Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Adhesion and residual stress of plasma sprayed Alumina–titania coatings

Plasma-sprayed coatings are formed by the impacting of particles onto a fixed substrate layer-by-layer. Residual stresses inside the coatings are essential for their influencing on the coatings’ performance and durability during service life. In the present work, heat transfer and elastic–plastic residual stresses generation during plasma spraying in Al₂O₃–13wt.%TiO₂/NiCrAl (AT13) coating system were analyzed by finite element analysis (FEA). The sophisticated spraying process was simulated and the laminated structure of the coating was modeled under three-dimension. In this simulation, radial and axial compressive stresses were concentrated at the interfaces and inside the bond layer. Besides, at the specimen corner of the free edge, there were high tensile radial and axial stress concentrations. Such remarkable stresses, no matter tensile or compressive, may lead to the delamination and failure of coatings. Comparing with the numerical results, X-ray diffraction measurement was conducted on the AT13 coatings. As a result, the tested values matched well with the FEA simulated results.     

Effect of cerium concentration on microstructure,morphology and corrosion resistance of cerium–silica hybrid coatings on magnesium alloy AZ91D

The aim of this work was to investigate the effect of cerium concentration on microstructure, morphology and anticorrosion performance of cerium–silica hybrid coatings on magnesium alloy AZ91D. Vinyltriethoxysilane (VETO) and γ-glycidoxypropyltrimethoxysilane (GPTMS) were employed as precursors to prepare sol–gel based silica coating. Cerium nitrate hexahydrate as dopant in five different concentrations was added into the silica coatings. Fourier transform infrared (FT-IR) spectrum analysis, viscosity measurements and scanning electron microscopy (SEM) were employed to characterize the microstructure and morphology of these coatings. It was found that with the increase of cerium concentration, the degree of decomposition of silane chains in the coating network increased. The corrosion resistance of the cerium–silica hybrid coatings was estimated by electrochemical impedance spectroscopy (EIS) measurements and potentiodynamic polarization tests. The results demonstrated that corrosion resistance of coatings initially increases and then decreases as cerium concentration goes up. When the cerium concentration is 0.01 M, corrosion resistance reaches its maximum.     

The effect of SiC addition on the crystallization kinetics of atmospheric plasma–sprayed basalt-based coatings

The crystallisation kinetics of the conversion of a glass coating layer made from a mixture of natural basalt volcanic rock and SiC into glass-ceramic have been investigated. The process depends on the crystallisation temperature, time and amount of the SiC added. Coating powders were prepared from pure basalt and from basalt containing 10–50 wt% SiC. The powders were coated by an atmospheric plasma spray technique on the pre-coated AISI 1040 steel substrate with Ni–Al. The coating layer was vitrified by sudden cooling. The amorphous structure of the coatings was verified by X-ray diffraction (XRD) analysis. To obtain glass-ceramic, coatings were subjected to crystallisation heat treatment in an argon atmosphere. Crystallisation heat treatment temperatures of 800 °C, 900 °C and 1000 °C were chosen by using DTA. After the heat treatment process, augite, ferrian-diopsite, diopside, albite, andesine, and moissonite phases formed in the coating layer and were verified by XRD analysis. The crystallisation activation energies were determined to be between 323.4 kJ/mol and 253.2 kJ/mol, depending on SiC addition. The crystallisation activation energies decreased with increasing amounts of SiC addition. The Avrami parameters of the crystallisation process varied between 1.60 and 3.33, which indicates that internal crystallisation dominated for all of the compositions.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Improving hardness and toughness of plasma sprayed Ti–Si–C nano-composite coatings by post Ar-annealing

Ti–Si–C (TSC) composite coatings were fabricated by plasma spraying using Ti/Si/graphite agglomerates as feedstock. Ar-annealing was carried out to reduce the intrinsic defects and increase the performance of the as-sprayed TSC coating. The effects of the annealing temperature (500–900 °C) on the microstructures and mechanical performances of the TSC coatings were investigated. All TSC coatings consisted of TiC, Ti₅Si₃ and MAX phase Ti₃SiC₂. With the increase in temperature (>700 °C), TiC became predominant, while the Ti₃SiC₂ phase content increased, which was accompanied by a decrease in Ti₅Si₃ content. The high -temperature annealing (>700 °C) led to a homogenous microstructure with a relatively low porosity and increased number of micro-cracks. Notably, the hardness and fracture toughness of the TSC coating were simultaneously increased after the annealing, from 1164 HV to 1.96 MPa m^1/2 to 1560 HV and 3.45 MPa m^1/2, respectively. The formation of nanoscale TiC and Ti₅Si₃ with a network distribution, uniform and dense microstructure, and toughening effects of Ti₃SiC₂ and micro-cracks provided the high mechanical performances of the TSC composite coatings.     

Microstructures and tribological properties of vacuum plasma sprayed B4C–Ni composite coatings

A promising wear resistant coating has been fabricated via vacuum plasma spray (VPS) technique by using electroless plating composite powders comprised of B₄C and different amounts of Ni (10 and 20 vol.%). Tribological evaluation from the ball-on-disk test showed that the wear resistance of the composite coatings was superior to that of the pure B₄C coating, and the composite deposit containing 10 vol.% Ni demonstrated the optimum tribological properties. This mainly attributed to the more uniform microstructures of the composite coatings, and the higher thermal conductivity of the composite coating also contributed to its distinguished wear behaviors. For the coatings investigated, the dominant wear mechanism was determined to be oxidation and the formation of a transfer layer on the worn surface.     

Phase stability of plasma sprayed YAG–YSZ composite beads/coatings at high temperature

Beads and coatings of YAG–YSZ composite ceramic were prepared by plasma spray. YAG was applied as an additive in the hope of improving the phase stability and oxygen impermeability of YSZ. To achieve this aim, some basic research about crystallization behavior and chemical compatibility of the composite were carried out. Plasma sprayed YAG tended to form amorphous state. With the different content of YAG in the composite, crystallization of YAG and YSZ was delayed by each other to different extent due to the barrier effect of heavy atoms. The relative low melting point of YAG led to dense coatings without obvious splat structure and further distinctive vertical cracks during crystallization within the coatings. The cracks became less severe when YAG content was lower. With the addition of YAG, the development of monoclinic ZrO₂ was suppressed while Y-rich phases were promoted at higher temperature.     

Structure and thermal properties of heat treated plasma sprayed ceria–yttria co-stabilized zirconia coatings

Thick plasma sprayed thermal barrier coatings are suitable for thermal and hot corrosion protection of metal components in land-based turbine and diesel engines. In this work, ceria–yttria co-stabilized zirconia coatings were deposited by atmospheric plasma spraying in a mixture of non-transformable tetragonal t′ and cubic c zirconia phases. Free-standing coatings were isothermally annealed at 1315 °C for different times and their crystal structure was studied by XRD. No phase decomposition occurred. Columnar grains grew in the molten splats with increasing annealing time according to a preferential direction and, after 50 h of heat treatment, they were partially replaced by equiaxed grains. Both in-plane and out-of-plane thermal expansion coefficients (CTEs) were measured from coating expansion during heating. The CTE was slightly sensitive to thermal exposure in out-of-plane direction, whereas it kept almost constant in plane direction. The specific heat capacity Cp of annealed coatings, measured by differential scanning calorimetry (DSC), decreased in comparison with as-sprayed coating, due to high-temperature sintering.     

Weatherability of hybrid organic–inorganic silica protective coatings on glass

Currently, there is a growing interest in the application of silicon-based technologies for the development of advanced hybrid organic–inorganic coatings with strong weatherability. In this study, the sol–gel process is used to prepare such coatings on glass and their resistance to weathering effects is assessed afterwards. Various sols were prepared by mixing a silica-based inorganic matrix (tetraethyl orthosilicate) with different quantities of silica alkoxides functionalised with various organic groups. Subsequently, the sols were dip-coated onto glass samples at low temperatures without any heat treatment. The coatings prepared were analysed before and after three model ageing tests simulating various weathering parameters. After ageing, the best performing coatings showed good overall homogeneity and transparency (optical microscopy, SEM), improved water repellency and adhesion to the glass substrate (static contact angle measurements, cross-cut tape tests) and no colour or chemical composition changes (UV–VIS, FTIR). Compared with commercial hybrid silica products, the alkyl- and methacryloxy-functionalised silica coatings particularly displayed improved homogeneity, elasticity and barrier properties. Thus, these low temperature coatings, easily applicable to thin films, appear to fulfil the main requirements for the protection of the glass exposed to weathering phenomena.     

Atmospheric plasma spraying coatings from alumina–titania feedstock comprising bimodal particle size distributions

In this work, Al₂O₃–13 wt% TiO₂ submicron-nanostructured powders were deposited using atmospheric plasma spraying. The feedstocks were obtained by spray drying two starting suspensions of different solids content, prepared by adding nanosized TiO₂ and submicron-sized Al₂O₃ powders to water. The spray-dried granules were heat-treated to reduce their porosity and the powders were fully characterised in both untreated and thermally treated state. Comparison with two commercial feedstocks was carried out. Characterisation allowed a temperature for the thermal treatment to be chosen on the basis of the sprayability of the feedstock and the preservation as much as possible of the submicron-sized structure of the unfired agglomerates.Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates was also observed.Moreover, microhardness, toughness, adhesion and tribological behaviours were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows controlling the coating quality and properties. In general, good mechanical properties were obtained using a feedstock comprising a binary mixture of submicrometric Al₂O₃ and nanometric TiO₂ particles in the spray-dried powder.     

Enhanced corrosion-resistant performance of the PEO coatings on AA7075 alloy by a sol–gel-derived silica layer

The paper aimed to increase the corrosion-resistant performance of AA7075 alloy by the incorporation of sol–gel with plasma electrolytic oxidation (PEO) technologies. The AA7075 alloy substrate was first coated by an oxide ceramic layer with the help of the PEO process and then covered by a sol–gel-derived SiO₂ layer. The results from an X-ray diffraction and scanning electron microscope clearly show that a layer of sol–gel-derived SiO₂ uniformly covered and sealed the surface microporous structure of PEO coatings on AA7075 alloy, effectively enhancing the corrosion-resistant performance of PEO-coated AA7075 alloy in an aqueous solution containing 3.5-wt.% NaCl. The lower current density of 2.09 × 10⁻⁷ A/cm² and the larger polarization resistance of 5.35 × 10⁴ Ω cm² were found for sol–gel/PEO-coated AA7075 alloy as compared with 3.68 × 10⁻⁶ A/cm² and 9.81 × 10³ Ω cm² for individual PEO-coated AA7075 alloy. Thus, it is an effective strategy to improve the corrosion-resistant performance of the AA7075 alloy by combining the PEO and sol–gel techniques.     

HA/β-TCP plasma sprayed coatings on Ti substrate for biomedical applications

The present work focuses on the fabrication of βTCP (β-tricalcium phosphate) and HA/βTCP (hydroxyapatite/β-tricalcium phosphate) composite coatings by plasma spraying. The starting powders were produced via solid-state method using 2 wt% MgO to stabilize βTCP phase. The synthesized powders were preliminarily granulated to be used by the plasma spray process. Coatings obtained on titanium substrates are uniform and well adherent but due to the high temperature and cooling rate typical for plasma spraying process, βTCP phase is almost totally transformed into the α allotrope. Thermal treatment at 800 °C allows the reconversion of the phase αTCP→ βTCP. It is therefore possible to produce coatings with tuneable dissolution properties by selecting the proper initial powder mixture and the specific thermal treatment.     

Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol–gel coatings

This study evaluated the corrosion resistance of AZ91D magnesium alloy coated by composite coatings which consisted of a molybdate conversion coating and three layers of silicon sol–gel coatings. For molybdate conversion treatment, various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements. The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters (7.3 g/L (NH₄)₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30 °C).     

Porous water repellent silica coatings on glass by sol–gel method

Development of the solid surfaces with water-repellent and self-cleaning ability has attracted much research interest in recent years. In the present research work, we have prepared water repellent silica coatings on glass at room temperature (~27 °C) by sol gel process and surface silylation technique. Coating sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS), methanol (MeOH) and water (H₂O) constant at 1:12.36:4.25, respectively, with 0.01 M NH₄F. The dip coated silica films were surface silylated using two different silylating agents namely hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ). The HMDSO and HMDZ in hexane solvent were varied from 0 to 1 vol.% and silylation period was varied from 1 to 3 h. The HMDSO and HMDZ modified films showed dense and porous surface morphology, respectively. The HMDSO modified silica films showed static water contact angle of 122° whereas HMDZ modified films showed 165°. The HMDZ modified films displayed the extreme water repellency comparing with that of lotus leaves. The silica films were characterized by surface profilometer, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared microscopy, thermal and chemical aging tests, optical transmission and static water contact angle measurements.     

Anatase–rutile transformation of TiO2 sol–gel coatings deposited on different substrates

Titanium dioxide is widely used in a lot of applications. The properties of TiO₂ strongly depend on its phase composition. The transformation temperature between phases is influenced by a lot of factors. One of them is a type of substrate under the TiO₂ film. In presented work, thin films of TiO₂ were deposited by the sol–gel method on silicon, stainless steel (304 L) and Co–Cr–Mo alloy (Vitallium). The process of anatase–rutile phase transformation was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies of deposited coatings. The results were compared with anatase–rutile transformations temperature of TiO₂ powders obtained by analogous sol–gel process. The temperature of anatase–rutile phase transformation changed in the range of 700–1000 °C and strongly depends on a kind of substrate. It was found that anatase–rutile transformation of TiO₂ coating proceeded at a higher temperature than rutilization of titania powders.     

Selective photoepoxidation of propylene over hydroxyapatite–silica composites

Two types of hydroxyapatite (HAP)–silica composites were prepared from aqueous solutions of Ca(NO₃)₂ and (NH₄)₂HPO₄ in the presence of SiO₂ in a buffer (pH 7.5) and an alkaline condition. Photooxidation of propylene with oxygen was carried out over the composites at 303 K. The major product was propylene oxide which exceeded 80% among C3 oxygenated products. The conversion increased with an increase in HAP content, but the selectivity of propylene oxide decreased. This was caused by that propylene oxide was strongly adsorbed on HAP followed by consecutive photooxidation. The thin-layered HAP in the HAP–SiO₂ composite is more effective for photoepoxidation of propylene than the bulk HAP.     

Preparation of organic–inorganic hybridized dual-functional antifog/antireflection coatings on plastic substrates

Dual-functional antifog/antireflection coatings with a special bi-layer structure for plastic substrates were prepared. The superhydrophilic top layer was a crosslinked network of dipentaerythritol hexaacrylate (DPHA), hydrophilic agent (synthesized from Tween 20, isophorone diisocyanate, 2-hydroxyethyl methacrylate), and organically modified silica. The high-refractive-index bottom layer was composed of TiO₂ nanoparticles and DPHA. The two layers were chemically bonded through a UV-curing process. By tuning the thicknesses of the two layers, a series of coatings were prepared. These coatings were highly transparent and able to reduce reflectance. In addition, they adhered well to the substrate, and demonstrated superb antifogging capability on steam tests. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48822.     

Failure of plasma sprayed nano-zirconia-based thermal barrier coatings exposed to molten CaO–MgO–Al2O3–SiO2 deposits

Recently, nanostructured thermal barrier coatings have received considerable attention because of some superior properties in comparison with their conventional counterpart. In this study, nanostructured 8 wt% yttria-stabilized zirconia (n-YSZ) coatings were deposited by atmospheric plasma spraying, and the degradation behavior caused by molten calcium-magnesium-aluminon-silicate (CMAS) attack was investigated. Results showed that the thermo-chemical reaction product between CMAS and YSZ (both powders and coatings) is different with the change of CMAS content. At low CMAS concentration, a cubic phase is generated by the diffusion of Ca into YSZ grains. As compared to the conventional YSZ, less C-ZrO₂ is detected for n-YSZ. When CMAS reaches a certain concentration (eg 15 mg/cm²), disruptive phase transformation from tetragonal to monoclinic will occur and the reaction is more readily for n-YSZ. Two different chemical reaction mechanisms governing the CMAS content effect were proposed. It should be noted that the nanozone in the coatings plays an important role in the CMAS degradation process, which enhances CMAS infiltration rate and accelerates the chemical reaction, leading to a poor CMAS resistance of the nanostructured coating than that of the conventional counterpart.