Key Engineering Materials Vols. 512-515

Abstract: The corrosion resistance of liquid phase sintered (LPS) alumina ceramics in aqueous environments strongly depends on composition and chemistry of grain boundary glass formed during sintering. The chemical durability of model alumino-silicate glasses with various contents of CaO in aqueous solutions was therefore evaluated. Prepared glasses were corroded under hydrothermal conditions in deionized water under static conditions. The examination of surface morphology of corroded specimens after the contact with deionized water, together with the analysis of corrosion solution provided information on mechanism of dissolution of grain boundary glasses in LPS aluminas and confirmed that dissolution process is hindered due to saturation of solution with respect to leached elements. The initial dissolution rates for studied glasses were determined. The results are applicable for optimization and enhancement of corrosion resistance of LPS alumina under hydrothermal conditions.

Abstract: Transparent oxyfluoride tellurite glass with the composition of TeO₂-SiO₂-AlF₃-CaO-KF system were prepared by conventional melting and annealing technique, and the transparent oxyfluoride glass-ceramics containing CaF₂ nano-crystallines were obtained by controlled heat-treatment. The effect of heat-treatment schedules on the crystallization behavior and the microstructure of the glass-ceramics were analyzed by differential scanning caborimetry (DSC) analysis, X-ray diffraction (XRD) analysis, infrared (IR) spectrum and scanning electron microscopy (SEM). The sole CaF₂ crystalline phase was confirmed by XRD, the spherical CaF₂ nano-crystallines at about 20~100nm embed homogeneously among the glassy matrix after crystallization by SEM observation. The size of CaF₂ grains grew with the increase of crystallization temperature and holding time. Te and Si atoms existed in [TeO₃] and [SiO₄] forms. Part of Al atoms existed in the form of [AlO₄] and formed network together with [SiO4] tetrahedron. The other Al atoms acted as modifiers in the form of [AlO₆]. The transmittance of glass-ceramics can reach 85% in visible light and 90% in infrared waveband. The novel transparent oxyfluoride tellurite glass-ceramics are excellent matrix materials in up-conversion luminescence field.

Abstract: In this study,we prepared two kinds of glass frits include CAS and Bi-based glass frit as inorganic binders.Analysis the thermal properties and particle size of the prepared glass frits through DTA analyzer and laser mastersizer.Through sessile drop to research the wetting property between the prepared glass frits and alumina substrate in high temperature,and observe the cross-sections of the sintered samples through scanning electron microscope(SEM) to study their adhesion strength.

Abstract: The main objective of this paper was to investigate the addition of iron tailing sintering brick production, in the presence of clay, coal refuses and bentonite. Mixtures containing raw materials of sintering brick and iron tailings were prepared at different proportions (up to 55 wt %), fired at 980°C. Freeze/thaw durability, drying and firing shrinkages were investigated as well as the loss on ignition, bulk density and compressive strength of the fired samples. Their mechanical and microstructure properties were also investigated by differential thermal analysis (DTA/TG), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that compressive strengths of the brick samples are higher than that required by the standards MU15 of GB5101-2003, up to 21.79Mpa with 40% iron tailings corresponding to its higher bulk density completely. Moreover, the results showed that it has such advantages as no lime blowing, uniform color, good freeze/thaw resistance and slight universal frost.

Abstract: Nano-structrued SiO2 coating was prepared on metal substrate by sol-gel processing and the dip-coating technique using tetraethyl orthosilicate (TEOS) as precursor. And the superhydrophobicic SiO2 coating was obtained after modified with fluoroalkylsilane (FAS, CF3(CF2)7CH2CH2Si(OCH3)3). The morphology of obtained SiO2 coating was characterized by scanning electronic microscope (SEM). And the superhydrophobicity of SiO2 coating modified with FAS was characterized by contact angle meter. It was observed that the SiO2 coating showed superhydrophobicity with water contact angle 154.7° after modified with fluoroalkylsilane, and the superhydrophobicity was corrosion-resistance to acid or alkali to some extant.

Abstract: Nano-structured TiO2 coating was constructed through sol-gel process and dip-coating method on the stainless steel surface using tetra-n-butyl titanate as precursor. The phase and the crystallographic structure of the TiO2 coating were characterized by an X-ray diffractometer (XRD), and the surface topography and structures of the TiO2 coating were characterized by a scanning electron microscope (SEM). The superhydrophobic property of the TiO2 coating modified with the fluoroalkylsilane (FAS, CF3(CF2)7CH2CH2Si(OCH3)3) was characterized by the water contact angles. It was observed that the TiO2 coating showed superhydrophobicity with water contact angle 155.3° after modifying with FAS, and the superhydrophobicity was corrosion-resistance.

Abstract: Ceramic coatings were fabricated by plasma arc oxidation on Ti₆Al₄V alloy in phosphate –containing solution. The growth rate, microstructure and phase composition of ceramic coatings were investigated using coating thickness gauge, scanning electron microscope (SEM) and X-Ray diffraction (XRD). After plasma arc oxidation treatment, the microstructure of the remained Ti₆Al₄V alloy substrate remains unchanged and no hardening zone in the substrate is found. Ceramic coating formed on Ti₆Al₄V alloy surface by alternating-current plasma arc oxidation contains two layers: an outer layer and an inner layer. The outer layer coating consists of a large amount of rutile and a little anatase. But in the inner layer, anatase becomes a main phase while the rutile fraction obviously decreases, which indicates that phase transformation of anatase into rutile occurs in the duration of oxidizing process. In the mean time, with increasing treatment duration, coating growth varies from rapidness to tardiness accompanied by gradually roughening in appearance.

Abstract: The effects of intrinsic feature of feedstock in air plasma-sprayed (APS) coatings on the microstructure and mechanical properties of vertical-cracked thermal barrier coatings (TBCs) were investigated in thermal exposure. The microstructure after the thermal exposure for 400 h is densified, while after the thermal exposure for 800 h various defects such as interlamellar cracks and the vertical and horizontal cracks are newly developed, even though the total porosity is decreased. The microstructure of the TBC prepared with 204 C-NS is more porous than that of the TBC with 204NS, showing higher mechanical properties and better thermal stability in the TBC with 204 NS.

Abstract: Thermal conductivity was the main thermal parameter that greatly influenced temperature distribution of materials especially in coatings. Some materials such as plasma-sprayed coatings that had a lamellar microstructure displayed the anisotropy of thermal conductivity. Thermal conductivities of isotropy and anisotropy metallic ceramic coatings were calculated and analyzed using Markworth method and parallel plate model, respectively. The influence of anisotropy on heat transfer and temperature distribution of coatings using finite element analysis and Gaussian heat source modal applied locally was investigated. Results showed that in anisotropic coatings the ratio of transverse and vertical thermal conductivity had a maximum value when metal volume fraction was 50%. The temperatures on the edge of heat sources (Th) had a maximum value, and the highest temperatures of substrate (Ts) had a minimum value with the same fraction. The value decreased by 79°C compared to that in the isotropy coatings. The highest temperatures of coatings (Tc) decreased expectedly with metal volume fraction rised. The maximum width of heat affected zone (HAZ) of coating and substrate was 4.52 mm at the metal content of 40%. The minimum HAZ depth was 0.27mm which was very small at the content of 50%, compared with 1.25 mm in isotropic coatings. It revealed that anisotropic conductive properties caused more heat transfer to outer regions and provided better protection for substrate.

Abstract: Reticulated ceramic/metal composites (RCMC) are characterized by the ceramic skeleton and metallic matrix both continuous in three-dimensional space, and this special structure makes full use of the ceramic phase’s high strength and the metal phase’s toughness. Correspondingly, the deformation and damages mechanisms of RCMC under dynamic load conditions are very complicated and the related studies are quite important. In the current study, the dynamic properties of RCMC are investigated by using finite element methods based on the factitious microstructures of SiC/Al composites. The ceramic/metal interface is induced by employing a “constrained-tie-break” method, so as to study the effects of interface when suffering from a dynamic impact loads. Effects of the phase distribution, shape, ratio, as well as the size are systematically analyzed. The methods proposed in this paper would be helpful for predicting the dynamic behaviors of RCMC, and developing new ceramic/metal composites.