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Abstract: A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB2 and SiC, the volatility diagram of a ternary ZrB2‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB2‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: Oxidation behavior and effect of oxidation on the room-temperature flexural strength were investigated for ZrB2–10vol% SiC (ZB10S) and ZrB2–30vol% SiC (ZB30S) in air at 1500°C with times ranging from 0.5h to 10h. The oxide scale of both ZB10S and ZB30S was composed of an outer glassy layer and an inner extended SiC-depleted layer. The changes in weight gain, glass layer thickness, and extended SiC-depleted layer thickness with oxidation were measured. Analysis suggested that the extended SiC-depleted layer was most indicative for evaluating the oxidation resistance. Compared to the ZB10S, the improved oxidation resistance in ZB30S was attributed to the viscosity increase of glassy layer and the lower number of ZrO2 inclusions in the glassy layer. Because of the healing of surface flaws by the glassy layer, the strength increased significantly by ∼110% for ZB10S and by ∼130% for ZB30S after oxidation for 0.5h. [Copyright &y& Elsevier]

Highly c-axis oriented ZrB2-based ultrahigh-temperature ceramics (UHTCs) were prepared via strong magnetic field alignment during slip casting, followed by spark plasma sintering, using ZrB2 and β-SiC as starting materials. Results showed that hardness was superior on the surface perpendicular to the grain alignment. Oxidation resistance was obviously better on the surface of (001) direction. The anisotropic properties of the textured materials would provide an opportunity to the material design and selection of UHTCs. [Copyright &y& Elsevier]

Toughened ZrB 2 –SiC composites have been prepared by an introduction of amorphous carbon spheres. After hot pressing, microstructure observation showed that the carbon phases were present in the form of thin layer at grain boundaries, or in the form of agglomerates at the multigrain junctions. The carbon spheres led to significant improvement of fracture toughness from 4.96 ± 0.46 MPa m 1/2 to 7.52 ± 0.19 MPa m 1/2 . The toughening mechanism was attributed to the weak interfaces, which occurred due to the presence of carbon layer formed at grain boundaries after sintering. [ABSTRACT FROM AUTHOR]

Abstract: ZrB2–20vol% SiC composites with 3vol% Re2O3 rare-earth oxide (Re=La, Nd, Y or Yb) were hot pressed to near full-density at 1900°C. The La2O3 and Nd2O3 additions not only caused the formation of an amorphous grain boundary phase and enhanced densification, but also resulted in substantial ZrB2 and SiC grain growth. In contrast, the Y2O3 and Yb2O3 additions resulted in the formation of crystalline (Y/Yb)2Zr2O7 and enhanced densification without ZrB2 and SiC grain growth. The hardness was improved by the rare-earth oxide addition, especially with Y2O3 and Yb2O3. The La2O3 and Nd2O3 only had a minor effect on the fracture toughness, whereas the Y2O3 and Yb2O3 additions increased the fracture toughness. The type of Re2O3 addition was found to influence the nature of the grain boundary and the concomitant fracture and toughening mechanisms. [Copyright &y& Elsevier]

A pressureless densification mechanism and microstructure development in ZrB2–20vol.% SiC ceramics with boron additive were investigated. Results showed that the boron addition removed oxygen impurity and improved the densification process. By adding 1wt.% boron, fully dense ZrB2–SiC ceramics with a fine-grained microstructure was obtained. However, a higher boron addition would lead to fast grain growth of both SiC and ZrB2 grains and a deteriorated densification behavior by forming a liquid phase. The mechanical properties of the sintered ceramics with 1wt.% boron addition were also evaluated. [Copyright &y& Elsevier]

Abstract: Three ZrB2–SiC composites were prepared from polycarbosilane (PCS)-coated ZrB2 powders. Powders with or without additions of Si or Zr were densified by hot pressing at 1800°C. Finally, the oxidation resistance of the composites was studied at 1500°C in air. The phase composition and microstructure were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with energy-dispersive spectroscopy (EDS). Results showed that Si addition, which reacted with free carbon coming from the pyrolyzed PCS to produce the SiC, improved the oxidation resistance of ZrB2–SiC composites with PCS as a precursor, whereas Zr addition, which reacted with the carbon to produce the ZrC, decreased it. [Copyright &y& Elsevier]