High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

Joining of Cf/SiC composites and Si3N4 ceramic with Y2O3–Al2O3–SiO2 glass filler for high-temperature applications

C_f/SiC composites and Si₃N₄ ceramics are candidate materials for applications in thermal protection system. This paper investigated the joining of C_f/SiC and Si₃N₄ using Y₂O₃–Al₂O₃–SiO₂ glass. The reliability of joints was evaluated by thermal shock tests. In this present work, the typical joint structure was C_f/SiC-glass-Si₃N₄. The results demonstrate that Direct bonding has been identified as the interfacial bonding mechanism at the SiC/glass and glass/Si₃N₄ interfaces. The maximum shear strength of the C_f/SiC–Si₃N₄ joint was ~34 MPa, which delivered an effective method to achieve strong, reliable bonding between the dissimilar materials. In addition, after thermal shock for 10 cycles, the residual strength remained ~13 MPa. Bubbles instead of microcracks formed in the glass filler, which was the main factor causing the degradation of the joint performance. It is suggested that improving the high temperature resistance of joining materials is the key to realize the application of this joint structure.     

Degradation behavior of SiO2f/Al2O3-SiO2 composites: Anti-oxidation behavior and high-temperature flexural strength

2.5-Dimensional SiO₂ fiber-reinforced Al₂O₃-SiO₂ (SiO_2f/Al₂O₃-SiO₂) composites were prepared by the sol-gel method, using diphasic SiO₂ sol as the precursor into which Al₂O₃ powders were added. Their antioxidative behaviors and flexural strengths at high temperature were tested and compared. In an oxidation atmosphere, the composites showed high oxidation resistance, with a flexural strength retention ratio of over 90.00% at 1200?°C. After oxidation at 1500?°C, the mass retention ratio and flexural strength were 97.49% and 65.0?MPa respectively. The oxidation resistance of SiO_2f/Al₂O₃-SiO₂ composites was higher than that of SiO_2f/SiO₂ composites. After high-temperature test, the flexural strength retention ratios of SiO_2f/SiO₂ and SiO_2f/Al₂O₃-SiO₂ composites were 86.18% and 94.80% respectively, and the latter had a flexural strength of 134.9?MPa. SiO_2?f/Al₂O₃-SiO₂ composites worked better than SiO_2f/SiO₂ composites did in the flexural strength test at 1200?°C. The mechanical performance degradation and mass variations of the composites during tests were closely associated with their microstructural evolutions.     

Design and fabrication of Al2O3f/SiCN composite with excellent microwave absorbing and mechanical properties

A new kind of structural and functional integration ceramic matrix composite material was prepared from high-performance alumina (Al₂O₃) fibers and absorbing silicon carbonitride (SiCN) ceramics via a combination of polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The Al₂O₃ fiber annealed at its cracked temperature had enhanced permittivity, because the sizing agent on the Al₂O₃ fiber surface was cracked into pyrolysis carbon. For PIP + CVI Al₂O_3f/SiCN composites, PIP SiCN matrix with low conductivity was used as the matching phase, while CVI SiCN matrix with medium permittivity and dielectric loss was regarded as the reinforcing phase distributed in porous PIP SiCN matrix and inter-bundles of Al₂O₃ fiber to improve their mechanical and microwave absorption properties. The fracture toughness and flexural strength of Al₂O_3f/SiCN composite were determined to be 9.4 ± 0.5 MPa m^1/2 and 279 ± 28 MPa, respectively. Based on the design principles for impedance matching, the Al₂O_3f/SiCN composites before and after oxidation were used as loss and impedance layers, respectively. It was found that the optimized composite had the lowest reflection coefficient (RC) of −70 dB and the effective absorption bandwidth covering the whole X-band. In conclusion, Al₂O_3f/SiCN composite can serve as a high-temperature structural material with excellent microwave absorption properties for aerospace applications.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

Microstructure,high-temperature mechanical and dielectric properties of novel Si3N4 f/SiNO wave-transparent composites

Continuous Si₃N₄ fiber reinforced SiNO matrix composites (Si₃N_4 f/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si₃N_4 f/SiNO composites were investigated in detail. The as-fabricated Si₃N_4 f/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si₃N_4 f/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 ℃ for the surface reactions between the SiNO matrix and Si₃N₄ fibers. After heat-treated at 1550 ℃, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si₃N₄ fibers at this temperature. Si₃N_4 f/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7–18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10^?3. The dielectric constants and dielectric loss of Si₃N_4 f/SiNO composites are relatively stable from RT to 800 ℃. The as-fabricated Si₃N_4 f/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.     

Thermal shock resistance of Al2O3/SiO2 composites by sol-gel

In this paper, the SiO₂ ceramic matrix composites were reinforced by the two-dimensional (2D) braided Al₂O₃ fibers by sol-gel. To develop the high performance aeroengine with excellent resistance to thermal shock for advanced aerospace application, two different thermal shock temperatures (1100?°C and 1300?°C) and three different thermal shock cycles (10, 20 and 30 cycles) were tested and compared in this paper; besides, the thermal shock resistance of Al₂O₃/SiO₂ composites was investigated in air. Our results suggested that, the flexural strength of the untreated composites was 78.157?MPa, while the residual strength of Al₂O₃/SiO₂ composites under diverse thermal shock cycles and temperatures had accounted for about 95% and 50% of the untreated composites, respectively. Meanwhile, the density and porosity of the composites were gradually increased with the increase in test temperature. Moreover, the changes in fracture morphology and micro-structural evolution of the composites were also observed. Our observations indicated that, the fracture morphology of the composites mainly exhibited ductile fracture at the thermal shock temperature of 1100?°C, whereas brittle fracture at the thermal shock temperature of 1300?°C. Additionally, Al₂O₃/SiO₂ composites belonged to the Oxide/Oxide CMCs, so no new phase was formed after thermal shock tests. Above all, findings of this paper showed that Al₂O₃/SiO₂ composites had displayed outstanding thermal shock resistance.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

Microstructure evolution and cooperative reinforcement mechanisms of Al2O3/Al2O3 joints brazed by low-melting borosilicate glass

The Al₂O₃/SiO₂–B₂O₃–Al₂O₃–Na₂O glass/Al₂O₃ joints reinforced cooperatively by glass matrix and in-situ Al₄B₂O₉ whiskers were obtained via a low-melting borosilicate glass braze. The composition of glass seam transformed from SiO₂–B₂O₃–Na₂O to SiO₂–B₂O₃–Al₂O₃–Na₂O due to continuous diffusion and dissolution of Al₂O₃. An appropriate amount of [AlO₄] units introduced into the glass braze played a vital role in strengthening the glass network structure resulting to considerably improved mechanical strength of the glass seam. Meanwhile, plenty of in-situ Al₄B₂O₉ whiskers growing from the Al₂O₃/glass braze interface to the center of glass seam in various directions generated. Three-dimensional crisscross structures were fabricated at the Al₂O₃/glass braze interface domains, where were enhanced by crack-bridging and pull-out effect of the whiskers. Generally, ascribed to the cooperative reinforcement of the glass matrix in the seam and in-situ Al₄B₂O₉ whiskers at Al₂O₃/glass braze interface domains through reactions of Al₂O₃ and borosilicate glass braze, strength of the as-brazed joints was promoted prominently. The shear strength of the joints reached a maximum of 61 MPa brazed at 1050 °C for 60 min.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

More effective crack self-healing capability of SiCf/SiC-B4C with Al2O3 modified under wet environment

Aluminum oxide has been introduced into SiC_f/(SiC + B₄C) composites to improve the crack self-healing property in O₂/H₂O atmosphere. The observation of the surface and interior morphologies of the oxidized composites reveal that Al₂O₃ can lead to rapid self-healing of cracks, which will effectively block the oxidation of interior interphase and fiber. Further investigation reveals that Al₂O₃ plays an important role in impeding the crystallization of SiO₂ and limiting the volatilization of B₂O₃. It is beneficial to form self-healing fluid glass phase and improve glass phase stability. Under these circumstances, the pre-crack can be healed more effectively, which is advantageous for composites to remain high strength retention rate.     

Enhanced high-temperature dielectric and microwave absorption properties of SiC fiber-reinforced oxide matrix composites

Because of outstanding performances of the SiC fiber-reinforced ceramic matrix composites in aircraft/aerospace systems, two silicon carbide fiber-reinforced oxide matrices (SiC_f/oxides) composites have been prepared by a precursor infiltration and sintering method. Results indicate that the flexural strength of the SiC_f/Al₂O₃–SiO₂ composite reaches 159 MPa, whereas that of the SiC_f/Al₂O₃ composite is only 50 MPa. The high-temperature microwave absorption properties of the composite are significantly enhanced due to choosing Al₂O₃ and SiO₂ as the hybrid matrices. Particularly, the minimum reflection loss (RL) value of the SiC_f/Al₂O₃–SiO₂ composite reaches −37 dB in the temperature of 200 °C at 8.6 GHz, and the effective absorption bandwidth (RL ≤ −5 dB) is 4.2 GHz (8.2–12.4 GHz) below 400 °C. The superior microwave absorption properties at high temperatures indicate that the SiC_f/Al₂O₃–SiO₂ composite has promising applications in civil and military areas. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47097.     

Thermal stability and oxidation resistance of C/Al2O3 composites fabricated from a sol with high solid content

To improve fracture toughness of monolithic Al₂O₃ ceramics, three-dimensional carbon fiber preform was used as reinforcement, and the C/Al₂O₃ composites without interfacial coating were fabricated through vacuum impregnation-drying-heat treatment route with an Al₂O₃ sol as starting material. Characteristics of the Al₂O₃ sol with high solid content were firstly analyzed. Then thermal stability and oxidation resistance of the C/Al₂O₃ composites were investigated. It is found that the Al₂O₃ sol is an appropriate raw material for the fabrication of C/Al₂O₃ composites. The C/Al₂O₃ composites with a total porosity of 15.5% show a flexural strength of 208.5 MPa and a fracture toughness of 8.1 MPa m^1/2. Strength loss is observed after the composites were annealed at 1400 °C and 1600 °C under inert atmosphere. Oxidation resistance of the C/Al₂O₃ composites is unsatisfactory because of the existence of open pores and microcracks. When Al₂O₃ matrix was modified with SiO₂, the oxidation resistance is remarkably improved due to the viscous flow effect of SiO₂.     

Mechanical and Dielectric Properties of SiCf/AlPO4 Composites with Multi‐Walled Carbon Nanotubes

SiC_f/AlPO₄ composites were prepared by laminating method. The slurry comprising Al(H₂PO₄)₃ solution, multi‐walled carbon nanotubes (MWCNTs), and Al₂O₃ powder was used as matrix precursor. The complex permittivities of the composites were measured in the X‐band (8.2–12.4 GHz). The reflection loss was calculated using the transmission line theory. It was found that the addition of 2.5 wt% MWCNTs noticeably improved the absorption property of 3 mm thick composites with the assistance of 15 wt% Al₂O₃, which also exhibited an obvious toughened fracture behavior with 190 MPa fracture strength.     

Additive manufacturing of high mechanical strength continuous Cf/SiC composites using a 3D extrusion technique and polycarbosilane-coated carbon fibers

A novel additive manufacturing approach is herein reported for manufacturing high mechanical strength continuous carbon fiber-reinforced silicon carbide (C_f/SiC) composite materials. Continuous carbon fibers were coated with polycarbosilane (PCS) using a colloidal evaporative deposition process and then coextruded with high solid content SiC ink. The zeta potential of the SiC ink was adjusted to optimize the printing ability of the suspension. During sintering, small SiC grains and whiskers were generated in the gaps in and around the PCS-coated carbon fibers, which led to the improved flexural strength and density of the composites. Meanwhile, the PCS coating on the surface of the carbon fibers prevented the carbon fibers from reacting with SiO gas generated by reactions between the SiC matrix and SiO₂ and sintering additives (Al₂O₃ and Y₂O₃), effectively preserving the structural integrity of the carbon fibers. Compared to the SiC specimens containing uncoated carbon fibers, the density of the specimens fabricated with coated carbon fibers was increased from 2.51 to 2.85 g/cm³, and the strength was increased from 190 to 232 MPa.     

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si₃N₄) fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si₃N_4f/BN/Si₃N₄ composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m^1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m^1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si₃N_4f/BN/Si₃N₄ composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.     

Processing and flexural properties of 3D,seven-directional braided (SiO2)f/SiO2 composites prepared by silica sol-infiltration-sintering method

Three dimensional (3D), seven-directional braided (SiO₂)_f/SiO₂ composites were prepared by silica sol-infiltration-sintering (SIS) method. The flexural properties were evaluated by three-point bending test. The flexural strength and flexural elastic modulus were found to be about 107 MPa and 17.5 GPa, respectively. The results of stress vs. deflection curve and SEM examination revealed that the fracture mechanism of the (SiO₂)_f/SiO₂ composite was a mixed mode of brittle and ductile. The bond strength of fiber/matrix was weak at low temperature, causing the extensive fiber pull-out. On the other hand, the brittle fracture of some fibers maybe caused by the propagation of micro defect or crack, which existed in the as-prepared composites for the ten-cycle process.     

Fabrication and mechanical properties of short ZrO2 fiber reinforced NiFe2O4 matrix composites

Short ZrO₂ fibers (ZrO_2(f)) reinforced NiFe₂O₄ ceramic composites were fabricated by cold pressing process. The phase composition, microstructure, mechanical properties and fiber/matrix interface of the composites were investigated by X-ray diffraction, scanning electron microscopy and mechanical testing machines. ZrO_2(f) show good thermodynamic and chemical compatibility with NiFe₂O₄ ceramic matrix and effectively enhanced the mechanical properties. The toughening mechanisms are fiber bridging, interfacial debonding, fiber pullout, phase transformation and the matrix constraint effect. By incorporation of 3 wt% fibers with the average length of 5～6 mm, the bending strength and fracture toughness of the composites reached 88.92 MPa and 4.62 MPa m^1/2, respectively, while the strength conservation ratio after thermal shock increased from 48.85% to 75.86%. The weak interface bonding built up between ZrO_2(f) and NiFe₂O₄ facilitates the reinforcing effects of the fibers to operate.     

Effects of post-sintering annealing on the microstructure and toughness of hot-pressed SiCf/SiC composites with Al2O3-Y2O3 additions

This study examined the effects of post-sintering heat treatment on enhancing the toughness of SiC_f/SiC composites. Commercially available Tyranno^® SiC fabrics with contiguous dual ‘PyC (inner)-SiC (outer)’ coatings deposited on the SiC fibers were infiltrated with a SiC + 10 wt% Al₂O₃-Y₂O₃ slurry by electrophoretic deposition. SiC green tapes were stacked between the slurry-infiltrated fabrics to control the matrix volume fraction. Densification of approximately 94% ρ_theo was achieved by hot pressing at 1750 °C, 20 MPa for 2 h in an Ar atmosphere. Sintered composites were then subjected to isothermal annealing treatment at 1100, 1250, 1350, and 1750 °C for 5 h in Ar. The correlation between the flexural behavior and microstructure was explained in terms of the in situ-toughened matrix, phase evolution in the sintering additive, role of dual interphases and observed fracture mechanisms. Extensive fractography analysis revealed interfacial debonding at the hybrid interfaces and matrix cracking as the key fracture modes, which were responsible for the toughening behavior in the annealed SiC_f/SiC composites.     

Densification,flexural strength and dielectric properties of CaO-MgO-ZnO-SiO2/Al2O3 glass ceramics for LTCC applications

Novel glass ceramics for LTCC applications with high flexural strength can be achieved by CaO-MgO-ZnO-SiO₂(CMSZ) glass cofiring with Al₂O₃. The sintering shrinkage behavior, crystalline phases, mechanical and dielectric properties, and thermal expansion of the CMZS/Al₂O₃ glass ceramic were determined. The X-ray diffraction results revealed that multiphases (CaMgSi₂O₆, Al₂Ca(SiO₄)₂ and ZnAl₂O₄) formed in the sintering process of the CMZS/Al₂O₃ glass ceramic. The flexural strength of CMZS/Al₂O₃ glass ceramics first increases and then decreases with increasing Al₂O₃ content. The CMZS/Al₂O₃ glass ceramic with 50 wt % Al₂O₃ sintered at 890 °C for 2 h achieved the best performance, with a maximum flexural strength of 256 MPa, dielectric constant (ε_r) of 7.89, dielectric loss (tan δ) of 3.41 × 10⁻³ (12 GHz), temperature coefficient of resonance frequency (τ_f) of −29 ppm/°C, and the CTE value of 7.93 × 10⁻⁶/°C.