Substrate Surface Research Articles

Wettability and lubricity of borosilicate glass to H13 steel and TZM alloy at 900 ℃

Lubrication can reduce friction, wear and oxidation of mechanical moving parts to achieve the purpose of maintaining the working stability, high efficiency and long life of the device. Melt lubricants have excellent anti-friction, anti-wear and anti-oxidation properties and are used in the hot metal forming process for steels. TZM alloy is a promising forging die to replace H13 steel in the hot metal forming process. In this study, the high temperature wettability and high temperature tribological performance of borosilicate glass on H13 steel and TZM alloy were studied. The results suggest that the glass has good wettability on the hot H13 and promising lubricating with coefficient of friction of 0.06 at 900 ℃. However, the same glass shows poor wettability and lubricating for the TZM alloy. It indicates that high temperature wettability is also one of the key parameters that influence the tribological performance, rather than the well- known high temperature viscosity. Surface and interface characterization indicates that the Fe2O3 induced enrichment of the network modifier Na+ near the substrate surface, the dense and shearable low-oxygen permeable glass film led to the excellent oxidation resistance and lubrication properties in the friction system of H13 steel (pin). The erosion of the borosilicate melt by MoO3 leads to poor adhesion, the phase separation behavior of the glass polymerization zone and the molybdate depolymerization zone. The melting state of the low melting point species make the TZM alloy (pin) friction system exhibit complex friction behavior. This work will provide some guideline for the structure and wettability control for promising high temperature lubricating.

Formation of planar tellurium nanowire networks on substrate surfaces

Formation of planar tellurium nanowire networks on substrate surfaces

Effects of various laser cladding materials on the tribological properties of damaged wheel treads under various temperatures and humidities

Laser cladding technology is used to repair damaged wheels and plays an important role in prolonging their service life. Additionally, the temperature and humidity of the environment have important effects on the operation of the repaired wheels. In this study, widely used 316L and 420 stainless steel alloy powders were selected as cladding materials, and three environmental conditions (25℃-RH60%, 50℃-RH60%, and 50℃-RH90%) were set according to changes in temperature and humidity in different areas of China. Friction and wear tests were performed under different temperature and humidity conditions on a damaged wheel after local repair by laser cladding. The results clearly show that there are demarcated equiaxed and directional columnar grain regions in the 316L cladding, whereas there are uniform plate and strip martensite structures in the 420 cladding, which are well formed by the metallurgical bonding of the wheel substrate. From 25℃-RH60% to 50℃-RH90%, the friction coefficient, plastic deformation thickness, and wear rate of the wheel–rail tended to decrease. However, the wear rates of the 420-coated wheels increased as the environmental conditions increased from 25°C-RH60% and 50°C-RH60%, and the wear rates of the corresponding rail samples remained high. The main forms of surface damage are fatigue cracks and material spalling. With increasing temperature and humidity, the damage to the bonding zone between the cladding and substrate surface decreases. The damage to the cladding profile is caused mainly by cracks at different angles and spalling pits. In the 25°C-RH60% environment, cracks initiate in the profile of the bonding zone and propagate along the plastic rheological line to the interior of the material. In comparison, the locally repaired wheel with 316L stainless steel alloy powder as the laser cladding material has better tribological properties and is more suitable for repairing damaged wheels.

Molecular Dynamics Simulation of the Hydration Lubrication Mechanism of CS-MPC/CS-ChS NPs.

Osteoarthritis (OA) remains a significant clinical challenge, with current treatments like sodium hyaluronate injections offering limited efficacy due to suboptimal lubrication and rapid degradation. In this study, we explored an advanced solution to these issues by investigating the colubrication mechanism of a composite biolubricant consisting of 2-methacryloyloxyethyl phosphorylcholine-modified chitosan (CS-MPC) and chondroitin sulfate-modified chitosan nanoparticles (CS-ChS NPs) using molecular dynamics (MD) simulations. Results show that the composite lubricant outperforms individual CS-MPC or CS-ChS NPs, exhibiting a lower coefficient of friction (COF) and a superior load-bearing capacity. The CS-MPC/CS-ChS NP system maintains a consistent compression ratio of -0.5% under different external pressures and exhibited a low coefficient of friction (COF) of 0.041 across varying shear velocities. The sulfate groups on CS-ChS NPs interact with CS-MPC chains, stabilizing the system through electrostatic interactions and enabling the effective dispersion of normal stress, thereby protecting the substrate surface from wear. This enhanced performance is attributed to the formation of a multilayered hydration shell around the lubricant molecules. The hydration shells provide superior lubrication, contributing to the system's robustness under varying loads. These findings offer critical insights into designing high-performance biolubricants for joint protection, presenting a promising avenue for future OA treatments.

Enhancing fretting performance of 316L biomedical alloys through duplex TiN and DLC coatings

This study investigates the use of duplex coatings, TiN/DLC, on improving the tribological performance of stainless steel 316L which is commonly used for femoral stems in total hip replacement implants. A range of substrate pre-treatment variants (nitriding and polishing) were applied to investigate the impact on the mechanical and tribological performance of the coatings. The tribological performance of plasma Vapour Deposition (PVD) duplex coating variants were tested using a bespoke in-house fretting tribometer with an electrodynamic shaker utilising a ball-on-plate configuration. Fretting was replicated by applying micro-motion to the Ø12 Al2O3 ball relative to duplex coated SS 316L plates under a dead weight normal load. Un-polished substrates prior to the coating deposition led to improved tribological performance likely due to improve coating adhesion to the substrate surface. The hydrogenated DLC sample variants showed lower friction performance compared to hydrogen-free DLC variants most likely due to higher amounts of graphitisation.

Characterization of Additively Manufactured Titanium-Based Alloy with a Micro-Arc Oxidation Coating and Overlying Polyurethane Layer

Titanium alloys are widely used in the aerospace, automotive, chemical, and biomedical industries due to their excellent corrosion resistance, mechanical properties, and biocompatibility. However, the surface properties of titanium alloys are often insufficient to meet the increasingly complex requirements of certain applications. Therefore, enhancing the surface performance of titanium alloys in physiological environments has become a key focus of research. In this study, a porous oxide layer was generated on the surface of a titanium substrate through micro-arc oxidation (MAO). This layer served as an intermediate layer for a subsequently deposited polyurethane (PU) coating, providing a strong foundation for adhesion. The high porosity of the MAO layer not only facilitated the adhesion of the PU coating but also protected the titanium alloy, further enhancing its corrosion resistance. The surface microstructure after MAO treatment and the morphological changes after application of the PU coating were characterized using scanning electron microscopy. The PU layer uniformly covered the surface of the MAO layer, significantly improving the smoothness and uniformity of the surface. The increase in surface smoothness due to the PU coating on top of the MAO layer was verified through white light interferometry. Additionally, surface hydrophobicity was assessed through water contact angle measurements. The PU layer over the MAO coating significantly enhanced the hydrophobicity of the titanium alloy’s surface, which is crucial for reducing biofouling and improving the effectiveness of biomedical implants. Finally, electrochemical analysis was conducted to study the corrosion resistance of the titanium alloy after MAO and PU treatment. The titanium alloy with an MAO–PU composite coating exhibited the highest corrosion resistance. The findings revealed that the combination of the MAO layer and PU coating provides an excellent multifunctional protective layer for titanium alloys, not only enhancing their durability but also their ability to adapt to physiological and harsh environments.

Dose-dependent enhancement of in vitro osteogenic activity on strontium-decorated polyetheretherketone

Polyetheretherketone (PEEK) is widely used in orthopedic and dental implants due to its excellent mechanical properties, chemical stability, and biocompatibility. However, its inherently bioinert nature makes it present weak osteogenic activity, which greatly restricts its clinical adoption. Herein, strontium (Sr) is incorporated onto the surface of PEEK using mussel-inspired polydopamine coating to improve its osteogenic activity. X-ray photoelectron spectroscopy and ion release assay results confirm that different concentrations of Sr are incorporated onto the PEEK substrate surfaces. The strontium-modified PEEK samples show a stable Sr ion release in 35 days of detection. Better results of MC3T3-E1 pre-osteoblasts adhesion, spreading, and proliferation can be observed in strontium-modified PEEK groups, which demonstrates strontium-modified PEEK samples with the improved MC3T3-E1 pre-osteoblasts compatibility. The boosted osteogenic activity of strontium-modified PEEK samples has been demonstrated by the better performed of ALP activity, extracellular matrix mineralization, collagen secretion, and the remarkable up-regulation of ALP, OCN, OPN, Runx2, Col-I, BSP, and OSX of the MC3T3-E1 pre-osteoblasts. Additionally, the strontium-modified PEEK samples exhibit a dose-dependent enhancement of osteoblasts compatibility and osteogenic activity, and the PEEK-Sr10 group shows the best. These findings indicate that strontium-decorated PEEK implants show promising application in orthopedic and dental implants.

Hydrogel Strain Sensors for Integrating Into Dynamic Organ-on-a-Chip.

Current hydrogel strain sensors have never been integrated into dynamic organ-on-a-chip (OOC) due to the lack of sensitivity in aqueous cell culture systems. To enhance sensing performance, a novel strain sensor is presented in which the MXene layer is coated on the bottom surface of a pre-stretched anti-swelling hydrogel substrate of di-acrylated Pluronic F127 (F127-DA) and chitosan (CS) for isolation from the cell culture on the top surface. The fabricated strain sensors display high sensitivity (gauge factor of 290.96), a wide sensing range (0-100%), and high repeatability. To demonstrate its application, alveolar epithelial cells are cultivated on the top surface of the hydrogel strain sensor forming alveolar barriers, and then integrated into dynamic lung-on-a-chip (LOC) systems. This system can sensitively monitor normal physiological breathing, pathological inflammation stimulated by lipopolysaccharide (LPS), and alleviated inflammation through drug intervention.

Cold-Spray Deposition of Antibacterial Molybdenum Coatings on Poly(dimethylsiloxane).

Despite their widespread utilization in biomedical applications, these synthetic materials can be susceptible to microbial contamination, potentially compromising their functionality and increasing the risk of infection in patients. In this study, molybdenum (Mo), an essential metal in biological systems, was investigated as a Mo-based cold-sprayed coating on poly(dimethylsiloxane) (PDMS) for its potential use as biocompatible and antimicrobial surfaces for biomedical applications. Various cold-spray parameters were employed in the fabrication of Mo-embedded PDMS surfaces to alter the surface structure of the substrate, Mo loading density, and embedding layer thickness. Specifically, relatively low nozzle scanning speeds were used to develop high-density Mo-embedded PDMS surfaces. A comprehensive analysis was conducted to investigate how cold-spray processing parameters affect the surface topography, wettability, and chemical properties. The ability of the Mo-embedded PDMS to inhibit the colonization of Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, and Pseudomonas aeruginosa bacterial species was demonstrated by both live/dead staining and disk diffusion methods. Surfaces with higher Mo loading densities significantly reduced the level of bacterial attachment and enhanced the bactericidal activity upon contact. Also, the level of Mo ion release over a 14-day period was measured and correlated to the properties of the substrate surface. Furthermore, attachment, viability, and proliferation of osteoblast-like MG63 cells were assessed to investigate the effect of Mo ion release on the biocompatibility of fabricated coatings. A notable decrease in cell viability and delayed growth of MG63 cells became evident after 7 days of incubation with the highly Mo-loaded samples. While this study enhanced our understanding regarding the engineering of composite materials for combatting microbial infections, the findings also suggest that the release of Mo ions may detrimentally affect osteoblast survival, potentially compromising the long-term functionality of orthopedic implants produced using this technique.

Research on Laser Cleaning of Graphite Lubrication Coating on the Magnesium Alloy Surface

The lubricating coating must be removed from the forged or stamped workpieces. Developing environment-friendly and high-precision cleaning technology is necessary. In this study, a nanosecond pulsed laser was used to clean the graphite lubricating coating of 15 μm thickness on the surface of an MB15 magnesium alloy. The effects of various laser cleaning parameters on the cleaning quality and the cleaning mechanism were studied. When the laser fluence (F) increases from 1.27 to 7.64 J/cm2, the clearance rate increases, and the surface roughness initially decreases before increasing. When the pulse frequency (f) increases from 10 to 30 kHz, the single-pulse energy decreases, the clearance rate decreases, and the surface roughness increases. When the scanning speed (v) increases from 1000 to 5000 mm/s, the spot overlap rate decreases, the clearance rate decreases, and the surface roughness firstly decreases and then increases. The optimal cleaning parameter combinations are F = 3.82 J/cm2, f = 10 kHz, and v = 3000 mm/s. The graphite lubrication coating was almost completely removed without damaging the substrate surface, and the surface carbon content of the sample was decreased to 6.42%. The laser cleaning mechanism of the graphite lubricating coating on the magnesium alloy surface is dominated by thermal ablation. As the laser fluence increases, the physical and chemical reactions become more violent.

Zincophilic CuO as electron sponge to facilitate dendrite-free zinc-based flow battery

Zinc (Zn)-based batteries have been persistently challenged by the critical issue of inhomogeneous zinc deposition/stripping process on substrate surface. Herein, we reveal that zinc electrodeposition behaviors dramatically improved through the introduction of highly zincophilic copper oxide nanoparticles (CuO NPs). Strong electronic redistribution between Zn and CuO explains the high Zn affinity on CuO, with negligible nucleation overpotential. Additionally, CuO exhibits remarkable electron-accepting and -donating capabilities in electron-rich and electron-deficient environments, resembling a sponge. This ‘Electron Sponge’ effect emerges from stable Zn-O bonding in CuO, enhancing electron duality in the Zn-O bond region. This unique strategy is pivotal in mitigating dendritic growth, fostering dendrite-free zinc-based flow batteries with enhanced rate performance and cyclability. It presents significant performance with not only high energy density (180 Wh L−1) but also the long cycle stability (> 2500 cycles) at high current density (140 mA cm−2).

Patterning of Photochromic Diarylethene Crystals by Sublimation for Morphological Controls.

A patterned growth of crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (1a) on the glass substrate with convex guides is reported by sublimation methods. The lower supersaturation of substrate surfaces with higher temperatures can facilitate the vapor-to-liquid process rather than the vapor-to-crystal process in the early stage of the sublimation. Micro-droplets of melts of 1a are generated on the sidewalls of the convex guides, then crystallized into the microcrystals, accompanied by the rearrangements of the crystallographic in-plane orientations. Moreover, the crystalline patterns fringed with the rod crystals are colored red upon irradiation with ultraviolet light. This well-controllability of crystal morphologies in a simple use of sublimation methods will pave the way for large-sized photomechanical materials with the desired morphologies.

Optimization of Micro-Pillars Electroplating Bonding Processes and Additives

Abstract With the increasing interconnect density of electronic components, copper-copper direct bonding technology has garnered increasing attention from researchers. The electroplating bonding method is an efficient copper pillar interconnection technique that can be implemented at room temperature and atmospheric pressure. COMSOL simulation results show that under convective conditions, the plating layer primarily deposits on the convection exit side of the copper plate. Under weak convection and low current density, the plating exhibits deposition characteristics that conform to the substrate surface. As convection intensity increases, preferential deposition begins to occur, although the overall deposition rate decreases. At this point, when the current density is increased, the deposition pattern predominantly shows preferential deposition; however, excessively high current density can lead to copper deposition in non-bonding areas. Orthogonal experimental results indicate that, within the accelerator bis(3-sulfopropyl) disulfide (SPS) – inhibitor polyethylene glycol (PEG) – leveling agent Jenner Green B (JGB) – chloride ion (Cl-) system, the influence of the four additives on bonding strength follows this order: JGB > Cl- > SPS > PEG. The optimal formulation derived from the orthogonal experiments is SPS 2 ppm, PEG (8000) 150 ppm, JGB 5 ppm, and Cl- 30 ppm, which results in a shear strength of 123.2 MPa. These findings suggest that high-strength copper pillar interconnections can be achieved by adjusting physical parameters such as the electric field, flow field, and additive concentrations.

Interface Acoustic Waves in 128° YX-LiNbO3/SU-8/Overcoat Structures.

The propagation of interface acoustic waves (IAWs) in 128° YX-LiNbO3/SU-8/overcoat structures was theoretically studied and experimentally investigated for different types of overcoat materials and thicknesses of the SU-8 adhesive layer. Three-dimensional finite element method analysis was performed using Comsol Multiphysics software to design an optimized multilayer configuration able to achieve an efficient guiding effect of the IAW at the LiNbO3/overcoat interface. Numerical analysis results showed the following: (i) an overcoat faster than the piezoelectric half-space ensures that the wave propagation is confined mainly close to the surface of the LiNbO3, although with minimal scattering in the overcoat; (ii) the presence of the SU-8, in addition to performing the essential function of an adhesive layer, can also promote the trapping of the acoustic energy toward the surface of the piezoelectric substrate; and (iii) the electromechanical coupling efficiency of the IAW is very close to that of the surface acoustic wave (SAW) along the bare LiNbO3 half-space. The numerical predictions were experimentally assessed for some SU-8 layer thicknesses and overcoat material types. The propagation of the IAWs was experimentally measured in LiNbO3/SU-8/fused silica, LiNbO3/SU-8/(001)Si, and LiNbO3/SU-8/c-Al2O3 structures for an SU-8 layer about 15 µm thick; the velocities of the IAWs were found in good agreement with the theoretically calculated values. Although the interest in IAWs was born many years ago for packageless applications, it can currently be renewed if thought for applications in microfluidics. Indeed, the IAWs may represent a valid alternative to standing SAWs, which are strongly attenuated when travelling beneath the walls of polydimethylsiloxane (PDMS) microfluidic channels for continuous flow particle manipulation, provided that the channel is excavated into the overcoating.

Microwave Surface and Lamb Waves in a Thin Diamond Plate: Experimental and Theoretical Investigation.

Microwave surface and Lamb waves in a multilayered piezoelectric "Al-IDT/(Al0.72Sc0.28)N/(001)[110] diamond" structure designed as a SAW resonator were studied using both the experimental and modeling methods. In this structure, it is possible to generate Rayleigh, surface horizontal (SHn) and Lamb waves simultaneously. The successful excitation of Lamb waves at operating frequencies up to 20GHz has been obtained. We have developed a method for identifying mode types based on an analysis of the elastic displacement fields and the frequency dependences of the dispersive phase velocities of each wave, as the frequency increases. The experimental data on the frequency response is in close agreement with the calculated values. The influence of Pt film deposition on the shifts in the resonant frequency of Lamb wave modes has been studied in detail. Since the Pt film was deposited on the free surface of a diamond substrate, shifts in the resonant frequency of Rayleigh-type waves are absent, as expected. This serves as an additional indication to distinguish these types of waves from others. The effect of film deposition on the frequency response of Lamb waves could be taken into account when designing acoustoelectronic sensors that use this type of wave as an operational mode.

Binding Kinetics of Self-Assembled Monolayers of Fluorinated Phosphate Ester on Metal Oxides for Underwater Aerophilicity.

The term "aerophilic surface" is used to describe superhydrophobic surfaces in the Cassie-Baxter wetting state that can trap air underwater. To create aerophilic surfaces, it is essential to achieve a synergy between a low surface energy coating and substrate surface roughness. While a variety of techniques have been established to create surface roughness, the development of rapid, scalable, low-cost, waste-free, efficient, and substrate-geometry-independent processes for depositing low surface energy coatings remains a challenge. This study demonstrates that fluorinated phosphate ester, with a surface tension as low as 15.31 mN m-1, can form a self-assembled monolayer on metal oxide substrates within seconds using a facile wet-chemical approach. X-ray photoelectron spectroscopy was used to analyze the formed self-assembled monolayers. Using nanotubular morphology as a rough substrate, we demonstrate the rapid formation of a superhydrophobic surface with a trapped air layer underwater.

Solution for Environmentally Friendly Silver Surface Magnetron Sputtering Color Titanium Film Layer Technology

Silver is an elegant white precious metal, but it is easily oxidized by O3, SO2, and H2S in the air, turning yellow or dark, which affects its decorative effect. The existing silver coating, primarily prepared through the electroplating process, poses serious environmental pollution problems. It is necessary to seek new, green, and environmentally friendly coating processes while also enhancing the color palette of silver jewelry coatings. Titanium film layers were deposited on Ag925 and Ag999 surfaces using magnetron sputtering coating technology. The effects of sputtering time, substrate surface state, reaction gas type and time, and film thickness on the color of the film layers were studied, and the anti discoloration performance of the obtained film layers under the optimal process was tested. The experimental results show that when the sputtering time varies from 5 to 10 minutes, injecting argon, oxygen, and nitrogen into the coating chamber yields rich colors such as purple with a red tint, blue, yellow green, yellowish purple, and blue purple. The precise control of gas injection time has a significant impact on the color of the film layer. In terms of anti tarnish performance, the film showed good stability in the artificial sweat immersion test. From an environmental perspective, the magnetron sputtering titanium film process has no harmful gas or liquid emissions, which aligns with the sustainable development trend of the jewelry industry and holds great promise for application. This study has improved the visual effect and practical performance of the product, providing important theoretical basis and experimental data support for the application of environmentally friendly silver surface vacuum magnetron sputtering titanium thin film coating technology.

Perovskite Nanocrystal Self-Assemblies in 3D Hollow Templates.

Highly ordered nanocrystal (NC) assemblies, namely, superlattices (SLs), have been investigated as materials for optical and optoelectronic devices due to their unique properties based on interactions among neighboring NCs. In particular, lead halide perovskite NC SLs have attracted significant attention owing to their extraordinary optical characteristics of individual NCs and collective emission processes like superfluorescence (SF). So far, the primary method for preparing perovskite NC SLs has been the drying-mediated self-assembly method, in which the colloidal NCs spontaneously assemble into SLs during solvent evaporation. However, this method lacks controllability because NCs form random-sized SLs at random positions on the substrate, rendering NC assemblies in conjunction with device structures, such as photonic waveguides or microcavities, challenging. Here, we demonstrate template-assisted self-assembly to deterministically place perovskite NC SLs and control their geometrical properties. A solution of CsPbBr3 NCs is drop-casted on a substrate with lithographically defined hollow structures. After solvent evaporation and removal of excess NCs from the substrate surface, NCs remain only in the templates, thereby defining the position and size of these NC assemblies. We performed photoluminescence (PL) measurements on these NC assemblies and observed signatures of SF, similar to those in spontaneously assembled SLs. Our findings are crucial for optical devices that harness embedded perovskite NC assemblies and enable fundamental studies on how these collective effects can be tailored through the SL geometry.

Cold-spray repair of CuCrZr crystallizers: Effect of substrate pretreatment

Cold spraying is a promising technique for repairing damaged CuCrZr crystallizers, and the interface between the repair coating and the substrate plays a critical role in determining the heat transfer efficiency of the repaired crystallizers. In this study, two coating/substrate interfaces were prepared through polishing and grit-blasting the CuCrZr substrate surface. Effect of substrate pretreatment on microstructure and performance of the CuCrZr coating was studied using electron backscatter diffraction, adhesive shear strength and thermal conduction tests. Results show that the surface preparation method does not affect the microstructure and hardness of the CuCrZr coating, but the coating sprayed on the grit-blasted substrate has a lower shear adhesive strength and heat conductivity than the coating sprayed on the polished substrate. This difference is attributed to the formation of a work hardening zone during the grit-blasting process. After releasing this work hardening by annealing at 500°C, the difference in heat conductivity then disappears, but the average shear adhesive strength of the annealed coating on the polished substrate is still higher than that on the grit-blasted substrate. Therefore, polishing the substrate could serve as a more viable option for the cold spray repair of CuCrZr crystallizers.

Probing Functional Thin Films with Grazing Incidence X-Ray Scattering: The Power of Indexing

Grazing incidence small- and wide-angle X-ray scattering (GISAXS, GIWAXS) has been widely applied for the study of functional thin films, be it for the characterization of nanostructured morphologies in block copolymers, nanocomposites, and nanoparticle assemblies, or for the packing and orientation of aromatic molecules or conjugated polymers. Solution-processed thin films are typically uniaxial powders, with a specific crystallographic plane oriented parallel to the substrate surface while ordered domains assume random orientations laterally. The convenient GISAXS/GIWAXS scattering geometry facilitates obtaining complete information about thin film structure as well as the ability to study samples in well-defined sample environments, as controlled by temperature, exposure to solvent vapor and drying, or coating processes. Moreover, with suitable X-ray sources and detectors, information about the ordering kinetics and phase transitions can be obtained down to the millisecond scale. The scattering geometry and an interactive graphical tool to index such scattering patterns will be discussed here. Furthermore, it will be demonstrated that proper indexing of the X-ray scattering patterns can provide deep insight into thin film structure–property relationships and the kinetics of structure formation. Recent examples of nanostructures and molecular organization in thin films will be discussed, as well as self-assembly processes leading to such structures.