Key Engineering Materials Vols. 434-435

Abstract: ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposites incorporated with and without Ag addition have been fabricated by spark plasma sintering (SPS) to evaluate their friction and wear properties in sliding against alumina ball from room temperature to 600 oC. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. The ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposite exhibits low and stable friction coefficients of 0.2 to 0.3 and wear rates in the order of 10-6 mm3/Nm at high temperatures. With the addition of Ag into the composite, the intermediate temperature lubricating property is greatly improved. Plastic deformation of SrSO4 and sliver during sliding plays an important role in formation of lubricating films on worn surfaces of nanocomposites. These lubricating films reduce the friction and wear of the ZrO2(Y2O3)-Al2O3 matrix composites.

Abstract: Recent ideas on the oxidation of ZrB2-SiC are presented, emphasizing the behavior of the boria-silica-zirconia liquid (BSZ liquid) which forms during oxidation and flows to transport silica and zirconia to the surface of the scale. Oxide film microstructure is strongly influenced by the boron oxide, which acts as a volatile flux.

Abstract: MAX phases, include Ti3SiC2, Ti2AlC etc, are machinable ternary carbides or nitrides with excellent properties. These materials, however, have obvious nonlinear elastic deformation due to nano- layered crystal structure. The stress-strain curves of cyclically load-unload test have obvious hysteretic loops. Because of this mechanical hysteresis, the strain of MAX phases at one time is not determined only by the stress applied to the sample at this time. Here the influence of grain size, chemical composition and porosity on the nonlinear elastic strain was introduced. Because of two properties of this hysteresis: wiping out and congruency, the classic hysteretic mathematic model (Preisach-Mayergoyz model, P-M model) can be applied to calculate the strain of MAX phases after any complex deformation history.

Abstract: According to the practical physical dimension and processing parameters of ladle with bottom gas blowing in a factory, a geometrical model was established and mesh structure was completed, Eulerian multiphase model and porous media model were adopted for the two-phase flow in ladle, which was coupled, the effect of temperature field and thermal stress in course of working process of permeable- gas refractory ceramic was studied by finite element approach. The correlations between processing, structure, and high-temperature mechanical properties were discussed, and the material and structure of permeable-gas ceramic were designed to optimum. The results show that: The steady gas flow rate can avoid wearing of permeable-gas refractory ceramic from cataclysm of temperature-stress; After the material and structure of permeable-gas refractory ceramic are optimized, the effect on ceramic wearing of temperature-stress in condition of oxygen washing is weakened. It is an important guiding for prepara- tion of permeable-gas refractory ceramic with long life, which will make for highly active refining ladle.

Abstract: A NFE model is constructed to analyze the convective heat transfer steady thermal stress in a ZrO2/FGM/Ti-6Al-4V composite ECBF plate with temperature-dependent material properties. From numerical calculation, when , T0=300K, Ta=550K and Tb=1800K, the stress distributions in the plate were obtained. The results are as follows. With the increase of the FGM gradient layer thickness, the stress distribution in the composite plate is more reasonable. With the increase of M, the thermal stress change increases obviously in metal and ceramic layers, and the stress on the ceramic surface reduces. With the increase of porosity, the change of stress at the bonding interfaces increases, and the tensile stress on the surface of ceramics reaches the maximum. Compared with , when , the stress on the metal surface reduces by 25.1%, and the maximum compressive stress on the surface of ceramics increases about 1.2GPa. Compared with the nongraded two-layered ceramic/metal composite plate, the thermal stress of ZrO2/FGM/Ti-6Al -4V composite ECBF plate is very gentle. When we consider the temperature dependency of the material properties, the thermal stress in graded three-layered composite plate becomes small obviously. The results provide the foundations of theoretical calculation for the design and application of the composite plate.

Abstract: Nano silicon carbide ceramics were prepared via precursor pyrolysis. Polycarbosilane (PCS) and divinylbenzene (DVB) were selected as a precursor to silicon carbide ceramics and a cross-linking reagent for PCS, respectively. The cross-linking properties and pyrolysis of PCS and DVB were investigated by changing the mass ratios of PCS/DVB. The mass ratio of PCS/DVB has a great effect on silicon carbide ceramic yield. The cured PCS/DVB with a mass ratio of 1:0.5 has the highest SiC ceramic yield (63.1%) at the temperature up to 1500 °C and its pyrolyzates consiste of nano silicon carbide with a diameter of 10-40 nm. The microstructures of the nano silicon carbide ceramics were characterized by SEM and XRD. The pyrolysis behavior of the cured PCS/DVB was characterized by thermogravimetry in nitrogen atmosphere.

Abstract: Dense bulk ZrB2 ceramic was synthesized by mechanical alloying (MA) and followed spark plasma sintering (SPS) using zirconium and boron as initial materials. It was found that MA process was effective to fragment the coarse metal zirconium particles from 45 m to less than 1 m within 20 hours. In comparison with the commercial ZrB2 powder, the as-obtained zirconium and boron mixture powders showed higher sinterability. When the sintering was carried out at 1800oC, the relative density of synthesized ZrB2 samples using mixture powder was above 95%, higher than that of ZrB2 sample prepared using commercial powder (73%). Vickers hardness of those ZrB2 samples was at the same level of 15 GPa. However, the fracture toughness of ZrB2 samples seemed to depend on the heating rate of the SPS process. Corresponding to the heating rates of 10, 50, and 100oC/min, the fracture toughness of as-prepared ZrB2 samples were 3.83, 3.19, and 2.74 MPa•m1/2, respectively.

Abstract: The use of secondary-ion mass spectrometry (SIMS), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) to detect the existence of amorphous silica in Ti3SiC2 oxidised at 500–1000°C is described. The formation of an amorphous SiO2 layer and its growth in thickness with temperature was monitored using dynamic SIMS. Results of NMR and TEM verify for the first time the direct evidence of amorphous silica formation during the oxidation of Ti3SiC2 at 1000°C.

Abstract: Ternary ZrC-SiC-ZrB2 ceramic composites were prepared by hot pressing at 1900 °C for 60 min under a pressure of 30 MPa in argon. The influence of ZrB2 content on the microstructure and mechanical properties of ZrC-SiC-ZrB2 composites was investigated. Examination of SEM showed that the microstructure of the composites consisted of the equiaxed ZrB2, ZrC and SiC grains, and there was a slight tendency of reduction for grain size in ZrC with increasing ZrB2 content. The hardness increased considerably from 23.3 GPa for the ZS material to 26.4 GPa for the ZS20B material. Flexural strength was a strong function of ZrB2 content, increasing from 407 MPa without ZrB2 addition to 627 MPa when the ZrB2 content was 20vol.%. However, the addition of ZrB2 has little influence on the fracture toughness, ranging between 5.5 and 5.7 MPam1/2.

Abstract: We have investigated the electronic structures and chemical bonding of four tungsten borides, including two WB2 compounds with different crystal structures, α-W2B5 and ε-WB2.5, by ab initio calculations based on density function theory (DFT). The calculated density of state (DOS) shows that all compounds are metallic. The DOS at Fermi level is mainly contributed from 5d states of W atoms. The strong covalent bonds of boron atoms make these compounds stable. Due to a lack of electrons in boron sublattices, weak ionic bonds are generated. The charge density distributions indicate the solid B layers or B polyhedrons are interleaved by the W layers.