Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

Effect of Nb2O5 on ZnBiMnO varistor ceramic prepared by solid-state sintering at 850°C

Nb₂O₅ is a commonly used donor dopant for ZnO-based varistor ceramics, but its effect, especially on the low-temperature sintered ZnO varistor ceramics, is not fully understood. To provide a possible answer to this problem, ZnO–Bi₂O₃–MnCO₃ (ZnBiMnO) based varistor ceramics with 0.05%–0.3% (in mole ratio) Nb₂O₅ were fabricated by solid-state sintering at 850 °C for 3 h. Their microstructure and nonlinear electrical properties were studied by XRD, SEM and the standard current-voltage (I–V) tests to reveal the effect of Nb₂O₅. With the increase of Nb₂O₅ from 0.05 mol% to 0.3 mol%, more solid Bi₅Nb₃O₁₅ inter-granular particles form within the ceramic during sintering, thereby decreasing the Bi-rich liquid phase. As a result, the average size of ZnO grain decreases from 4.35 μm to 1.67 μm. This microstructural change leads to the increase of the breakdown voltage in the range of 821 V/mm to 1851 V/mm. The ZnBiMnO varistor ceramic with 0.1 mol% Nb₂O₅ shows the best nonlinear properties. The optimum nonlinear coefficient is 35.81, the breakdown voltage is 907.51 V/mm, and the leakage current is 7.72 μA/cm². The result of this study provides a promising candidate material for manufacturing the multilayered low-voltage varistor that may use Ag, Ni or even Cu as the inner electrodes.     

Effect of Si content on the microstructure and properties of Ti–Si–C composite coatings prepared by reactive plasma spraying

In this study, Ti–Si–C composite coatings were synthesized via plasma spraying of agglomerated powders prepared by a spray drying/precursor pyrolysis technology using Ti, Si, and sucrose powders. The influence of Si content, ranging from 0 wt% to 24 wt%, on the microstructure, mechanical properties, and oxidation resistance of the composite coatings was investigated. Results show that the phase composition of the Ti–Si–C composite coatings changes with the increasing Si content. The coatings without Si addition consist of TiC and Ti₃O; the coatings with 6–18 wt% Si are composed of TiC, Ti₅Si₃, and Ti₃O; the coatings with Si content of 24 wt% form only TiC and Ti₅Si₃ phases. As the Si content increases, the hardness of the Ti–Si–C composite coatings increases first and then decreases, depending on the intrinsic hardness of the ceramic phases, the brittleness of Ti₅Si₃, and the defects such as pores and cracks. The Ti–Si–C composite coatings have high wear resistance due to the in-situ synthesized high-hardness TiC and Ti₅Si₃. Owing to the high brittleness of Ti₅Si₃, the increasing Si content leads to higher wear volume loss at room temperature, which can be partially improved in high-temperature wear tests. The oxidation resistance of Ti–Si–C composite coatings increases with the increase of Si content, and the higher the oxidation temperature, the more obvious the influence of the Si addition on oxidation resistance.     

Electrodics of methanol oxidation on Pt-f-multiwalled carbon nanotube composite, prepared by γ-radiolysis

We report the electrodics of methanol oxidation on Pt-multiwalled carbon nanotube composites (Pt-f-MWCNTs), prepared by γ-radiolysis of K₂PtCl₆ in the presence of HOOC-functionalized multiwalled carbon nanotubes. The electrocatalytic activity for the methanol oxidation was studied using cyclic and linear sweep voltammetric techniques on the stationary indium tin oxide and rotating gold disc electrodes, respectively. Higher values of oxidative (anodic) current were obtained using Pt-f-MWCNTs compared to the polycrystalline Pt electrode. This phenomenon is attributed to the synergistic effect of oxy groups on MWCNTs, which alleviate CO poisoning. The electrodics of the reaction at various temperatures was studied using linear sweep voltammetry (LSV) on a rotating disc gold electrode, modified with the composite. From the Koutecky-Levich plots, the standard rate constant (k⁰) was determined to be 7.9 ± 1.9 × 10⁻⁸ cm s⁻¹.     

Electrothermal parameters and useful life of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C under electric load

The electrothermal behavior of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C was investigated. The analytical expressions correlating the linear density with the electrical resistance, heat capacity, thermal conductivity, temperature of the surface of the fibre, predicted useful life, and electric load were obtained. A method was developed for conducting and mathematically interpreting the experiments on determination of the lifetime before combustion of conducting carbon fibre of different linear density as a function of the strength of the electrical current passed through it. The lifetime of the carbon fibre in air with no electric load was equal to 6.3·10¹⁰ sec and decreased exponentially with an increase in the current strength. The specific resistance is approximately 5.2·10⁻⁵ Ω·m at 20°C, the specific heat capacity varied from 0.64 to 0.93 J/(g·K), and thermal conductivity of 83 to 120 W/(m·K) in the 0–100°C temperature range. UVIKOM, Mytishchi. Translated fromKhimicheskie Volokna, No. 1, pp. 55–58, January–February, 2000.     

Ablation resistance of HfC–SiC coating prepared by supersonic atmospheric plasma spraying for SiC-coated C/C composites

In order to improve the ablation properties of carbon/carbon composites, HfC–SiC coating was deposited on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC–SiC coating were characterized by SEM and XRD. The ablation resistance test was carried out by oxyacetylene torch. The results show that the structure of coating is dense and the as-prepared HfC–SiC coating can protect the C/C composites against ablation. After ablation for 30 s, the linear ablation rate and mass ablation rate of the coating are −0.44 μm/s and 0.18 mg/s, respectively. In the ablation center region, a Hf–Si–O compound oxide layer is generated on the surface of HfC–SiC coating, which is conducive to protecting the C/C composites from ablation. With the ablation time increasing to 60 s, the linear ablation rate and mass ablation rate are changed to −0.38 μm/s and 0.26 mg/s, respectively. Meanwhile, the thickness of the outer Hf–Si–O compound layer also increases.     

Effect of plasma treatment of aluminum on the bonding characteristics of aluminum–CFRP composite joints

The effect of surface treatment of aluminum on the bonding characteristics of aluminum- CFRP (carbon fiber reinforced plastic) composite joints has been investigated. The surface of the aluminum panel was treated by DC plasma. The plasma treatment was carried out at different treatment times and volume ratios of acetylene gas to nitrogen gas. The volume ratios used were 1 : 9, 3 : 7, 5 : 5, 7 : 3, and 9 : 1. The treatment times used were 10, 20, 30, 40, 50, and 60 s. Optimal plasma treatment conditions were determined by measuring the T-peel strength and water contact angle as functions of the treatment time and gas volume ratio. Single lap shear tests and T-peel tests were performed using plasma-treated aluminum-CFRP composite joints and regular aluminum-CFRP composite joints to determine the effect of treatment on the shear strength and T-peel strength. The results showed that the water contact angle was minimum and the peel strength was maximized when the aluminum was plasma-treated for 30 s at a volume ratio of 5 : 5. The shear strength of aluminum-CFRP composite joints plasma-treated under the optimal treatment conditions was 33% higher than that of regular aluminum-CFRP composite joints. The T-peel strength of plasma-treated aluminum-CFRP composite joints was almost six times larger than that of regular aluminum-CFRP composite joints. SEM examination showed that the improvement in bond strength was attributed to a uniform spread of the epoxy adhesive due to the surface energy increase of aluminum and this resulted in a cohesive failure of the epoxy adhesive.     

Effect of carbon nanotubes on sintering behavior of alumina prepared by sol–gel method

Alumina (Al2O3)/carbon nanotube (CNT) (99/1 by weight) composite was prepared by mixing CNT dispersion with AlCl₃-based gel, followed by high temperature sintering at a temperature up to 1150 °C in argon. Composite alumina precursor showed phase transition order from amorphous to γ-Al₂O₃ after sintered at 900 °C for 2 h, partially to θ-Al₂O₃ after sintered at 1000 °C for 2 h, and then partially to α-Al₂O₃ after sintered at 1150 °C for 2 h. By comparison, control alumina precursor directly transformed from amorphous to α-Al₂O₃ after sintered at a relatively low temperature of 600 °C for 2 h. Composite alumina showed porous structure with pore diameter ranging from 100 nm to 2 µm, whereas control alumina was relatively pore-free. The elevated alumina-crystal phase transition temperatures and the formation of porous structure were ascribed to the presence of CNTs in alumina precursor. The composite alumina sintered at 900 °C for 2 h containing only γ-Al₂O₃ had a BET surface area of 138 m²/g, which was significantly higher than that of control alumina sintered at 1150 °C for 2 h containing only α-Al₂O₃, ~15 m²/g.     

Nanostructured ceramic composite coating prepared by reactive plasma spraying micro-sized Al–Fe2O3 composite powders

Nanostructured ceramic matrix composite coating was prepared in-situ by reactive plasma spraying micro-sized Al-Fe₂O₃ composite powders. The microstructure, toughness and Vickers hardness of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the coating exhibited nanostructures which consisted of FeAl₂O₄, Al₂O₃, Fe (or Fe solid solution) and a little FeAl. The composite coating showed significantly higher toughness and wear resistance than the conventional Al₂O₃ coating.     

Study of the ablation of a carbon/carbon composite at ∼25 MW/m2 with a nitrogen plasma torch

Ablation of carbon/carbon (C/C) composites was investigated in a nitrogen plasma torch with a heat flux of ∼25 MW/m². The reaction products of carbon in C/C composites and nitrogen plasma jet were calculated based on the principle of free energy minimum. The calculated results show that the thermal chemical ablation and sublimation of carbon occur and C_n(g) (n = 1–3), CN(g), C₂N(g) and C₂N₂(g) may be the major reaction products consuming carbon. Ablation is apt to begin at the interfaces, especially the fibre-matrix interface and interfaces inside the matrix. Ablation of C/C composites is mainly controlled by the thermal chemical ablation, sublimation of carbon, and mechanical breakage. The formation of needle-shaped fibres and shell-shaped matrices is attributed to both the thermal chemical ablation and sublimation of carbon, while carbon fragments and fractured fibres or matrices result from the mechanical breakage.     

Degradation at 1200°C of a SiC coated 2D-Nicalon/C/SiC composite processed by SICFILL® method

The thermal stability of a 2D-Nicalon/C/SiC composite was studied through the variation of both mechanical properties and microstructure occurring during heat treating. The composite was processed by infiltration of SiC preforms according to SICFILL® method. The material toughness was enhanced by a carbon interphase put between the fibers and the matrix. In order to improve the thermal stability a CVI layer was deposited on the carbon interphase and the specimen surfaces were CVD covered by an external SiC seal coating about 165 μm thick. The aging tests were carried out at 1200°C in air or in non oxidizing environment (vacuum). Other specimens were thermally cycled between 25 and 1150°C. Three point bending tests and Charpy impact measurements were performed before and after these treatments. The composite microstructure was investigated by scanning electron microscope (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), reflectance infrared spectroscopy (FTIR) and surface area BET measurements. The as-processed material showed a modulus of rupture (MOR) of 483 MPa and appreciable toughness. These characteristics were retained after aging (200 h at 1200°C) under vacuum. Air thermal treatments caused heavy loss of strength and increase of brittleness. Strong oxidation occurred during these last treatments at both the carbon interlayer and the matrix, while the SiC external sample coating was not oxidized. The oxygen needed for composite bulk oxidation flowed through the SiC coating due to the occasional presence of very few structural defects.     

TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Characterization and properties of iron-incorporated gismondine prepared at 80°C

Iron-incorporated zeolites were successfully synthesized at a low temperature such as 80°C by choosing appropriate starting materials and characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), wide-angle X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and magnetic susceptibility. ICP-AES showed␣that Fe component can be readily incorporated␣up to a maximum extent of Fe substitution, Fe/(Fe + Al) × 100 = 22.7%. XRD measurements suggested that the zeolites obtained have a crystal structure of gismondine type. The characterizations identified that the Fe component present in the products is all incorporated into the zeolite framework. The ammonia and water desorption profiles were compared for Fe-free and 22.7% Fe-zeolites ion-exchanged for NH₄⁺ by means of TG-MS and DSC. The ammonia desorption peak temperatures considerably shifted toward lower temperatures by the introduction of Fe, suggesting decreased solid acidity. DSC thermograms of the as-synthesized gismondines revealed that they do not contain free water (i.e., water not coordinated to cations) in the pores irrespective of the Fe content. The enhanced catalytic reactivity of the Fe-incorporated gismondines was also confirmed from the decomposition of hydrogen peroxide. An apparent activation energy of 43 kJ mol⁻¹ was obtained independent of the Fe contents in zeolites. This value was much lower than 70 kJ mol⁻¹ for the same reaction in the homogeneous solution containing iron alum as a reference sample.     

Influence of countersurface materials on dry sliding performance of CuO/Y-TZP composite at 600 °C

Dry sliding wear tests on 5 wt.% copper oxide doped yttria stabilized zirconia polycrystals (CuO–TZP) composite have been performed against alumina, zirconia and silicon nitride countersurfaces at 600 °C. The influences of load and countersurface materials on the tribological performance of this composite have been studied. The friction and wear test results indicate a low coefficient of friction and specific wear rate for alumina and zirconia countersurfaces at F = 1 N load (maximum Hertzian pressure ～0.5 GPa). Examination of the worn surfaces using scanning electron microscope/energy dispersive spectroscopy confirmed the presence of copper rich layer at the edge of wear scar on the alumina and zirconia countersurfaces. However, Si₃N₄ countersurface sliding against CuO–TZP shows a relatively higher coefficient of friction and higher wear at 1 N load condition. These results suggest that the countersurface material significantly affect the behavior of the third body and self-lubricating ability of the composite.     

Cyclization during heat bodying of safflower oil at 300°C

Summary Safflower oil was heat-bodied at 300°C., and its methyl esters were fractionated by vacuum-distillation and with urea. A monomeric cyclic compound was isolated as the non-adduct-forming distillate. It was presumably a cyclized product of methyl linoleate as it has a mean molecular weight of 293.7 (theo. 295) and a mean unsaturation of approximately one double bond per mole of methyl ester corresponding to a hydrogen deficiency of 1.96 to 2.01.     

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi₂ composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi₂ interlayer and Nb₅Si₃ interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi₂ composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm² and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO₂ layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm² and 15 μm, respectively. The Si–NbSi₂ composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.     

Effect of Al addition on creep resistance of MgO-Al2O3 composite for sliding plate at 1400 °C

MgO-Al₂O₃ composites were prepared using fused magnesia, α-Al₂O₃ micropowder, tubular alumina, metal aluminum and sintered magnesia as raw materials, magnesium aluminate sol as the binder. The creep resistance test was conducted at 1400 °C under 0.2 MPa with insulation for 50 h. The specimens after creep resistance test were characterized and analyzed by XRD, SEM, and EDS to investigate the effect of adding metal aluminum powder on the creep resistance of magnesia based composites as well as its mechanism. The results show that Al-MgO-Al₂O₃ composite has better creep resistance than MgO-Al₂O₃ composite. Power Al is more reactive than AlN, adding Al function is to accelerate both spinel solid solution(containing MgAlON) and AlN-polytype, both are reinforcement phase for MgO-Al₂O₃ composite. The mechanism that metal Al powder improves the creep resistance of the specimen M₂ can be expressed as follows：An oxygen concentration gradient exists in the specimen M₂ from the external areas to the internal areas. The Al in external areas is oxidized into Al₂O₃ and further solid-solves with MgO forming MgAl₂O₄. The oxidation of Al leaves shell structures which can bear some compressive stress, restraining the volume shrinkage. When the oxygen concentration is low, Al is nitrided forming reinforcing phases such as AlN polytype and lamellar-structure MgAlON with reticular distribution, restraining the volume shrinkage of specimen M₂.     

Enhanced corrosion resistance of layered double hydroxides/steam coating composite coating prepared by the hydrothermal method on LA43M magnesium–lithium alloy substrates

An excellent anticorrosion Mg–Al layered double hydroxide (LDH) composite coating was successfully fabricated on LA43M magnesium alloy substrates via an in situ steam coating (SC) process and a subsequent hydrothermal treatment at different temperatures. The microstructure, composition and phase formation of the composite coatings were studied via X-ray diffractometer, energy disperse spectroscopy, and scanning electron microscope, respectively. The corrosion resistance of composite coatings was further investigated using electrochemical measurements and corrosion test. The results showed that LDH/SC composite coating has typical nanosheets microstructure, which effectively seal the defects of SC. As the hydrothermal temperature increases, the thickness and density of nanosheets increases, and the corrosion resistance was significantly improved. Especially, the Mg–Al LDH/SC composite coating prepared at 100°C was the most dense and thickness, and exhibited the optimal and long-term anticorrosion resistance in 3.5 wt.% NaCl soultion. It has the lowest I_corr (1.767 × 10⁻⁸ A/cm²), which decreased by three and two orders of magnitude compared with the bare substrate and SC. Furthermore, it can maintain good chemical stability after immersion in the corrosion medium for 192 h and its hydrogen evolution rate (0.00416 mL·cm⁻²·h⁻¹) and weight lost rate (0.00266 mg·cm⁻²·h⁻¹) were the lowest compared with other samples.     

Effect of fiber architecture and density on the ablation behavior of carbon/carbon composites modified by Zr–Ti–C

Three carbon/carbon (C/C) composites modified by Zr–Ti–C, with different fiber architecture in preforms and the same density, were prepared using chemical vapor infiltration and reactive melt infiltration methods. Two other samples with the same architecture in preforms and different density were also fabricated by the same methods. Their ablation behaviors were examined by oxy-acetylene flame. The results showed that the samples with chopped web needled perform had better ablation resistance than that of the samples with needle-integrated and fine-weave pierced perform. In the models of ablation behaviors, the sealing time of pores and gaps on the ablated surfaces has been defined to indirectly estimate the ablation property. The analysis of models also indicated that high density of the composites and appropriate small diameter of bundles of carbon fibers led to the short sealing time and good ablation resistance of the C/C–carbide composites.     

A Si–SiC oxidation protective coating for carbon/carbon composites prepared by a two-step pack cementation

To prevent carbon/carbon (C/C) composites from oxidation, a Si–SiC coating has been prepared by a two-step pack cementation technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis show that the coating obtained by the first step pack cementation is a porous β-SiC structure, and a dense structure consisting α-SiC, β-SiC and Si is obtained after heat-treatment by the second step pack cementation. By energy dispersive spectroscopy (EDS) analysis, a gradient C–SiC transition layer can be formed at the C/C-coating interface. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 166 h at 1773 K in air. The weigh loss of the coated C/C is due to the formation of bubble holes on the coating surface and through-coating cracks in the coating.