Key Engineering Materials Vols. 336-338

Abstract: To obtain high performance ceramic matrix composites (CMCs), fiber coatings are often fabricated as the interphase between fiber and matrix. The SiC coating was synthesized at low temperature and reduced pressure in the present experiment. SiC was derived from a gaseous methyltrichlorosilane (MTS)/H2 precursor by chemical vapor infiltration (CVI). The thickness of the coating was inspected by SEM. The correlation between the coating thickness and the depositing conditions, i.e. the deposition temperature, the pressure, the deposition time per pulse and the pulse number were investigated. Based on these work, the C/SiC double-layer coating was fabricated.

Abstract: The in situ TiB whisker reinforced titanium matrix composites were prepared by mechanical alloying followed by spark plasma sintering. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the TiB whiskers. The effect of sintering temperature on morphologies of in situ TiB whiskers was evaluated. With the increase of spark plasma sintering temperature, the average diameter of in situ TiB whiskers increased. The in situ TiB whiskers exhibited a hexagonal shape with (100), (101) and (10 1 ) planes at the transverse section and a growth orientation of [010]TiB direction.

Abstract: Mullite whiskers were prepared by sol-gel method. The preparing conditions, the morphology and crystallite phase of the mullite whiskers were studied by Thermal analysis, XRD and SEM. The results demonstrate that the uniform mullite whiskers with high aspect ratio were obtained in properly preparing conditions and appropriate amount of fluorides.

Abstract: Mesoporous MCM-41 materials were prepared using the conventional thermal and microwave heating method. Both in conventional thermal and microwave heating synthesis of MCM-41, tetraethyl orthosilicate (TEOS) and cetyltrimethyl ammonium bromide (CTAB) were used as Si source and template, respectively. Microwave heating was carried out in a modified 400 W, 2.45 GHz microwave oven. In both methods, the crystallization temperatures was 70 °C and crystallization time was 3~12 h. The prepared MCM-41 samples were characterized by XRD, TEM and SEM. The XRD patterns of MCM-41 samples prepared from both the methods were typical of a relatively well ordered material. TEM results also revealed that the MCM-41 samples prepared from both the methods possess the regular hexagonal array of mesoporous channels. On the contrary, SEM has shown the difference in morphologies of the MCM-41 samples prepared from both the methods. The samples prepared by conventional thermal heating method mainly consist of irregular and small particles, whereas the samples prepared by microwave heating method have elongated rope-like structure and their length extends longer than 10 μm.

Abstract: In this paper, the mechanical properties of porous Si3N4/SiC composite prepared by different forming method (uniaxial pressing and gel-casting) were compared. Using gelcasting and two steps sintering technology, the porous SiC-particle/Si3N4 composites with a high strength, uniform structure and a relative high porosity were obtained by adding a little amount of nano carbon in slurry. The flexural strength of optimized material reaches to 100MPa and its porosity is more than 60%. But using uniaxial pressing and sintering at N2 atmosphere, because the green bodies have the high density, and the density distribution of a powder compact was not uniform, the flexural strength is not high, and at the same time, the microstructure is not uniform also.

Abstract: To study the effect of second sintering on intragranular structure and mechanical properties of 10vol%Al2O3/3Y-TZP composites, the composites were prepared by ball-milling nano-γ-Al2O3 and nano-ZrO2(3mol%Y2O3), pressing unidirectionally, cold-pressing isostatically, then first sintering at 1400°C for 2h and second sintering at 1600°C for 5h in air. The composition and microstructure of the composites were examined by X-ray diffractometer and scanning electron microscope, transmission electron microscope. The results showed that the second sintering changed the microstructure and fracture mode. After second sintering, the structure of fine grains was changed into the mixed one of large grains and small grains . Many large grains contained small grains, forming intragranular stucture that could be produced by the interblending of not only Al2O3 and ZrO2 but also ZrO2 itself. The intergranular fracture after first sintering was changed into transgranular fracture and intergranular fracture after second sintering . The second sintering led to grinds growing up and intragranular forming, which enhanced the contribution of the phase transformation toughening. Although the second sintering resulted in abnormal increase of grain size, the fracture strength and the fracture toughness of the composite were 667 MPa and 12.9 MPa⋅m1/2, respectively, due to the intragranular structure and the enhanced phase transformation toughening.

Abstract: The synthesis of aluminum titanate solid solution (Al2(1-x)MgxTi1+xO5, x=0, 0.1, 0.2) powders was realized by solid state reaction method using fine commercial α-Al2O3, TiO2 and MgO powders as raw materials. The effect of the introduced amount of MgO on the phase formation of aluminum titanate solid solution was investigated by XRD following the phase evolution. It was revealed that the introduction of MgO strongly affects the formation of Al2(1-x)MgxTi(1+x)O5 solid solution. The easily formed MgTi2O5 acts as the nucleus prompting the formation and lowering the synthesis temperature. The microstructure and properties of the solid solution ceramics were also studied by SEM and measuring the fracture strength and the thermal expansion coefficient.

Abstract: The effect of aluminium sol on the properties and microstructure of corundum-mullite ceramics was investigated. The sample with 1.5 wt% aluminium sol possessed higher room-temperature and high-temperature strengths, while the sample with 3 wt% aluminium sol had higher bulk density, lower apparent porosity and better thermal shock resistance. Aluminium sol not only acts as a binder instead of some polyvinyl alcohol (PVA), but also reacts with silica to form mullite, which improves the properties and microstructure of the ceramics by increasing corundum and mullite phases.