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In this study, PyC coating with different thickness was prepared on SiC fibers via chemical vapor deposition method. Uncoated and PyC coated SiC fibers were heat treated in Ar atmosphere under different annealing condition. The effects of PyC coating on the mechanical properties of SiC fibers at room temperature were investigated. The morphology, composition, structure and mechanical properties of SiC fibers after high temperature treatment were also studied. The results showed that PyC coating could reduce the dispersion of tensile strength for SiC fibers. Weibull modulus increased from 4.19 (SiC fiber) to 5.59 (PyC700 fiber), while the tensile strength decreased a little with the increment of PyC coating thickness. Furthermore, PyC coating could remarkably retard Si sublimation from SiC fiber at high temperature. Uncoated SiC fiber was converted to porous pure-carbon fiber after being annealed at 2000 °C for 5 h, while PyC700 fiber still showed relative dense SiC core after the same treatment. To a certain extent, the inhibition ability was improved with increment of PyC coating thickness. In addition, PyC coated SiC fibers could maintain more tensile strength after being annealed at 1800 °C for 1 h. The tensile strength retention of uncoated SiC fiber was 27.3%, while that of PyC700 fiber could reach up to 43.2%. [ABSTRACT FROM AUTHOR]

A bi-layer environmental barrier coating (EBC) consisting of silicon(Si) bond coat/mixed ytterbium disilicate (Yb 2 Si 2 O 7) and ytterbium monosilicate (Yb 2 SiO 5) topcoats has been successfully prepared to completely wrap up the SiC f /SiC composites and the protective effects of such EBC have been evaluated by soaking them in a mixed 50% O 2 and 50% H 2 O corrosive gases at 1300 °C for various times. In topcoats, Yb 2 Si 2 O 7 is the major phase, providing good thermal expansion coefficient (CTE) matching with composite substrate and thus excellent thermal shock resistance, whereas Yb 2 SiO 5 is the dispersing minor phase, providing improved water vapor corrosion resistance. The completely wrapping up of SiC f /SiC composites by above EBC system is employed to avoid direct exposure to the corrosive conditions, making it possible to evaluate the genuine protection effects of current EBCs. Under 1300 °C water vapor corrosion, the mass change, the phase composition and the evolution of microstructure are investigated, which suggest that the bi-layer EBC has excellent performance on protecting SiC f /SiC composites from water vapor corrosion at 1300 °C. [ABSTRACT FROM AUTHOR]

In this paper the long-term oxidation resistance of SiC f /SiC composites manufactured by a single PIP technique and a hybrid CVI-PIP technique at 1200 °C in a static air has been systematically studied. It has been found that after oxidation at 1200 °C for 200 h, SiC f /SiC composites by the hybrid CVI-PIP technique can retain flexural strength as high as 76% and still exhibit a typical quasi-plastic fracture. In contrast, after oxidation for 100 h, SiC f /SiC composites produced by the single PIP technique can only keep 31.8% of their initial flexural strength and demonstrate a typical brittle fracture. An insertion of a dense and crystalline CVI derived SiC layer between fibers/interphase and PIP derived matrix makes the difference. To be specific, for those single PIP derived composites, i.e. without this CVI SiC layer, the fibers/interphase are totally exposed to oxidative environments due to the nature of PIP derived matrix that is porous and full of micro-cracks. Worse still, the oxidized products of PIP derived matrix, viscous and sticky SiO 2 , tends to tightly wrap up fibers, forming a strong fiber/matrix bond. On the contrary, for those hybrid CVI-PIP derived composites, the dense CVI SiC layer serves as an oxidation barrier, not only blocking the inward flowing of oxygen to invade interphase/fibers, but also precluding the viscous and sticky SiO 2 reach fiber surface. The superior properties of SiC f /SiC composites produced by hybrid CVI-PIP technique suggest it be a desirable manufacture technique for industrial applications. [ABSTRACT FROM AUTHOR]

Abstract The yttrium heavily doped La 2 Zr 2 O 7 solid solutions coatings, with a Y to La molar ratio of 1:1, have been successfully prepared by air plasma spraying technique. The evolution of phase composition, phase structure and thermal conductivity of such coatings with annealing at 1300 °C has been investigated. The results show that, a single pyrochlore structure can be retained for coating after annealing up to 48 h, beyond which the fluorite phase begins to precipitate out. By comparing thermal conductivities to those undoped counterparts at a similar porosity level, we find a considerably flat thermal conductivity versus temperature (k-T) curve, suggesting the existence of a strong phonon scattering source, which is inferred as rattlers. In addition, after the segmentation of the fluorite phase, the thermal conductivity of corresponding coatings rises considerably, indicating that the fluorite phase has a higher thermal conductivity than that of pyrochlore phase. Moreover, while the as-sprayed coatings show a clear indication of radiative thermal conduction beyond 1000 °C, the thermal conductivity of annealed coatings do not show such an uprising trend after 1000 °C, suggesting that the radiative thermal conduction has been greatly suppressed. The reason is proposed as the formation of local dipoles due to local enrichment of certain elements influences the propagation of electromagnetic waves and thus suppresses the radiative thermal conduction. [ABSTRACT FROM AUTHOR]

The poor fracture toughness limits the wide application of the cubic lanthanum zirconate (La 2 Zr 2 O 7 ) ceramics. The current study reports a dramatic fracture toughness improvement of lanthanum zirconates by the introduction of lanthanum aluminates (LaAlO 3 ), a ferroelastic second phase. A composite ceramic with the composition of x LaAlO 3 / (1- x ) La 2 Zr 2 O 7 , where x  = 10, 20, 30, 40, 50 mol%, has been fabricated by a spark plasma sintering method. It is found that, the ferroelastic domains of LaAlO 3 in x LaAlO 3 /(1- x )La 2 Zr 2 O 7 composite ceramics clearly switch towards a direction that is normal to a propagating crack. In addition, the processing zone width of ferroelastic domains is found to decrease with an increase of LaAlO 3 inclusion, attributing to that the residual tensile stress in LaAlO 3 caused by the thermal expansion coefficient mismatch, which helps to expand the processing zone, decreases with more LaAlO 3 included. [ABSTRACT FROM AUTHOR]

The poor fracture toughness of La 2 Zr 2 O 7 hinders its practical application as a thermal barrier coating topcoat material. To toughen La 2 Zr 2 O 7 , metastable tetragonal 8 wt% yttria-stabilized zirconia ( t ’-8YSZ), has been introduced into the La 2 Zr 2 O 7 matrix by plasma spray quenching of 8YSZ powders and spark plasma sintering. An evident fracture toughness improvement has been achieved due to ferroelastic domains switching. The resulting toughness increase has a quadratic relationship with t ’ phase volume fraction below a critical value (30 vol%), beyond which it is far smaller to what expected owing to the residual stress being smaller than the ferroelastic coercive stress. [ABSTRACT FROM AUTHOR]

Polymer-derived SiC fibers are critical structural and functional materials for high-temperature applications, whose service performance can be further improved by introduction of certain heterogeneous elements. Herein, flexible and thermal stable SiC fiber mats were prepared from boron-doped polyaluminocarbosilane (PACS) precursor by electro-spinning, curing and pyrolysis. Two boron-containing compounds o-carborane (oCB) and phenylboronic acid (PBA) were used as the boron sources and their effects on the properties and structure of the SiC fiber mats were investigated. The introduced oCB and PBA had obviously improved the ceramic yield of the cured fiber mats and the flexibility of the SiC fiber mats. However, PBA introduced more boron, but also more oxygen, which leaded to formation of more amorphous SiC x O y phases. Although only a small amount of boron was introduced by oCB, a more significant improvement in the thermal stability of the SiC fiber mat was obtained. This work provided a practical approach for fabrication of flexible and thermally stable SiC fiber mats. [ABSTRACT FROM AUTHOR]

C f /SiC–ZrC ultra-high temperature ceramic matrix composites prepared by precursor impregnation and pyrolysis (PIP) method were exposed to laser with energy density of kW level and the ablation behavior and mechanism were investigated. The results show that under different ablation times, the composites exhibited similar ablation region divisions, namely, central ablation pit, ablation transition zone, oxidation zone and non-ablated zone. Sample subjected to laser ablation of 5 s has the largest linear ablation rate of 0.0946 mm/s, which decreased to 0.0748 mm/s for 10 s ablation sample, and after that increased to 0.0876 mm/s for 15 s ablation sample. Under high-energy laser ablation, ZrO 2 was the main anti-ablative component among the oxides formed by the oxidation of the matrix, which underwent morphology changes from sheets, fragments to micro and nano particles. Dense structure and continuous anti-ablation layer with high melting point components are the key factors to improve the anti-laser ablation performance of materials. [ABSTRACT FROM AUTHOR]

Y–Si–O ceramics are widely used as good thermal barrier coatings but barely made for structural/functional bulks. In this study, we introduced Al 2 O 3 into Y 2 Si 2 O 7 ceramics by sol-gel method to form novel Y–Al–Si–O ceramics for high-temperature resistant microwave transmission application. The compositions, microstructures, mechanical, thermal and dielectric properties of the composites were investigated. The results show that density of the composites can achieve 2.4 g cm−3 with a porosity of 27.4%. The average compressive strength is 94.8 MPa, which is 113% higher than the Y 2 Si 2 O 7 matrix. The strength retention rate at 1400 °C is 54.9%. Due to the bridging effect between Al 2 O 3 particles and Y 2 Si 2 O 7 matrix together with the interlock effect among Al 2 O 3 grains, the mechanical strength is significantly enhanced. Additionally, excellent thermal property and oxidation resistance are indicated. The composites exhibit excellent dielectric properties, with average dielectric constant and loss tangent being 4.85 and 0.0016 (8.1–16.5 GHz, room temperature). [ABSTRACT FROM AUTHOR]

Abstract 3D needle-punched C/C-SiC composites were fabricated from carbon fiber reinforced carbon (C/C) preforms, with densities of 1.05 g/cm3 and 1.28 g/cm3, by the gaseous silicon infiltration (GSI) method at fabrication temperatures from 1500 °C to 1800 °C. The compressive strengths and elastic moduli in transverse direction are larger than those measured under longitudinal compression except that samples fabricated from 1.28 g/cm3 density exhibit lower elastic moduli in transverse direction than in longitudinal direction. The compressive strength and modulus increase with fabrication temperature at 1500 °C and 1600 °C, and then decrease with higher fabrication temperature. Samples fabricated from the lower density C/C preforms have greater compressive strength and modulus. X-ray tomography was applied before and after the mechanical tests to characterize the microstructure and damage patterns, and the results indicated that for C/C-SiC composites fabricated at 1700 °C from 1.28 g/cm3 density C/C preform the matrix has a volume fraction (vol%) of 36.9%, and the initial intra-bundle cracks (0.6 vol%) display a space crossing structure while the inter-bundle pores (6.0 vol%) are special irregularly distributed. [ABSTRACT FROM AUTHOR]

Abstract In situ observations of damage development within 3-dimensional 5-directional braided carbon fiber reinforced carbon and silicon carbide (C/C-SiC) ceramic composites, fabricated by gaseous silicon infiltration (GSI) and precursor infiltration pyrolysis (PIP), have been obtained using laboratory X-ray computed tomography during in situ flexural tests. The GSI composite has a denser structure than that fabricated by PIP, but contains initial defects within the fiber bundles. The GSI composite ultimately failed due to fracture across the fiber bundles, while failure of the higher strength PIP composite propagated along the interface between the fiber bundle and matrix with a greater degree of fiber pullout. These differences arise from the higher process temperature and greater degree of matrix-fiber reaction of GSI compared to PIP. Digital volume correlation (DVC), applied to the tomographs, measured the 3-dimensional deformations and hence the specimen curvature. This demonstrated the significant reduction in elastic modulus caused by the development of internal cracking with tensile strain in both materials. [ABSTRACT FROM AUTHOR]

SiC ceramic materials were prepared using a combination of direct ink writing (DIW) and liquid silicon infiltration (LSI). In this study, we investigated the effects of the content of a copolymer of isobutene and maleic anhydride (ISOBAM) on the rheological properties of the slurry, microstructure of green bodies and SiC ceramics, and bending properties of SiC ceramics. ISOBAM significantly improved the rheological properties of the slurry and formability of the SiC green body. The yield stress and equilibrium modulus of the slurry without ISOBAM were 160.3 Pa and 1.12 × 105 Pa, respectively, and increased to 607.7 Pa and 2.37 × 106 Pa, respectively, after the addition of 0.46 wt% ISOBAM. LSI was effective in improving the density of the green body, and the densities of the sintered SiC ceramic materials ranged from 2.71 to 2.86 g/cm3. The bending strength of SiC ceramic materials initially increased and then decreased when the ISOBAM addition increased from 0.15, to 0.30, and 0.46 wt%, and the slurry with 0.30 wt% ISOBAM exhibited the highest bending strength of 287.7 ± 9.1 MPa. [ABSTRACT FROM AUTHOR]

Lanthanum hafnate (La 2 Hf 2 O 7) with a pyrochlore structure has excellent high temperature stability and low thermal conductivity, which is promising for thermal/environmental barrier coatings (T/EBCs) applications. To reduce its thermal expansion coefficient (TEC) so as to better match SiC f /SiC composites, a smaller tetravalent dopant Ti4+ has been introduced in the Hf-sites to form La 2 (Hf 1-x Ti x) 2 O 7 (x ≤ 0.20). The phase composition and microstructure confirms that La 2 (Hf 1-x Ti x) 2 O 7 solid solutions possess a pure pyrochlore structure. With an increase of x, their TECs are decreasing consistently, whilst their thermal conductivities of La 2 (Hf 1-x Ti x) 2 O 7 are slightly increasing at high temperature but still much lower than those of meta-stable yttria partially stabilized zirconia, both of which are attributing to an increase of elastic modulus after Ti4+ doping on Hf-sites. The extremely excellent high temperature stability, relatively low thermal conductivities and low TECs suggest that La 2 (Hf 1-x Ti x) 2 O 7 is a prospective candidate material for T/EBC applications. [ABSTRACT FROM AUTHOR]

C f /Hf 0.5 Zr 0.5 C-SiC composites were prepared by introducing Hf 0.5 Zr 0.5 C matrix (11 cycles) and SiC matrix (9 cycles) into the carbon cloth preform through precursor impregnation and pyrolysis (PIP) process. The influence of the introduction time of SiC matrix on the microstructure and mechanical properties of C f /Hf 0.5 Zr 0.5 C-SiC composites was studied, and the results show that with the increase of the PIP cycles of the SiC matrix introduced before Hf 0.5 Zr 0.5 C matrix, the composite open porosity decreased, and the flexural strength and modulus presented an obvious upward trend. CS45 sample, which has 4 cycles of PIP SiC introduced in advance, has the highest flexural strength, flexural modulus and interfacial shear strength of 402.73 ± 35.73 MPa, 56.92 ± 3.97 GPa and 100.88 ± 7.79 MPa, respectively. Hf 0.5 Zr 0.5 C matrix has a loose and porous structure, so when more SiC matrix was introduced in advance, its covering effect on the surface of fibers led to less intra-bundle pores and thusly denser composite structure, and due to the compactness of SiC matrix, better overall bonding of fiber, interface and matrix was achieved, as well as better load transfer effect, which led to obvious interfacial debonding and cracking based on the in-situ SEM observation during flexural tests. While in the sample without pre-introduced SiC, the cracking occurred mainly between the interface and porous matrix and the overall performance of the material was poor. [ABSTRACT FROM AUTHOR]

The SiC f /SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a 'surface sealing' effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t 0 , which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t 0 , closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC f /SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC f /SiC composites for industrial applications. [ABSTRACT FROM AUTHOR]

Monoclinic (m) and monoclinic-prime (m′) phases of Yb(Ta x Nb 1− x)O 4 ceramics were synthesized successfully, and the relationships between the crystal structures and the thermophysical properties were investigated by various methods. A ferroelastic domain existed in the m-Yb(Ta x Nb 1- x)O 4 ceramics, as observed by a scanning electron microscopy, which suggests that a ferroelastic phase transition occurred. The minimum thermal conductivity of the Yb(Ta x Nb 1− x)O 4 ceramics was 1.49 W m−1·K−1, whereas the maximum thermal expansion coefficient (TEC) was 10.6 × 10−6 K−1 (1200 °C). The m-phase had a lower Young's modulus (110–132 GPa) than the m′-phase (133–140 GPa). In view of the low thermal conductivity and high TEC, Yb(Ta x Nb 1− x)O 4 ceramics have the potential to be high-temperature structural ceramics. [ABSTRACT FROM AUTHOR]

3D needled carbon fiber reinforced composites (C/C-SiC) were fabricated by gaseous silicon infiltration (GSI) for different reaction times, followed by further densification with liquid silicon infiltration (LSI). The density, flexural modulus and flexural strength increased as a spatially connected SiC structure developed, but the flexural strength could be degraded by a long total reaction time. The composites all exhibited quasi-brittle fracture with fiber bundle pull-out and interfacial debonding. Fracture could propagate within the needle bundles that were perpendicular to the tensile stress. [ABSTRACT FROM AUTHOR]

A novel, easy and cost-effective way, infiltration and pyrolysis of phenolic resin solution, was exploited to prepare pyrolytic carbon (PyC) interlayers for carbon fiber/silicon carbide (Cf/SiC) mini-composites. X-ray photoelectron spectroscopy, dynamic contact angle measurement and scanning electron microscope were carried out to characterize chemical structure of carbon fibers (CFs), wetting properties between CFs and phenolic resin solution and microstructure of CFs and their composites, respectively. Remarkably, SEM results showed regulation of uniformity and thicknesses of PyC interlayer could be achieved through controlling the concentration of phenolic resin solution and oxidation condition of CFs. When CFs were treated by 10 min' oxidation with 40 mg/L ozone followed by dip-coating with 4 wt% phenolic solution, uniform PyC interlayer with approximately 120 nm were prepared on CFs. The corresponding Cf/SiC specimens had the largest increase in tensile strength and work of fracture with the improvement of 26.2% and 71.6% from the PyC-free case. [ABSTRACT FROM AUTHOR]