Advanced Materials Research Vols. 194-196

Abstract: The POB/PTFE composites were prepared by blending-cold pressing-sintering-hot pressing method. Influence of POB content on mechanical and tribological properties of the composites was investigated. Worn surfaces were analyzed by scanning electron microscope (SEM). The results indicated that ball indentation hardness and compressive strength of POB/PTFE increase with increasing POB content in composites. Tensile strength, elongation and impact strength of POB/PTFE decrease with increasing POB content in composites. Friction coefficient of composites increases compared to PTFE. Wear resistance of POB/PTFE composites was improved with filling POB. The POB/PTFE composite incorporating 20% POB has best comprehensive mechanical properties. Wear mechanism of PTFE are abrasive wear. Wear mechanism of POB/PTFE composites mainly are fatigue wear and flaking.

Abstract: In this work, the composites were fabricated by in-situ synthesis reaction and hot-pressing using Ti, Al, TiO₂ and Eu₂O₃ as starting materials. Effects of the sintering temperature on the microstructures and properties of the Al₂O₃/TiAl composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and universal properties tests. The result shows that the sintering temperature had effect on improving the mechanical properties of TiAl composites. The phase of the composites composed of γ-TiAl/_α2-Ti₃Al matrix and reinforcing phases of Al₂O₃ and EuAlO₃. The grain size of composite was smaller than 0.5 µm at 1200 °C. And the mechanical properties reached to the maximum value (477.96 MPa and 9.73 MPa•m^1/2). The properties decreased with grain growth at 1300 °C.

Abstract: Al₂O₃/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al, TiO₂ and Cr₂O₃. The effect of Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites has been characterized. The results show that the Rockwell hardness, flexural strength and fracture toughness of the composites increase with increasing Cr₂O₃ content. When the Cr₂O₃ content is 3.5 wt %, the flexural strength and the fracture toughness reach the maximum value of 658.7 MPa and 7.2 MPa•m^1/2, respectively. The improvement of mechanical properties is firstly associated with a more homogeneous and finer microstructure developed by addition of Cr₂O₃; secondly, it is related to the increase of the ratio of α₂-Ti₃Al/γ- TiAl matrix phases.

Abstract: Flax fabric was woven and composites were produced by using the VARI technique with flax fabric as the reinforcement and unsaturated polyester as the matrix. Laminates with two, three and four layers were fabricated respectively. After saturated in the water for different durations of time (7, 14, 21 and 30 days), the tensile strength of the composites was tested. After being soaked in the water for 7, 14 and 21 days, the tensile strength of the two-layer composites was decreased. For the three and four layers specimens, the tensile strength was increased initially with water treatment for 7 and 14 days,and decreased for 21 and 30 days. Scanning electron microscopy (SEM) confirmed that it might be contributed to the thickness of the two-layer composites. The thinner specimen is easier to be damaged by the penetrated moisture owing to the delamination between the fiber and the matrix after water immersion. For the three and four layers specimens, their contradictory tensile strength suggests that the thicker specimen can delay the moisture permeation and is of better water durability.

Abstract: The Al₂O₃-Fe₂O₃ composite was prepared at 1550°C for 3h with alumina powder and ferric oxide powder as raw materials and with magnesia powder as additive. The effect of molar ratio of Al₂O₃ to Fe₂O₃ on the sintering and thermal shock resistance of the composite materials was studied. The results showed that the apparent porosity and bulk density of the samples both decreased with the molar ratio of Al₂O₃ to Fe₂O₃ increase, as a result, the linear change ratio and bending strength both increased. When the molar ratio of Al₂O₃ to Fe₂O₃ equals to 3, the sample exhibits excellent thermal shock resistance. The XRD and SEM analysis results indicated that the mechanical and thermal proprieties are relative to the microstructure and crystal phases of the composite materials.

Abstract: A processable polyborazine precursor was synthesized via substitution reaction and deamine condensation reaction by using methylamine, propylamine and B-trichloroborazine (TCB) as the starting materials. The chemical composition, structure and properties of the obtained polymer were investigated using element analysis, FT-IR, NMR, XPS and TG. The results indicated that the backbone of the polymer was approximate linear arrangement of boron-nitride ring molecules with B-N(CH₃)-B structure as bridge bond, meanwhile minor amount of methylamino and propylamino remained in polyborazine as the pendant or end group. The melting point, number average molecular weight and polydispersity index of polyborazine was 90°С , 1002 and 1.355, respectively. The polyborazine showed good processability and polyborazine precursor fibers with diameter of 10~15 µm could be obtained by melt-spinning in N2 atmosphere. The ceramic yield of the polyborazine at 1000°С in nitrogen atmosphere was 53.2 wt%, which suggested that the as-achieved polymer can be used as a precursor to boron nitride ceramic fibers and membranes.

Abstract: The composites in the MgO-Al₂O₃-Fe₂O₃ system were prepared using high pure magnesia and alumina as raw materials and ferric oxide powder as additive. The effect of sintering temperatures on the sintering performance and thermal shock resistance of the composites was studied. The results showed that both the apparent porosity and linear change ratio of the samples decreased with the increase of sintering temperatures, and their bulk density and bending strength increased accordingly. The sample sintered at 1550°C exhibits excellent thermal shock resistance. The XRD and SEM results indicated that the crystal phase of the samples remained the same, but their microstructure became denser as the sintering temperatures increased from 1500°C to 1600°C. As a result, the composites could be sintered at about 1550°C, which has a potential application in cement rotary kiln linings.

Abstract: Cotton fibers and gypsum were taken as the raw materials to manufacture cotton fiber reinforced gypsum based composites. Optimum length and dosage of cotton fiber and mixing method were confirmed in the experiments. The effect of cotton fiber on mechanical properties and waterproof properties of composites were studied. The reinforcement mechanism of cotton fibers on composites was discussed.

Abstract: The process of non-isothermal oxidation of AlN powder (d₅₀=15μm, 5μm and 0.5μm) were studied by TG-DTA and XRD in atmosphere of 80%N₂ and 20%O₂ mixture at flowrate of 55ml/min and heating rate of 10 /min. The experimental results reveal that the AlN powder obviously began to be oxidated and its weight increased at 900-1000°С . The process of dynamics of oxidation reaction between AlN and O₂ was controlled alternately by reaction and diffusion. Non-isothermal oxidation dynamic parameters (activation energy E, reaction orders n and pre-exponential factor A) of d₅₀=5μm AlN powder were obtained by the experimental data. E equals about 417.4kJ/mol, n about 2.9 and A about 3.47×10¹². Due to smaller activation energy E and larger pre-exponential factor A in dynamic parameters of AlN powder non-isothermal oxidation, powdery AlN is easily oxidated at high temperature and in air and has a larger oxidation reaction rate. If AlN material with high thermal conductivity characteristics is introduced to prepare composite with high thermal shock resistance, the AlN powder must be treated for oxidation resistance.

Abstract: In order to improve the fracture strength of G_p/SiC composites, the mean size (d50) 50 μm high-strength graphite particles, TiCl₄ used as starting materials, and the graphite particles coated TiO₂ film were obtained using co-precipitation method. Then the G_p/SiC composites were prepared with the “roll the snowball method” and hot-pressing sintering technology. The microstructure and phase constitution is measured by scanning electron microscopy (SEM) and electron energy spectrum (EDS). It is found that the coated graphite cores are embedded in the SiC matrix as the islands. The apparent porosity increases, density decreases, with graphite content increasing. The fracture strength and hardness decrease, fracture toughness increases with the graphite content increasing. The apparent porosity, density, fracture strength, fracture toughness, hardness of the composite are 3.8%, 2.61 g•cm^-3,135 MPa, 2.82 MPa•m^1/2,22.1 GPa, respectively, while the volume percent of ceramic and graphite is 6︰4 and sintered at 2000°C. The TiO₂ film can effectively lower the sintering temperature and impart composite better mechanical properties. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip.