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3D3d, 3D4d, 3D5d-braided Cf/SiC composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor. The coefficient of thermal expansion (CTE) of C/SiC composites was measured in longitudinal and transversal directions in the temperature range from -150°C to 25°C. The longitudinal CTE varies in the range (0.09-0.68) × 10/°C, and the transversal CTE varies in the range (0.21-1.95) × 10/°C. Various CTEs of 3D-braided C/SiC composites were mainly determined by different braided structures of carbon fibers. The longitudinal CTE is lower than transversal CTE for the negative axial expansion of carbon fibers at cryogenic temperature. Microcracks were examined to understand the effect of structure on the thermal expansion of composites. [ABSTRACT FROM AUTHOR]

Highlights: [•] C/ZrC, C/SiC and C/ZrC–SiC with different SiC content are fabricated by PIP process. [•] C/ZrC has the superior anti-ablation property but the poor anti-oxidation property. [•] The introduction of SiC improves oxidation resistance but degrades ablation property. [•] The C/ZrC–SiC with a certain content of SiC shows superiority over other composites. [•] A high temperature self-healing mechanism has been achieved by C/ZrC–SiC composite. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) needle-punched C/ZrC–SiC composites were successfully fabricated by infiltration and pyrolysis of a novel liquid ZrC precursor and polycarbosilane (PCS) separately. The effects of PIP cycles of PCS on the microstructures and mechanical property of the composites were studied. The C/ZrC–SiC composite with more PIP cycles of PCS had a high performance in mechanical property due to its higher density. The C/ZrC–SiC composite with 6 PIP cycles of PCS had a flexural strength of 142.4±17.5MPa and a fracture toughness of 7.6±0.3MPam1/2, which was superior to C/ZrC composites with the same total 20 PIP cycles. The ablation behaviors of the C/ZrC–SiC composite were studied using an oxyacetylene torch. The formation of continuous molten ZrO2 layer contributed mainly the high ablation resistance in the center region, while the SiC matrix accounted for the improved oxidation resistance beyond the center and transition regions. Moreover, the ZrC matrix homogeneously distributed in composite can act as a reservoir of the ZrO2 phase, and then supplemented the ZrO2 loss continuously in the whole testing. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional (3D) Cf/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The Cf/ZrC–SiC composite with relatively more PIP cycles of PCS has a higher density and a lower porosity, which account for the improved mechanical property of the composite. The composite with 6 PIP cycles of PCS has a flexural strength of 136.0±13.1MPa, and a fracture toughness of 7.3±0.3MPam1/2, which is superior to the monolithic ultrahigh temperature ceramics. The relatively low flexural strength and elastic modulus of the Cf/ZrC–SiC composites are attributed to the small stiffness of the matrix because of the debonding feature of the ZrC matrix. The high temperature oxidation testing of the S6-Cf/ZrC-SiC composite reveals that the surface molten ZrO2 layer contributes mainly to the improved anti-ablation property. [Copyright &y& Elsevier]

Abstract: Chitosan–zirconium complex, obtained by chelation of chitosan to zirconium, formed a novel precursor for zirconium carbide (ZrC). ZrC was synthesized via carbothermal reduction reaction using zirconium oxychloride (ZrOCl2·8H2O), and chitosan as sources of zirconium and carbon. The initiation of carbothermal reduction process was at about 1400°C, and the reaction was substantially completed at a relatively lower temperature (∼1550°C), compared with that using ZrOCl2·8H2O and phenolic resin as a carbon source. The conversions from as-synthesized preceramic precursors to ceramics were studied by means of FTIR, DSC-TG, SEM, EDS and XRD. [Copyright &y& Elsevier]

Zirconium carbide ( ZrC) was synthesized by solution-based processing using versatile starting reagents including zirconium oxychloride octahydrate ( Zr O Cl2·8 H2 O, ZOC), acetylacetone (Hacac), and phenolic resin. Polyzirconoxane ( PZO), obtained by chelation of Hacac to zirconium, was used to combine with phenolic resin to form a precursor for ZrC. It generated ZrC at a relatively low temperature (1550°C) and in a low C/ Zr molar ratio of 2 assuming that phenolic resin was the only carbon source. As a comparison, synthesis of ZrC only using ZOC and phenolic resin was also conducted. Besides, the pyrolysis of PZO was carried out and the transformation mechanism during the pyrolysis was studied, during which ZrC was obtained at 1550°C in flowing argon. The conversions from as-synthesized preceramic precursors to ceramics were studied by means of FTIR, DSC- TG, SEM, EDS, and XRD. [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]

Abstract Damage development has been observed in situ within a 3-dimensional needle-punched carbon fiber reinforced carbon and silicon carbide (C/C-SiC) ceramic composite, as compressive loads were applied longitudinally, transversely and obliquely to its anisotropic structure. Laboratory X-ray computed tomographs were analyzed using digital volume correlation to measure the local strains. Cracks exist within the microstructure before the application of load, due to thermal strains, and their propagation is affected by the heterogeneous microstructure and the direction of loading. The punched carbon fiber needles are observed to restrict the propagation of damage. [ABSTRACT FROM AUTHOR]

Abstract The thermal shock properties of mixed Yb 2 Si 2 O 7 and Yb 2 SiO 5 composite environmental barrier coating (EBC) topcoat, with various Yb 2 Si 2 O 7 -to-Yb 2 SiO 5 ratios, are investigated between room temperature and 1500 °C. The results show that, with more Yb 2 Si 2 O 7 introduced, the mixed Yb 2 Si 2 O 7 and Yb 2 SiO 5 composite topcoats exhibit an increasing thermal shock property. This may be attributed to that, while Yb 2 Si 2 O 7 mole fraction increases, the thermal expansion coefficient of the mixed Yb 2 Si 2 O 7 and Yb 2 SiO 5 composite ceramics becomes smaller, more matched to substrates and the fracture toughness of the mixed Yb 2 Si 2 O 7 and Yb 2 SiO 5 composite ceramics becomes larger, both of which are favoring the improvement of thermal shock resistance. Highlights • Yb 2 Si 2 O 7 and 3Yb 2 Si 2 O 7 •Yb 2 SiO 5 exhibit better durability compared with other EBC samples. • TEC and fracture toughness for five compositions of ytterbium silicates have been studied. • The failure mechanisms are probably unveiled as TEC and TGO. [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 cubic pyrochlores has severely blocked their wide application. To toughen them, a series of yttrium dopants have been introduced to lanthanum zirconate, creating atomic or phase-scale compressive stress dependent on the concentration of dopants. We find bigger substitutional dopants, rather than smaller ones, can toughen cubic ceramics, attributing to the dominance of an imposed atomic compressive stress to surroundings over the local one near normal point defects. For special point defects such as oxygen vacancies or interstitials, the local stress in their vicinity could become dominant given an adequate concentration. Despite of the pyrochlore/fluorite intermixture possessing evidently reduced grains, it only exhibits intermediate fracture toughness, suggesting negligible grain refining effects on toughening. Further, a random distribution of atomic compressive stress induced by disordered oxygen vacancies is beneficial to toughening. This research highlights the atomic compressive stress on toughening, providing guidance for future design of novel thermal barrier coatings. [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]

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D C f /HfC–SiC composites by precursor impregnation and pyrolysis (PIP) process were investigated systematically. Both strength (flexural and compressive strength) and modulus increase as the SiC/HfC ratio are improved. The compact and stiff HfC-SiC matrix in addition to the carbon fiber and PyC interphase with less reaction damage accounts for the improved mechanical properties of C f /HfC-SiC with higher SiC/HfC ratios. Meanwhile, both weight loss and erosion depth of C f /HfC-SiC are improved with the increased SiC/HfC ratios. Therefore, in order to balance the ablation and mechanical properties, an appropriate SiC/HfC ratio should be considered. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) braided C/ZrC–SiC composites with various SiC/ZrC ratios were fabricated by precursor infiltration and pyrolysis (PIP) process. The effects of SiC/ZrC ratios on the ablation and mechanical properties were investigated systematically. The flexural strength of 3D C/ZrC–SiC grows with the increase of SiC/ZrC ratios. The compact and stiff ZrC-SiC matrix, moreover, the carbon fiber and PyC interphase with the less reaction damage account for the improved flexural strength of C/ZrC–SiC with higher SiC/ZrC ratios. As for the ablation properties, both mass and linear ablation rates drop with the decrease of SiC/ZrC ratios at the beginning. However, the ablation rates start to increase with the further decrease of SiC/ZrC ratio, where the high open porosity deteriorates the ablation properties. Herein, in order to obtain a balance between ablation and mechanical properties, an appropriate SiC/ZrC ratio should be considered. [ABSTRACT FROM AUTHOR]

MB 2 -Al 2 O 3 in situ composite powders were synthesized at 1500 °C via a novel solid-state precursor route using Group IV–V metal (M, M = Ti, Zr, Hf, Nb, Ta) oxide, Al, and BN as raw materials. As a moderate cost boron source, BN has a well-defined stoichiometry and low impurity. In these precursor powder systems, the TiO 2 -Al-BN is an exception, which gives rise to the final TiB 2 -Al 2 O 3 -AlN product, whereas the rest produce the diboride-Al 2 O 3 composite powders. As to HfB 2 –Al 2 O 3 and TaB 2 –Al 2 O 3 composite powders, the diboride phases have a smaller particle size (submicrometer) while the Al 2 O 3 phase possesses a larger size (several micrometers). Besides, these diboride-alumina products produced by the solid-state precursor route form an interwoven network and are intimately mixed on a microscopic scale. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) C f /ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C f /ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m 1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C f /ZrC–SiC composites slightly and the ZrO 2 scale acts as an anti-ablation component during the testing. [ABSTRACT FROM AUTHOR]

ZrB 2 –Al 2 O 3 in situ composite powders were synthesized at 1550 °C via a novel solid-state precursor route using ZrO 2 , Al, and BN as raw materials. The final ZrB 2 –Al 2 O 3 composite powders, consisting of large crystal Al 2 O 3 particles (several micrometers) and fine ZrB 2 particles (100–400 nm), form an interwoven network and are intimately mixed on a microscopic scale. The formation mechanism for ZrB 2 –Al 2 O 3 composite powders comprises two steps: aluminothermic reduction of ZrO 2 , which generates Zr–Al alloy compounds and the coproducts Al 2 O 3 , then followed by a series of solid reactions between Zr and BN to give the ZrB, ZrN, and ZrB 2 products. The aluminothermic reduction is a fast reaction, while the reaction between Zr and BN to obtain the final ZrB 2 product is the limiting step in the conversion process of the precursor powders. [ABSTRACT FROM AUTHOR]

Submicrometer (200-800 nm) ZrB2 powders have been successfully prepared via a new ZrO2-C- BN precursor powder system at a relatively low temperature (1550°C) for 1.5 h. As a moderate cost boron source, BN has a well-defined stoichiometry and low impurity. Both thermodynamic and experimental results indicated that ZrC was formed below 1300°C, a temperature required for ZrB2 formation. Moreover, the reaction of ZrC- BN mixture at 1300°C indicated that the ZrC acted as an effective direct reducing reagent for BN to form ZrB2, indicating that the pathway involving the formation of intermediate phase ZrC determined the formation mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiCf/SiC composites were studied. The results show that the density and open porosity of the as prepared SiCf/SiC composites are 2.1g/cm3 and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiCf/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270MPa, fracture toughness of 11.4MPa·m1/2 and shear strength of 25.7MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200°C. In addition, the thermal conductivity is 10.6W/mk at room temperature. [Copyright &y& Elsevier]