Key Engineering Materials Vols. 602-603

Abstract: Silicon powder and phenolic resin were used as raw materials to produce porous silicon carbide (SiC) ceramics, with SiC whiskers (SiC_w) as reinforcement additionally. Starting with the preparation of core-shell structure precursor powder through coat-mix method, and then by carrying out molding, carbonization and sintering processes, SiC_w/SiC ceramic examples were produced. The phase composite, fracture surface morphology, pore size and porosity, bending strength and thermal expansion coefficient of the final product were measured. Results show that the addition of SiC_w apparently improved the intensive property of the products and the changing pattern were quantitatively analyzed; while little influence was observed on some other properties such as phase composition and thermal expansion coefficient. It means that SiC_w can strengthen porous silicon carbide ceramics without weakening their thermal properties, which is particularly important because of its application in the field of high temperature fluid filtration. Incorporated SiC_w is supposed to work in accordance with some toughening mechanism such as load transferring and matrix prestressing. Microstructure, pore evaluation and weight loss rate during carbonation and sintering were also noted to describe the procedure better.

Abstract: Yttrium aluminium garnet (YAG) possesses many good properties, and usually uesd as sintering additives for silicon carbide ceramics. The YAG precursor solution was preformed by the coprecipitation process, then 8wt% prepared precursor solution was added and uesed as binder for samples' green bodies dry mould pressing. The dried pressed samples were directly sintered at 1850¡æ under vaccum atmosphere, and the precursor Al₂O₃-Y₂O₃ could be formed YAG during sintering process. The oxidation resistance property was performed at 1500¡æ in air. Bulk density and porosity were about 2.42g/cm³ and 24.98% respectively, and three-point bonding strength was up to 90MPa. Phase composition of sintered specimen was determined by X-ray diffraction and fracture morphology was detected by scanning electron microscope (SEM). The results showed that the as-fabricated porous SiC ceramics exhlibted excellent oxidation resistance abilities, its oxidation resistance improvement was benefited from silicon carbide particle coated with a compact YAG layer. The relationship between microstructure and properties was analyzed, and the finally formed phase composition was discussed.

Abstract: The densification of α-SiC occurred by liquid-phase sintering mechanism with AlN-RE₂O₃ (RE=Nd, Gd, Y, Lu) was studied. The total additive content was fixed at 15 wt%. Cold isostatically pressed samples were sintered at 1800-1950 °C under N₂ atmosphere for 1 h. The linear shrinkage and weight loss of the samples were about 17-20% and 2-5%, respectively. The mechanical properties and microstructure of sintered samples were investigated. The experimental results showed that the fracture toughness of samples was 6-8 MPa·m^1/2, the hardness was in the range of 18-21 GPa and the bending strength was in the range of 400-500 MPa. It was found that a decrease in the cationic radius of the rare-earth oxides was accompanied by an increase in hardness and flexural strength of the SiC ceramics, whereas the fracture toughness was improved by incorporating rare-earth oxides of larger cationic radius. The morphology (SEM) of sintered sample showed a fine grained microstructure with equiaxed grains. Fracture mode was intergranular fracture.

Abstract: High thermal conductivity and dense SiC coating were fabricated by chemical vapor deposition (CVD) on surface of different graphite at temperature of 1100°C. The results indicated that the thickness of the coating influences the quality of coating. There was no crack in coating when thickness of the coating is less than 0.3mm. The structure of graphite affected the reaction badly. When the graphite is coarse and porous the reaction is chemical vapor infiltration (CVI) and the interface of coating and substrate isnt obvious. When the graphite is dense and smooth, the coating is covered in surface of graphite, the interface is found between the coating and substrate. Optical microscope and SEM were used to observe the surface and cross-section morphology and microstructure of coatings. It shown coating and substrate had a good combination and the deposited grains mainly showed equiaxed form with the crystallite size of 30 nm at 1100°C.

Abstract: Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature applications and nuclear applications. In this study, SiC/SiC composites were fabricated via polymer impregnation and pyrolysis (PIP) process and studied by X-ray tomography. The SiC/SiC composites were first scanned using a Metris X-tek 320 kV source at the Henry Moseley X-ray Imaging Facility at the University of Manchester, the closed porosities were investigated after three dimensional (3D) imaging of the samples. Furthermore, high-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite at Diamond Light Source. Digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite, quantifying damage by measurement of the displacement fields within the material. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite. Graded microstructures, textures and multiple phases were simulated and a mesh-free framework was developed to compute the damage development through the microstructure. The results indicated that we could study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure with this method.

Abstract: continuous carbon fiber reinforced silicon carbide (C_f/SiC) ceramic matrix composites were prepared by precursor infiltration pyrolysis and chemical vapor infiltration (PIP-CVI process), in which the honeycomb sandwich structure preforms were fabricated by the three dimensional braid method. In this paper, the microstructure and the bending strength were observed and analyzed by SEM and three point bending method. The results of the study show that: The C_f/SiC ceramic matrix composites, which were lightweight and high strength, were prepared by that technique. The composite samples have a fiber volume fraction of 20%, a density of 0.38 g/cm³ and a flexural strength of 3.81 MPa. The honeycomb sandwich fiber reinforced ceramic matrix composite with a light weight, corrosion resistance and excellent physical and mechanical properties is a kind of structure and functional ceramic materials, which can realize the structure and the requirement of heat integration.

Abstract: C_f/SiC composites were fabricated by chemical vapor infiltration with CH₃SiCl₃ as precursor and H₂ as carrier gas. The carbon preform was noodled by continuous carbon fiber and network tires layer. The screws were processed with C_f/SiC composites by diamond tool. The mechanical properties of screws were studied in this paper. Results show that the tensile and double shear strength reach separately 128.15 Mpa and 58Mpa. The microstructure of fracture surfaces from tensile and shear test were analysized.

Abstract: Continuous carbon fibre reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) process, a mullite/yttrium silicate multilayer coating was prepared by plasma spray method as the oxidation protective coating. The efficiency of the coating against oxidation was characterized by means of heat treatment of the C/SiC composites at 1500 °C in static air for 1 hour. The results indicated that the weight loss of the coated composites was only 3.4 %, and the flexural strength of the composites retained 95.3 % of the original strength.

Abstract: C_f/SiC-ZrB₂ composites were fabricated by chemical vapor infiltration and precursor infiltration pyrolysis. Specimens were then tested for mechanical properties , thermo-physical properties and microstructure. Results show flexural strength, tensile strength and compressive strength reach 430MPa,120 MPa and 365 Mpa, respectively. The mechanical properties at high temperature were tested. And the fractograph and microstructure of composites were analysized.

Abstract: Fully dense (ZrB₂+ZrC)/Zr₃[Al (Si)]₄C₆ composites with ZrB₂ content varying from 0 to 15 vol.% and fixed ZrC content of 10 vol.% were successfully prepared by in situ hot-pressing in Ar atmosphere using ZrH₂, Al, Si, C and B₄C as raw materials. With the increase of ZrB₂ content, both the bending strength and fracture toughness of the composites increase and then decrease. The synergistic action of ZrB₂ and ZrC as reinforcements shows significant strengthening and toughing effect to the Zr₃[Al (Si)]₄C₆ matrix. The composite with 10 vol.% ZrB₂ shows the optimal mechanical properties: 516 MPa for bending strength and 6.52 MPa·m^1/2 for fracture toughness. With the increase of ZrB₂ content, the Vickers hardness of the composites shows a near-linear increase from 15.3 GPa to 16.7 GPa. The strengthening and toughening effect can be ascribed to the unique mechanical properties of ZrB₂ and ZrC reinforcements, the differences in coefficient of thermal expansion and modulus between them and Zr₃[Al (Si)]₄C₆ matrix, fine grain strengthening and uniform microstructure derived by the in situ synthesis reaction.