Materials Science Forum Vols. 546-549

Abstract: CVD (chemical vapor deposition) SiC continuous fiber is used as reinforcement of metal matrix composites because of its high strength, high stiffness and elevated temperature stability. In this work, anodic oxidation method was used to treat the surface of carbon coated SiC fiber. Then the surface of treated fiber was compared with untreated one by scan electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). SiC was detected on the surface of untreated carbon-rich coating. It is proved by XPS that SiC was transformed into SiO2 after treatment. About 100~200nm thick oxidation layer was detected on the surface of carbon-rich coating and the structure of double coatings formed. Because the surface of fiber became smooth and the tensile stress on it was relaxed after treatment, the tensile strength of fiber was improved by 8%~20% average and the dispersing ratio of it was reduced.

Abstract: This study concerned with the optimum design, microstructure and mechanical properties analysis of a multi-layered metal/intermetallic materials consisting of Ti and Ti3Al prepared by the electron beam physical vapor deposition (EB-PVD) technology. Based on fracture mechanics and numerical simulation method, the optimized microstructure of Ti-Ti3Al multi-layered materials has been obtained by analyzing the relation curve between structural parameters and work of fracture of materials, then dual-target evaporating method was used to evaporate Ti and Ti-47Al bar alternately to form Ti/Ti3Al thin sheet about 0.12mm thickness. Pattern and phase analysis by SEM and XRD showed that there was homogeneous and continuous interface between layers and the intermetallic layers were made up of α2 phase alloy. The tensile curve of Ti/Ti3Al microlaminates represented the characteristic of multi-layered materials and the maximal extensibility of sample as deposited reached 5.83% and the fracture appearance showed ductile rupture feature.

Abstract: Silica aerogel/epoxy composite, a kind of efficient thermal insulation material, was prepared by doping silica aerogel of different sizes into epoxy resin through thermocuring process. The results of thermal experiments showed that silica aerogel/epoxy composite had a lower thermal conductivity (0.105W/(m·k) at 60 wt% silica aerogel) and higher serviceability temperature (Martens heat distortion temperature: 160°C at 20 wt% silica aerogel). In addition, the composite doping larger size (0.2-2mm) of silica aerogel particle had lower thermal conductivity and higher Martens heat distortion temperature. Based on the results of SEM and FT-IR, the thermal transfer model was established. Thermal transfer mechanism and the reasons of higher Martens heat distortion temperature have been discussed respectively.

Abstract: Three-dimensional (3D) carbon fiber reinforced silicon carbide matrix composites (C/SiC) were prepared by a low-pressure chemical vapor infiltration method. The thermal shock behaviors of the composites in different environments were researched using an advanced acoustic emission (AE) system. Damage initiation and propagation were easily detected and evaluated by AE. The thermal shock damage to C/SiC composites mainly occurred at the process of cooling and was limited at argon but unlimited at wet oxygen atmosphere. Also correlations have been established between the different damage mechanisms and the characteristics of acoustic emission signals obtained during thermal shock tests. In this way, the paper contributes to the development of the acoustic emission technique for monitoring of damage development in ceramic-matrix composites.

Abstract: As for 2.5-D layer-to-layer angle interlock braided composites, the cross section of the warp tow was represented in double-convex lens form, and the center line of the warp tow was along the sinusoid. The arranging characteristic of weft tow fibers along the cross section outline of the longitude fibers was studied in detail. A novel finite element model for 2.5-D braided composites was established to predict elastic modulus. The finite element software ANSYS was adopted to study the mechanical properties of the model and presented its stress nephogram, and the influence of the braided structure parameters on the elastic modulus of this material was analyzed in detail. To validate this model, qualified experimental samples were made by VARTM technique, and then tensile tests were performed to determine the mechanical properties. The results show that the conclusions of finite element method (FEM) fit well with the experimental values, and this model can be used to predict effectively the macro modulus of 2.5-D braided composites.

Abstract: High volume fraction (63vol.%) SiCp/Al composite for electronic packaging fabricated by pressureless infiltration was characterized by microscopy, physical and mechanical tests. Microscopy observations and XRD analysis indicated that the composite appeared to be free of porosity and macroscopically homogeneous, SiC-Al interface was atomic bonding interface with low thermal resistivity and electrical resistivity, no interfacial reaction products was detected. Examination of the fracture surfaces of the composites revealed that the cracks passed through the SiC particles and Al matrix, no debonding of SiC-Al interface was observed. The fracture mode indicated that the bonding between SiC-Al was sufficient strong. The properties of the composite were noted for its ultrahigh thermal conductivity of 235W/m·K and specific modulus (79.9×105m), low coefficient of thermal expansion (7.2×10-6/K) and density of 2.99g/cm3. The advantages of the composite over traditional materials used as the electronic packages for aerospace applications were analyzed.

Abstract: In this study, multi-objective optimization of radar absorbing structure with circuit analog structure using the genetic algorithm was investigated, at the basis of the study on the influence of the size of the circuit analog, the electromagnetic parameter and the thickness of the medium ply on the properties of the microwave-absorbing composite. Based on the concept of Pareto optimality, Sharing and Niche technology was applied in the algorithm(NSGA), and the calculating results converged at the Pareto front dividedly. The study have been showed that the calculating values fit well with that of the experiment, which indicate that this algorithm is proper and has extensive adaptability. The results also showed that introducing circuit analog structure(CAS)into the radar absorbing structure composite design can improve its wave-absorbing properties. So, the radar absorbing structure composite with CAS is a promising radar absorbing structure composite form.

Abstract: Two kinds of DMLC which have same mesomorphic unit but different spacer chain were used to toughen brittle DAIP. In order to get the best toughening effect, influences of curing procedure, especial the curing temperature on the formation of mesomorphic structure and miscibility between DAIP and DMLC were studied. The results showed that when DMLC was used to toughen brittle resins, the initiation curing temperature of the modified system was desired to be higher than it’s Ti and lower than DMLC’s Ti. When cured as the above procedure, the modified DAIPs showed macroscopic disordered but microscopic ordered structures. At the same time, most of modified DAIP systems showed great improvement of impact strength, as well as bend strength, modulus and heat resistance. The results also showed that when the DMLC with spacer chain was used, the modified DAIPs will have better toughness than DAIPs modified with no spacer DMLC. Fracture surfaces of the modified DAIPs were studied with SEM further. The results revealed that it was the ductile and strong DMLC mesomorphic domains to modify the toughness of brittle DAIP matrixes.

Abstract: DSC/TG analysis was used to investigate the change of TiSi2-SiC-N2 system in the temperature range from room temperature to 1400°C. The relationships between $G and T about the reactions possible occurring in TiSi2-N2 system during combustion synthesis were calculated. In order to study the combustion process in detail, the reaction of TiSi2-SiC-N2 system was stopped at different stages by quenching the samples. The composition and microstructure in different reaction zones were analyzed by means of XRD and SEM, accordingly, reaction kinetic model of TiSi2-SiC-N2 combustion system was established. The result showed that TiSi2 firstly reacted with N2 to form TiN and Si, and subsequently Si were nitrified.

Abstract: By introducing ZrO2(4Y) powder of suitable content into the thermit and based on oxidation-reduction reaction of the thermit and liquid-liquid phase separation of ceramics/metal under gravity, Al2O3/ZrO2(4Y) composite ceramics have been prepared through melt-growth with eutectic reaction under high degree of undercooling from SHS process. It has been found that the ceramics were composed of major rod-shaped sapphires with 8.0~12.0 aspect ratios and minor plate-like α-Al2O3 grains, and within the sapphires t-ZrO2 nano-micron fibers were embedded. Flexural strength and fracture toughness were measured to be 1256MPa and 13.2MPa·m1/2. It was obtained that a number of low energy interfaces (the interface distance on nano-micron scale) between two Al2O3/ZrO2 (4Y) phases and the strong compressive residual stress in the sapphires made the sapphires reinforced, forcing the crack to propagate along the rod-shaped sapphires; meanwhile, crack-bridging and pull-out of the sapphires, crack - bridging and frictionally interlocked effects of plate-like α-Al2O3 grains in the wake of crack tip also rendered the crack stabilization to arise.