Layer Thickness Research Articles

Layer thickness dependent strengthening and strain delocalization mechanism in CuTa nanopillars with nanoscale amorphous/amorphous interfaces

Introducing amorphous/amorphous interfaces in metallic glasses offers an effective strategy to address the strength-plasticity trade-off by suppressing the strain localization. Elucidating the underlying size-dependent deformation mechanisms is crucial for designing strong and ductile amorphous nanolayered materials. Herein, molecular dynamics simulations are conducted to investigate the deformation behaviors of Cu70Ta30/Cu30Ta70 amorphous/amorphous nanopillars (AANPs) under compression, focusing on the intrinsic size effect of amorphous layer thickness. The results indicate a critical layer thickness of approximately 6.7 nm, below which an inverse Hall-Petch relation occurs. This transition is attributed to a shift in dominant deformation mode from the necking and localized deformation to the relatively homogeneous deformation as the layer thicknesses decreases. These A/A interfaces could be considered as a third medium phase in nanolaminates, especially when the thickness is very low, down to only a few of nm. The mechanical properties and deformation behavior of transitional interface phase lie between those of soft and hard phases, balancing heterogeneous deformation between hard and soft layers and resulting in homogeneous flow deformation of specimen with thinner amorphous layer. These findings provide insight for designing ductile amorphous materials through architecting nanoscale amorphous/amorphous interfaces.

Optimizing Mechanical Properties of Recycled 3D-Printed PLA Parts for Sustainable Packaging Solutions Using Experimental Analysis and Machine Learning

Abstract: Increasing environmental concerns and the need for sustainable materials have driven a focus towards the utilization of recycled polylactic acid (PLA) in additive manufacturing as PLA offers advantages over other thermoplastics, including biodegradability, ease of processing, and a lower environmental impact during production. This study explores the optimization of the mechanical properties of recycled PLA parts through a combination of experimental and machine learning approaches. A series of experiments were conducted to investigate the impact of various processing parameters, such as layer thickness and infill density, as well as annealing conditions, on the mechanical properties of recycled PLA parts. Machine learning algorithms have proven the possibility to predict tensile behavior with an average error of 6.059%. The results demonstrate that specific combinations of processing parameters and post-treatment annealing differently improve the mechanical properties (with 7.31% in ultimate tensile strength (UTS), 0.28% in Young’s modulus, and 3.68% in elongation) and crystallinity (with 22.33%) of recycled PLA according to XRD analysis, making it a viable alternative to virgin PLA in various applications such as sustainable packaging solutions, including biodegradable containers, clamshell packaging, and protective inserts. The optimized recycled PLA parts exhibited mechanical properties and crystallinity levels comparable to those of their virgin counterparts, highlighting their potential for reducing environmental impact and saving costs. For both as-built and annealed samples, the optimal settings for achieving high composite desirability involved a 0.2 mm layer thickness, with 75% infill for the as-built samples and 100% infill for the annealed samples. This study provides a comprehensive framework for optimizing recycled PLA in additive manufacturing, contributing to the advancement of sustainable material engineering and the circular economy.

Enhancing the printing quality and mechanical properties of 3D-printed cement composites with date syrup-based graphene coated sand hybrid

This study addresses the influence of adding date syrup-based graphene-coated sand hybrid (D-GSH) as a partial substitute for sand in 3D-printed cement composites, exploring its potential to enhance both fresh and hardened properties. Incorporating 0.5% D-GSH significantly improved the printing quality compared to the control mix, as evidenced by the reduction in the variance in thickness of double layers from 38% to 28%. In addition, buildability was enhanced by 116% with an open time of 27 min. The rheological properties showed reduced viscosity and shear stress against shear rates, thereby enhancing flowability. Moreover, the addition of 0.5% D-GSH increased the compressive strength, flexural strength, and elastic modulus by 62%, 118%, and 40%, respectively, after 28 days of curing, compared with a mix containing silica fume. Microstructural analysis revealed that D-GSH effectively fills gaps, bridges cracks, and bonds with the hydration products of cement matrix, thus improving the mechanical properties.

The New Interpretation of the Photothermal Spectra of CdTe Samples After Different Surface Treatments

This article presents new research on the surface condition of bulk crystal samples after the following stages of surface treatment: grinding, polishing, and etching. Furthermore, it shows how the surface condition affects the photothermal signal’s spectral amplitude and phase characteristics (PZE). A new theoretical interpretation of the photothermal spectra of CdTe samples after different surface treatments is proposed. We demonstrate that the piezoelectric method is susceptible to the surface condition, and it allows for the estimation of the thickness of surface-damaged layers of samples, and for the analysis of their thermal parameters. The roughness of surfaces obtained from the AFM pictures is estimated and compared to the photothermal results.

Plume discharge strategies for artificial downwelling in stagnant linear stratified environments

Artificial downwelling (AD) potentially mitigates oceanic hypoxia by utilizing pipelines or other artificial structures to transport oxygen-rich seawater from the surface to greater depths. However, limited research has focused on determining the discharge parameters for surface waters, which can often rise back to the upper layer or scour the seafloor sediments if discharged at inappropriate parameters. To address this issue, this study proposes a theory for determining the optimal discharge height of oxygen-rich plumes based on their hydrodynamic characteristics. Laboratory experiments were conducted in a linearly stratified flume to validate the theoretical model. The results show that the proposed theory can effectively calculate the important parameters of the AD jet, including the maximum depth, spread depth, and thickness of the intrusion layer. Furthermore, based on the theory of optimal discharge height, this paper provides plume discharge strategies for a typical AD device in field applications to regulate the operating parameters of the AD equipment, which can effectively keep the oxygen-enriched seawater completely in the hypoxic layer, avoid sediment-suspended secondary pollution, and maximize the oxygenation efficiency in different sea conditions.

The first implementation of repetitive translucent photocatalyst structures to enhance the energy efficiency of photoelectrochemical applications

Improving the energy efficiency of photoelectrochemical cells (PECs) yields an important tool to address the current carbon circularization problems. However, the efficiency of these cells was held back due to mass, electron, and photon transfer limitations. In this work, the use of translucent repetitive thin catalyst structures within the same light path was introduced in PEC cells, allowing to absorb more irradiated light due to an increased total catalyst loading, while still benefiting from the good transfer phenomena of thin catalyst films. The results unequivocally showed that thinner catalyst layers in a repetitive structure could produce more than seven times the amount of photocurrent than a single thick catalyst layer while utilising less catalyst material. This work proved that not only the nature of the photocatalyst but also the use of repetitive thin translucent structures should be considered when designing a photoelectrochemical reactor to maximise energy efficiency.

Formation Mechanism and Influence of Heating Temperature and Holding Time on Surface Decarburization of 40Cr Steel during Hot Forging Process

During hot forging and heat treatment of 40Cr steel, a decarburized layer emerges on the surface, affecting fatigue life. However, the decarbonization of 40Cr steel has not been reported so far. The experiment mainly studies the effect of holding temperature and time on decarbonization. The 40Cr steel undergoes complete decarburization at 700 and 750 °C while experiencing a combination of complete and partial decarburization at 800 °C, transitioning solely to partial decarburization above 850 °C. The decarburized layer thickness measured at 750 and 800 °C is 140 and 200 μm respectively; the total decarburization layer thickness exhibits an exponential increase with the temperature. The thickness of the completely decarburized layer, influenced by varying factors across temperature ranges, initially rises and subsequently declines with the escalation of temperature, reaching its maximum at ≈750 °C. The maximum value of complete decarburization is 146 μm. Simultaneously, the total depth of decarburization is found to be proportional to the square root of time, adhering to Fick's second law. The fitting result is expressed as x = 165.47t1/2 at 750 °C. Notably, the existence of the decarburized layer results in a decrement in both the tensile strength and yield strength of the component, albeit accompanied by a marginal improvement in ductility.

Localization and Delocalization During Seismic Slip

AbstractThe thickness of a seismic slip layer controls style and rate of rupture propagation, frictional heating, weakening, and energy budget of earthquakes. Slip layer thickness changes dynamically with feedbacks between temperature rise, roughness, damage, and fluid pressurization. Natural faults have complex slip histories, ambiguating which layer thicknesses represent a record of seismic slip. The thickness of proven paleoseismic slip layers in nature are 1 mm–1 cm. Thicker slip layers do not get hot enough to retain coseismic frictional temperature anomalies and thereby prevent the detection of big earthquakes using current methods. We suggest that delocalization—the spontaneous increase in slipping layer thickness during slip—plays a role in coseismic healing and cause biases in the rock record of earthquakes. Recognizing the importance of feedbacks affecting slipping layer thickness is critical to understanding strength and stress variations during earthquakes and to correctly interpreting earthquake source parameters from exhumed faults.

A computational study of AlScN-based ferroelectric tunnel junction

Ferroelectric (FE) AlScN materials have been experimentally explored for memory and neuromorphic computing device applications. Here a computational study is performed to simulate the device characteristics and assess the performance potential of a ferroelectric tunnel junction (FTJ) based on AlScN. We parameterize an efficient k⋅p Hamiltonian from the complex band structure of AlScN from ab initio density-functional theory calculations to enable efficient quantum transport simulations of the FTJ device. Using a metal–FE–graphene structure enhances the barrier height modulation and the tunneling electroresistance (TER) ratio, compared to a metal–FE–semiconductor FTJ device structure. The barrier height modulation between ON and OFF states can reach ∼ 0.7eV with a FE polarization of 25 μC/cm2. Reducing the AlScN tunnel layer thickness is important for increasing the device ON current and reducing the read latency. The results indicate the importance of contact designs and FE layer thickness in the design of AlScN-based FTJ devices, and highlight the potential of AlScN FTJ for future memory device technology applications.

A new approach to reducing slagging based on laser modification of heating surfaces: Field tests and mathematical modeling

The article presents the results of field tests of a new approach to reducing slagging of a boiler furnace. The approach involves laser radiation treatment of metal surfaces to obtain a coating with a high degree of passivation and a special texture (microchannel or anisotropic). Due to the texture and formation of the oxide layer, the resistance of the heating surfaces to adhesion of hydrocarbon fuel combustion products, including slag, is increased. Field tests were conducted over 60 days in an operating boiler burning brown coal, the combustion products of which form solid ferrous and sulfate–calcium deposits. The study used scanning microscopy, X-ray fluorescence spectral analysis and mass analysis of slag deposits. The results showed that steel samples with the Microchannels texture formed by laser radiation are characterized by increased resistance to adhesion of brown coal slag. Using the original software code, a mathematical model has been developed to describe the slag deposition process on heat exchange surfaces modified by laser radiation. A numerical simulation was performed under conditions of dynamic change in the thickness of the slag layer on the surface of the heat exchanger pipe to assess the heat transfer characteristics in the “hot flue gases – slag layer – pipe wall – liquid coolant” system. The results showed that the steel surface modified by laser radiation, compared to the untreated surface, is resistant to slagging at the initial stages of interaction with molten ash from different types of fuel. The use of laser-modified surfaces in boiler furnaces will increase the intervals between procedures for cleaning the furnace from slag by up to 64 %. Also, due to the increase in absorbed heat per unit area by 1.6–2.2 times, the use of laser-modified surfaces increases the efficiency of energy production.

Optical coherence tomography as an adjunct tool to assess response to treatment in cerebral venous thrombosis: A prospective longitudinal study

IntroductionPapilledema, due to raised intracranial pressure (ICP) is commonly seen in cerebral venous thrombosis (CVT) and can have serious complications. This study reports the quantitative assessment of papilledema through the measurement of the Peripapillary choroidal thickness (ChT) and Peripapillary retinal nerve fiber layer thickness (RNFLT) using optical coherence tomography (OCT). MethodsWe conducted a prospective longitudinal study of 30 subjects with CVT. Age and gender-matched healthy individuals were included as controls. In addition to history, RNFLT and ChT measurements were obtained at presentation, discharge, and follow-up two months after discharge. ResultsThe mean age of the patients was 32.8 ± 9.9 years. Papilledema was seen in 14 (46.66 %) patients. The peripapillary RNFLT and ChT were significantly greater in patients with CVT [RNFLT 121.5 μm (105,169) in the right eye (RE), 120 μm (103,148) in the left eye (LE); ChT 159 μm (143,171) in the RE, 155.5 μm (140,166) in the LE] as compared to controls [RNFLT 106.5 μm (102,111) in RE, 103.5 μm (98,107) in LE; ChT 130 μm (119,134) in RE, 124 μm (119,136) in LE]. There was a significant decrease in the peripapillary RNFLT and ChT in patients with papilledema at follow-up after two months in all segments in both eyes. ConclusionIn patients with CVT, RNFLT, and ChT reduced significantly with treatment. OCT is potentially useful to objectively assess response to treatment in patients with CVT.

Extracting full information from OCT scans-signs of early age-related macular degeneration within inner retinal layers by local neighbourhood statistics. Part II: Results.

Associations between the occurrence of early age related macular degeneration (AMD) and alterations in retinal layer thicknesses have been reported, based on classical processing of optical coherence tomography (OCT) data by noise removal and subsequent image segmentation. However, speckle noise within OCT data itself bears a substantial part of the total information. For this reason, we designed an omics-type approach for full exploitation of OCT data, which was able to identify signs of early AMD throughout the retina as a whole. A nested case-control study was designed with 200 early AMD cases and 200 healthy controls. For each participant, within a randomly selected OCT scan and a randomly selected column therein, manual grading was performed for 26 retinal feature positions. At every position, a total of 3792 descriptors were computed, based on nonlinear transformations of OCT data, first-order neighbourhood statistics and Haralick features. Equivalence and differences between cases and controls were tested for each descriptor at every graded position. Results of multiple testing were expressed in terms of false and true discovery rates controlled by the Benjamini-Yekutieli procedure. In terms of the amount and disparity of true discoveries, overall non-equivalence was found for early AMD and healthy groups. Strong difference signals were observed at the internal limiting membrane and two central retinal positions, particularly for descriptors emphasising speckle noise. Between the retinae of healthy controls and early AMD patients, significant differences were observed at the level of local neighbourhood statistics within OCT data. Thus, independent evidence was obtained for AMD affecting not only the outer retinal layers but the retina as a whole, even in the early stages of the disease. Within OCT data, both cartoon and speckle bear essential parts of the total information. We pursued a constructive, completely documented, traceable and repeatable approach without invoking artificial intelligence methods.

Optimizing the Material Extrusion Process for Investment Casting Mould Production

This study investigates the optimization of the Material Extrusion (MEX) process for producing polylactic acid (PLA) patterns used in investment casting moulds, specifically targeting the casting of non-ferrous alloys such as brass. Key MEX process parameters—layer thickness, wall thickness, infill density, and post-processing with dichloromethane vapour for surface enhancement—were systematically analyzed for their impact on mould quality. Results indicate that an optimized combination of MEX parameters yields moulds with high dimensional accuracy, low surface roughness, and minimal pattern residue within the mould cavity. These optimized moulds were subsequently used in brass casting, with the final cast parts evaluated for dimensional precision and surface finish. The study concludes that PLA patterns manufactured via optimized MEX parameters provide a precise, cost-effective, and easy-to-implement solution for industry applications. Additionally, this process is environmentally friendly and presents clear advantages over other pattern-making methods, offering a sustainable alternative for producing complex metal parts with reduced environmental impact. The findings underscore the significant role of post-processing in enhancing mould quality and, consequently, the quality of the cast parts.

Deciphering the mechanical strengthening mechanism: Soft metal doping in ceramic matrices-A case study of TiN-Ag films

Soft metals have been widely added into ceramic-based films for fully meeting the demanding requirements of green tribological applications. However, the resulting considerable increase of the mechanical strength by adding a soft metal below 5 at.%, which reversed the rule-of-mixture, was still not fully revealed. In this paper, a case study of TiN-Ag films was carried out to investigate the strengthening mechanism induced by adding soft metal in TiN-Ag composite/multilayered films deposited by magnetron sputtering. The results showed that dual-phases of fcc-TiN and fcc-Ag co-existed in the composite films with the Ag particles embedded in the matrix. In some areas of the Ag particles, with a size below 4 nm, epitaxial growth with the TiN template was detected, which obliged the lattice to be distorted and shrunken. Consequently, both hardness and elastic modulus were enhanced from 21 and 236 GPa, for the reference TiN film, to 26 and 323 GPa for the TiN-Ag composite film with 2.4 at.% Ag. The possibility of having the epitaxial growth of Ag within TiN were also confirmed by designing a TiN/Ag multilayered film with an Ag layer thickness of ∼3 nm.

Extracting full information from OCT scans-signs of early age-related macular degeneration within inner retinal layers by local neighbourhood statistics. Part I: Methodology.

Associations between the occurrence of early age-related macular degeneration (AMD) and alterations in retinal layer thicknesses have been reported based on classical processing of optical coherence tomography (OCT) data by noise removal and subsequent image segmentation. However, speckle noise within OCT data itself bears a substantial part of the total information. For this reason, an omics-type approach was designed for full exploitation of OCT data, which was able to identify signs of early AMD throughout the retina as a whole. A nested case-control study was designed with 200 early AMD cases and 200 healthy controls. For every participant, within a randomly selected OCT scan and a randomly selected column therein, manual grading was performed for 26 retinal feature positions. At each position, a total of 3792 descriptors were computed, based on nonlinear transformations of OCT data, first-order neighbourhood statistics and Haralick features. Equivalence and differences between cases and controls were tested for every descriptor at each graded position. Results of multiple testing were expressed in terms of false and true discovery rates controlled by the Benjamini-Yekutieli procedure. In terms of the amount and disparity of true discoveries, overall non-equivalence of early AMD and healthy groups was found. Strong difference signals were observed at the internal limiting membrane and two central retinal positions, particularly for descriptors emphasising speckle noise. Between retinae of healthy controls and early AMD patients, significant differences were observed at the level of local neighbourhood statistics within the OCT data. Thus, independent evidence was obtained for AMD affecting not only the outer retinal layers but also the retina as a whole, even in the early stages of the disease. Within OCT data, both cartoons and speckle bear essential parts of total information. A constructive, completely documented, traceable and repeatable approach was pursued without invoking artificial intelligence methods.

How smectic-A and smectic-C liquid crystals resolve confinement-induced frustration in spherical shells.

The layered structure of smectic liquid crystals cannot develop unobstructed when confined to spherical shells with layers extending in the radial direction, since the available cross section area increases from the inside to the outside of the shell yet the number and thickness of layers must be constant. For smectic-A (SmA) liquid crystals, with the layer normal m parallel to the director n, the frustration breaks up the texture into spherical lune domains with twist deformations of alternating sense, overlaid with a herringbone-like secondary modulation and mediated via localized bend regions where the boundary conditions are violated. The SmC phase has more degrees of freedom to resolve the frustration thanks to its non-zero tilt angle τ between n and m, but its response to tangential shell confinement was never studied. We show experimentally and theoretically that the lunes in shells undergoing a SmA-SmC transition become twice as wide and half as many and they lose the secondary modulation, adopting a configuration with no layer twist but uniform layer bend if τ reaches a large enough value. Our study expands our understanding of how smectics respond to spherical confinement and it opens new soft matter research opportunities, given the rich diversity of phases with SmC-like symmetry, including chiral and spontaneously polarized phases.

Preparation methodology for the microstructural characterization of diffusion layers in a titanium/steel composite

Abstract Hot pressing of titanium and carbon steel leads to the formation of a diffusion layer at the interface. Depending on the carbon content of the steel used, it either exclusively contains TiC or additional other phases. In the case of steel with a medium carbon concentration (0.67 wt.% C), a pure TiC layer forms. A preparation methodology was developed to make statements about functional and microstructural properties of the respective layers such as layer thickness, porosity, or grain size. However, apart from the diffusion layer’s microstructure, it also reveals the microstructure of the two base substrates. A comparison based on electron backscatter diffraction (EBSD) examinations yields similar results in terms of microstructure. A micrograph analysis based on the new preparation methodology also allows confirming the element distribution measurement by glow discharge optical emission spectroscopy (GDOES). The methodology therefore provides a way of quickly and reliably controlling the layer formation during the hot pressing process of titanium and carbon steel.

Effects of plunge depth on the intermetallic compounds and mechanical properties of Al/Cu joints fabricated by micro friction stir welding

0.5 mm thick 6061-T4 Al and T2 Cu were butt-joined by micro friction stir welding (μFSW) at different plunge depths (D1=0.04 mm, D2=0.08 mm, D3=0.12 mm), the temperature distribution was studied by numerical simulation. The results showed that the control of thickness reduction and intermetallic compounds (IMCs) are the keys to obtain a joint with excellent performance. When the plunge depth is 0.04 mm, the peak temperature and residence time are both minimum, the reduction rate of the weld thickness and the proportion of agglomerated IMCs are minimum, which are 4 % and 9 % respectively, the average thickness of IMCs layer is only 112 nm. The tensile strength is the largest (94.1 % of Al base material), the joint fails in the heat-affected zone (HAZ) of Al side due to lowest hardness. Agglomerated IMCs (including Al2Cu and Al4Cu9) are formed in the upper nugget zone (NZ), and IMCs layer (including Al2Cu, AlCu and Al4Cu9) are formed in the lower NZ. As the plunge depth increases, the proportion of agglomerated IMCs and the thickness of IMCs layer increases. So, preferential sites of crack nucleation appear at the agglomerated IMCs. Subsequently, crack growth is exacerbated by the inherent brittleness of AlCu, resulting in ductile fracture in the upper NZ and cleavage fracture in the lower NZ.

Predictive risk scores for visual prognosis after photodynamic therapy for central serous chorioretinopathy.

To comprehensively evaluate baseline characteristics of patients with central serous chorioretinopathy (CSC) and develop predictive risk scores to identify visual prognosis. This single-institute, retrospective cohort study included 144 eyes of 144 patients with CSC who underwent photodynamic therapy and achieved serous retinal detachment resolution. We developed and assessed the performance of several risk scores for best-corrected visual acuity (BCVA) outcomes six months post-treatment: i) BCVA improvement (≤-1.0 logMAR), and ii) BCVA deterioration (≥+ 1.0 logMAR). The BCVA improvement models used photoreceptor outer segment thickness, loss of photoreceptor outer segment, and neurosensory retinal thickness (NSRT), while the BCVA deterioration models included outer nuclear layer thickness and NSRT. The BCVA improvement models demonstrated a corrected area under the curve (AUC) of 0.786 (95% confidence interval [CI]: 0.699-0.864), with 80.4% sensitivity, and 71.2% specificity. The BCVA deterioration models achieved a corrected AUC of 0.864 (95% CI: 0.742-0.958), with 85.7% sensitivity, and 83.5% specificity. The predictive models for CSC exhibited favorable performance in predicting individual visual prognoses. A thinner outer nuclear layer may be associated with BCVA deterioration, whereas preservation of the photoreceptor outer segment may be correlated with BCVA improvement. WHAT IS KNOWN : Pre-treatment best-corrected visual acuity, thickness of each sensory retinal layer, time fromonset to treatment, and macular atrophy were each found to be associated with visualprognosis for patients with central serous chorioretinopathy (CSC). The current study comprehensively assessed potential prognostic factors and precisely identified individual likelihood of visual prognosis. The study found that different regions of the sensory retina were associated with either worsening or improving visual acuity. Accurately predicting visual outcomes after photodynamic therapy for CSC would help healthcare providers create personalized treatment plans and enable patients to make informed decisions about their treatment based on their expected visual results.

Electrochemical synthesis and carbon doping of nanostructured iron fluorides from the selection of metal current collectors in lithium-ion batteries

The use of materials that rely on conversion reactions as electrodes in lithium-ion batteries is extensively investigated due to their potential for enhanced capacity compared to traditional electrode materials. Iron fluorides, in particular, present a promising alternative in terms of specific capacity. However, these materials often face challenges related to low intrinsic conductivity. This issue is typically addressed in the literature by doping the active material with carbon particles and reducing the particle size of the active material. This study explores the feasibility of directly integrating conductive carbon from the substrate into the fluoride-based active material during the synthesis process. The synthesis employs a simple anodic method conducted directly on the chosen metallic substrate, which then functions as the current collector in the devices. This approach simplifies the synthesis process, reduces processing time, and eliminates the need for additives and binders at the conventional active material-current collector interface. Two FeF3 layers were electrochemically synthesized on steel substrates with different carbon contents. These layers were evaluated as cathode-active materials for rechargeable lithium-ion batteries. The influence of carbon on the conductivity of the conversion layer was assessed using Electrochemical Impedance Spectroscopy (EIS) with a model based on conductive porous electrodes. Morphological and thickness analyses of the layers showed a strong correlation between increased pore size and layer thickness and the carbon content in the metallic substrate. The optimal performance was observed with the layer on the substrate with higher carbon content. The electrochemical performance of the active material was further evaluated using electrochemical impedance spectroscopy and galvanostatic tests in pouch cells. The conversion layers derived from carbon steel exhibited reduced resistivity and enhanced specific capacitance and cyclability compared to layers formed on pure iron.