Organic-inorganic Composite Coatings Research Articles

Chloride Penetration of Surface-Coated Concrete: Review and Outlook.

Concrete coatings show significant promise in shielding concrete substrates from corrosion by effectively resisting harmful ions and moisture. Thanks to their practicality, high efficiency, and cost-effectiveness, coatings are considered a potent technique for enhancing the chloride resistance of reinforced concrete structures. Over recent decades, extensive research has concentrated on employing coatings to bolster concrete's ability to withstand chloride penetration. This paper provides a holistic review of the current studies on chloride infiltration in concrete surfaces treated with coating materials, primarily focused on chloride resistance improvement efficiency and chloride transport modeling. Firstly, by comparing the functions of assorted coatings, four inherent protection mechanisms are summarized and elaborated thoroughly. Afterwards, the chloride resistance improvement efficiency of assorted coatings reported in current studies are reviewed and compared in great detail, with a specific focus on inorganic, organic, and organic-inorganic composite coatings. Furthermore, the theoretical research about methodologies for chloride transport behavior prediction is summarized. Finally, this paper outlines the potential research directions in this field and the theoretical, technical, and practical application challenges. This review not only identifies critical areas necessitating further investigation and problem-solving in this domain but also aids in selecting appropriate coating materials and refining corrosion management strategies.

Synergistic effect of an organic hydrophobic film deposited on MAO containing NaH2PO4 for enhanced corrosion resistance of 6063 aluminum alloy

Aluminum alloys play a significant role in aviation, medical equipment, electronic device and automotive industry. Nevertheless, the corrosion is a critical problem that must be solved before applications. In this work, synergistic effect of an organic hydrophobic film (2-mercaptobenzothiazole (MBT), sodium stearate (SS) or 8-hydroxyquinoline (8-HQ)) deposited on micro-arc oxidation (MAO) which was post-sealing treated by NaH2PO4 was investigated on corrosion resistance. The morphology and composition of the resulting organic hydrophobic films were analyzed by FESEM, EDS and FTIR. The results revealed that the organic hydrophobic films were successfully deposited on the MAO coating with NaH2PO4 (MAO-NaH2PO4), and the pores and cracks of the MAO layer were effectively sealed. Moreover, the contact angle of the sample with 8-HQ treating reached the hydrophobic level (136.8°), which was much higher than that of the MAO-NaH2PO4 (58.06°). The PDP and EIS measurements showed that the sample with 8-HQ treating possessed superior corrosion resistance with a low corrosion current (1.924 × 10−8 A cm−2) and the decreased corrosion rate (1.8 × 10−4 mm·a−1), which was two order of magnitude higher than that of the MAO-NaH2PO4 sample (3.8 × 10−2 mm·a−1). This facile and low-cost strategy to form organic-inorganic composite coatings could have promising application in the corrosion protection for the light alloys.

Poly(l-lactic acid) (PLLA)/MgSO4·7H2O Composite Coating on Magnesium Substrates for Corrosion Protection and Cytocompatibility Promotion.

Magnesium (Mg) and its alloys show excellent potential as orthopedic implantable materials, with their in vivo degraded Mg ions (Mg2+) known to promote the growth of new bone. However, the swift corrosion process during implantation has greatly hindered its clinical applications. A method to counter the high rate of corrosion is to coat Mg substrates on surfaces with a thin layer of biodegradable polymer. Although such a coating reduces the long-term corrosion rate, it also prevents the short-term release of bone-simulating Mg2+ after orthopedic operations. To balance these contradicting short- and long-term characteristics, we present a polymer-inorganic composite coating on pure Mg substrates that enables Mg-based implants to achieve controllable release of Mg2+ and high corrosion resistance simultaneously. The coatings were fabricated by adding an appropriate amount of inorganic magnesium sulfate heptahydrate (MgSO4·7H2O) salt particles into a biodegradable poly(l-lactic acid) (PLLA) polymer matrix, such that they can percolate inside to form an interconnected morphology during the phase separation between Mg salt and PLLA polymers as solvent evaporates during the drying process, resulting in the formation of an organic-inorganic composite coating. The in vitro corrosion tests indicated that the composite coatings with lower Mg salt loading had the best degradation behavior, resulting in controllable release of Mg2+ and alkaline shift. Cytocompatibility of bare and coated Mg were investigated via MC3T3-E1 preosteoblasts through MTT assay and LIVE/DEAD imaging, along with the observation of cell distribution and adhesion behaviors. The results further demonstrated that the incorporation of a suitable amount of Mg salt particles could further improve the cytocompatibility as compared to the pristine PLLA coating. We believe such fabricated organic-inorganic composite coatings could have great potential for application on Mg substrates to obtain Mg-based biomaterials with higher practical value in clinical treatments.

Preparation and Properties of TiO2-Coated Hollow Glass Microspheres as Thermal Insulation Materials for Energy-Saving Buildings

A hollow glass microsphere (HGM)/TiO2 composite hollow sphere was successfully prepared via a simple precipitation method. The TiO2 coating layers grew on the surface of the HGMs that range from 20 to 50 μm in diameter as nanoparticles with the formation of the Si‒O‒Ti bonds. The growth mechanism accounting for the formation of the TiO2 nanolayers was proposed. The morphology, composition, thermal insulation properties, and visible–near infrared (VIS–NIR) reflectance of the HGMs/TiO2 composite hollow spheres were characterized. The VIS–NIR reflectance of the HGMs/TiO2 composite hollow spheres increased by more than 30% compared to raw HGMs. The thermal conductivity of the particles is 0.058 W/(m K). The result indicates that the VIS–NIR reflectance of the composite hollow spheres is strongly influenced by the coating of TiO2. The composite hollow spheres were used as the main functional filler to prepare the organic–inorganic composite coatings. The glass substrates coated by the organic–inorganic coatings had lower thermal conductivity and higher near infrared reflectivity. Therefore, the HGMs/TiO2 composite hollow spheres can reflect most of the solar energy and effectively keep out the heat as a thermal insulation coating for energy-saving constructions.

Enhanced atomic oxygen resistance and tribological properties of PAI/PTFE composites reinforced by POSS

The atomic oxygen (AO) in the low Earth orbit (LEO) severely damages the traditional polymer materials used for surface protective coatings in spacecraft. To solve this problem, organic-inorganic composite coatings with excellent AO resistance were prepared by covalently grafting octa- and mono-amino polyhedral oligomeric silsesquioxane (OMPOSS) onto the polyamide-imide (PAI) matrix in this paper. The pristine PAI/PTFE and OMPOSS/PAI/PTFE composite coatings before and after AO exposure were detail investigated by Fourier transform infrared spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. Compared with the pristine PAI/PTFE coating, the mass loss of OMPOSS/PAI/PTFE coating reduced by about 80% significantly. The findings indicate that AO attack can lead to the oxidation of the OMPOSS on the surface of the composite coatings, furthermore generate a SiO2 passivating layer. The formation of the passivating layer can effectively prevent further erosion and degradation of the underlying composites. In addition, the wear tests prove that the octa- and mono-amino POSS impart the PAI/PTFE coating with outstanding tribological properties under AO irradiation. Therefore, such OMPOSS-containing PAI/PTFE composite coatings have great potential for the application in the space environment.

Synergistic effect of hydrophobic film and porous MAO membrane containing alkynol inhibitor for enhanced corrosion resistance of magnesium alloy

Magnesium and magnesium alloys have important applications in aviation, electronic devices, medical equipment, and automotive industries, while the corrosion is a common and real problem that must be solved for these applications. In this paper, a novel and effective anti-corrosive composite coating on AZ31 Mg alloys was prepared by the combination of micro-arc oxidation (MAO) layer, corrosion inhibitor and hydrophobic wax film technologies. And among these, the MAO treated micro- and nanopores as the container of corrosion inhibitor and the solid hydrophobic wax as the multifunctional sealing isolating agent. The morphology and phase composition of the resulting composite coatings were investigated by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively, indicating the successful synthesis of the organic-inorganic composite coating. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements showed that compared with the MAO membrane covered Mg alloys, the organic-inorganic composite coating has superior corrosion resistance with the a lower corrosion current (5.764 × 10−9 A/cm2) and a higher protection efficiency (99.7%) after immersion in 3.5 wt% NaCl solution, attributing to the synergistic effect of effective inhibition of inhibitor and the physical barrier ability of hydrophobic film and MAO membrane. This simple and low cost strategy to fabricate organic-inorganic composite anticorrosion coatings would have promising applications in the corrosion protection of the light metals and their alloys.

Tailoring the surface characteristics of electrophoretically deposited chitosan-based bioactive glass composite coatings on titanium implants via grit blasting

Chitosan, chitosan/45S5 bioactive glass (mBG), chitosan/nanobioglass (nBG), and chitosan/mBG/nBG coatings were deposited on grit-blasted and mirror-polished Ti6Al4 V alloy substrates by electrophoretic deposition (EPD). This study discusses how the surface characteristics of the substrates influence the 3D surface topography, areal surface roughness, morphology, and wettability of chitosan-based thin composite coatings while focusing on tuning the surface characteristics by grit-blasting. The surfaces of coated and uncoated specimens were characterised by SEM analysis with EDX, image analysis, contact angle measurements, 3D surface topography analysis, FTIR analysis, and in-vitro bioactivity studies. The coatings deposited on grit-blasted substrates presented rougher surface topography, higher areal surface roughness values and wettability compared to the mirror-polished surfaces. A deposition mechanism was suggested to clarify the deposition of chitosan/mBG coatings on grit-blasted substrates. Apatite crystals were formed on chitosan/mBG coatings after immersion in SBF for three days. A robust image analysis method was applied to reveal the distribution and area coverage percent of deposited mBG, which demonstrated the increase of mBG coverage with the increases in voltage. The present study shows that grit blasting could be a favourable surface treatment to tailor the surface characteristics of organic-inorganic composite coatings by influencing their surface topography, roughness, and wettability.

Electrophoretic deposition and characterization of HA/chitosan nanocomposite coatings on Ti6Al7Nb alloy

Nano-hydroxyapatite/chitosan (nc-HA/chitosan) composite coatings were produced on two phase (α+β) Ti6Al7Nb titanium alloy substrates by electrophoretic deposition (EPD). The microstructure of the coatings was examined by scanning- and transmission electron microscopy methods as well as by X-ray diffractometry. The coatings, 770 nm–800 nm thick, were uniform, without any cracks or presence of large voids and they exhibited good adhesion to the titanium alloy substrate. The microstructure of the coatings consisted of nc-HA needle-like particles homogeneously embedded in a chitosan matrix. The deposited coatings exhibited good adhesion to the substrate. The best adhesion to the titanium alloy was determined for the coating deposited from suspensions containing 4 g/L of HA at 10 V during 240 s. The results confirm EPD as a convenient method to develop uniform and crack-free nanoscale organic-inorganic composite coatings on two phase titanium alloy substrates with potential application in orthopedic and dental implants.

Polyelectrolyte multilayer-calcium phosphate composite coatings for metal implants.

The preparation of organic-inorganic composite coatings with the purpose to increase the bioactivity of bioinert metal implants was investigated. As substrates, glass plates and rough titanium surfaces (Ti-SLA) were employed. The method comprises the deposition of polyelectrolyte multilayers (PEMLs) followed by immersion of the coated substrate into a calcifying solution of low supersaturation (MCS). Single or mixed PEMLs were constructed from poly-L-lysine (PLL) alternating with poly-L-glutamate, (PGA), poly-L-aspartate (PAA), and/or chondroitin sulfate (CS). ATR-FTIR spectra reveal that (PLL/PGA)10 multilayers and mixed multilayers with a (PLL/PGA)5 base contain intermolecular β-sheet structures, which are absent in pure (PLL/PAA)10 and (PLL/CS)10 assemblies. All PEML coatings had a grainy topography with aggregate sizes and size distributions increasing in the order: (PLL/PGA)n < (PLL/PAA)n < (PLL/CS)n. In mixed multilayers with a (PLL/PGA)n base and a (PLL/PAA)n or (PLL/CS)n top, the aggregate sizes were greatly reduced. The PEMLs promoted calcium phosphate nucleation and early crystal growth, the intensity of the effect depending on the composition of the terminal layer(s) of the polymer. In contrast, crystal morphology and structure depended on the supersaturation, pH, and ionic strength of the MCS, rather than on the composition of the organic matrix. Crystals grown on both uncoated and coated substrates were mostly platelets of calcium deficient carbonate apatite, with the Ca/P ratio depending on the precipitation conditions.

AC electrophoretic deposition of organic–inorganic composite coatings

Alternating current electrophoretic deposition (AC-EPD) of polyacrylic acid (PAA)–titanium oxide (TiO2) nanoparticle composites on stainless steel electrodes was investigated in basic aqueous solution. AC square wave with duty cycle of 80% was applied at a frequency of 1kHz. FTIR–ATR spectra showed that both AC and direct current (DC) EPD successfully deposited PAA–TiO2 composites. The deposition rate using AC-EPD was lower than that obtained in direct current DC-EPD. However, the microstructure and surface morphology of the deposited composite coatings were different depending on the type of electric field applied. AC-EPD applied for not more than 5min led to smooth films without bubble formation, while DC-EPD for 1min or more showed deposits with microstructural defects possibly as result of water electrolysis. AC-EPD was thus for the first time demonstrated to be a suitable technique to deposit organic–inorganic composite coatings from aqueous suspensions, showing that applying a square wave and frequency of 1kHz leads to uniform PAA–TiO2 composite coatings on conductive materials.

Fabrication of bio-inspired composite coatings for titanium implants using the micro-dispensing technique

Bio-inspired composite coating, which resembles the unique nano-structured composite bone tissue, is greatly needed in biomedical implant applications. The target of this coating is to create a continuous transition from tissue to implant surface, and modulate the biological response between the implant and host tissue. Our study on this bio-inspired composite coating is to fabricate organic---inorganic composite coatings (OICCs) and functionally graded coatings (FGCs). In this paper, a few commonly used coating methods have been evaluated on their capabilities on OICCs and FGCs fabrication. Compared with other available methods, the drop-on-demand (DoD) micro-dispensing technique enables us not only to flexibly fabricate multi-material coating at micron scale, but also to reliably construct multi-layer structures with varied material property distribution within a layer and/or among layers. This DoD micro-dispensing technique has been characterized based on three type of biomaterials (hydroxyapatite, titanium oxide and collagen) and dispensing parameters. The micro-fabrication process has been further investigated with regards to its capabilities on OICCs and FGCs. The fabricated samples on titanium substrate are characterized in terms of material distribution, adhesion, layer thickness and uniformity. The results show that the DoD micro-dispensing technique is capable to fabricate OICC and FGC samples in a single process. A comprehensive study on fabrication process and biological test will be conducted in the next stage.

Bio-Inspired Organic-Inorganic Composite Coatings for Implants via a Micro-Dispensing Technique

nanohydroxyapatite (nHA) and collagen were utilized to fabricate the bio-inspired organic-inorganic composite coating (OICC) via the Drop-on-Demand (DoD) micro-dispensing technique, which could flexibly construct multi-layer structures with varied materials composition within a layer and /or among layers reliably. This technique has been further investigated on its capability of OICC fabrication with regards to various materials (hydroxyapatite and collagen) as well as its dispensing parameters. A four-layered structure was formed, with the sequence of nHA-collagen-nHA-collagen from bottom to top. The dispensing parameters were also investigated with regards to the characteristics of the OICC fabrication. The coating was then subjected to various characterizations including scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and adhesion test. SEM and XRD results revealed that the DoD micro-dispensing technique did not change the morphology and phase of these two coating materials. And the results of EDS further demonstrated the corresponding elemental distributions within the four-layered coating structure which demonstrated the feasibility of the DoD micro-dispensing technique for the fabrication of thin-layered OICC.

Self-Healing Effect by Zinc Phosphate and Calcium Silicate Included in Organic-Inorganic Composite Coating on 55%Al-Zn Coated Steel Sheet

Recently, it has become necessary to improve the corrosion resistance of zinciferous coated steel sheets, especially 55% Al-Zn alloy coated steel sheets. However, it is known that cracks are easily generated on the surface of 55% Al-Zn alloy coated steel sheet during production processes. This is due to the hardness of the metallic coating. We studied non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that the composite coating containing both Zn3(PO4)2 and CaSiO3 had the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by the analysis of the specimens after corrosion test using SEM, AFM, TEM and EDX. It was found that the amorphous surface layer, which was approximately 30 nm in thickness and contained Ca, Cr, P, Si, C, and O, was newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca2+ and HPO42- are formed by dissolution of the CaO component in CaSiO3 and hydrolysis of ZnO3(PO4)2, respectively, and they react each other to form an insoluble Ca-phosphoric compound, which might inhibit the corrosion of metals exposed in the cracks.

55%Al-Zn 合金めっき鋼板上の有機-無機複合皮膜中に含有したリン酸亜鉛とケイ酸カルシウムによる自己修復効果

Recently, it has become necessary to improve the corrosion resistance of zinciferous coated steel sheets, especially 55% Al-Zn alloy coated steel sheets. However, it is known that cracks are easily generated on the surface of 55% Al-Zn alloy coated steel sheet during production processes. This is due to the hardness of the metallic coating. We studied non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that the composite coating containing both Zn3(PO4)2 and CaSiO3 had the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by the analysis of the specimens after corrosion test using SEM, AFM, TEM and EDX. It was found that the amorphous surface layer, which was approximately 30 nm in thickness and contained Ca, Cr, P, Si, C, and O, was newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca2+ and HPO42- are formed by dissolution of the CaO component in CaSiO3 and hydrolysis of ZnO3(PO4)2, respectively, and they react each other to form an insoluble Ca-phosphoric compound, which might inhibit the corrosion of metals exposed in the cracks.

Electrophoretic deposition of composite hydroxyapatite–chitosan–heparin coatings

Electrophoretic deposition method has been developed for the fabrication of organic–inorganic composite coatings for biomedical applications. The coatings were obtained as multilayer and functionally graded materials (FGM). Needle-shaped hydroxyapatite (HA) particles were prepared by a wet chemical method and used for the fabrication of chitosan–HA layers. The HA particles in the chitosan matrix showed preferred orientation of c-axis parallel to the substrate surface. Multilayer coatings were obtained, which contained pure chitosan layers and composite HA–chitosan layers. The thickness of the multilayer coatings was in the range of 2–100 μm. The thickness of the individual layers can be varied by the variation of deposition conditions. The feasibility of co-deposition of chitosan and heparin has been demonstrated. The proposed mechanism of heparin deposition is based on the use of non-stoichiometric chitosan–heparin complexes. The deposition yield and coating composition have been studied at various deposition conditions. The addition of heparin to chitosan solutions resulted in increasing deposition rate. Composite chitosan–heparin layers were used for the surface modification of HA–chitosan coatings. Obtained results pave the way for the electrophoretic fabrication of novel FGM coatings for biomedical implants with improved blood compatibility. The coatings were studied by X-ray diffraction analysis, thermogravimetric and differential thermal analysis, electron microscopy, and Fourier transform infrared spectroscopy.

Microstructure of organic–inorganic composite coatings studied by TEM and XANES

Chromate coatings on Zn or Zn alloy coated steel sheets often include silica for the aim to improve corrosion resistance. In the case of dry-in-place chromate coatings containing acrylic resin (hereafter referred to as an organic–inorganic composite coating), an addition of silica, however, did not show an improvement in corrosion resistance. The microstructures of the organic–inorganic composite coatings were observed by transmission electron microscopy (TEM) and the chemical states of Cr were investigated by the total electron yield X-ray absorption near edge structure (TEY-XANES) method. TEM samples were successfully prepared by dry ultramicrotomy preventing water-soluble components in the coatings fromdissolving out. TEY-XANES revealed the chemical states of components even in the organic matrix. Using these methods, it was found that the addition of silica changed just the morphology of the chromium compound in the organic–inorganic composite coating but not the chemical state of Cr. This is a reason for the addition of silica being not effective at improving corrosion resistance. The combination of dry ultramicrotomy-TEM and TEY-XANES spectroscopy was proven to be a powerful tool for characterizing organic–inorganic composite coatings.

Oxygen Permeability of Sol-Gel Coatings

Sol-gel processes are applied to prepare organic-inorganic composite coatings. The purpose of this report is to evaluate the possibility of tuning the oxygen permeability of the resulting coatings. It is proposed that if the oxygen permeability is largely controlled by the hydrophobic character and the pore sizes of membranes, then the permeability of the solgel coatings could be tuned by adjusting the composition of the organosilane precursor. The oxygen permeability is measured by the selective oxygen quenching of phosphorescent probes, such as platinum octaethylporphine (PtOEP). The application of such sol-gel coatings for optical fiber oxygen sensors will also be discussed.