Synthesis of hexagonal prisms and hexagonal plates of Na2SiF6 microcrystals

Hexagonal Na₂SiF₆ prisms and hexagonal or dodecagonal Na₂SiF₆ plates were prepared using Na₂CO₃, SiO₂, and HF under microwave irradiation for the large-scale production of ice-analog materials. The morphology of the Na₂SiF₆ crystals changed from long thin hexagonal prisms to flat hexagonal or dodecagonal plates with pyramidal faces as the Na₂CO₃ concentration was increased. X-ray diffraction studies confirmed that the axes of the hexagonal Na₂SiF₆ prisms were aligned along the c axis. The microwave irradiation time played an important role in controlling the morphology and aspect ratio of the Na₂SiF₆ microcrystals. The mechanisms of formation of the various Na₂SiF₆ microcrystalline morphologies are discussed.     

Corrosion resistance property of micro–arc oxidation coatings on Mg–Li alloy obtained in different systems

通过三种优化工艺体系在Mg--5%Li合金表面上生长陶瓷膜层, 分析了膜层的厚度、显微结构、相组成和耐蚀性. 结果表明, 三种膜层都含有MgO相, 微弧氧化试样的耐蚀性能都明显提高. 使用Na₃PO₄体系制备的膜层含有MgF₂, 膜层最厚、表面有大量裂纹; 使用Na₂SiO₃体系制备的膜层含有橄榄石型Mg₂SiO₄, 耐点蚀性能最好; 使用Na₂SiO₃--Na₃PO₄体系制备的膜层含有MgSiO₃, 致密性最好, 膜层耐均匀腐蚀性能最好.     

Analysis of degradation process during the incorporation of ZrO2:SiO2 ceramic nanostructures into polyurethane coatings for the corrosion protection of carbon steel

Three different molar ratios of ZrO₂:SiO₂ mixed oxides (25:75, 50:50, 75:25) were produced by the sol–gel technique and sintered at different temperatures (400, 600, 800, and 1000 °C) in order to analyze differences in mechanical and electrochemical properties of a wide variety of organic–inorganic hybrid coatings on AISI 1018 commercial carbon steel. For this purpose, 2, 4, and 6 wt% of the obtained ZrO₂:SiO₂ nanoparticles were incorporated to the polymeric matrix under vigorous stirring and spread on metallic substrates to reach between 40 and 55 μm of dry film. Light microscopy, scanning electron microscopy, confocal laser scanning microscopy studies, atomic force microscopy, and nanoindentation tests were used to evaluate morphological, topographical, and mechanical properties; whereas atmospheric corrosion and electrochemical impedance spectroscopy (EIS) were performed using a 3 wt% NaCl medium in continuous immersion for 226 days. The crystallite size of the as-prepared ZrO₂:SiO₂ nanoparticles changed according to the sintering temperature from 4 to 9 nm. It was found that an adequate dispersion and homogeneity was achieved when 2 wt% of sintered ZrO₂:SiO₂ nanoparticles were mechanically mixed with polymer (MDI) to produce hybrid coatings on the metallic substrate. Free-bubble surface and hardness enhancement can be achieved by adding nanostructures assuming fact that the particles are capable of occupying the gas bubble sites. Atmospheric corrosion in the coatings without reinforced particles was more severe than that observed in hybrid coatings, and for these, corrosion was higher according to the increasing zirconia molar ratio. The EIS studies indicated that the synergetic effect between the organic–inorganic phases to seal the surface enhances the barrier properties on this metallic substrate.     

Controlled synthesis of microarc oxidation coating on Ti6Al4V alloy and its antifriction properties

Abstract

Ceramic coatings were fabricated on a Ti6Al4V alloy surface by microarc oxidation (MAO) in Na₂ SiO₃ – (NaPO₃)₆ aqueous solutions with and without NaAlO₂ additive using an AC power supply. The effect of NaAlO₂ on microstructure, composition, and homogeneity of ceramic coatings were characterised using SEM, XRD, and EPMA. The antifriction property of the coatings with optimised microstructure sliding against SAE 52 100 steel ball was investigated on a pin-on-disc friction and wear tester. The results show that the addition of NaAlO₂ into Na₂ SiO₃ – (NaPO₃)₆ solution assists the formation of more dense, uniform, and thicker coatings and increases rutile TiO₂ content in the coatings. The optimised coating sliding against the steel has a friction coefficient as low as 0.2 – 0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of material from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterised by slight abrasive wear and adhesive wear.     

In situ fabrication and wettability of Ca2SiO4/CaTiO3 biocoating by laser cladding technology on Ti–6Al–4V alloy

Abstract

Wetting is the first and foremost event when a biomaterial is implanted into the biological system. Hence, it is very essential to investigate the wettability of a biomaterial before further biological studies. A textured coating with different relative amounts of Ca₂SiO₄ and CaTiO₃ was in situ fabricated by varying laser scan speed. The wettability of different coatings was investigated. The results indicate that the relative amount of Ca₂SiO₄ phase increased with decreasing laser scan speeds and reached the highest value of 48·17±2·10 mJ mm^?2, and the geometrically textured topography with a surface roughness of 9·17 μm was obtained at a laser scan speed of 2 mm s^?1. The microstructure in the coating can be characterised as fine dendrites. Surface energy values varied with the relative amount of Ca₂SiO₄ phase. The coating obtained at the laser scan speed of 2 mm s^?1, which contains more relative amount of Ca₂SiO₄, presents the highest surface energy, indicating most desirable wettability. This resulted in an increase in contact angle in simulated body fluid solution for improved wettability. The microhardness presented a gradient distribution from the coating surface (1072 HV) to the substrate (260 HV).     

Electrodeposition of silicon from molten salts

Silicon coatings were electrodeposited at 750 °C from alkali metal fluoride melts containing K₂SiF₆. The deposit grows three dimensionally from nuclei formed simultaneously. The surface structure of the coating is voltage dependent. On Inconel, extensive diffusion of the alloy constituents into the silicon takes place.     

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO<Subscript>2</Subscript>–Na<Subscript>2</Subscript>CrO<Subscript>4</Subscript> electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti₂AlNb alloy in various NaAlO₂ electrolytes containing 2 g L^?1, 4 g L^?1, and 6 g ^?1 Na₂CrO₄ additive, respectively. The influence of Na₂CrO₄ additive in NaAlO₂ electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na₂CrO₄ additive in the NaAlO₂ electrolyte not only promotes the formation of γ-Al₂O₃ phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr₃O and (Al_0.948Cr_0.052)₂O₃ phases. The PEO ceramic coatings formed in NaAlO₂ electrolytes with 2 g L^?1 and 4 g L^?1 Na₂CrO₄ additive show better oxidation resistance than those PEO coatings formed in a NaAlO₂ basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO₂ electrolyte with 4 g L^?1 Na₂CrO₄ additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO₂ basic electrolyte without any Na₂CrO₄ additive.     

Effect of zirconia sol in electrolyte on the characteristics of microarc oxidation coating on AZ91D magnesium

The microarc oxidation coatings were prepared on AZ91D magnesium alloy in a Na₂SiO₃-KOH electrolyte with and without zirconia sol, respectively. The effect of zirconia sol as an additive in the electrolyte on the surface morphologies, compositions, structures, and corrosion resistances of the coatings were investigated. It was found that the coating formed in the Na₂SiO₃-KOH electrolyte with zirconia sol has more uniform morphology, less micropores and cracks than that formed in an electrolyte without zirconia sol. The phase compositions of the coatings also varied after addition of zirconia sol in the electrolyte, owing to the participation of zirconia sol in the reaction and its incorporation into the oxide coating, and Zr existed in the form of Mg₂Zr₅O₁₂. The results of potentiodynamic polarization analysis show that the coating formed in the electrolyte with zirconia sol increases significantly corrosion resistance for magnesium alloy.     

Effects of SiO<Subscript>2</Subscript> substitution on wettability of laser deposited Ca-P biocoating on Ti-6Al-4V

Silicon (Si) substitution in the crystal structure of calcium phosphate (CaP) ceramics has proved to generate materials with improved bioactivity than their stoichiometric counterpart. In light of this, in the current work, 100 wt% hydroxyapatite (HA) precursor and 25 wt% SiO₂-HA precursors were used to prepare bioactive coatings on Ti-6Al-4V substrates by a laser cladding technique. The effects of SiO₂ on phase constituents, crystallite size, surface roughness, and surface energy of the CaP coatings were studied. Furthermore, on the basis of these results, the effects and roles of SiO₂ substitution in HA were systematically discussed. X-ray diffraction analysis of the coated samples indicated the presence of various phases such as CaTiO₃, Ca₂SiO₄, Ca₃(PO₄)₂, TiO₂ (Anatase), and TiO₂ (Rutile). The addition of SiO₂ in the HA precursor resulted in the refinement of grain size. Confocal laser microscopy characterization of the surface morphology demonstrated an improved surface roughness for samples with 25 wt% SiO₂-HA precursor compared to the samples with 100 wt% HA precursor processed at 125 cm/min laser speed. The addition of SiO₂ in the HA precursor resulted in the highest surface energy, increased hydrophilicity, and improved biomineralization as compared to the control (untreated Ti-6Al-4V) and the sample with 100 wt% HA as precursor. The microstructural evolution observed using a scanning electron microscopy indicated that the addition of SiO₂ in the HA precursor resulted in the presence of reduced cracking across the cross-section of the bioceramic coating.     

钛合金微弧氧化一步制备含石墨的减摩涂层

为了进一步提高钛合金表面微弧氧化陶瓷涂层的摩擦磨损性能,在石墨分散的Na2CO3-Na2SiO3-KOH电解液溶液中一步制备了含自润滑微粒的微弧氧化复合涂层.利用X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)和扫描电子显微镜(SEM)研究了未添加和添加石墨微弧氧化涂层的相组成和微结构,采用往复式球-盘试验机评价了两种涂层的摩擦学性能.结果表明:加入到电解液中的石墨在微弧氧化过程中进入到涂层中,从而得到含有固体润滑微粒的复合涂层;在干摩擦条件下,含石墨的微弧氧化涂层相比于不含石墨的涂层具有更小的摩擦系数.     

Theoretical Analysis of Spectral Selective Transmission Coatings for Solar Energy PV System

Spectral selective transmission coatings can adjust the spectrum of incident solar radiation to reduce the waste heat generation in solar cells and to improve solar photovoltaic conversion efficiency. The ideal spectral transmissivity of the coatings should match the spectral response of solar cells, which means the ideal spectral transmissivity should be equal to 1.0 in the range of the spectral response and 0 in the other spectrum. The reflection performance of the three kinds of spectral selective transmission coatings for silicon solar cells are designed and analyzed. The results indicated that the ZnS/Na₃AlF₆ coating systems have a wider reflected infrared (IR) region than the TiO₂/SiO₂ coating systems, but lower transmissivity in the wavelength range of 0.5 μm to 1.1 μm. Furthermore, an Nb₂O₃/SiO₂ coating system is proposed and optimized, which has 31 layers with a smaller total thickness of 2.675 μm. The radiative properties including reflectivity and transmissivity of spectral selective transmission coatings are investigated and theoretically analyzed.     

Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation

Plasma-sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings were successfully fabricated on titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma gun. The microstructure, phase constituents and mechanical properties of the titanium alloys before and after ablation were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-sectional microstructure and hardness of titanium alloys with coatings are similar before and after ablation. In contrast, the microstructure and mechanical properties of the titanium alloy without coating are significantly changed after ablation. The surface coating is found to serve as a protective coating during ablation.     

The outward-inward growth behavior of microarc oxidation coatings in phosphate and silicate solution

Microarc oxidation (MAO) coatings have a unique outward-inward growth behavior, which is crucial to the dimensional matching of precision-matching parts after surface treatment. Here, a notably different outward-inward growth behavior of MAO coatings was observed in phosphate solution and silicate solution, respectively. The results indicate that the outward-growth dimension of MAO coating in Na₂SiO₃ solution is notably larger than that in (NaPO₃)₆ solution, especially at the initial stage of MAO treatment, but as an increase of oxidation time, the inward-growth behavior enhances in both solutions. It is considered that the different adsorptive capacity of solute anions leads to the various outward-inward growth behaviors of MAO coatings in phosphate solution and silicate solution.     

Techniques for High Temperature Fatigue Testing

The influence of small concentrations of SO₂, HCl or Na₂SO₄ vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO₂ in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO₂ at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO₂ and 100ppm HCl, but SO₂ reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 DS + Hf and CMSX-6 in the presence of Na₂SO₄ vapour in air at 1000 and 1100°C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarified and it is still an open question whether the corrosion is caused by acidic fluxing of a liquid Na₂SO₄?MoO₃ film or by the reaction between Na₂SO₄ vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant extension rate tests with IN 792 DS + Hf and CMSX-6 at 1000°C did not reveal any influence of 100 ppm HCl on the mechanical properties at strain rates of 1 × 10^?6 and 3 × 10^?8 S^?1.     

Effect of Na2SiO3 on synthesis of TiO2 nanopowders by thermal processing of the precursor

TiO₂ nanopowders could be prepared by thermal processing of the precursor of titanium hydroxide, urea and Na₂SiO₃. The powders were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results show that Na₂SiO₃ has an important effect on the average crystallite size and dispersity of TiO₂ nanopowders, and other phases (Na₂SO₄ and SiO₂) will be introduced. However, Na₂SO₄ has distinctive intercalation ability and catalytic activity; SiO₂ coating layers can effectively inhibit the agglomeration of TiO₂ nanopowders. TiO₂ (anatase) powders with well dispersity can be prepared at ～600 °C for 2 h with addition of 2–6 wt.% Na₂SiO₃, and the average crystallite size is 15.5–22.1 nm. The surface of the sample mainly consists of Ti, O, C, Si, Na and S six species elements.     

Fabrication and characterization of ZrO2–CaO–P2O5–Na2O–SiO2 bioactive glass ceramics

SiO₂–CaO–Na₂O–P₂O₅–ZrO₂ based bioactive glasses with different compositions of SiO₂ and yttrium stabilized ZrO₂ were prepared by the conventional melt quenching technique. The effects on the chemical–mechanical properties of bioactive glasses due to the addition of ZrO₂ by replacing SiO₂ were investigated. Microstructure and phase behavior were studied by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction analysis. Compressive strength, porosity, Vickers hardness, and Young’s modulus were measured as mechanical properties. Bioactivity and cell viability were investigated by immersion in simulated body fluid and MTT assay analysis. Osteosarcoma cell proliferation on the specimen surfaces was examined by confocal laser scanning microscopy. The results showed that replacing SiO₂ with ZrO₂ helps the bioactive glass to be completely vitrified at comparatively lower sintering temperature than conventional Bioglass^®. The mechanical properties were also improved without compromising biocompatibility. Bioactive glass containing 10 wt% ZrO₂ and 35 wt% SiO₂ showed compressive strength of 399.71 MPa, Young's modulus of 22.3 GPa, Vicker’s hardness of 502.54 HV, and porosity of 26 vol%.     

The Degradation Behavior of SiC<Subscript>f</Subscript>/SiO<Subscript>2</Subscript> Composites in High-Temperature Environment

SiC_f/SiO₂ composites had been fabricated efficiently by Sol-Gel method. The oxidation behavior, thermal shock property and ablation behavior of SiC_f/SiO₂ composites was investigated. SiC_f/SiO₂ composites showed higher oxidation resistance in oxidation atmosphere, the flexural strength retention ratio was larger than 90.00%. After 1300 °C thermal shock, the mass retention ratio was 97.00%, and the flexural strength retention ratio was 92.60%, while after 1500 °C thermal shock, the mass retention ratio was 95.37%, and the flexural strength retention ratio was 83.34%. After 15 s ablation, the mass loss rate was 0.049 g/s and recession loss rate was 0.067 mm/s. The SiO₂ matrix was melted in priority and becomes loosen and porous. With the ablation going on, the oxides were washed away by the shearing action of the oxyacetylene flame. The evaporation of SiO₂ took away large amount of heat, which is also beneficial to the protection for SiC_f/SiO₂ composites.     

Microstructure developments of F-doped SiO2 thin films prepared by liquid phase deposition

This study presents a systematic investigation of the microstructure dependence of liquid phase deposition (LPD) of SiO₂ films on solution parameters and deposition temperature. The corresponding deposition rate and film roughness were also evaluated under various deposition conditions. Smooth and sufficiently dense SiO₂ films, which are the prerequisite for reliable low-k dielectric applications, were deposited on both silicon and fluorine-doped tin oxide coated glass substrates from supersaturated hydrofluorosilicic acid (H₂SiF₆) solution with the addition of boric acid (H₃BO₃). It is shown that H₂SiF₆ acid controls the surface morphology and grain structure through surface reaction while H₃BO₃ acid prompts bulk precipitation in solution. For the 208-nm thick SiO₂ film, the breakdown field exceeded 1.9 MV/cm and the leakage current density was on the order of 10^− 9 A/cm² at 4 V, indicating excellent insulating properties of LPD SiO₂ films. The strong presence of Si-O-Si and some Si-F with little Si-OH bond as shown in FT-IR spectra indicate that the LPD SiO₂ films have mostly a silica network with some fluorine (F) content. F-doping was self-incorporated into the silica films from the H₂SiF₆ solution during deposition process.     

In-Situ TiB2–TiC–Al2O3 Composite Coating on Aluminum by Laser Surface Modification

To improve the surface hardness of aluminum, in-situ TiB₂–TiC–Al₂O₃ composite coating was deposited on it by pre-placed laser coating process using precursor mixture of (TiO₂ + B₄C) and (TiO₂ + B₄C + Al). Pulsed Nd:YAG laser was used to produce coating track by scanning a laser beam in overlapped condition. Multiple tracks again overlapped to get a wider coating area. Phase constituents and microstructure of the deposited coating were studied by XRD and FESEM analysis. Vickers micro-hardness tester was used to measure micro-hardness of the coating. Results indicate that, in appropriate laser processing condition, coating was obtained with metallurgical bonding to aluminum substrate. XRD and microstructure analysis confirms the formation of TiB₂, TiC, and Al₂O₃ in the coating layer through in-situ reaction of reactant powders. Micro-hardness of the coating was found appreciably higher in comparison to the as-received aluminum substrate, due to presence of hard ceramic particles produced during in-situ reaction and their grain refinement for rapid cooling.     

Osteoblast interaction with laser cladded HA and SiO2-HA coatings on Ti–6Al–4V

In order to improve the bioactivity and biocompatibility of titanium endosseous implants, the morphology and composition of the surfaces were modified. Polished Ti–6Al–4V substrates were coated by a laser cladding process with different precursors: 100 wt.% HA and 25 wt.% SiO₂-HA. X-ray diffraction of the laser processed samples showed the presence of CaTiO₃, Ca₃(PO₄)₂, and Ca₂SiO₄ phases within the coatings. From in vitro studies, it was observed that compared to the unmodified substrate all laser cladded samples presented improved cellular interactions and bioactivity. The samples processed with 25 wt.% SiO₂-HA precursor showed a significantly higher HA precipitation after immersion in simulated body fluid than 100 wt.% HA precursor and titanium substrates. The in vitro biocompatibility of the laser cladded coatings and titanium substrate was investigated by culturing of mouse MC3T3-E1 pre-osteoblast cell line and analyzing the cell viability, cell proliferation, and cell morphology. A significantly higher cell attachment and proliferation rate were observed for both laser cladded 100 wt.% HA and 25 wt.% SiO₂-HA samples. Compared to 100 wt.% HA sample, 25 wt.% SiO₂-HA samples presented a slightly improved cellular interaction due to the addition of SiO₂. The staining of the actin filaments showed that the laser cladded samples induced a normal cytoskeleton and well-developed focal adhesion contacts. Scanning electron microscopic image of the cell cultured samples revealed better cell attachment and spreading for 25 wt.% SiO₂-HA and 100 wt.% HA coatings than titanium substrate. These results suggest that the laser cladding process improves the bioactivity and biocompatibility of titanium. The observed biological improvements are mainly due to the coating induced changes in surface chemistry and surface morphology.