Porous Coatings Research Articles

Metal–Organic Frameworks with a Bioinspired Porous Polymer Coating for Sieving Separation

Polymer/metal-organic framework (MOF) composites have been widely studied for their favorable combination of polymer flexibility and MOF crystallinity. While traditional polymer-coated MOFs maximize the polymer properties at the surface, the dramatic loss of MOF porosity due to blockage by the nonporous polymeric coating remains a problem. Herein, we introduce intrinsically microporous synthetic allomelanin (AM) as a porous coating on the zirconium-based MOF (Zr-MOF) UiO-66 via an in situ surface-constrained oxidative polymerization of the AM precursor, 1,8-dihydroxynaphthalene (1,8-DHN). Transmission electron microscopy images verify the formation of well-defined nanoparticles with a core-shell morphology (AM@UiO-66), and nitrogen sorption isotherms indicate the porosity of the UiO-66 core remains constant and is not disturbed by the AM coating. Notably, such a strategy could be adapted to MOFs with larger pores, such as MOF-808 by generating porous AM polymer coatings from bulkier DHN oligomers, highlighting the versatility of this method. Finally, we showed that by tuning the AM coating thickness on UiO-66, the hierarchically porous structures of these AM@UiO-66 composites engender excellent hexane isomer separation selectivity and storage capacity.

Synthesis of Porous MgF2 Coating by a Sol–Gel Method Accompanied by Phase Separation

Surfaces with translucent and wear-resistant effects have a wide range of applications, especially as protective layers. In this work, a simple and convenient method for the preparation of porous magnesium fluoride (MgF2) coatings was proposed. Nano-porous MgF2 powder was prepared with sol–gel and phase separation methods by optimizing the polymer amount and used for the preparation of thick layers onto PVC substrates. The automated surface area and porosity analyzer (BET) and scanning electron microscopy (SEM) confirmed that the layers containing 0.028‰ PEO presented a 3D structure with pore sizes in the range of 16 nm. The layer reached 93% transmittance in the visible region, a Vickers hardness value of 2889.1 kg/mm2, and a friction coefficient of 0.2 in the wear test.

Numerical analysis of residual stresses induced by cold spray fabricating cBN-reinforced Ni matrix composites

Metal particles used as cold spraying feedstock undergo severe plastic deformation during application. During this process, residual stresses are generated. The presence of porous metal particles and ceramic particles further complicates the deformation process. Numerical models have been proposed to describe the formation process for residual stresses induced by cold spray fabricating cBN-reinforced Ni matrix composites on a 42CrMo substrate. The feedstocks to be sprayed are mixtures of porous Ni particles and Ni-coated cBN particles. The analysis of residual stresses for the cBN-reinforced Ni matrix composites was accomplished by depositing cBN particles on the cold-spayed porous Ni particles and multi-layered Ni coatings. Firstly, local residual stresses are generated near the large number of closed pores inside the cold-sprayed porous Ni particles. Simultaneously, the residual stresses at the bottom of the multi-layered Ni coatings are changed from a compressive stress state to a tensile one as the number of cold-sprayed Ni layer increases. Finally, the deposited cBN particles aggravate the transformation of residual stresses during the cold spraying fabrication of multi-layered cBN-reinforced Ni matrix composites. The concentrated compressive stresses are generated when the cBN particles are deposited on the Ni particles. Understanding these phenomena may provide a guideline for tailoring the residual stresses for cold-sprayed cBN-reinforced Ni matrix composites and thereby improve service performance.

Experimental studies on pressure drag and aerodynamic noise of cylinder with porous coatings

Previous numerical studies hold inconsistent viewpoints of the porous coating on a circular cylinder reducing or increasing drag, and the present experimental study aims to clarify this argument. Additionally, a common viewpoint is that the porous coatings can suppress both the near-field wall pressure fluctuations and far-field acoustic pressure, but optimal parameters of the porous materials are still unclear. In this paper, the experimental studies reveal effects of three parameters, i.e., incoming flow velocity, pores per inch, and thickness of metal foams, on the pressure drag, wall pressure fluctuations, and associated noise level.

Evolution of colloidal coatings due to a wetting and drying process

Paints and coatings are usually made by depositing a volatile liquid containing dispersed colloidal particles. The dry film is obtained through the evaporation of the volatile liquid. Depending on the ability of the particles to deform under capillary effect, we show that the drying can yield continuous coatings with no porosity, uniform porous coatings, or the formation of singularities, such as cracks causing the final film to be non-uniform. The evolution of the resulting coatings is then subjected to a wetting and drying process. Wetting leads to an increase in the water content of the unsaturated porous coating while drying results in water reduction. The response of the coatings to such a process can exhibit slight or significant changes in the morphology of the coatings that are related to their rheological properties. In particular, the growth of blisters is reported during the wetting and drying process.

Surface Characteristics, Osteogenic Activity and Antibacterial Effect of Magnesium-Doped Porous Coating on Titanium Surface

Biomedical titanium and its alloys have become the preferred materials for bone and joint implants due to their excellent biocompatibility, high specific strength, and low elastic modulus. However, poor biological activity and bacterial infection on the surface of titanium alloys have become major obstacles to their clinical application. Improving the osteogenic and antibacterial properties of titanium through surface modification is currently an important research area. In this study, a microporous magnesium-doped titanium dioxide coating was prepared on a titanium surface via microarc oxidation. This coating not only has a good porous surface morphology, with magnesium distributed on the surface of the coating, but also exhibits good resistance to corrosion and wear. In vitro cell experiments showed that the coating promoted proliferation, differentiation and mineralization in bone marrow mesenchymal stem cells, while bacterial experiments showed that the coating inhibited the growth of Staphylococcus aureus and Escherichia coli, with good antibacterial activity. This study provides a new theoretical and experimental basis for improving the biological activity of titanium.

Inversion of the Boiling Curve on Microstructured Porous Coatings

Inversion of the Boiling Curve on Microstructured Porous Coatings

A new approach to protect and extend longevity of the thermal barrier coating by an impermeable layer of silicon nitride

AbstractA sample representation of a gas turbine engine blade, consisting of a nickel superalloy substrate with a deposited thermal barrier coating (TBC), was covered with silicon nitride, Si3N4, as an impermeable layer using plasma enhanced chemical vapor deposition (PECVD). The silicon nitride layer was used to seal the topcoat of yttria‐stabilized zirconia (YSZ) surface of the TBC to mitigate calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS testing was carried out on the covered and uncovered surfaces by melting a ratio of 25 mg/cm2 of CMAS powder onto the surface of each sample in a furnace at 1100°C for 1 h. The conformal surface reaction of the sealed layer confirmed no cracking or delamination at high temperatures. Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ was successfully sealed. The new coating of silicon nitride was shown to be a viable solution and technique to significantly block CMAS infiltration in porous thermal barrier coatings.

Synthesis of PMMA Microspheres with Tunable Diameters: Evaluation as a Template in the Synthesis of Tin Oxide Coatings.

The synthesis of polymethyl methacrylate (PMMA) spheres with different sizes has been a challenge. PMMA has promise for future applications, e.g., as a template for preparing porous oxide coatings by thermal decomposition. Different amounts of SDS as a surfactant are used as an alternative to control PMMA microsphere size through the formation of micelles. The objectives of the study were twofold: firstly, to determine the mathematical relationship between SDS concentration and PMMA sphere diameter, and secondly, to assess the efficacy of PMMA spheres as templates for SnO2 coating synthesis and their impact on porosity. The study used FTIR, TGA, and SEM techniques to analyze the PMMA samples, and SEM and TEM techniques were used for SnO2 coatings. The results showed that PMMA sphere diameter could be adjusted by varying the SDS concentration, with sizes ranging from 120 to 360 nm. The mathematical relationship between PMMA sphere diameter and SDS concentration was determined with a y = axb type equation. The porosity of SnO2 coatings was found to be dependent on the PMMA sphere diameter used as a template. The research concludes that PMMA can be used as a template to produce oxide coatings, such as SnO2, with tunable porosities.

Fabrication and Experimental Study of Micro/Sub-Micro Porous Copper Coating for Anti-Icing Application.

Micro and sub-micro-spherical copper powder slurries were elaborately prepared to fabricate different types of porous coating surfaces. These surfaces were further treated with low surface energy modification to obtain the superhydrophobic and slippery capacity. The surface wettability and chemical component were measured. The results showed that both the micro and sub-micro porous coating layer greatly increased the water-repellence capability of the substrate compared with the bare copper plate. Notably, the PFDTES-fluorinated coating surfaces yielded superhydrophobic ability against water under 0 °C with a contact angle of ~150° and a contact angle of hysteresis of ~7°. The contact angle results showed that the water repellency of the coating surface deteriorated with decreasing temperature from 10 °C to -20 °C, and the reason was probably recognized as the vapor condensation in the sub-cooled porous layer. The anti-icing test showed that the ice adhesion strengths of the micro and sub-micro-coated surfaces were 38.5 kPa and 30.2 kPa, producing a 62.8% and 72.7% decrease compared to the bare plate. The PFDTES-fluorinated and slippery liquid-infused porous coating surfaces both produced ultra-low ice adhesion strengths of 11.5-15.7 kPa compared with the other non-treated surfaces, which showed prominent properties for anti-icing and deicing requirement of the metallic surface.

Fast Fabrication of Porous Amphiphilic Polyamides via Nonconventional Evaporation Induced Phase Separation.

In the present work, we report a facile approach for the fast fabrication of porous films and coatings of long-chain polyamides through a nonconventional evaporation induced phase separation. Because of its amphiphilic nature, polyamide 12 can be dissolved in the mixture of a high-polarity solvent and a low-polarity solvent, while it could not be dissolved in either solvent solely. The sequential and fast evaporation of the solvents leads to the formation of porous structures within 1 min. Moreover, we have investigated the dependence of the pore structures on composition of the solutions, and have demonstrated that our approach can be applied to other long-chain polycondensates, too. Our findings can provide insight on the fabrication of porous materials by using amphiphilic polymers.

Total Specific Energy as a New Figure of Merit for the Design of Porous TiO2 Surfaces Synthesized via Plasma Electrolytic Oxidation

The plasma electrolytic oxidation (PEO) is a variation of the anodic oxidation technique, one of the simplest and most economical methods to obtain crystalline and porous TiO2 coatings. Beyond the conventional approach of establishing a relationship between the variables of synthesis and the characteristics of the TiO2 coatings, a new figure of merit was defined in the present work for the synthesis of TiO2 coatings by PEO: the total energy per unit of surface (TSE). A strong relationship was found between TSE and the TiO2 anatase fraction. The application of TSE, together with the known influence of the variables of synthesis on the coating characteristics, allows a prediction, previously to the anodization and with reasonable precision, of the anatase fraction and the pore diameter of the coating. Applying the TSE new concept, three coatings were successfully designed. This is the first step toward a surface design using the PEO technique.

Effect of porous coatings on the nonlinear evolution of Mack modes in hypersonic boundary layers

The influence of a porous wall on the nonlinear evolution of Mack modes in a hypersonic boundary layer is studied by solving the nonlinear parabolized stability equations. The fundamental resonance of the second mode is particularly considered. It is found that the porous effect leads to (1) a much stronger mean-flow distortion in an indirect way, (2) a greater suppression of the saturated fundamental mode, and (3) slower amplification rates of the secondary instability modes, which eventually delays the transition onset. Detailed explanations of the three mechanisms are provided.

Interfacial delamination of porous thermal barrier coatings based on SEM image processing in finite element model

Numerical analysis of interfacial delamination and the effect of microstructural pores and rough interfaces on the field variables in thermal barrier coatings (TBCs) based on a real scanning electron microscope (SEM) image is performed. The interfacial delamination is modelled by the utilization of cohesive zone method (CZM) in finite element (FE) analysis. The empirical TBC sample considered in this study is constructed via air plasma spray (APS) method and comprises ZrO27-8wt.%Y2O3 ceramic top coat (TC), thermally grown oxide (TGO) made of Al2O3 and NiCrAlY metallic bond coat (BC). A compatible geometry is provided by applying image processing technique to SEM image of the sample's cross-section. SEM image-based geometry imported into FE software to scrutinize the interplay effects of porosity and rough interfaces on temperature fields, thermal stresses and interfacial debonding of TC/TGO and TGO/BC interfaces of the TBC structure. FE analysis reveal that porosity distribution has a prominent role on the stress concentration zones at the sharp corners of pores especially adjacent to undulations of the TC/TGO interface, which is responsible for possible internal cracks and interfacial fracture, respectively. The main findings demonstrate that the porosity near the TC/TGO interface has detrimental effect as leads to aggravation in the TC/TGO interface damage, which causes an increment in TC/TGO interfacial crack length. While, the porosity within the BC has the ability to reduce the stress and damage level along the TGO/BC interface. In addition, considering creep for all layers leads to lower stress levels up to 24.82 % and consequently alleviates the TC/TGO interface damage.

A Study of Tibial Cyst Formation in Modular Stemmed Total Ankle Arthroplasty: Exploring a Possible Relationship to Smooth and Porous Coating on the Stem Segments

Vertical fixation through stemmed components has been a successful strategy in total ankle arthroplasty. Research in hip replacement surgery has demonstrated increased rates of stress shielding, aseptic loosening, thigh pain, and cystic formation around stemmed femoral implants extensively coated with porous surfaces. While some ankle prostheses have integrated porous coating technology with stemmed tibial implants, there is little to no research investigating the potential negative effects of bone bonding to the tibial stems and possible impact on tibial cyst formation. We performed a retrospective cohort study comparing the incidence of periprosthetic tibial cyst formation in smooth versus fully porous-coated stemmed tibial implants after undergoing total ankle implant arthroplasty. Radiographs were compared for postoperative rates of tibial cyst formation and bone bonding to the tibial stems. Relative risk for reoperation between the smooth and porous-coated implants was investigated. The smooth-stem group showed no incidence of tibial cyst formation nor signs of significant bone bonding to the tibial stems; however, the follow-up matched porous-coated group showed a rate of 63% of cystic formation with associated evidence of bone bonding on final radiographic follow-up (p < .01). Relative risk for reoperation was 0.74. Despite a higher incidence of tibial cyst formation in the stemmed ankle arthroplasty groups with porous coating, reoperation rates were similar. We theorize that the proximal bonding to the porous stem surface could impact the distal stems and result in the observed increase in cyst formation.

Preparation and mechanical and biological performance of the Sr-containing microarc oxidation layer on titanium implants

A Strontium-doped titanium oxide coating was successfully prepared on a TA4 substrate with one-step microarc oxidation (MAO) technology. Then bioactivity of the coating was performed with the inducation test of bone-like apatite formation immersed in simulated body fluid (SBF). The biological activity of the coating was evaluated with cell culture experiments, and its corrosion resistance was characterized with electrochemical tests in the SBF. The composition and morphology of the coatings before and after SBF immersion were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the coating had a rough porous structure, which was mainly composed of anatase, rutile and Ti phases. The SBF immersion test showed that abundant 3D micro nanopore structures were formed on the surface of the porous titanium oxide coatings, which enabled the coatings to exhibit good apatite nucleation ability. Faster precipitation of bone-like apatite by MAO-Sr coatings in SBF. Finally, in the biological compatibility test, compared with TA4, the contact angle between the MAO-Sr coating and SBF solution was lower, and the proliferation ability of bone marrow stromal cells (BMSCs) in the biological coating was stronger, indicating the good biocompatibility of the coating.

Study on microstructure and properties of AA7075 coated by micro arc oxidation

Effect of ageing temperature on the properties of 7075 aluminum alloy coated by micro arc oxidation (MAO) technology has been investigated. The precipitates formed at various ageing temperatures are observed by transmission electron microscope (TEM), which would significantly influence the properties of the MAO coating. Coherent distortion, size and volume fraction of precipitates are related to the changing of electrical conductivity during ageing treatment. Full coherent and dense Guinier Preston zones (GP zones) formed at 90 °C can act as scattering centers to impede electron mobility, so decrease the electrical conductivity. η and η′ phases produced at 120 °C and 150 °C are semi-coherent and incoherent, respectively, meanwhile, the precipitate size increases and the volume fraction decreases with increasing the ageing temperature, therefore, the electrical conductivity increases. Higher electrical conductivity contributes to thicker coating with better surface morphology. Double cantilever beam (DCB) tests show that the thicker and less porous coatings prepared at 150 °C aged specimens own the best stress corrosion cracking (SCC) resistance. After pores are broke and coatings are ruptured, high Cu and low Mg segregation at the grain boundary of 150 °C aged specimens founded from high-angle annular dark field scanning transmission electron microscopy (HADDF-STEM) are beneficial to anodic dissolution and hydrogen assisted crack.

Facile fabrication of durable and fluorine-free liquid infused surfaces on aluminum substrates with excellent anti-icing, anticorrosion, and antibiofouling properties

Marine biofouling has a significant impact on marine industry and activities. Various slippery liquid-infused porous surfaces (SLIPS) for marine antibiofouling have been reported in recent years. However, these surfaces are limited to a few substrates, require a complicated construction procedure, and have a low durability. In this regard, simple, efficient and eco-environmental-friendly methods were used to fabricate SLIPS coatings on aluminum substrates with durable and efficient properties. The chemically etched and grinded aluminum substrates were modified to fabricate hydrophobic surfaces by the application of stearic acid (SA) and polydimethylsiloxane (PDMS) porous coatings. Both SA-modified and PDMS-modified porous hydrophobic surfaces were infused with silicone oil to obtain SLIPS (SLIPS-SA and SLIPS-PDMS). The qualities of two hydrophobic surfaces (SA and PDMS) and their corresponding slippery surfaces (SLIPS-SA and SLIPS-PDMS) were investigated. Wetting studies revealed that the as-prepared slippery surfaces have hydrophobic properties and a low sliding angle, suggesting the formation of a homogeneous and smooth oil layer. Furthermore, results showed that both SLIPS are durable and could retain their slippery properties even after 30 days of immersion in artificial acid rain and saturated saline solution. The optimum SLIPS with very low ice adhesion prolonged the water droplet freezing time over two times compared to the bare aluminum substrate. The hydrophobic surfaces displayed inferior anti-icing, anticorrosion, and antibiofouling performance; however, the lubricant-infused surfaces revealed considerably enhanced anti-icing, anticorrosion, and antibiofouling performance. This approach can be applicable to a wide variety of metal substrates and alloys because of its simplicity, which is critical for the production of SLIPS and expanding their applications in marine environments.

Core-Shell Polyvinyl Alcohol (PVA) Base Electrospinning Microfibers for Drug Delivery.

In this study, electrospun membranes were developed for controlled drug release applications. Both uniaxial Polyvinyl alcohol (PVA) and coaxial fibers with a PVA core and a poly (L-lactic acid) (PLLA) and polycaprolactone (PCL) coating were produced with different coating structures. The best conditions for the manufacture of the fibers were also studied and their morphology was analyzed as a function of the electrospinning parameters. Special attention was paid to the fiber surface morphology of the coaxial fibers, obtaining both porous and non-porous coatings. Bovine serum albumin (BSA) was used as the model protein for the drug release studies and, as expected, the uncoated fibers were determined to have the fastest release kinetics. Different release rates were obtained for the coated fibers, which makes this drug release system suitable for different applications according to the release time required.

Powder Metallurgy Methods in Technologies for Producing Titanium Orthopedic and Dental Implants

Today, the ability to form porous structures and coatings from titanium powder on implants parts of specified porosity, size and morphology of pores into which newly formed bone tissues and, thus, they contribute to the osseointegration and high and long-term stability of the implant functioning in the body is widely used. The ability to obtain work pieces of complex geometry by powder metallurgy methods with a minimum of mechanical rework by cutting, it also promotes the diffusion of this technology in the implants manufacturing as the titanium alloys are classified as difficult to process. The future progress in the development of new materials and types of implants is directly related to powder metallurgy - there is information about the creation of a highly solid biocompatible composite based on the intermetallic system β-Ti3Au, from which rib replacement implants are made