Formation and thermal stability of dual glass phases in the h-BN/SiO2/Yb-Si-Al-O composites

The glass phases formation and microstructural evolution in the h-BN/SiO₂/Yb-Si-Al-O composite were investigated. Owing to the introduction of Al₂O₃, SiO₂ could transform into amorphous silica at a lower temperature (∼1600 °C). On the other hand, with increasing temperature, Al³⁺ gradually dissolved into ytterbium silicate (Yb₂Si₂O₇) and then accelerated the formation of amorphous Yb-Si-Al-O glass phase. Up to 1880 °C, fine spherical Yb-Si-Al-O glass particles and irregular amorphous silica distributed uniformly, which contributes to the excellent mechanical properties of the composite. The thermal stability study disclosed that more [AlO₄] units could effectively inhibit crystallization of the Yb-Si-Al-O glass phase, but mechanical properties of composite still decreased slowly with increasing the heat treatment temperature. For example, the flexural strength of the composite with 1.5 wt.% Al₂O₃ decreased from 297 ± 30 to 284 ± 22 MPa as the treatment temperature rose from 800 to 1200 °C.     

Effect of h-BN on the microstructure and fracture behavior of low carbon Al2O3-C refractories

h-BN is an ideal substitution candidate for graphite due to its similar crystal structure, better oxidation resistance. In this work, the effect of h-BN on microstructure and comprehensive properties of Al₂O₃-C refractories are investigated, and the specimen containing 0.5 wt% h-BN (G0.5N0.5) possesses the best comprehensive properties. The addition of h-BN could reduce the diameter of SiC whiskers, which leads to the highest strength of specimen G0.5N0.5 (42.63 ± 3.10 MPa). Moreover, the fracture behavior of the specimens is demonstrated using wedge splitting test. The results show that the specimen G0.5N0.5 possesses the highest crack initiation and propagation resistance, which could be attributed to the collaborative effect of h-BN and SiC whiskers. Noteworthily, the addition h-BN could improve the thermal shock resistance. The specimens containing h-BN possess the higher residual ratio, compared with the specimen containing no h-BN (G1N0), and the specimen G0.5N0.5 shows the highest residual strength (14.12 ± 0.67 MPa). Furthermore, the oxidation resistance could be enhanced with introducing the h-BN.     

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.     

Novel Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> aerogel/porous zirconia composite with ultra-low thermal conductivity

Highly porous zirconia fibers networks with a quasi-layered microstructure were successfully fabricated using vacuum squeeze moulding. The effects of inorganic binder content on the microstructure, room-temperature thermal and mechanical properties of fibrous porous zirconia ceramics were systematically investigated. Al₂O₃–SiO₂ aerogel was impregnated into fibrous porous ceramics, and the microstructures, thermal and mechanical properties of Al₂O₃–SiO₂ aerogel/porous zirconia composites were also studied. Results show that the Al₂O₃–SiO₂ aerogel/porous zirconia composites exhibited higher compressive strength (i.e., 1.22 MPa in the z direction) and lower thermal conductivity [i.e., 0.049 W/(m/K)]. This method provides an efficient way to prepare high-temperature thermal insulation materials.     

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.     

Mullite-bonded SiC-whisker-reinforced SiC matrix composites: Preparation,characterization, and toughening mechanisms

A novel mullite-bonded SiC-whisker-reinforced SiC matrix composite (SiCw/SiC, SiC whisker-to-SiC powder mass ratio of 1:9) was designed and successfully prepared. Before preparing the composite, the inexpensive lab-made SiCw was first modified by an oxidation/leaching process and then coated with Al₂O₃. The kinetics results indicate that the oxidation process can be described by improved shrinking-cylinder models. The aspect ratio of SiCw improved after modification. Subsequently, raw materials with a SiC–SiO₂–Al₂O₃ triple-layered structure were obtained after the Al₂O₃-coating process and used as feedstocks during the subsequent hot-pressing sintering. Finally, the characterization of the composites indicates that the mullite-bonded sample performs better (relative density of 93.8?±?1.4%, flexural strength of 533.3?±?18.2?MPa, fracture toughness of 13.6?±?2.1?MPa?m^1/2, and Vickers hardness of 20.6?±?2.5?GPa) than the reference sample without the mullite interface. The improved toughness could essentially be attributed to the moderately strong interface bonding and effective load transfer effects of the mullite interface.     

Effects of cubic BN addition and phase transformation on hardness of Al2O3-cubic BN composites

The Vickers hardness of dense Al₂O₃-cubic BN (cBN) composites prepared by spark plasma sintering under a moderate pressure of 100 MPa at 1200-1600 °C was investigated at indentation loads of 0.098-19.6 N. The BN grains in the Al₂O₃-BN composite prepared at 1300 °C showed no transformation from the cBN to hBN phase, and the hardness was 59 GPa at 0.098 N. The hardness of the Al₂O₃ matrix in the Al₂O₃-BN composites containing 10-30 vol% cBN prepared at 1300-1400 °C was around 25 GPa at 0.098 N, which was higher than monolithic Al₂O₃ bodies prepared at the same temperatures. The hardness of the Al₂O₃ matrix in the Al₂O₃-BN composites decreased with increasing sintering temperature. The increase in the hardness of the Al₂O₃ matrix may be due to the decrease in the size of Al₂O₃ grains in the Al₂O₃-BN composites owing to the addition of cBN particles and the decrease in sintering temperature. The Meyer exponents of the monolithic Al₂O₃ bodies and Al₂O₃-BN composites were 1.90-1.94 independent of cBN content.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Degradation of Al2O3f/SiO2 composites exposed to Na2SO4 environment and MMH/N2O4 bipropellants test

Al₂O_3f/SiO₂ composites were fabricated efficiently using sol-gel process. The degradation behavior exposed to Na₂SO₄ environment at 1100℃ and MMH/N₂O₄ bipropellants test were investigated and compared. The results showed that the strength of Al₂O_3f/SiO₂ composites gradually decreased as the ratio of Na₂SO₄:water increased; the strength of the composites was only 23.56 MPa at 20% (Na₂SO₄:water), which suggested that the composites maintained lower strength. Cracks began to appear in SiO₂ matrix, and the structure of Al₂O₃f/SiO₂ composites could be corroded which would corrode the SiO₂ matrix, leaving naked fibers. Developing a protective layer with higher stability for Al₂O_3f/SiO₂ composites would be considered for long time use. The composites showed higher ablation resistance to MMH/N₂O₄ bipropellant test; the flexural strength was (77.15 ± 4.56) MPa and the retention ratio was 98.7%. The degradation of Al₂O_3f/SiO₂ composites was promoted owing to the thermal-mechanical and chemical factors. SiO₂ matrix became weak and fragile at elevated temperature; some SiO₂ matrix became loosened and porous after being washed away through the shearing of MMH/N₂O₄ bipropellants, which prevented cracks from penetrating Al₂O₃ fibers. With ongoing test, the fibers were worsened by thermal-mechanical corrosion.     

Mechanical and thermal properties of h-BN based composite containing dual glass phases at elevated temperatures

High-temperature mechanical and thermal properties of h-BN based composite containing amorphous silica and Yb-riched silicate glass phases were systematically investigated in this work. Owing to anisotropic microstructure of h-BN matrix, the obtained composite demonstrates anisotropic mechanical and thermal properties. The composite possesses higher elastic modulus at 1673?K than that at room temperature and presents excellent high-temperature stiffness. Flexural strengths in parallel and perpendicular directions reach 496?±?22 and 258?±?21?MPa at?1073?K, respectively, and increases by 74 and 66% compared with the room-temperature strengths of 285?±?4?and 155?±?5?MPa. The composite containing dual glass phases shows lower coefficients of thermal expansion in the temperature range of 473–900?K, the values are ?1.4?×?10^?6 and 0.3?×?10^?6 ?K^?1 for the perpendicular and parallel directions, respectively. Thermal conductivities in the perpendicular and parallel directions at 373?K are 24.8 and 14.8?W?m^?1?K^?1, respectively, and then decrease to 14.9 and 9.3?W?m^?1?K^?1 at 1473?K.     

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.     

Alumina/copper foams produced by replica using a double impregnation process

Rheological behaviour of Al₂O₃ and CuO suspensions was characterised in order to fabricate Al₂O₃/Cu foams as an alternative way to produce foams with better mechanical properties than common Al₂O₃ foams. Different quantities of raw materials were combined to select the formulations for the ceramic-metal foams. Foams were produced by a double impregnation process, followed by a thermal cycle and a reduction cycle. In this work, materials were characterised trough zeta potential, SEM and optical dilatometry. Foams microstructure and compressive strength were evaluated. Final structures had a compressive resistance of 0.44?±?0.14?MPa.     

Boron nitride–silicon carbide interphase boundaries in silicon nitride–silicon carbide particulate composites

Interphase boundaries between SiC and h-BN grains in hot isostatically pressed Si₃N₄–SiC particulate composites made from both as-received powders and deoxidised powders, in which sub-micron size h-BN particles occur as a contaminant, have been characterised using transmission electron microscopy techniques. Most of the h-BN grains observed were aligned with respect to SiC grains so that (111) 3C SiC and (0001) α-SiC planes were parallel to (0001) h-BN planes. The h-BN–SiC interphase boundaries in the composites made from as-received powders were covered with thin silica-rich intergranular films, in contrast to the interphase boundaries in the composites made from deoxidised powders. These observations are discussed in the light of models for the formation of intergranular amorphous films in ceramic materials, geometric considerations for low interfacial energies and the possible bonding at h-BN–SiC interphase boundaries free of intergranular films.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.     

Preparation of mullite from alumina/aluminum nitrate and kaolin clay through spark plasma sintering process

In the present study, the in-situ synthesized mullite has been prepared successfully by mixing kaolinite with alumina and aluminum nitrate nonahydrate (ANN) powders through high energy milling followed by spark plasma sintering (SPS). Using a high-energy ball-mill, the stoichiometric compositions of the starting powders, considering their final transformation to Al₂O₃ and SiO₂, have been mixed. The SPS process has been performed at 1400 and 1375?°C for the specimens containing Al₂O₃ and ANN, respectively. XRD patterns of the milled powders after 30?h showed the formation of quartz from kaolinite for both starting batches. The displacement-temperature-time (DTT) curves and the corresponded vacuum changes indicated the dehydration and phase transformation of ANN and kaolinite at different stages of the sintering process. The XRD patterns of the sintered samples revealed the formation of mullite alongside un-reacted Al₂O₃ and crystobalite for the batches containing Al₂O₃ and ANN, respectively. The results of the physical and mechanical properties tests showed higher amounts of bending strength (397?±?18?MPa), Vickers hardness (16.32?±?0.21?GPa) and fracture toughness (3.81?±?0.24?MPa?m^?1/2) alongside a lower porosity (0.070?±?0.02%) for the prepared sample containing Al₂O₃, than those of the sample containing ANN.     

Injection‐molded high‐density polyethylene–hydroxyapatite–aluminum oxide hybrid composites for hard‐tissue replacement: Mechanical,biological, and protein adsorption behavior

In this article, we report the mechanical and biocompatibility properties of injection‐molded high‐density polyethylene (HDPE) composites reinforced with 40 wt % ceramic filler [hydroxyapatite (HA) and/or Al₂O₃] and 2 wt % titanate as a coupling agent. The mechanical property measurements revealed that a combination of a maximum tensile strength of 18.7 MPa and a maximum tensile modulus of about 855 MPa could be achieved with the injection‐molded HDPE–20 wt % HA–20 wt % Al₂O₃ composites. For the same composite composition, the maximum compression strength was determined to be 71.6 MPa and the compression modulus was about 660 MPa. The fractrography study revealed the uniform distribution of ceramic fillers in the semicrystalline HDPE matrix. The cytocompatibility study with osteoblast‐like SaOS2 cells confirmed extensive cell adhesion and proliferation on the injection‐molded HDPE–20 wt % HA–20 wt % Al₂O₃ composites. The cell viability analysis with the 3(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay revealed a statistically significant difference between the injection‐molded HDPE–20 wt % HA–20 wt % Al₂O₃ composites and sintered HA for various culture durations of upto 7 days. The difference in cytocompatibility properties among the biocomposites is explained in terms of the difference in the protein absorption behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Lamellar-interpenetrated Al−Si−Mg/Al2O3−ZrO2 composites prepared by freeze casting and pressureless infiltration

Using freeze casting and pressureless infiltration methods, we prepared lamellar Al−Si−Mg/Al₂O₃−ZrO₂ composites with initial ceramic loading of 30 vol% and different Al₂O₃:ZrO₂ weight ratios (Al₂O₃:ZrO₂=1:9, 3:7, 5:5, 7:3 and 9:1). The resultant composites inherited the lamellar structure of the Al₂O₃−ZrO₂ scaffolds, and the thickness of both metal and ceramic layers showed a trend of first increase and then decrease with increasing Al₂O₃ content. During pressureless infiltration, multiple chemical reactions took place between ZrO₂ and the Al−12Si−10Mg alloy and the main reaction products were (Al_1−m, Si_m)₃Zr, Al₂O₃ and ZrSi₂ phases. The degree of the reaction depended on the ZrO₂ content in the ceramic composition. In general, the compressive strength of the composites decreased with increasing Al₂O₃ content, but three-point bending strength showed a first decrease and then increase. When Al₂O₃:ZrO₂=1:9, the compressive and bending strength of the composites reached about 997±60 MPa and 426±10 MPa, respectively. A simple model was proposed to illustrate the fracture mode and toughening mechanism of the composites.     

In situ synthesis,mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT

Ti₃AlC₂/Al₂O₃ composite materials were successfully fabricated from TiO₂/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al₂O₃ content. The relative density of Ti₃AlC₂/Al₂O₃ composites exhibits a declining tendency with Al₂O₃ content especially exceeds 10 vol.?%. The Ti₃AlC₂/Al₂O₃ composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti₃AlC₂/10 vol. % Al₂O₃ are 461 ± 20?MPa and 6.2?±?0.2?MPa m^1/2, respectively. The cyclic oxidation behaviour of resistance of Ti₃AlC₂/10 vol. % Al₂O₃ composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al₂O₃ and TiO₂, forming a dense and adhesive protect layer. The result indicates that the Al₂O₃ can greatly improve the oxidation resistance of Ti₃AlC₂.     

High performance polymer composites on PEEK reinforced with aluminum oxide

A study on high performance poly(ether‐ether‐ketone) (PEEK) composites prepared by incorporating aluminum oxide (Al₂O₃), 0 to 50 wt % by hot compaction at 15 MPa and 350°C was described. Density, thermogravimetric analysis/differential scanning calorimetry, and scanning electron microscopy (SEM) were employed to evaluate their density, thermal stability, crystallinity, and morphology. Experimental density was found higher than theoretical density, which indicates that composite samples are sound. It was found that the addition of micron sized (< 15 μm) Al₂O₃ increased the peak crystallization temperature by 12°C when compared with neat PEEK with insignificant increase in melting temperature. Half‐time of crystallization is reduced from 2.05 min for the neat PEEK to 1.08 min for PEEK incorporated with 30 wt % Al₂O₃ because of the strong nucleation effect of Al₂O₃. The thermal stability of composites in air atmosphere was increased by 26°C. However, thermal stability in nitrogen atmosphere decreases at lower concentration of Al₂O₃ but increases above 20 wt % of Al₂O₃. Uniform dispersion of Al₂O₃ particles was observed in PEEK polymer matrix by SEM. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4623–4631, 2006     

Strength improvement and purification of Yb2Si2O7‐SiC nanocomposites by surface oxidation treatment

A two‐step processing was developed to prepare Yb₂Si₂O₇‐SiC nanocomposites. Yb₂Si₂O₇‐Yb₂SiO₅‐SiC composites were first fabricated by a solid‐state reaction/hot‐pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb₂SiO₅ into Yb₂Si₂O₇. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.