Key Engineering Materials Vols. 512-515

Abstract: After fiber reinforced ceramics occur a crack, its bridging fibers must engender, moreover the crack usually runs in the format of similarity. In order to analyze easily dynamic problems of fiber reinforced ceramics, bridging fiber segment is substituted for loads. When a crack moves at high speed, its fibers will break continuously. By application of the built dynamic model and the ways of self-similar functions, analytical solutions of the displacements, stresses and dynamic stress intensity factors under the action of a running increasing force Px2/t2 and a motive unit step load, respectively, can be gained, and it is also used to attain the concrete solution by means of superposition theorem.

Abstract: Adhesion is one of the most important mechanical properties of ceramic coatings. Scratch testing is considered as a simple and effective method to evaluate adhesion of ceramic coatings. In this paper, the critical normal forces and scratch morphologies for different coating-substrate systems were studied by scratch testing. It is shown that the critical normal force obtained by acoustic emission (AE) signals decreases from 12 N to 7 N when the applied normal force rate increases from 20 N/min to 100 N/min for CVD SiC on C, and the failure area of this scratched sample increases with increasing maximum normal force; Based on scratch morphologies, spallation or delamination can be observed for hard-brittle coatings on glass or metal, while discontinuous or continuous ductile perforation can be observed for ductile coatings on metal; The critical normal force for hard-brittle coatings can be effectively obtained by AE signals.

Abstract: Polycarbosilane (PCS) is one of precursor systems applied in industry for the Polymer Infiltration Pyrolysis (PIP) processing of C or SiC fiber reinforced SiC matrix composites (Cf/SiC or SiCf/SiC CMC) materials. In this work, a series of PCS-derived SiC specimens treated at 300, 550, 800, and 1100 °C was studied by XRF, IR, XRD and XPS methods. The results show that a slight amount of PCS molecules with low softening point evaporates below 300 °C; the cross-link between PCS chains takes place to form a 3D -Si-C-Si- network via the reaction between -Si-CH3, -Si-CH2-Si- and -Si-H groups in the temperature range of 300-550 °C; an amorphous phase of SiC was observed when PCS was heated up to 800 °C, which indicates the completion of transformation from organic to inorganic; above 800 °C, β-SiC domain grows. Furthermore, a surface structure evolution of PCS-derived SiC specimens with temperature is proposed after the analyzing the XPS spectra.

Abstract: The sp³C doped SiC superhard nanocomposite films had been deposited on stainless steel (SS) substrates at different temperature by electron beam-physical vapor deposition (EB-PVD). The sp3C doped SiC film was studied by grazing incidence X-ray asymmetry diffraction (GIAXD), and X-ray photoelectron spectroscopy (XPS). The results of GIAXD showed that the sp³ doped SiC nanocomposite films were not perfect crystalline, which was composed with fine SiC nanocrystals, and a second phase very similar with diamond like carbon (DLC). XPS analysis showed that the excess C existing in films and turned from diamond into DLC from the surface to inner of film.

Abstract: For a long time since the anisotropy basically confined to a single crystal, used as a polycrystalline ceramic materials generally considered to be isotropic. In this paper, the anisotropic mechanical, thermal expansion coefficient and thermal conductivity of a double perovskite slab-rocksalt layer Nd₂SrAl₂O₇ was studied by first principles as an example. The method is using density functional theory (DFT) and crystal parameters, which has been used to calculate the modulus of elasticity of anisotropic in a three-dimensional space. While combined with traditional thermal conductivity theory, we have obtained the tensor of thermal diffusion, thermal conductivity in 3D space for the first time in the world within no. The results are in good agreement with the experiment. The advantage of method is avoiding the difficulty of experimental measurement, reducing the time and obtaining relatively accurate results.

Abstract: The fatigue crack growth tests for nickel-based GH4133B superalloy used in turbine disk of a type of aero-engine are carried out at room temperature. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding threshold stress intensity factor ranges are determined. Using the Paris formula, the experiment data of fatigue crack growth are analyzed. It is shown that the fatigue crack growth rate increasing with increasing stress intensity factor range and stress ratio, and a modified Paris formula considering threshold stress intensity factor range can describe the fatigue crack growth behavior precisely. The fracture surface morphologies are investigated using a scanning electron microscope. It is shown that in the crack initiation region, steady growth region and rapid growth region, the fracture surface exhibits a cleavage fracture mode, fatigue striations and an intergranular fracture mode, respectively. Finally, the von Mises stresses and stress intensity factors at the crack tip of specimen of GH4133B superalloy at various external loads and crack lengths are simulated using the finite element method, and the threshold stress intensity factors under different maximal external loads at a certain crack length are calculated. The comparison between test and simulation indicates that the stress intensity factors at the crack tip calculated by the finite element method agree well with experimental data.

Abstract: The carbonated hydroxyapatite (CHA) was synthesized by precipitation-calcination method. The influences of carbonate subsitution on high-temperature sintering, thermal expansion coefficient (CET) and flexural strength were investigated by the high-temperature dilatometer, scanning electron microscopy (SEM) and universal testing machine. The results showed that the sintering temperatures of CHA were related to the initial carbonate contents. The sintering temperature decreased with increasing initial carbonate contents. The CET of CHAs decreased with the increase of carbonate content, due to the stoma caused by the partially decompostion of CHAs. The CHA ceramics tested were as strong in flexure strength when compared to non-carbonated hydroxyapatite.

Abstract: Oxyfluoride tellurite glass with the composition of TeO₂-AlF₃-LaF₃-ZnO/ZnCl₂/ZnF₂ was prepared successfully. The thermal stability and structure of TeO₂-AlF₃-LaF₃ system tellurite glass were studied by_{Subscript text} DSC and IR spectra. The results indicated that glass transition temperature of TeO₂-AlF₃-LaF₃-ZnO/ZnCl₂/ZnF₂ glass was higher than that of (1-x)TeO₂-xAlF₃ (x=10%, 20%, 30%, 40%, 50%, in mol%) binary glass system slightly. A small number of ZnF₂/ZnCl₂/ZnO (5mol%) improved glass thermal stability. After adding 5mol% ZnF₂/ZnO/ZnCl₂ into fluoride tellurite glass respectively, glass transition temperature increased in turn. The introduction of 5mol% different zinc compounds had a little impact on the glass network structure.

Abstract: Light-weight insulation materials were prepared by extrusion processing using forsterite raw tailings, light-burned magnesia and fused magnesia as main raw materials with addition of rice hull powder as a pore-forming agent. The results show that the plasticity of green compact and the cold compressive strength of bricks are improved with the addition of 2-3 wt% dextrin as plasticizer. Light weight insulation materials, which meet different application requirements, were prepared by controlling the addition amount of rice hull powder by firing at 1550 °C with volume weight of obtained materials varies from 1.4-1.8 g/cm3, thermal conductivity at 600 °C is less than 0.48 W/(m•K), cold compressive strength varies from 8-24MPa and reheating linear change rate is less than 1% (1400°C × 6h).

Abstract: In order to reduce energy costs, light-weight alumina-spinel refractories with tri-modal pores were prepared by foaming process. The influence of foaming agent and magnesia on the properties of light-weight alumina-spinel refractory was investigated. The rheology of the original slurry and the density, mechanical properties and microstructure of samples were evaluated. The results showed that light-weight alumina-spinel refractory contained a highly interconnected network of spherical cells, consisting of the large-sized cells (larger than 40 um), moderate-sized pores( averaging about 20 um) and small-sized voids, which mainly attributed to the foaming agent and in-situ formation of spinel. The cold compressive strength of the samples varied within the range of 2-12MPa, corresponding to densities of 0.8~1.2g/cm3,and the thermal conductivity changed from 0.273W/(m•K) to 0.315 W/(m•K) at the 500 °C. It was found that the density of the sintered samples was dependent both on the content of foaming agent and magnesia. With the help of magnesia, light-weight alumina-spinel refractories with good volume stability were prepared by foam method.