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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]

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]