Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Study of SiC fiber axial compressive behavior using tensile recoil measurement

SiC fibers have been widely investigated as reinforcements for advanced ceramic matrix composites owing to their excellent high-temperature properties. However, the axial compressive strength of SiC fibers has not been thoroughly studied. In this study, the compressive behavior of two SiC fiber types containing different compositions and thermal degradation were characterized by tensile recoil measurements. Results illustrated that the SiC fiber compressive strength was 30%–50% of its tensile strength, after heat treatment at 1200℃–1800℃ for 0.5 h in argon. The fiber compressive failure mechanism was studied, and a “shear-bending-cleavage” model was proposed for the recoil compression fracture of pristine SiC fibers. The average compressive and tensile strengths of the pristine SiC-II fiber were 1.37 and 3.08 GPa, respectively. After treatment at 1800℃ for 0.5 h in argon, the SiC-II fiber compressive strength decreased to 0.42 GPa, whereas the tensile strength reduced to 1.47 GPa. The mechanical properties of the fibers degraded after high-temperature treatment. This could be attributed to SiC grain coarsening and SiC_xO_y phase decomposition.     

High strength,low thermal conductivity and collapsible of Y2O3-stablized HfO2 crystalline fibrous membranes

HfO₂ is an important high temperature resistant material, which has a high melting point (2810 °C), good chemical stability and high thermal radiation. Fibrous material has some advantages for heavy refractory matter, such as large aspect ratio, light weight, more energy saving, flexibility and further can be processed into a variety of product forms. In this paper, high strength HfO₂ crystalline nanofibers were prepared through self-synthesis polyacetylcatonahafnium (PAHf) with electrospinning method. Phase variation process was discussed in detail, and the HfO₂ fibers that use Y₂O₃ as stabilizer with phase stability were prepared finally. The fibers were composed of nanocrystals with good flexibility even folded in half, tensile strength of 1.6 MPa @ 1000 °C and can lifting up 1288 times its own weight, low density around 35 mg/cm³. The thermal conductivity was stable around 26 mW/(m·K) @ 1200 °C and the temperature can be cooled from 1200 °C to 396 °C after 180 s through 7 mm fibrous membranes. And after treated at 1200 °C for 5 h, the fibrous membranes also maintain high strength and good flexibility. By this kind of precursor method, high strength oxide fibers can be obtained, which has a wide application prospect in field of high temperature thermal protection.     

Controlled anisotropy in 3D printing of silica-based ceramic cores through oxidization reaction of aluminum powders

Ceramic cores are key components to form inner hollow structures in aero-engine blades, and 3D printing is an ideal molding technology for ceramic cores. In this work, silica-based ceramic cores are fabricate via 3D printing of digital light processing (DLP) stereolithography, and the anisotropy in microstructure and property are controlled by aluminum powders. The ceramic cores without aluminum powders exhibit anisotropic microstructure with interlayer gaps, which get narrower and disappear with doping of 7.5–10 wt% of aluminum powders, due to the volume expansion during oxidization reaction of aluminum powders filling the interlayer gaps. The anisotropy in mechanical property is rely on the printing direction, and the ratio of strength in different directions (σ_V/σ_H) is put forward to value the mechanical anisotropy; the ratios rise from 0.40 to 0.92 at room temperature and 0.51 to 0.97 at 1540 °C, as 7.5 wt% of aluminum is doped, and the optimized ceramic cores show high-temperature strengths of 16.6 MPa and 16.1 MPa in different printing directions. Even though ceramic cores with 10 wt% of aluminum show uniform microstructure and higher σ_V/σ_H ratio, the weak particle bonding within printing layers limits their mechanical property, and the strengths decrease to 13.8 MPa and 13.4 MPa at 1540 °C. This work inspires a new technique to excellent high-temperature mechanical properties with anisotropy control in 3D printing of ceramic cores.     

Thermal stability and decomposition behavior of Cr2TiAlC2 at elevated temperature

As a good candidate for high-temperature uses in aerospace vehicles, the thermal behaviors of Cr₂TiAlC₂ in three different circumstances were studied. It exhibited good thermal-structural stability up to 1450 °C in flowing argon atmosphere, while heat treated to 1300 °C in Ar + CO/CO₂ atmosphere, it decomposed to Cr₂AlC resulting from the Ti consumption. The thermal stability in vacuum was also studied by in-situ X-ray diffraction methods, which showed a new decomposition mechanism based on the multi-scale bonding characteristics in its crystal structure. To deep understand the relationship between the crystal structure, chemical composition, and high-temperature behaviors, the hierarchical bonding characteristics were revealed. The out-plane ordering of M atoms together with their apparent negativity differences may result in the large M − C bonding strength, i.e., increased Cr–C strength and decreased Ti–C strength. The present work based on Cr₂TiAlC₂ helps better understand the composition-structure-property relationship of M_n+1AX_n compounds.     

High-temperature oxidation induced layering process for Si–C–B–N ceramic fibers with SiC nanograins

High-temperature oxidation resistance is important for Si–C–B–N ceramic fibers when reinforcing ceramic matrix composites with superior reliability and faulting tolerance. At present, few studies have investigated on the high-temperature oxidation behavior of Si–C–B–N fibers, limiting their further applications. In this work, we analyzed the high-temperature oxidation process of Si–C–B–N ceramic fibers with SiC nanograins (SiBCN-SiCn fibers) at 1000–1500 °C in air. SiBCN-SiCn fibers stated to be oxidized at 1000 °C, with the formation of thin oxide layer. After oxidizing at 1300 °C, obvious oxide layer that mainly consisted of amorphous SiO₂ could be detected. Further oxidizing at 1500 °C caused the thickness increment of oxide layer, which could inhibit the oxidation products (CO, N₂) to release away from the fibers. The remained CO and N₂ may react with SiC nanograins to form SiO₂ and graphite-like g-C₃N₄, causing the formation of additional transition layer. Our finding may support useful information for the applications of SiBCN-SiCn fibers under harsh environment.     

Oxidation resistance and thermal stability of the SiC-ZrB2 composite ceramic fibers

In this study, continuous SiC-ZrB₂ composite ceramic fibers were synthesized from a novel pre-ceramic polymer of polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam cross-linking, pyrolysis, and finally sintering at 1800°C under argon. The ZrB₂ particles with an average grain size of 30.7 nm were found to be uniformly dispersed in the SiC with a mean size of 59.7 nm, as calculated using the Scherrer equation. The polycrystalline fibers exhibit dense morphologies without any obvious holes or cracks. The tensile strength of the fibers was greater than 2.0 GPa, and their elastic modulus was ~380 GPa. After oxidation at 1200°C for 1 hour, the strength of the fibers did not decrease despite a small loss of elastic modulus. Compared to the advanced commercial SiC fibers of Tyranno SA, the fibers exhibited improved high-temperature creep resistance in the temperature range 1300-1500°C.     

Zirconia-alumina multiphase ceramic fibers with exceptional thermal stability by melt-spinning from solid ceramic precursor

Zirconia-alumina multiphase ceramic fibers with 80 wt% (Z80A20 fiber) and 10 wt% (Z10A90 fiber) proportions of zirconia were prepared via melt-spinning and calcination from solid ceramic precursors synthesized by controllable hydrolysis of metallorganics. The zirconia-alumina multiphase fibers had a diameter of about 10 µm and were evenly distributed with alumina and zirconia grains. The Z80A20 and Z10A90 ceramic fibers had the highest filament tensile strength of 1.78 GPa and 1.87 GPa, respectively, with a peak value of 2.62 GPa and 2.71 GPa. The Z80A20 ceramic fiber has superior thermal stability compared to the Z10A90 ceramic fiber and a higher rate of filament strength retention due to the stability in grain size. After heat treatment at 1100 °C, 1200 °C, and 1300 °C for 1 h respectively, the filament tensile strength retention rate of Z80A20 ceramic fibers was 87 %, 80 %, and 40 %. While Z10A90 ceramic fiber was fragile after being heated at 1300 °C. The results showed that the high zirconia content facilitated the fiber's thermal stability.     

Fabrication of lightweight ZrO2 fiberboards using hollow ZrO2 fibers

ZrO₂ fiberboards with ultra-low densities (0.34–0.40 g/cm³) were fabricated using biomorphic ZrO₂ hollow fibers, which have a lower density and better thermal insulation than traditional ZrO₂ solid fibers. The effects of sol binder content, sintering temperature, and proportion of solid fibers on the density, microstructure, compressive strength, linear shrinkage, and thermal conductivity of lightweight ZrO₂ fiberboards were investigated. The results showed that the hollow features of biomorphic ZrO₂ fibers were successfully maintained after they were made into ZrO₂ fiberboards, which made them less dense and thermally conductive. The best conditions were found to be a sol binder content of 30 vol%, sintering temperature of 1400 °C, and 20 wt% sintered solid fibers to balance thermal insulation and compressive strength. The results show that the density and thermal conductivity of lightweight ZrO₂ fiberboard gives it obvious advantages as a heat-insulating ceramic. Specifically, when the sintering temperature was 1400 °C, the sample had an ultra-low density of 0.34–0.40 g/cm³, a thermal conductivity of 0.101–0.116 W/(m·K) (at 500 °C), a compressive strength of 0.05–0.24 MPa, and a linear shrinkage of 9.4–13%.     

Effects of ZrSiO4 content on properties of SiO2-based ceramics prepared by digital light processing

SiO₂-based ceramic cores are widely used in the preparation of gas turbine engine hollow blades due to their excellent chemical stability and easy removal after casting. In this paper, ZrSiO₄ reinforced SiO₂-based ceramics were fabricated using digital light processing (DLP) technology. The results showed that the addition of ZrSiO₄ reduced the cure depth due to its high UV light absorptivity and refractive index. When the content of ZrSiO₄ increased to 15 wt%, the cristobalite content reached the maximum, and radial shrinkage reached the minimum of 1.4%. ZrSiO₄ grains could hinder the propagation of cracks, enhancing the room-temperature flexural strength. At 1550 °C, fracturing across SiO₂ grains in SiO₂-based ceramics led to the great improvement of high-temperature flexural strength. When the content of ZrSiO₄ reached 15 wt%, the flexural strength at room temperature and high temperature was 11.5 MPa and 36.7 MPa, respectively. Therefore, the SiO₂-based ceramics prepared using DLP technology have good room temperature and high temperature properties, and are expected to be used for hollow blade casting.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Influence of Y2O3 on densification,flexural strength and heat shock resistance of cordierite-based composite ceramics

In order to fabricate a heat transfer ceramic-based pipeline for concentrated solar power, rare earth Y₂O₃ was utilized as a modifying agent to improve the physico-chemistry properties of the cordierite-based composite ceramics. The influences of the sintering temperature and Y₂O₃ additive on the densification, flexural strength, and thermostability were investigated. The research results indicate that the densification degree of the composite ceramics gradually increases with elevated temperature, and the initial sintering temperature decreases with the addition of Y₂O₃. In addition, the flexural strength and heat shock resistance of the ceramic materials were improved with the addition of Y₂O₃. In particular, a sample containing 7 wt% Y₂O₃ (sample E4) sintered at 1360 °C showed the best properties with a relative density of 92.49%, a flexural strength of 126.81 MPa, and strength loss rate of -7.74% after 30 heat shock cycles. X-ray diffraction and scanning electron microscopy analysis showed that parts of Y³⁺ ions dissolving into high-temperature liquid phases could reduce liquid viscosity to accelerate grain crystallization and pore elimination. The second phase of yttrium silicate properly impeded the generation of β-spodumene with lower strength during the heat shock process. Overall, a cordierite-based composite ceramic with low porosity was obtained with high mechanical strength and heat shock resistance and can be regarded as a highly potential material for solar heat transfer pipelines.     

Preparation and mechanical properties of the 2.5D ASf/ZrO2 composites after high-temperature heat treatment

In the paper, the aluminosilicate fiber-reinforced zirconia (AS_f/ZrO₂) ceramic composites were successfully fabricated by polymer impregnation and pyrolysis (PIP) method. The microstructure and high-temperature mechanical properties of the original composites were well studied. The results revealed that the composites could maintain the stability of microstructure at 1000 °C. The flexural strength increased from 58.82 ± 2.83 MPa to 88.74 ± 6.20 MPa and the flexural modulus increased from 29.26 ± 4.67 GPa to 40.76 ± 8.76 GPa. The thermal exposure improved the interfacial bonding and made the load transfer more effective. After heat treatment from 1200 °C to 1400 °C, the flexural strength gradually declined due to the crystallization of the AS fibers and ZrO₂ matrix, while the flexural modulus increased in a completely different trend. After heat treatment at 1400 °C, the composites could maintain a flexural strength of 66.95 ± 4.24 MPa with a flexural modulus of 60.42 ± 7.25 GPa. But the fracture mode gradually evolved to brittleness.     

Phase transformation and thermal conductivity of the APS Y2O3 doped HfO2 coating with hybrid structure

Hafnia-based materials are very promising to serve as thermal protecting coatings at temperature above 1200 °C. In this work, two kinds of 8 mol% Y₂O₃ stabilized HfO₂ ceramic coatings (YSH-SN and YSH-MX) with conventional and hybrid structures were prepared by air plasma spray (APS) method. The microstructure, thermal conductivity and the mechanical properties of the coatings before and after thermal exposure at 1300 °C were compared in detail. Results show that the as-sprayed YSH-MX has a hybrid laminated structure of monoclinic HfO₂ and cubicY₂O₃ splats, and transforms to monoclinic HfO₂ and cubic YSH after thermal exposure, while the YSH-SN is composed of major tetragonal YSH phase and transforms to monoclinic HfO₂ and cubic YSH afterward. Thermal conductivities at ultra-high temperature (1600 °C) before and after thermal exposure for those two coatings are close, and the fracture toughness in the direction parallel to the interface exceeds 2.1 MPa m^0.5. The YSH-MX coating with a hybrid structure provides insights to conveniently prepare gradient coating or other coatings with complex structures.     

Densification of copper oxide doped alumina toughened zirconia by conventional sintering

The effect of copper oxide doping (0.05–1 wt%) on the densification, microstructure evolution and mechanical characteristics of alumina toughened zirconia (ATZ: 80 wt% Y-TZP + 20 wt% Al₂O₃) ceramic composites was investigated. Green samples were pressureless sintered using a short hold time of 12 min at temperatures varying from 1250 °C to 1500 °C. The incorporation of up to 0.2 wt% copper oxide was beneficial in promoting densification at low sintering temperature and improving the mechanical properties of ATZ without affecting the tetragonal phase stability. It was found that 0.2 wt% copper oxide addition was most efficacious, and the samples could attain a relative density of approximately 92% at 1250 °C, approximately 97% dense at 1350 °C and above 99% dense at 1450–1500 °C. This approach was also accompanied by an improvement in the Vickers hardness (12.7 GPa) and fracture toughness (6.94 MPam^1/2) when consolidated at 1450 °C/12 min. In comparison, the undoped composite exhibited relative densities of approximately 80% at 1250 °C, 87% at 1350 °C and approximately 97%–98% at 1450 °C-1500 °C. However, the study also found that higher dopant levels (0.5 wt% and 1 wt%) was not beneficial because the tetragonal zirconia phase was disrupted upon cooling from sintering, resulting in the monoclinic phase formation. In addition, low densification and poor mechanical properties were obtained.     

Influence of yttrium oxide addition and sintering temperature on properties of alumina-based ceramic cores

Al₂O₃-based ceramic cores with a uniform microstructure were fabricated successfully by a traditional pressing forming method, in which Al₂O₃ powders were used as matrix and yttrium oxide as additive. The influences of yttrium oxide content and sintering temperature on properties of ceramic cores were studied carefully. Results indicated that a higher sintering temperature benefited the preparation of ceramic cores with excellent properties. As the temperature was above 1400°C, the reaction of Al₂O₃ and yttrium oxide occurred, leading to the formation of YAG phases. And, YAG was uniformly adhered on the surface of Al₂O₃ particles, exerting a good role in connecting Al₂O₃ particles. Based on XRD analyses, it was found that the increase in the sintering temperature could promote the formation of more YAG phases. When sintering temperature was adjusted to 1600°C, with the increase in the yttrium oxide content, their relative density developed a trend of decreasing first and then increasing, while the apparent porosity had an opposite change tendency. With the increase in the sintering temperature, the line shrinkage and bending strength of Al₂O₃-based ceramic cores both increased gradually. In our research, their bending strength reached to 53.5 MPa and apparent porosity was 33.9% when the ceramic cores were prepared with 9 wt% yttrium oxide at 1600°C.     

Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

Polar bear hair inspired ternary composite ceramic aerogel with excellent interfacial bonding and efficient infrared transmittance for thermal insulation

Highly porous, heat resisting ceramic aerogels are considered as promising materials for high-temperature insulation. However, the general structural characteristics of ceramic aerogel, such as poor mechanical strength and transparency to infrared radiation, pose a major obstacle to their practical application. In this paper, we report a general strategy to prepare hollow mullite fiber (HMF) structures by coaxial electrostatic spinning and grow TiO₂ nanorods (TiO₂/NAs) in situ on HMF. The ternary composite ceramic aerogel material was prepared by filling the pores of HMF-TiO₂/NAs with SiCN aerogel. The TiO₂/NAs increased the fiber/aerogel interfacial bonding of the composite (0.392 MPa, 30% strain) and improved the IR transmittance (∼0%, 1200 ℃) without sacrificing their low density and thermal conductivity. In addition, low thermal conductivity (0.041 W/(m·K), 1200 °C) and excellent high-temperature insulation properties allow the composite aerogel to meet the urgent need for lightweight, high-strength, high-temperature insulation systems for spacecraft.     

High-pressure structural stability,equation of state,and thermal expansion behavior of cubic HfO2

The structural stability, equation of state, and thermal expansion behavior of nanocrystalline cubic HfO₂, an ultra-high-temperature ceramic, have been investigated using X-ray diffraction at extreme conditions of pressures and temperatures. High-pressure studies show that the cubic structure is stable up to 26.2 GPa, while the high-temperature studies show the stability of the cubic structure up to 600°C. The Rietveld structure refinement of the high-pressure data reveals the progressive transition of secondary monoclinic phase to the cubic phase at higher pressures. The phase progression is accompanied by incompressibility along the b axis and a large compressibility along the c axis of the monoclinic structure. The second-order Birch-Murnaghan equation of state fit to the unit cell volume data yielded a bulk modulus of 242(16) GPa for the cubic structure. A linear thermal expansion value of α_a(c) = 8.80(15) × 10⁻⁶°C⁻¹ and a volume thermal expansion value of α_v = 26.5(4) × 10⁻⁶°C⁻¹ have been determined from the in situ high-temperature X-ray diffraction studies. The results are discussed by comparing with the high-pressure and high-temperature behavior of isostructural ZrO₂. To the best of our knowledge, this is the first experimental report on the structural stability of cubic HfO₂ at high pressures.     

Effect of silica sol on phase transition of alumina-mullite fibers

Duplex oxide ceramic fibers have excellent high temperature properties. However, controlling the microstructure of duplex oxide ceramic fibers is difficult and complexed. In this work, the effect of silica sols affecting the interaction of precursor sol-gel particles on the high-temperature phase transition and microstructure of alumina-mullite fibers was investigated. The results show that the silica sol affects the particle interactions and distribution in the mixed sol. Alumina-mullite fibers prepared from non-homogeneous precursor sols generated α-Al₂O₃ at 1400 °C, while fibers prepared from homogeneous precursor sols required higher temperatures. In addition, mullite (3Al₂O₃:2SiO₂) with an orthogonal structure is more likely to be generated in the non-homogeneous Al₂O₃-SiO₂ precursor sol. The prepared alumina-mullite fibers have a tensile strength of up to 1.68 GPa. Finally, the mechanism of the influence of silica-sol properties on the final organization of the alumina-mullite fiber was further discussed.