Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.     

Effect of different fibres on texture,rheological and sensory properties of acid casein processed cheese sauces

The objective of this study was the evaluation of different fibres (bamboo, acacia, potato or citrus) addition on texture, rheological and sensory properties of acid casein processed cheese sauces. Fibres used in production of sauces had an impact on the texture, viscosity, viscoelastic and sensory properties. The largest increase in viscosity was observed in products with addition of potato fibre, which have good water holding and adsorption capacity. Processed cheese sauce with the addition of citrus fibre was characterised by the highest values in general, and the increase of this feature in the tested samples was regular. Adhesiveness was the highest in products with 1% addition of every fibre. The lowest values of viscosity single shear, G′ and G″ moduli, among all tested, had sauces with acacia fibre. Moreover, they had the most thin liquid consistency, which was different from preferred one.     

A preliminary study on the preparation of nanostructured Ti-doped Li4SiO4 pebbles by two-step sintering process

Li₄SiO₄ has been widely studied as attractive tritium breeding materials due to its innate merits. Considering the potential advantages of nanostructure in tritium breeding materials, a distinctive process was developed to obtain nanostructured Li₄SiO₄ pebbles. In brief, ultrafine precursor powders were synthesized by solvothermal method without using surfactants, and then indirect wet method was adopted to generate the green spheres with homogeneous microstructure. After that, the suitable sintering conditions were defined by studying the effects of sintering parameters on the grain size evolution, and nanostructured Ti-doped Li₄SiO₄ pebbles were first obtained by two-step sintering method. This study will be expected to provide references for fabricating other Li-based tritium breeding materials.     

Influence of pore distribution on the equivalent thermal conductivity of low porosity ceramic closed-cell foams

The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory.     

Research advances in residual thermal stress of ceramic/metal brazes

Brazing, as a common method of bonding ceramic and metal, has been applied in microelectronics, aerospace, machinery and other domains extensively. The residual thermal stress in the brazed joint has direct effects on the mechanical properties of the joint, so how to control the generation of residual thermal stress has become the vital point. In this paper, the methods of reducing residual thermal stress in the brazing process in recent years are reviewed. The generation and effects of residual thermal stress in the brazed joint are introduced. Besides, the methods of detecting residual thermal stress are discussed, and different methods of reducing residual thermal stress in brazed joints are also analyzed. Finally, the future development directions of reducing residual thermal stress in the brazed joint are proposed.     

Microstructure and mechanical properties of 3D-printed nano-silica reinforced alumina cores

Ceramic cores are an important component in the preparation of hollow turbine blades for aero-engines. Compared with traditional hot injection technology, 3D printing technology overcomes the disadvantages of a long production cycle and the difficulty in producing highly complex ceramic cores. The ceramic cores of hollow turbine blades require a high bending strength at high temperatures, and nano-mineralizers greatly improve their strength. In this study, nano-silica-reinforced alumina-based ceramic cores were prepared, and the effects of nanopowder content on the microstructure and properties of the ceramic cores were investigated. Alumina-based ceramic cores contained with nano-silica were prepared using the vat photopolymerization 3D printing technique and sintered at 1500 °C. The results showed that the linear shrinkage of ceramic cores first increased and then decreased as the nano-silica powder content increased, and the bending strength showed the same trend. The fracture mode changed from intergranular to transgranular. The open porosity and bulk density fluctuated slightly. The weight loss rate was approximately 20%. When the nano-silica content was 3%, the bending strength reached a maximum of 46.2 MPa and 26.1 MPa at 25 °C and 1500 °C, respectively. The precipitation of the glass phase, change in the fracture mode of the material, pinning crack of nanoparticles, and reduction of fracture energy due to the interlocking of cracks, were the main reasons for material strengthening. The successful preparation of 3D printed nano-silica reinforced alumina-based ceramic cores is expected to promote the preparation of high-performance ceramic cores with complex structures of hollow turbine blades.     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

Fabrication of ceramic particles from preceramic polymers using stop flow lithography

Stop flow lithography (SFL) combines aspects of microfluidic and photolithography to continuously fabricate particles with uniform planar shapes as dictated by a mask. In this work we aim to expand the palette of materials suitable for SFL processing by investigating the use of UV-crosslinkable preceramic polymers to make ceramic particles. A commercially available methacrylated-polysiloxane was used as the preceramic polymer and was mixed with 2.5 wt% Irgacure 651 photoinitiator. A simple SFL system was assembled to continuously fabricate UV-crosslinked preceramic polymer particles in the shape of hexagons, triangles, and gears with diameters ranging from 100 to 200 μm and thicknesses of 74 μm +/- 4 μm. Particles were harvested from the excess preceramic solution, cleaned and then pyrolyzed at 1000 °C to transform them into silicon oxycarbide ceramic particles. Particle shape was maintained during pyrolysis despite a ～80 % linear shrinkage due to the removal of acryl and methyl side groups, as confirmed via FTIR. After pyrolysis the outer diameters of the SiOC particles ranged from 20 to 40 μm with thicknesses of 10 μm–12 μm. Pyrolyzed particles were successfully recovered and dispersed in water. This work demonstrates a robust path for the fabrication of ceramic particles with specific shapes from preceramic polymers via SFL.     

Enhancement of super-hydrophilic/underwater super-oleophobic performance of ceramic membrane with TiO2 nanowire array prepared via low temperature oxidation

A gradient porous ceramic membrane with surface super-hydrophilic and underwater super-oleophobic performance was prepared by combining hydrogel directional freezing method and low temperature oxidation process. The effects of solid contents and sintering temperature on the ceramic membrane matrix were examined. The reaction time and synthesis temperature on the TiO₂ nanowire array were also evaluated. In addition, the related effects on pore size distribution, permeation flux, contact angle, and oil-in-water emulsion separation were systematically investigated. The ceramic membrane matrix pore size changed from 0.5 μm to 25 μm gradually, indicating the gradient structure controlled by the growth of ice. The super-hydrophilic and underwater super-oleophobic performance of ceramic membrane surface was obtained with surface modification by TiO₂ nanowire array, and the surface water contact angle and underwater oil contact angle were less than 5° and over 158°, respectively. The bonding strength between TiO₂ nanowire and ceramic membrane matrix was high enough to withstand ultrasonic waves. The ceramic membrane modified with TiO₂ nanowire array was used for 1000 ppm diesel oil-in-water emulsion separation, and the stable separation efficiency and flux were about 97% and 100–200 L/(m² h bar) even after 10 filtration cycles.     

Enhanced microwave absorbance of oxidized SiCf/AlPO4 composites via the formation of a carbon layer on the SiC fibre surface

A single-layer radar-absorbing structure active in the X-band (8.2?GHz to 12.4?GHz) was demonstrated by blending SiC fibres with an AlPO₄ matrix material. The as-prepared SiC_f/AlPO₄ composites were oxidized at 1273?K for several hours to investigate the effects of oxidation on the dielectric and wave-absorbing properties. Scanning and transmission electron microscopy were used to characterize the morphology and microstructure of the composites. The AlPO₄-SiO₂ solid solution during oxidation promoted the formation of a complete carbon layer on the SiC fibre surface. The real and imaginary parts of the SiC_f/AlPO₄ composites increased from 4.2–4.4 to 5.9–7.1 and from 0.08–0.2 to 3.9–5.2, respectively, with increasing oxidation time from 0 to 10?h, respectively. When the thickness of the composites increased from 2.9?mm to 3.3?mm, the wave-absorbing property noticeably improved due to the formation of a carbon layer on the SiC fibre surface after oxidation.