Effects of fiber aspect ratio and fabrication temperature on the microstructure and mechanical properties of elastic fibrous porous ceramics by press-filtration method

The inherent brittleness of fibrous porous ceramics (FPCs) results in their fragility, limiting their application in thermal protection. In this paper, a novel elastic fibrous porous ceramic (EFPCs) with quasi-layered structure were successfully prepared by facile press-filtration method. To further investigate the characteristics of EFPCs, the effect of fiber aspect ratio and fabrication temperature on the microstructures and properties were studied. Results demonstrated that both fiber aspect ratio and fabrication temperature had influence on the microstructure and mechanical properties on EFPCs. The prepared EFPCs exhibited low density (0.124–0.181 g cm^?3), relatively high compressive stress (0.096–0.377 MPa) compared to flexible fibrous ceramics, high porosity (91.73%–94.86%) and low thermal conductivity (～0.03 W m^?1 k^?1). According to these excellent properties, the EFPCs may have potential use in thermal insulation fields.     

Effect of the addition of polyvinylpyrrolidone as a pore-former on microstructure and mechanical strength of porous alumina ceramics

The effect of the solvent on the properties of porous alumina ceramics was studied when polyvinylpyrrolidone (PVP) was used as an organic pore-former. In particular, porous alumina ceramics were produced by dry-pressing of mixed PVP–alumina powder; the mixing of PVP and alumina powder was achieved via ball milling using water or acetone as solvent, or dry ball milling. Due to the different solubility of PVP in water and acetone, porous alumina ceramics with different pore structures and mechanical properties were obtained. Because of its cylindrical pores being aligned to some extent, the sample prepared using acetone as solvent exhibited the highest bending strength (140.2 MPa) and Young's modulus (57.4 GPa), which were 1.6 times and 3.4 times higher compared to that prepared without PVP. Moreover, the addition of PVP via wet ball milling led to more uniform dispersion of PVP in alumina, hence limiting the grain growth during sintering process and increasing the grain bonding.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Influence of phosphorus pentoxide on crystallisation,microstructure and mechanical properties of potassium fluorophlogopite glass ceramics

Abstract

Potassium fluorophlogopite glass ceramics were prepared. Differential thermal analysis showed that there were two exothermic peaks at ～750 and 950°C, corresponding to the Avrami exponent ～1·5 and 2·0. Spherical particles were found using a scanning electron microscope when the glass ceramics were heat treated at 750 and 800°C. With the increase in heat treatment temperature, a kind of lamellar crystals was gradually formed, and the shape of crystals changed from spherical to lamellar when the Avrami exponent changed from 1·5 to 2·0. Mechanical tests showed that material cutting and bending strength increased, while the Vickers hardness decreased with the increasing of P₂O₅ content and heat treatment temperature.     

Anisotropies in structure and properties of hot-press sintered h-BN-MAS composite ceramics: Effects of raw h-BN particle size

Effects of raw h-BN particle size from 0.5 μm to 11 μm on the phase compositions, texture degree, bending strength, fracture toughness and thermal conductivity of hot-press sintered h-BN-MAS composite ceramics were investigated. Larger h-BN grain can facilitate the nanocrystallization of MAS phase due to the inhibiting crystallization effect of h-BN on α-cordierite. Texture degree of h-BN-MAS composite ceramics increased significantly with increasing raw h-BN particle size, and the 11.0μmBN-MAS composite ceramic shows typical textured structure. The h-BN-MAS composite ceramics show anisotropy in mechanical properties and thermal conductivity, and the anisotropy increased significantly with increasing raw h-BN particle size. The 0.5μmBN-MAS sample shows excellent mechanical properties, and the 10μmBN-MAS sample shows strong anisotropy in thermal conductivity.     

Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids

Stereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO₂, CaCO₃, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO₂ content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO₂. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO₃ content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

Optimal design on the mechanical and thermal properties of porous alumina ceramics based on fractal dimension analysis

In order to investigate the relationship between pore structure and thermal conductivity as well as mechanical strength, porous alumina ceramics (PAC) with various pore structures were fabricated, using starch as the pore‐forming agent. Fractal theory was employed to characterize the pore size distribution more accurately than ever used parameters. The results show that the increase in starch content in PAC leads to an enhanced porosity, a higher mean pore size, and reduced fracture dimension, thermal conductivity and strength. The fractal analysis indicated that the fractal dimension decreases gradually and reaches its minimum value with increasing the starch content up to 25 wt%, but the further incorporation results in an opposite trend. It is suggested from micro‐pore fractographic analysis that the optimization of both thermal insulation performance and mechanical strength are positively correlated with the increase in the mean pore size and proportion of 2‐14 μm pores but negatively corrected with the porosity. These results provide a new perspective and a deeper understanding for fabrication of PAC with both excellent thermal insulation and mechanical performance.     

Influence of co-doped alumina on the microstructure and radioluminescence of SrHfO3:Ce ceramics

SrHfO₃:Ce and SrHfO₃:Ce,Al powders were prepared by the solid-state synthesis using the commercial HfO₂, SrCO₃, CeO₂ and α-Al₂O₃ as the starting materials. All the SrHfO₃:Ce and SrHfO₃:Ce,Al powders calcined at different temperatures within 1100 °C–1300 °C for 8 h in air exhibited a pure SrHfO₃ phase. Meanwhile, the effects of different calcination temperatures and co-doped alumina on the microstructure of SrHfO₃:Ce,Al powders were discussed. SrHfO₃:Ce and SrHfO₃:Ce,Al ceramics were successfully fabricated via the vacuum sintering at 1800 °C for 20 h. The alumina acted as the sintering aid, which promoted the densification and eliminated the intragranular pores of SrHfO₃:Ce,Al ceramics. The charge disbalance was effectively compensated by co-doped alumina. SrHfO₃:Ce,Al ceramics exhibited a higher light yield of 5700 ph/MeV compared with 4600 ph/MeV obtained in SrHfO₃:Ce ceramics. The charge traps in ceramics and their effects on scintillation properties were also investigated by thermoluminescence measurement.     

92黄色氧化铝陶瓷的研制及其显微结构分析

将ZrO2＋Y2O3复合添加剂加入到92白色氧化铝陶瓷基料中,经常压烧结制备了黄色氧化铝陶瓷。实验结果表明：当添加0．6％（质量分数,下同）ZrO2和0．3％Y2O3时,可以使氧化铝陶瓷呈现均匀的黄色。通过SEM检测对氧化铝陶瓷的显微结构进行了分析。     

Preparation of high strength lamellar porous calcium silicate ceramics by directional freeze casting

In this study, porous calcium silicate (CS) ceramics with oriented arrangement of lamellar macropore structure were prepared by directional freeze casting method. The lamellar macropores were connected by the micropores on the pore wall, which had good pore interconnectivity. The effects of solid loading of the slurry, freezing temperature, sintering additive content, and sintering temperature on the microstructures and compressive strength of the synthesized porous materials were investigated systematically. The results showed that with the increase of solid loading (≤20 vol%) and sintering additive content, the sizes of lamellar pores and pore walls increased gradually, the open porosity decreased and the compressive strength increased. The sintering temperature had little effect on the pore size of the ceramics, but increasing the sintering temperature (≤1050 °C) promoted the densification of the pore wall, reduced the porosity, and improved the strength. The decrease of freezing temperature had little effect on porosity, but it reduced the size of lamellar pore and pore wall, so as to improve the strength. Finally, porous CS ceramics with lamellar macropores of about 300–600 μm and 2–10 μm micropores on the pore wall were obtained. The porous CS ceramics had high pore interconnectivity, an open porosity of 66.25% and a compressive strength of 5.47 MPa, which was expected to be used in bone tissue engineering.     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Effects of nano-ZrO2 content on microstructure and mechanical properties of GNPs/nano-ZrO2 reinforced Al2O3/Ti(C,N) composite ceramics

Dense Al₂O₃/Ti(C,N) composite ceramics reinforced with GNPs/nano-ZrO₂ were fabricated by hot-press sintering. The effects of nano-ZrO₂ content on the microstructure and mechanical properties of the prepared Al₂O₃/Ti(C,N)/GNPs/ZrO₂ composites were investigated. Results showed that nano-ZrO₂ inclusions refined the matrix grains significantly and resulted in the formation of intra-granular structure. Excellent comprehensive mechanical properties were achieved via addition of combined GNPs and nano-ZrO₂. In particular, the fracture toughness of composites incorporating GNPs (0.4 wt%)/ZrO₂ (1 wt%) exceeded 11 MPa m^1/2, which was increased by more than 86 % compared with that of Al₂O₃/Ti(C,N) ceramic composites without GNPs/ZrO₂. The main toughening mechanisms have been identified as stress-induced phase transformation, crack bridging, deflection and pull-out of GNPs. The toughening effects originated from GNPs were enhanced with the introduction of nano-ZrO₂ because of not only the residual stress resulted from phase transformation but also the formation of intra-granular structure with uneven surface around GNPs.     

Densification,microstructure and mechanical properties of Ta4HfC5-based ceramics obtained from synthesized nanoscale powder

Solvothermal treatment was used to synthesize nanoscale 4TaC-HfC (Ta₄HfC₅) powder at a relatively low calcination temperature and in a short period (1400 °C, 2 h). The obtained powder had uniform size distribution and dispersion. Ta₄HfC₅ ceramics were then consolidated via spark plasma sintering at 2100 °C. Ceramics had a better densification and smaller mean grain size at a shorter sintering time compared with that of materials sintered using mechanical ball milling method. The densification behavior of ceramics deriving from synthesized or ball milled powders was analyzed and the mechanical properties of different samples were investigated. To further increase the mechanical properties, a nearly fully dense Ta₄HfC₅-MoSi₂ ceramic was sintered using the synthesized powder. The mechanical properties of the ceramic composite doubled the strength values. This processing route demonstrated to be a viable approach to synthesize nanoscale Ta₄HfC₅ powder with high purity and uniformity, and obtain higher performances ceramics once sintered.     

Preparation of elongated mullite self-reinforced porous ceramics

Elongated mullite was synthesized using mullite powder as a raw material and AlF₃·3H₂O as an additive, and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The effects of AlF₃·3H₂O content and reaction temperature on the formation of elongated mullite were investigated, and the relevant growth mechanism was discussed based on the experimental results and density functional theory (DFT) calculations. When the optimal amount of AlF₃·3H₂O (4?wt% in the present work) was used, the length and diameter of elongated mullite increased with increasing the reaction temperature, and elongated mullite of 22.3?µm in average length and 4.6?µm in average diameter was formed after 5?h at 1873?K. Based on the results, elongated mullite self-reinforced porous ceramics were prepared by a combined foam-gelcasting and solid-reaction method, and their mechanical properties were examined. Elongated mullite in-situ formed in the porous samples evidently enhanced their mechanical strength. The flexural strength of the elongated mullite self-reinforced porous sample with 67.0% porosity (prepared using 6?wt% AlF₃·3H₂O) was as high as 13.9?MPa, which was about 26.4% higher than that of a porous sample (11.0?MPa) prepared without AlF₃·3H₂O.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Influence of in-situ synthesized Zr-Al-C on microstructure and toughening of ZrB2-SiC composite ceramics fabricated by spark plasma sintering

Zr-Al-C was in-situ synthesized as a toughening component in ZrB₂-SiC ceramics by spark plasma sintering (SPS) ball-milled ZrB₂-based composite powders with SiC and graphite powders. The phase composition of Zr-Al-C toughened ZrB₂-SiC (ZSA) composite ceramics fabricated through the two-step process (ball milling and SPS) did not change dramatically with varying content of Zr-Al-C which shows a major phase of Zr₃Al₄C₆. With increasing Zr-Al-C content, the fracture toughness of the ZSA ceramics initially increased and then decreased when the content reached 40 vol%. The ZSA ceramic with 30 vol% Zr-Al-C exhibited a maximum fracture toughness value of 5.96 ± 0.31 MPa m^1/2, about 22% higher than that of the ZSA ceramic with 10 vol% Zr-Al-C. When the Zr-Al-C content goes beyond 30 vol%, the higher open porosity and component agglomeration led to the relatively lower fracture toughness. Crack deflection and bridging resulted from the weak interface bonding between Zr-Al-C and matrix phases and the weak internal layers of Zr-Al-C crystals, leading to longer crack paths and, hence, the toughened ZSA composite ceramics. Compared to the one-step in-situ synthesis process of Zr-Al-C and the direct incorporation process of synthesized Zr-Al-C grains, the two-step in-situ synthesis process not only led to the more uniform distribution of different components but also resulted in a much larger size of Zr-Al-C grains with a large aspect ratio causing longer crack propagation path as the result of crack deflection and bridging. The larger Zr-Al-C grains combined with the more homogeneous microstructure achieve the most substantial toughening of the ZSA composite ceramics. This work points out a promising approach to control and optimize the microstructure and improve the fracture toughness of ZrB₂-SiC composite ceramics by selecting the incorporation process of compound reinforcement components.     

A nitride whisker template for growth of mullite in SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer replica technique. The effects of nitride whisker template on the growth of mullite, the strut structure and mechanical properties of SiC RPCs were investigated. Prepolyurethane (PU) open-cell sponge was first coated by SiC slurry consisting of SiC, reactive Al₂O₃, microsilica and Si powder, then it was nitridized at 1400 °C in a flowing N₂ atmosphere to prepare SiC preforms. Subsequently, these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450 °C in air. The results showed that Si₂N₂O whiskers grew on the surface and in the matrix of SiC preforms after nitridation. The diameter of struts in SiC RPCs increased after vacuum infiltration process because alumina slurry was easily adhered by the surface nitride whiskers. In addition, such whiskers inside the strut of SiC preforms acted as the template to promote the growth of column-liked mullite in SiC RPCs. The mechanical properties and thermal shock resistance of SiC RPCs were greatly improved due to the special interfacial characteristics of multi-layered struts as well as better interlocked column-liked mullite in SiC skeleton.     

Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Directionally solidified Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12 eutectic high-entropy oxide ceramics with well-oriented structure,high hardness,and low thermal conductivity

Directionally solidified Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic high-entropy oxide ceramics (HEOCs) were successfully prepared with an optical floating zone furnace. The Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs were pure phases with uniform distribution of rare-earth elements. The preferred growth orientation relationships were <10−10 > {0001}Al₂O₃ // <110 > {211}(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂. The indentation fracture toughness and Vickers hardness were 6.8 ± 0.9 MPa·m^1/2 and 16.1 ± 0.3 GPa, which were higher than that of Al₂O₃/Y₃Al₅O₁₂ eutectic ceramics. The room temperature bending strength was 333 ± 42 MPa. Crack bridging, deflection and bifurcation were the main toughening mechanism. Hardness and reduced modulus mapping results illustrated that the hardness of (Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ was close to that of Al₂O₃. Thermal expansion coefficient of Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs was very similar to that of Al₂O₃/Y₃Al₅O₁₂ but thermal conductivity was as low as 4.9 Wm⁻¹ K⁻¹ due to strong lattice distortion. These results suggest that high-entropy Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic ceramics are promising candidates for structural components application in gas turbine engines.