Ablation behaviour of ultra-high temperature ceramic matrix composites: Role of MeSi2 addition

A new class of ZrB₂ composites reinforced with 40 vol% C short fibers and containing 5 vol% SiC in combination with 5 vol% MoSi₂, HfSi₂ or WSi₂ successfully withstood extreme conditions in a oxyacetylene torch. Different responses to the torch testing were recorded depending on which secondary phase was present; this was primarily a result of the final density which ranged between 83 and 94% of the theoretical value. The temperatures achieved on the surfaces of the samples tested also varied as a function of the residual porosity and ranged from 2080 to 2240 °C. HfSi₂ additions offered the best performance and exceeded that of the baseline material that contained only SiC. It is believed that this was due to its ability to promote the elimination of porosity during densification and to the refractory nature of its oxide, HfO₂. In contrast, MoSi₂ and WSi₂ formed highly volatile oxides on the surface, which did not offer better protection than the ZrO₂-SiO₂ scale that developed in the baseline.     

Development of a slurry injection technique for continuous fibre ultra-high temperature ceramic matrix composites

A simple and effective slurry injection method for producing dense and uniform ultra-high ceramic matrix composites from preforms of high fibre density was developed. As this method is based on slurry injection the homogeneity is not constrained to small preform sizes; dense components of high fibre volume can be produced in theoretically any size and shape. Samples produced by this method demonstrated high and consistent densities, with the injection method obtaining densities an average 27% higher and 87% lower in variability when compared to conventional vacuum impregnation. Tomography demonstrated no bias in the ceramic powder distribution for samples produced by injection, whereas samples produced by vacuum impregnation alone displayed poor powder penetration to the centre of large samples. The new approach yielded composites that were as strong and/or more consistent in strength compared to vacuum impregnation. Thermo-ablative testing demonstrated significant improvements in protective capability for materials produced by this route.     

Fabrication and characterisation of high-performance joints made of ZrB2-SiC ultra-high temperature ceramics

Joining is crucial for ultra-high temperature ceramics (UHTCs) to be used in demanding environments due to the difficulty in manufacturing large and complex ceramic components. In this study, ZrB₂-SiC composite UHTCs parts were joined via Ni foil as filler, and the mechanical properties and oxidation behaviour of the fabricated ZrB₂-SiC/Ni/ZrB₂-SiC (ZS/Ni/ZS) joint were investigated. Firstly, dense ZrB₂-SiC composites were prepared from nano-sized powders by spark plasma sintering (SPS). The ZrB₂-SiC parts were then joined using SPS. Furthermore, the elastic modulus, hardness, shear strength and high temperature oxidation behaviour of the ZS/Ni/ZS joint were examined to evaluate its properties and performance. The experimental results showed that the ZrB₂-SiC parts were effectively joined via Ni foil using SPS and the resultant microstructures were free from any marked defects or residual metallic layers in the joint. Although the elastic modulus and hardness in the joining zone were lower than those in the base ZrB₂-SiC ceramics, the shear strength of the joint reached ∼161 MPa, demonstrating satisfactory mechanical properties. Oxidation tests revealed that the ZS/Ni/ZS joint possesses good oxidation resistance for a wide range of elevated temperatures (800–1600 ^oC), paving the way for its employment in extreme environments.     

Structure,microwave dielectric properties,and infrared reflectivity spectrum of olivine type Ca2GeO4 ceramic

Ca₂GeO₄ dielectric ceramic was prepared using the conventional solid-state reaction method. Sintering behavior, crystal structure, microstructure, and microwave dielectric properties were analyzed by XRD, SEM, Raman, and Infrared reflectivity spectrum. Ca₂GeO₄ was found to crystallize in the olivine structure with a space group of Pnma. A dense and high-performance microwave dielectric property with permittivity ? 6.76 ± 0.02, Q×f value ? 82,400 ± 1800 GHz, and temperature coefficient ? -67 ± 3.4 ppm/°C were obtained in the sample sintered at 1420 °C. Infrared spectral analysis supported that the dielectric contribution for Ca₂GeO₄ at microwave region is dominated by absorption of phonons and there is no contribution from dipolar or other polarization mechanisms. The large negative τ_f values could be compensated by forming composite ceramics with CaTiO₃. A low-ε_r of 9.02 ± 0.03, a high Q×f of 49,880 ± 1400 GHz, and a near-zero τ_f value of +4 ± 0.6 ppm/°C were obtained for 0.92Ca₂GeO₄-0.08CaTiO₃ ceramic at 1420 °C for 4 h. This ceramic could be a good candidate for microwave substrate materials.     

红外节能涂料的研制和应用

论述了提高工业炉窑使用寿命和节能等多功能的低、中、高温远红外辐射涂料的机理、性能、制备、施工和应用。     

Compatible performance of low thermal conductivity and high infrared radiation of Sm2O3-HfO2 series ceramics applied for thermal protection coatings

Sm₂O₃-HfO₂ series ceramics were synthesized at high temperature using the solid-state reaction. The phase stability, thermo-physical and infrared emission properties of Sm₂Hf₂O₇ (SHO) and Sm₂Hf₂O₇-44.83 wt%HfO₂ (25S/H) composite ceramics were comparatively investigated. Furthermore, their calcium magnesium aluminosilicate (CMAS) corrosion was conducted at 1250°C for different times. The results reveal that both SHO and 25S/H ceramics have excellent phase stability at 1600°C as well as excellent sintering resistance. SHO still exhibits slightly lower thermal conductivity and lower hardness and Young's modulus, higher thermal expansion coefficient (CTE) and fracture toughness as well as higher infrared emittance (0.899 at 800°C) than 25S/H composite with the excessive HfO₂ inside. Both SHO and 25S/H ceramics react with CMAS to form a relatively compact reaction layer, which can effectively prevent the penetration of CMAS. These results preliminarily indicate that SHO ceramic can be proposed as an alternative material of the traditional YSZ for high-temperature thermal protective applications thanks to its compatible performance of low thermal conductivity and high infrared radiation, etc.     

Interaction of air plasma-sprayed spinel and yttria coatings with ultra-high temperature ceramic-metal hot-melt

Air Plasma Sprayed (APS) ceramic coatings are conventionally used for protecting metallic parts of hot section components in aero- and land-based gas turbines. Owing to their capability to withstand very high temperatures high specific heat capacity, and thermal shock resistance, such APS coatings are also investigated for the potential to protect the core catcher in nuclear reactors during the high-temperature core-meltdown accidents resulting in corium hot-melt. In this study, candidate plasma-sprayed spinel and yttria coatings on SS 316LN substrates were subjected to an ultra-high temperature (UHT) ceramic-metal hot-melt at ∼2500 °C. Uncoated steel substrates showed melting, whereas coated substrates were found un-attacked at the expense of degradation of the coatings such as surface melting, topcoat sintering, columnar grain growth and thermal shock cracks. The evolution of interfacial fusion was observed in the case of yttria coatings due to the formation of primary YAG and Al₂O₃-YAG eutectic at the interface. Spinel coatings with a comparatively lower thermal conductivity could generate a higher ΔT across the topcoat thickness with limited grain growth in the substrate. Evidence of local melting at the interface and evolution of temperature gradient between the hot-melt and substrate are comprehensively illustrated. Out of the two coatings tested, spinel was found to be more protective to the steel substrate.     

Dielectric characterization of a novel Bi2O3-Nb2O5-SiO2-Al2O3 glass-ceramic with excellent charge-discharge properties

A newly designed glass-ceramic system consisting of 15Bi₂O₃-15Nb₂O₅-40SiO₂-30Al₂O₃ was successfully prepared, which was followed by its controlled crystallization at different heating temperatures. The effects of crystallization temperature on the microstructure, phase evolution and the energy storage behaviors of the novel material were systematically investigated. A maximum theoretical energy storage density of up to 15.3 J/cm³ was found in the samples heated at 800 °C. The polarization-electric (PE) hysteresis loops of this material exhibited very good linear character and high energy efficiency. In addition, an approximate value of 25 ns for discharged period T has been obtained, which demonstrated that most of the energy stored in dielectric was released over a very short time. The maximum powder density exceeds a high value of 90 MW/cc in a 390 kV/cm electric field. Therefore, the new developed Bi₂O₃-Nb₂O₅-SiO₂-Al₂O₃ glass-ceramic can be used as an alternative, promising high-performance electrostatic capacitor material.     

镍掺杂铝酸镧涂层的制备及其红外辐射性能

采用溶胶?凝胶法合成了Ni2+掺杂的LaAlO3基红外辐射材料LaAl0.6Ni0.4O2.89 (LANO),以其为辐射基料,利用喷涂工艺在氧化铝陶瓷片表面制备红外辐射涂层,考察了磷酸二氢铝、铝溶胶、硅溶胶和钠水玻璃4种粘结剂对涂层物相组成、热稳定性和红外辐射性能的影响. 结果表明,以LANO为辐射基料、铝溶胶为粘结剂时,涂层红外辐射性能最佳,3?5 ?m波段红外发射率达0.93;所制涂层具有良好的抗热震性能,50次热震后涂层未明显剥落失效;涂层具有显著的强化辐射传热效果,节能率达31.7%.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb₂Si₂O₇ coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO₂ content with higher power and H₂ flow. Differences in microstructure and thermomechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb₂SiO₅ metastable phase was identified after thermal treatments at temperatures ～ 1000 °C that transformed to its stable isomorph at 1220 °C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the “healing” of microcracks present in the amorphous as-sprayed coating.     

Electrophoretic co-deposition of Fe2O3 and Mn1,5Co1,5O4: Processing and oxidation performance of Fe-doped Mn-Co coatings for solid oxide cell interconnects

Fe-doped Mn_1,5Co_1,5O₄ coatings on Crofer22APU were processed by an electrophoretic co-deposition method and the corrosion resistance was tested at 750 °C up to 2000 h.The “in-situ” Fe-doping of the manganese cobalt spinel was achieved by electrophoretic co-deposition of Mn_1,5Co_1,5O₄ and Fe₂O₃ powders followed by a two-step reactive sintering treatment. The effects on the coating properties of two different Fe-doping levels (5 and 10 wt.% respectively) and two different temperatures of the reducing treatment (900 and 1000 °C) are discussed. Samples with Fe-doped coatings demonstrated a lower parabolic oxidation rate and thinner oxide scale in comparison with both the undoped Mn_1,5Co_1,5O₄ spinel coating and bare Crofer 22 APU. The best corrosion protection was achieved with the combined effect of Fe-doping and a higher temperature of the reducing step at 1000 °C.     

Thermodynamic assessment of the group IV,V and VI oxides for the design of oxidation resistant multi-principal component materials

Multi-principal component materials (MPCMs) are currently being investigated for use in high and ultra-high temperature environments. The design of oxidation resistant multi-component materials requires as input the oxidation behavior of each of the components. FactSage free energy minimization software and databases were used to calculate the equilibrium oxide phases and free energies of formation for the oxides of the Group IV, V and VI refractory metals, and their carbides, nitrides and borides. The results are summarized in Ellingham diagrams. Periodic trends were noted; Group IV elements form the most stable oxides with the highest melting temperatures (T_m), Group V elements form oxides with low T_m, and Group VI elements form gaseous oxide species. Oxygen diffusion data from literature for some of these oxides were also reviewed and summarized. The results are utilized to identify strategies for optimizing oxidation resistance of MPCMs for service at temperatures above 1700°C.     

Fabrication and investigation of Ca/Tb co-doped HfO2 infrared coatings

In this study, Ca/Tb co-doped HfO₂ coatings were prepared by atmosphere plasma spraying. The chemical composition, morphology and infrared property of the coatings were characterized. The coatings possessed a layer-stacked morphology. When the Ca/Tb doping atomic ratio was 1:1, the phase of the coatings gradually changed from monoclinic to cubic with increasing the doping mass. The CTH2 coating had the highest emissivity which was 0.820 in 0.75–6.5 µm and 0.902 in 6.5–15 µm respectively. The enhancement in short band was mainly due to the introduction of Ca²⁺ and Tb³⁺ ions that generated oxygen vacancies in the lattice forming impurity levels within the forbidden band, moreover, the transfer of Tb³⁺ to Tb⁴⁺ increased the concentration of free electrons, which promoted the absorption of free carriers. The increase in long band attributed to the lattice distortion that reduced the lattice symmetry and strengthened the absorption of lattice polar vibration.     

Synthesis of Hf6Ta2O17 superstructure via spark plasma sintering for improved oxidation resistance of multi-component ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) have shown aspiration to overcome challenges in the thermal protection system (TPS) by designing new materials referred to as multi-component UHTCs (MC-UHTCs) in the compositional space. MC-UHTCs have shown remarkable improvement in oxidation resistance due to the formation of the Hf₆Ta₂O₁₇ superstructure during plasma exposure. Herein, the Hf₆Ta₂O₁₇ superstructure is synthesized via a solid-state reaction between HfO₂ and Ta₂O₅ powder mixtures during spark plasma sintering (SPS). The compositions chosen are 50 vol% of HfO₂ -50 vol% of Ta₂O₅ (50HO-50TO) and 70 vol% of HfO₂ -30 vol% of Ta₂O₅ (70HO-30TO). The phase quantification via Rietveld analysis showed Hf₆Ta₂O₁₇ as a principal phase with some residual Ta₂O₅ phase in both the samples. The high-temperature thermal stability of the samples was evaluated using high-velocity plasma jet exposure for up to 3 min. 50HO-50TO was able to withstand the intense plasma condition, which is attributed to the higher content of the Hf₆Ta₂O₁₇ phase (～84%) and lower strain in the Ta₂O₅ phase. The augmentation in the Hf₆Ta₂O₁₇ phase to 94.7% (in 50HO-50TO) post plasma exposure has been attributed to the invariant transformation from a liquid state to Hf₆Ta₂O₁₇ at temperatures >2500 °C during testing. The mechanical integrity is elucidated from the insignificant change in the hardness ～13.3 GPa before and 11.2 GPa after plasma exposure of the 50HO-50TO sample. As a result, the Hf₆Ta₂O₁₇ superstructure's thermo-mechanical stability suggests developing novel oxidation-resistant MC-UHTCs in compositional space for reusable space vehicle applications.     

Modified double-notched specimen for ultra-high temperatures shear-strength testing of carbon/carbon composites

In this paper, a modified double-notched specimen (MDNS) is proposed to investigate shear strength of carbon/carbon composites at ultra-high temperatures. The effects of surplus notch length on distribution of shear stress in the gauge area are studied using finite element method. Both standard Iosipescu method and the MDNS method are used to test shear strength of 3D needled, 3D woven, and 4D woven C/C composites at room temperature. The results are in good agreement with each other. Uniform shear strain is observed using digital image correlation (DIC) technology to confirm the proposed method. Additionally, the shear strength of these three types C/C composites are measured at 2000 °C, 2400 °C, and 2800 °C using the designed testing system. The temperature field at 2000 °C is measured using a thermal imaging system to demonstrate uniform distribution of temperature. The failure mechanisms at ultra-high temperatures are also characterized via optical microscopy.     

Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications

The effect of exposure temperature and time on the diffusion rate of rare-earth oxides applied on silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) have been investigated. Knowledge on diffusion mechanism between the deposited rare-earth (RE) slurry and silica layer is necessary to understand the process governing EBCs formation and their properties. SEM/EDS analysis were used to study the effect of microstructure on diffusivity. The diffusion coefficient increases with increasing sintering temperature and time. The measured diffusion coefficients of the RE-coating into silica layer were in the order of 10^?15-10^-17 m²/s revealing an overall good adhesion on the SiC/SiC CMCs.     

Reactive hot pressing route for dense ZrB2-SiC and ZrB2-SiC-CNT ultra-high temperature ceramics

The in-situ exothermic reactions between ZrC_0.8, B₄C and Si have assisted densification and allowed to obtain fully dense ZrB₂-31 wt.%SiC ultra-high temperature ceramics within 6 min at 1750 °C. The use of zirconium carbide instead of metallic zirconium in the green body obviated the possibility of in-situ SHS process and allowed to apply the pressure at low temperatures. The latter provided a first densification stage just above 1050 °C. A slight carbon excess was created in the green body to preserve the carbon nanotubes. The developed reactive hot pressing route (1830 °C, 3 min, 30 MPa) has been successfully used to obtain ZrB₂-SiC ceramics containing 8 vol.% of multi-wall carbon nanotubes (MW-CNT). The carbon nanotubes survived the thermal cycle and could be clearly observed in the sintered ceramics. The CNT addition improved the fracture toughness of the composite from 4.3 MPa m^1/2 for ZrB₂-31 wt.%SiC to 6.8 MPa m^1/2 for ZrB₂-29 wt.%SiC-CNT.     

Friction and wear properties of MoAlB against Al2O3 and 100Cr6 steel counterparts

The present work investigates, for the first time, the dry sliding friction and wear behaviour of fully dense, predominantly single-phase MoAlB ceramics against alumina (Al₂O₃) and 100Cr6 steel counterparts. Against Al₂O₃, the friction coefficient (μ) increased with increasing load and the wear was highly dependent on the load applied. A transition from mild wear under 1 N and 4 N to severe wear at 10 N occurred. Scanning electron microscopy revealed that abrasion is the dominant wear mechanism. Against steel, μ decreased with increasing load and the wear rates were low, under all applied loads. The morphologies of the worn surfaces against steel were characterized by the appearance of a rippled layers. Atomic force microscopy and Raman spectroscopy were used to propose a possible formation mechanism of such patterns. X-ray photoelectron spectroscopy revealed the rippled surfaces to be composed of Fe₂O₃ and a mixture of MoO_x.     

Structure and thermal properties of Al2O3-doped Gd3TaO7 as potential thermal barrier coating

In this paper, a series of solid solutions ceramics of (Al_xGd_1-x)₃TaO₇ (x = 0, 0.01, 0.03, 0.05) were synthesized via solid-state reaction. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the crystal structure of (Al_xGd_1-x)₃TaO₇ ceramics is weberite in spite of the content of Al³⁺ is up to 5 mol.%. The thermal conductivities of (Al_xGd_1-x)₃TaO₇ ceramics range from 1.37 W?m^?1 K^?1 to 1.47 W?m^?1 K^?1 at 900 ℃, which is much lower than that of 7–8 YSZ (about 2.5 W?m^?1 K^?1). The thermal expansion coefficients (TECs) of (Al_xGd_1-x)₃TaO₇ ceramics vary in the range of 6–10 × 10^-6 K^-1 within the temperature range 100–1200 ℃, and the values are close to the TECs of 7–8 YSZ. Given the low thermal conductivity and high thermal expansion coefficients of (Al_xGd_1-x)₃TaO₇ ceramics, they have the potential to be the next generational thermal barrier coating materials.