µm depth containing high-Al content oxide was observed at the boundary between the oxide layer and PFZ. However, high temperature oxidation formed on the thermally exposed blade surface consisted of the following steps: ① Ti-oxide formation in the center of the oxide layer, ② Cr-oxide formation surrounding the inner oxide layer, and ③ Al-oxide formation in the pits directly under the Cr oxide layer. It is estimated that the Cr content of Ni-based superalloys improves the oxidation resistance of the alloy by forming dense oxide layer, but produced the σ or µ phase of TCP precipitation with the high-Cr component resulting in material brittleness.

Mesoscale volumetric light-field (MesoLF) imaging of neuroactivity across cortical areas at 18 Hz.

Various implementations of mesoscopes provide optical access for calcium imaging across multi-millimeter fields of view in the mammalian brain; however, capturing the activity of the neuronal population within such fields of view near-simultaneously and in a volumetric fashion has remained challenging as approaches for imaging scattering brain tissues typically are based on sequential acquisition. Here we present a modular, mesoscale light-field (MesoLF) imaging hardware and software solution that allows recording from thousands of neurons within volumes of ⌀ 4 × 0.2 mm, located at up to 350 µm depth in the mouse cortex, at 18 volumes per second and an effective voxel rate of ~40 megavoxels per second. Using our optical design and computational approach we show recording of ~10,000 neurons across multiple cortical areas in mice using workstation-grade computing resources.

Multi-scale textured PEEK surfaces obtained by direct laser interference patterning using IR ultra-short pulses

In this work, we aimed at getting the first insight into the influence of the laser processing parameters on the production of functional multi-scale PEEK surfaces using the Direct Laser Interference Patterning technology. An infrared ultra-short pulsed laser source (1064 nm, 10 ps) and a two-beam interference setup were used. The fabricated structures morphology was analyzed by Scanning Electron Microscopy and Confocal Microscopy. As a result, line-like structures with periodic spatial repetition (p) at two levels were produced: main structures with p = 5 µm and mean depth values up to ∼2.5 µm (aspect ratio of ∼0.5) and secondary structures, perpendicularly oriented relative to the main structures, formed at DLIP maxima position (top part of ridges) with p≈1 µm (LIPSS).

Nanosecond pulsed laser surface modification of yttria doped zirconia for Solid Oxide Fuel Cell applications: Damage and microstructural changes

An effective strategy to reduce the cathode polarization in a Solid Oxide Fuel Cell (SOFC) is to enlarge the cathode-electrolyte interface, corrugating the electrolyte surface of zirconia doped with 8 mol% of yttria (8YSZ) by pulsed-laser machining. However, laser-material interaction using a nanosecond pulsed laser can involve thermal effects on the surface. The objective of this work was to analyze the microstructural and phase changes, and the collateral damage caused by laser machining on the 8YSZ electrolyte surface of the SOFCs, and compare it with the 3% molar in yttria (3Y-TZP). Several patterns consisting in parallel tracks below 10 µm depth were investigated. The results evidenced a heat affected zone (HAZ) limited to ⁓1–2 µm with microcracking and directional recrystallization, which was larger in 8YSZ than in 3Y-TZP. However, the mechanical response near the HAZ and chemical composition at the machined surface was not significantly changed.

Single-spot spiral phase plate for tracking 3D particle motion.

We have imaged and tracked submicron fluorescent particles in three dimensions by inserting a spiral phase plate (SPP) of our design into a conventional wide-field optical microscope. Our novel SPP consists of concentric annular zones that each impose a unique quantized orbital angular momentum (OAM) on the ring of fluorescent light it intercepts. Each ring of the SPP consists of a vortex surface that produces one additional unit of OAM than the smaller ring just inside it, with the seven successive rings each contributing n = 1 to 7 quantum units of OAM respectively. The light rings interfere to produce a single-spot rotating point spread function (SS-RPSF). This image ‘spot’ rotates with the axial depth of a particle, permitting 3D mapping of the particle position. The SPP is easily incorporated into existing microscopes. We used the DIC slider here for the SPP. 3D positions of fluorescent beads were retrieved using template matching. Bead positions could be inferred over a 6 µm depth range. Best results were obtained over a depth range of 2.3 μm, with a mean absolute error of 20 nm in this range. As proof-of-concept for live-cell imaging we tracked DNA loci and determined their diffusion coefficients in the 3D environment of the nucleus of U2OS cells. We are pursuing chromatin tracking experiments to better understand how DNA damage affects chromatin motions and, reciprocally, how chromatin dynamics impacts the DNA damage response. Beyond this specific application, our SPP-based imaging approach should enable researchers to track dynamic fluorescent objects in samples with high temporal resolution because x, y, and z are determined at exactly the same time.

Superhydrophilic and topography-regulatable surface grafting on PEEK to improve cellular affinity

Polyetheretherketone (PEEK) has been widely used in the preparation of orthopedic implants due to its biological inertness and similar mechanical modulus to natural bone. However, the affinity between biological tissue (bone and soft tissue) and PEEK surface is weak, leading to low osseointegration and an increased risk of inflammation. The situation could be improved by modifying PEEK surface. Surfaces with good hydrophilicity and proper microtopography would promote cellular adhesion and proliferation. This work presented a two-step surface modification method to achieve the effect. Polyacrylic acid (PAA) chains were grafted on PEEK surface by UV irradiation. Then, ethylenediamine (EDA) was added to introduce amino groups and promote the cross-linking of PAA chains. Furthermore, a mathematical model was built to describe and regulate the surface topography growth process semi-quantitatively. The model fits experimental data quite well (adjusted R2 = 0.779). Results showed that the modified PEEK surface obtained superhydrophilicity. It significantly improved the adhesion and proliferation of BMSCs and MFBs by activating the FAK pathway and Rho family GTPase. The cellular affinity performed better when the surface topography was in network structure with holes in about 25 μm depth and 20–50 μm diameter. Good hydrophilicity seems necessary for the FAK pathway activation, but simply improving surface hydrophilicity might not be enough for cellular affinity improvement. Surface topography at micron scale should be a more important cue. This simple surface modification method could be contributed to further study of cell-microtopography interaction and have potential applications in clinical PEEK orthopedic implants.

Characterization of an ablative fractional CO2 laser-induced wound-healing model based on in vitro 3D reconstructed skin.

This study describes the development and characterization of a novel in vitro wound-healing model based on a full-thickness reconstructed skin by exposing the tissue to fractional ablative laser treatment. A 3D full-thickness skin model was fabricated and treated with fractional ablative CO2 laser. Wound-healing process was characterized by HE staining, noninvasive OCT imaging, immunostaining, as well as transepidermal water loss measurement. Cytokines and proteins involved in the inflammatory and dermal remodeling process were studied by ELISA and protein array assays. Fractional ablative CO2 treatment induced a wound zone of 9 mm in diameter, containing 56 micro-wounds with 200 μm diameter and 500-700 μm in depth on reconstructed full-thickness skin model. HE staining revealed a typical wound morphology and healing process with migration of keratinocytes, formation and extrusion of necrotic tissue, and cell inclusion in dermis, which correlates with clinical observations. Based on OCT and TEWL measurements, the re-epithelialization took place over 2 days. Laser-triggered keratinocytes proliferation and differentiation were demonstrated by activated Ki67 and Filaggrin expression respectively. Injury-invoked cytokine ICAM-1 showed instant upregulation on Day 1. Decreased epidermis thickness and depression of IGFBP-2 protein level synergistically indicated the unavoidable thermal side effects from laser treatment. Downregulated DKK-1 protein level and upregulation of α-SMA together implicated the risk of potential fibrosis post-laser treatment. This in vitro laser wounded reconstructed skin model captured the key events of wound-healing process, could be used to investigate the mechanisms of wound-healing triggered by a commonly used beauty procedure, and also provides a valuable tool for evaluating the efficacy of novel actives for the post-procedure application.

Three-in-One Detector by 3D Printing: Simultaneous Contactless Conductivity, Ultraviolet Absorbance, and Laser-Induced Fluorescence Measurements for Capillary Electrophoresis.

We describe a 3-in-1 detector for simultaneous contactless conductivity (C4D), ultraviolet absorbance (UV-AD), and laser-induced fluorescence (LIF) measurements on a single detection point for capillary electrophoresis (CE). A key component of the detector was a rectangular detector head that was assembled with four 3D-printed parts. Two parts covering the detector head to function as a Faraday cage were fused deposition modeling printed using an electrically conductive material. The other two parts in between the conductive parts were stereolithography (SLA) printed with high-resolution (50 μm) constructions on the surface. After assembling the two SLA printed parts, several cavities were built with the surface constructions. Two electrodes and a Faraday shield for C4D were cast by injecting molten Wood's metal into the cavities. For UV-AD, a slit (100 μm width) was created by putting together two grooves (50 μm depth) on the surface of the SLA printed parts. A 255 nm UV-LED was used as the light source. The effective path length and stray light for a 50 μm id capillary were 39 μm and 13%, which were superior to those of other reported 3D-printed AD detectors. Confocal LIF detection was conducted by using an objective lens to focus the laser on the capillary via a through-hole. The detector was used to detect model analytes, including inorganic and organic ions, and fluorescein isothiocyanate labeled amino acids in a signal-run CE separation. In detecting fluorescein, LODs were 1.3 μM (C4D), 2.0 μM (UV-AD), and 1 nM (LIF). The calibration ranges covered from 0.01 μM to 500 μM.

Inducement of cell tissue arrangement and vascular smooth muscle differentiation using micro-grooved concaves

Vascular smooth muscle cells (VSMCs) in the normal vascular wall regulate vascular contraction and dilation due to their contractility. But they change their phenotype from contractile to synthetic state and actively remodel the vascular wall in pathological conditions. Findings on the phenotypic change mechanism of VSMCs have been reported by many in vitro studies, however, mechanical environments in vivo vascular wall are quite different from those of in vitro culture condition: VSMCs in vivo exhibit an elongated shape and form a tissue that aligns with the circumferential direction of the walls, while VSMCs in vitro spread randomly and form irregular shapes during cultivation on flat culture dishes, and dedifferentiate into synthetic phenotype. To clarify the mechanisms of the phenotypic changes in VSMCs, it is essential to develop a cell culture model that consider the mechanical environment of in vivo vascular wall. Here, we fabricated the polydimethylsiloxane (PDMS)-based micro-grooved substrate with 5 or 20 μm of groove width and 5 μm of groove depth to induce cell elongation and alignment observed in vivo. We established the coating method with cell adhesion protein only on the surface of groove concaves, and found that VSMCs adhering into the concaves formed uniform cell tissue and allowed remarkable elongation. In particular, the micro grooves with 5 μm groove width and depth facilitated a significant nuclear deformation and volume reduction of the nucleus due to a lateral compression by the side wall of the groove concaves that is relatively similar to a sandwich-like arrangement of in vivo elastic lamellae, resulting in the cell proliferation inhibition and VSMC differentiation. These results indicate that our cell culture model with the micro-grooved substrates can be useful to study the mechanisms of the phenotypic changes in VSMCs under consideration of in vivo vascular mechanical environment.

Implementation of structurally pre-stressed piezo actuator based active vibration isolation system for micro milling

This paper presents the implementation of a structurally pre-stressed piezo actuator based active vibration isolation system incorporated with tool based micromachining setup for analyzing and comparing the milled pocket depth surface before and after isolation. A carbide micro end mill tool with 1 mm diameter and 4 flutes has been used for carrying out pocket milling experiments with and without vibrations on copper work pieces having 3 mm thickness. The machining parameters selected were spindle speed of 16,000 rev/min with 64 mm/min feed rate and 50 μm depth of cut. Two sets of pocket milling experiments were carried out using the proposed vibration isolation setup one with vibration and the other without vibration. In the first set of experiment, the source actuator was actuated for generating vibrations during pocket milling whereas in the second set of experiment, both source and isolator actuators were actuated for nullifying the vibrations generated during pocket milling. The macroscopic lens output images of the pocket depth surfaces before and after isolation were then compared corresponding to various actuation voltages at different frequencies using the proposed vibration isolation setup. Based on the macroscopic lens output images it was observed that the milled pocket depth surface obtained by actuating only source actuator showed distinct rings wherein the ring count matched with frequency and feed rate that has been provided during machining. However, the milled pocket obtained by actuating both source and isolator actuators resulted in surface characteristics with less pronounced rings similar to that of regular machining. Also the rings that has been formed showed more defined edges with the increase in actuator voltage indicating that these distinct patterns were caused due to the vibrations generated by the source actuator alone and not other factors.

Surface integrity of SLM manufactured meso-size gears in laser shock peening without coating

The present work aimed to improve the surface integrity of the selective laser melting manufacturing (SLM) manufactured 10 mm sized meso gears using the unconventional Laser Peening without Coating (LPwC) technique. To accomplish LPwC on meso gears low energy in the range of 200 mJ to 1000 mJ at 10 Hz were applied underwater in the gear root and fillet gap generating significant surface compressive residual stresses (from +73 MPa to −298 MPa). Besides the surface, residual stresses, average surface roughness (Ra), arithmetical mean height (Sa) and parameters of the material ratio curve were also considered as a part of the study. Notable improvement was achieved in Sa, as it improved over 50 % while considerable improvement in Ra and the material ratio curve parameters has also been observed. Scanning electron microscopy confirmed that the void and porosity seen in the unpeened gears were filled. The electron backscatter diffraction analysis confirms the grain refinement in LPwC region up to 100 μm depth. It was observed that multifold dislocation occurs within the grain, and dislocation generates sub-grain. The sub-grain formation substantially inhibits further nucleation and crack propagation owing to an increase in grain boundary density. Such improvement justifies the set objectives, and the outcome of this research will pave a foundation to improve the overall performance of the micro/meso parts using the LPwC process.

Phase Changes in the Surface Layer of Stainless Steel Annealed at a Temperature of 550 °C.

Stainless steels have the advantage of forming a protective surface layer to prevent corrosion. This layer results from phase and structural changes on the steel surface. Stainless steel samples (1.4404, 316L), whose alloying elements include Cr, Ni, Mo, and Mn, were subjected to the study of the surface layer. Prism-shaped samples (25 × 25 × 3) mm3 were made from CL20ES stainless steel powder, using selective laser melting. After sandblasting with corundum powder and annealing at 550 °C for different periods of time (2, 4, 8, 16, 32, 64, 128 h), samples were studied by conversion X-ray Mössbauer spectroscopy (CXMS), conversion electron Mössbauer spectroscopy (CEMS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The main topics of the research were surface morphology and elemental and phase composition. The annealing of stainless steel samples resulted in a new surface layer comprising leaf-shaped crystals made of chromium oxide. The crystals grew, and their number increased as annealing time was extended. The amount of chromium increased in the surface layer at the expense of iron and nickel, and the longer the annealing time was set, the more chromium was observed in the surface layer. Iron compounds (BCC iron, mixed Fe-Cr oxide) were found in the surface layer, in addition to chromium oxide. BCC iron appeared only after annealing for at least 4 h, which is the initial time of austenitic-ferritic transformation. Mixed Fe-Cr oxide was observed in all annealed samples. All phase changes were observed in the surface layer at approximately 0.6 µm depth.