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•A toughened ZrO2/MgO nanocomposite coating is in-situ synthesized during the plasma electrolytic oxidation process (PEO).•The ZrO2/MgO toughening behavior occurs with dislocation ...slipping and pinning caused by semicoherent interface lattice distortion.•The toughness (KIC) of the ZrO2/MgO nanocomposite coating is 2.7 times that of the traditional PEO coating.
Ceramic coatings are in general a kind of brittle material because they are predominantly made up of ionic crystals that avoid dislocation motion caused by lattice distortion. In this regard, a remarkable toughened ZrO2/MgO nanocomposite coating is obtained by the plasma electrolytic oxidation (PEO) process and in-situ synthesized ZrO2 with quantitative control approach. It is revealed that the toughening behavior of the ZrO2/MgO coating is related to the coordination and diversion of lattice distortion at the metallic oxide interface, which induces distinct dislocation motion at the interface. The semicoherent interface between m-ZrO2 and MgO is verified to act as a buffer to realize toughening of the nanocomposite coating through dislocation slipping induced by lattice coordinated distortion. Simultaneously, significant interfacial lattice distortion transfer and dislocation pinning are discovered at the semicoherent interface between t-ZrO2 and MgO, which are beneficial to toughness enhancement of the nanocomposite coating. The results indicate that the toughening effect occurs along with dislocation slipping and pinning caused by lattice distortion of the ZrO2/MgO semicoherent interface, which enables the toughness of novel nanocomposite coating to reach 2.7 times of the traditional PEO coating.
Alumina ceramic is an ideal candidate for armor protection, but it is limited by the difficult molding or machining process. Three-dimensional printing imparts a superior geometric flexibility and ...shows good potential in the preparation of ceramics for armor protection. In this work, alumina ceramics were manufactured via 3D printing, and the effects of different monomers on the photosensitive slurry and sintered ceramics were investigated. The photosensitive slurries using dipropylene glycol diacrylate (DPGDA) as a monomer displayed the optimal curing performance, with a low viscosity, small volume shrinkage and low critical exposure energy, and each of the above properties was conducive to a good curing performance in 3D printing, making it a suitable formula for 3D-printed ceramic materials. In the 3D-printed ceramics with DPGDA as a monomer, a dense and uniform microstructure was exhibited after sintering. In comparison, the sample with trimethylolpropane triacrylate (TMPTA) showed an anisotropic microstructure with interlayer gaps and a porosity of about 9.8%. Attributed to the dense uniform microstructure, the sample with DPGDA exhibited superior properties, including a relative density of 97.5 ± 0.5%, a Vickers hardness of 19.4 ± 0.8 GPa, a fracture toughness of 2.6 ± 0.27 MPa·m1/2, a bending strength of 690 ± 54 MPa, and a dynamic strength of 3.7 ± 0.6 GPa at a strain rate of 1200 s−1.
In this work, Al2O3 nanoceramics were prepared by spark plasma sintering of amorphous powders and polycrystalline powders with similar particle sizes. Effective comparisons of sintering processes and ...ultimate products depending on starting powder conditions were explored. To ensure near-full density higher than 98% of the Al2O3 nanoceramics, the threshold temperature in SPS is 1450 °C for polycrystalline Al2O3 powders and 1300 °C for amorphous powders. The low SPS temperature for amorphous powders is attributed to the metastable state with high free energy of amorphous powders. The Al2O3 nanoceramics prepared by amorphous powders display a mean grain size of 170 nm, and superior mechanical properties, including high bending strength of 870 MPa, Vickers hardness of 20.5 GPa and fracture toughness of 4.3 MPa∙m1/2. Furthermore, the Al2O3 nanoceramics prepared by amorphous powders showed a larger dynamic strength and dynamic strain. The toughening mechanism with predominant transgranular fracture is explained based on the separation of quasi-boundaries.
Silica-based ceramic cores play key roles in the casting of aeroengine blades, but they are highly limited by the poor high-temperature mechanical property. Here, fused mullite (FM) and sintered ...mullite (SM) powders were modified in silica-based ceramic cores, and the microstructure evolution and crystallization kinetics of ceramic cores depending on mullite types were studied. The ceramic cores with FM showed a dense microstructure and superior mechanical properties compared to those with SM. The ceramic cores with 10 wt.% of FM showed a crystallization activation energy of 1119.5 kJ/mol and a crystallization exponent of 1.74, and the values of 938.4 kJ/mol and 1.86 as SM were employed; the decreased crystallization activation energy and the elevated crystallization exponent by SM suggested that the excess impurities of alkali oxides and alkaline-earth oxides significantly promoted the crystallization of cristobalite. Even though the ceramic cores with mullite powders decreased slightly in the room-temperature mechanical property, their high-temperature flexure strength and creep deformation resistance were enhanced. The ceramic cores with 10 wt.% of FM showed excellent comprehensive performance, with linear shrinkage of 0.69%, room-temperature strength of 18.9 MPa, and high-temperature strength of 15.5 MPa, which satisfied the demands for hollow-blade casting.
Ordering food through smartphones brings millions of laborers into a new occupation -food delivery rider. To date, insufficient research has focused on this group of population in platform economy. ...This paper examines the management mechanism of Chinese food delivery industry based on Marxist labor process theory and its extensions. Three main findings are revealed. First, the food delivery platform strictly set online and off-line institutions to manage food delivery riders; second, riders are involved in an illusion of flexitime but indeed provide more labor forces; third, riders are not free as they are constantly monitored by platform’s algorithm driven by big data. Given this, the conclusion suggests that all platform enterprises should abide professional ethnics and undertake social responsibility and to liberate food delivery riders’ nature.
Composite ceramic cores reinforced by different contents of mullite fibers were prepared via injection molding. With increasing mullite fibers, the porosity showed near-exponential increase due to ...the accumulation of fibers, and the leaching rate showed the similar tendency attributed to the increasing porosity for the permeation of alkali solution. At room temperature or 1550 °C, the flexural strength increased as the fibers contents were below 3 wt%, due to the micro-cracking and fiber extraction; however, the flexural strength degraded as the mullite fibers contents was 4 wt%, due to the high porosity. Compared to the others, the ceramic cores with 3 wt% mullite fibers showed excellent properties, including apparent porosity of 35.2%, linear shrinkage of 0.81%, flexural strength being 25.5 MPa at room temperature and 22.3 MPa at 1550 °C, creep deformation of 0.12 mm and leaching rate of 0.34 g/min, well meeting the demands of hollow blades casting. This paper may provide new guidance for high-property ceramic cores.
•Ceramic cores were reinforced by mullite fibers.•Ceramic cores with 3wt% mullite fibers exhibited superior properties.•The reinforcement was due to the micro-cracking and fiber extraction.
A ceramic core is the key component in the manufacture of the hollow turbine blades of aeroengines. Compared with the traditional injection molding method, 3D printing is more suitable for ...manufacturing ceramic cores with a complex geometry at high precision. However, the stair-stepping effect is inevitable in the 3D printing process and affects the surface roughness and strength of the ceramic core. In this study, to explore the influence of nano-silica content on the microstructure and properties of the ceramic core, silicon-based ceramic cores were fabricated with the addition of nano-silica powder by digital light processing and subsequent sintering at 1200 °C. The results showed that the apparent porosity and pore size of the ceramic core gradually decreased as both the nano-silica powder content and bulk density increased. Meanwhile, the printing interlayer spacing was significantly reduced, resulting in a low surface roughness, high flexural strength, and creep-resistance. To simulate the entire casting process of a superalloy blade, the thermal deformation behavior of the ceramic core was observed by heating and cooling cycles performed in a thermal dilatometer at 1540 °C. The total linear shrinkage decreased as the nano-silica powder content increased, which was mainly due to the phase transformation of cristobalite and the densification of the ceramic core sintered at 1200 °C. The low surface roughness and linear shrinkage as well as high flexural strength of the ceramic core can contribute to the excellent quality of cast superalloy blades.
Ceramic core is an essential component in the precise casting of hollow turbine blades, and the investigation on 3D printing of silica-based ceramic cores is crucial to the development of aviation ...industry; however, they are suffered from difficulty in high-temperature strength and structural anisotropy. In present work, silica-based ceramic cores were prepared via DLP stereolithography 3D printing, and the anisotropy management on microstructures and properties were explored based on the particle size of fused silica powders. In 3D printed ceramic cores with coarse powders, significant anisotropy was displayed exhibiting multilayer structure with large gaps in horizontal printing and uniform porous microstructure in the vertical direction, which was further explained by the particle deposition in printing. With finer silica powders, the uniformity in the microstructures was highly improved, attributed to the enhanced particle dispersion in ceramic slurries and promoted interlayer particle rearrangement during sintering. To evaluate the anisotropy in mechanical property, the ratio of vertical strength to horizontal strength (σV/σH) was proposed, which rose from 0.48 to 0.86 as the particle size decreased from 35 µm to 5 µm, suggesting enhanced mechanical uniformity. While the average particle size of silica powders was 5 µm, the flexure strengths of ceramic cores in different directions were up to 18.5 MPa and 16.3 MPa at 1540 °C with σV/σH ratio of 0.88, which well satisfied the demands for the casting of turbine blades. This work inspires new guidance on the anisotropy management in ceramic cores prepared by 3D printing, and provides new technology for fabrication of silica-based ceramic cores with superior high temperature mechanical properties.
In present work, Al2O3 nanoceramics toughened with 5 mol% ZrO2 and 10 mol% ZrO2 were prepared by pressureless sintering; polycrystalline ZrO2 powders and Al2O3-ZrO2 amorphous powders were used as ...dopants in conventional sintering and amorphous-phase-assisted sintering, respectively. In amorphous-phase-assisted sintering, the densification of Al2O3-ZrO2 nanoceramics was highly promoted, and nanoceramics with composition of Al2O3-5 mol% ZrO2 and Al2O3-10 mol% ZrO2 were almost fully densified at temperature as low as 1350 °C. The easy densification was attributed to the metastable state and phase transition of amorphous powders, which acted as sintering aids. The nanoceramics exhibited dense and homogenous mixture of Al2O3 and ZrO2 particles with fine grain size, and the microstructure refinement further contributed to superior mechanical properties with high Vickers hardness, bending strength and Weibull modulus values. This work may provide new guidance for low temperature fabrication of nanoceramics by employing amorphous powders as sintering aids.
•The amorphous powders promoted the densification of Al2O3-ZrO2 nanoceramics.•Full dense Al2O3-ZrO2 nanoceramics were obtained by pressureless sintering at 1350 °C.•This work provides new guidance for low temperature fabrication of nanoceramics.
Ceramic core (CC) is a key component in the casting of aero-engine turbine blades, and CC through 3D printing is crucial to the growing aviation technology. However, these silica CCs are highly ...limited by their high temperature softening and inevitable anisotropy. In this work, silica-based CCs reinforced by alumina powders are manufactured by 3D printing via digital light processing (DLP) stereolithography, and the anisotropy in microstructure and property was investigated. In the CC without alumina powders, multilayer structures with significant interlayer gaps are exhibited in horizontal printing direction suggesting significant anisotropy in microstructure, which is related to the particle deposition in printing. As the alumina content is 6 wt%, microstructures become uniform, due to the enhanced particle rearrangement between different layers. To assess the mechanical anisotropy, the ratio of vertical strength to horizontal strength (σV/σH) is raised, and the σV/σH ratios rise from 0.45 to 0.86 at room temperature and 0.52–0.89 at 1540 °C with increasing alumina content from 0 to 6 wt%. As the alumina content is 6 wt%, the CC exhibits high temperature flexure strengths as high as 17.9 MPa and 19.5 MPa in different directions, well satisfying the demands for turbine blades casting. This article inspires a profound understanding on the anisotropy control in 3D printing of CC with excellent high-temperature mechanical properties.
•3D printed Ceramic cores exhibit anisotropy in structure and mechanical property.•The microstructure anisotropy is featured by multilayer structures with interlayer gaps.•The mechanical anisotropy is assessed through the ratio of vertical strength to horizontal strength (σV/σH).•Moderate contents of alumina powders promote the uniformity in microstructure and high temperature mechanical property with σV/σH of 0.89.
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