Key Engineering Materials Vols. 336-338

Abstract: Silicate fireproof coatings for tunnels (FCT) have many eminent properties. But low adhesion strength and poor water/fire-resistance of this kind of materials largely limit its applications. Here we reported a new kind of FCT based on high alumina cement as principle adhesive, redispersible powder as assisting adhesive, ammonium polyphosphate as fire-retardant material, vermiculite as adiabatic padding, and magnesium hydroxide as assisting reagents. The influence of various experimental conditions on fire resistance, adhesion strength and water resistance were carefully studied. Results showed that dispersible emulsoid powder was a key component affecting adhesion strength and water resistance of FCT, whereas fire-retardant material posed significant effects on the fire resistance.

Abstract: Oxidation resistant SiC coating for graphites was prepared by a pack cementation process with mixture powders of Si, C, SiC and some infiltration additives containing MgO, Al2O3 and Cr2O3. The influence of different infiltration additives on the phase, microstructure and oxidation resistance of SiC coating was primarily investigated. The results showed that adding infiltration additive in the pack powders is helpful for depositing denser SiC coating on graphite substrate; and the coating prepared by using MgO as infiltration additive exhibited better oxidation resistance at high temperature. XRD analyses revealed that SiC coating prepared by adding of MgO as infiltration additive possessed more free silicon; and the SEM analyses displayed that the free silicon could effectively fill the small holes of the SiC coating, which resulted in a perfect dense surface structure of SiC coating. Oxidation test showed that the as-prepared SiC coating could effectively protect graphite materials at 1500oC for 200 hours and the corresponding weight loss of the coated graphite was only 0.13%.

Abstract: Lanthanum-cerium oxide (La2Ce2O7, LC) is considered as a new candidate material for thermal barrier coatings (TBCs) because of its low thermal conductivity and high phase stability between room temperature and 1673K. The LC coatings with different La2O3 contents were prepared by air plasma spraying (APS) and their lifetime was evaluated by thermal cyclic testing from room temperature to 1373 K. The structures of the coatings were characterized by XRD and SEM and the deviation of the composition from the powder was determined by EDS analysis. Long time annealing for the freestanding coating at 1673K reveals that the near stoichiometric LC coating is stable up to 240h, and the stability decreases with increasing the deviation from stoichiometric LC composition. During thermal cyclic testing, spallation was observed within the top coat near the bond coat. It is considered that the effect of intrinsic stress caused by the coefficient of thermal expansion (CTE) mismatch between top coat and bond coat is larger than that of thermally grown oxide (TGO) and the bond adherence of top coat with TGO.

Abstract: Sm1.9Ca0.1Zr2O6.95 ceramic was sintered at 1600°C for 10 h in air by solid-state reaction method. The phase structure and thermal expansion coefficient were measured by XRD and a high-temperature dilatometry, respectively. The results show that the crystal structure of Sm1.9Ca0.1Zr2O6.95 ceramic is still pyrochlore. The doping with calcium cation leads to a shift of the X-ray spectrum of Sm1.9Ca0.1Zr2O6.95 ceramic to lower 2θ values. The experiments also show that the thermal expansion coefficients of Sm1.9Ca0.1Zr2O6.95 ceramic are higher than those of Sm2Zr2O7 ceramic. These results are related to the vacancy induced by doped calcium cation in the samarium lattice.

Abstract: This paper summarizes the basic properties of a series of rare-earth zirconate ceramics (Gd2Zr2O7, Sm2Zr2O7, Dy2Zr2O7, Er2Zr2O7 and Yb2Zr2O7). The phases and microstructures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermal properties of these materials were determined. The results indicated that Sm2Zr2O7 rare-earth zirconate ceramics have the lower thermal conductivity and the highest thermal expansion coefficient than other rare-earth zirconate ceramics. The dielectric constant decreases with the increase of atomic number.

Abstract: The influence of YSZ addition on isothermal oxidation behavior of EB-PVD NiAl bond coatings on Ni based superalloy was investigated. The oxidation rate of the YSZ doped bond coat is almost the same as the bond coat without the doping of YSZ. Four different areas exist on the surface of the coating after 100 h oxidation, including the spalled area, the high Al-contained area, the high Cr&Ni-contained area and the high Ti-contained area.

Abstract: Thermal barrier coatings with one-layered/ two-layered NiAl bond coat were produced by electron beam physical vapor deposition (EB-PVD). Compared to the TBC with one-layered bond coat, the TBC with two-layered bond coat improved the thermal cycling resistance significantly. The failure mechanism of the two-layer NiAl bond coat TBC was investigated in this paper.

Abstract: Sm0.4Ce0.6O1.8 specimen with a defective fluorite structure was synthesized and its thermophysical properties were characterized for thermal barrier coatings (TBCs) application. At high temperature, Sm0.4Ce0.6O1.8 exhibited much lower thermal conductivity than 7wt% yttria-stabilized zirconia (7YSZ)-the commonly used composition in current TBCs. Sm0.4Ce0.6O1.8 also possessed large thermal expansion coefficient, which could help reduce the thermal mismatch between the ceramic coating and bond coat.

Abstract: Diamond-like carbon (DLC) films have been deposited by radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) with different Ar-CH4 mixtures. Nanocrystalline diamond films have been deposited by microwave plasma-enhanced chemical vapour deposition (MPCVD), using Ar-H2-CH4 mixtures. X-ray photoelectron spectroscopy (XPS) and nanotribological investigation (by scanning force microscopy) have been used to compare the mechanical properties and structures of these films. Highly orientated and non-orientated microcrystalline diamond films and MPCVD-produced amorphous carbon have also been studied by way of comparison. The diamond films exhibit a linear relationship between roughness and the coefficient of friction. The DLC and amorphous carbon have higher friction coefficients than the best performing diamond film, but may more easily be deposited as smooth coating. Possible applications for these various carbon-based films include microelectromechanical components, for which smooth, hard coatings are required.

Abstract: Corrosion experiments on a number of rare earth di-silicates containing Y, Yb and Lu have been conducted in flowing air with 30 vol.% H2O at 1500°C. Nominally the corrosion rate is slower by a factor 5 to 10 compared to silica. However, alumina impurity incorporation has a profound influence on the process of corrosion as it masks not only the gravimetric results but also changes the surface phase assemblages to contain rare earth garnets and liquid phases. It is presumed that silica loss occurs under those conditions via the liquid phase and does not necessarily follow the same kinetics as direct silicate corrosion. Furthermore, atmospheric silicon hydroxide saturation effects contribute to the corrosion process. Currently all determined corrosion rates of rare earth silicates are viewed as system specific only.