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Four ZrB 2 –SiC/SiC dual-layer coatings were prepared on the surface of graphite matrix by pack cementation to improve the oxidation and thermal shock resistance of graphite. The crystalline structure and morphology as well as the resistance to oxidation and thermal shock of these coatings were investigated. The results indicated that the weight loss of the ZS50 coating sample, whose pack powders contained 52.4 wt.% ZrB 2 , 39.2 wt.% Si and 8.4 wt.% graphite, was only 2.9% after oxidation in air at 1500 °C for 19 h and 6.5% after thermal shocks between 1500 °C and room temperature for 15 cycles. With the increasing silicon in pack powders, some residual silicon appeared in ZS50 coating, which was considered to be beneficial to oxidation resistance and thermal shock resistance because it can improve the density of coating and the SiO 2 formed by oxidation of residual Si can heal the microcracks at high temperature. [ABSTRACT FROM AUTHOR]

SiC nanoparticles were introduced in a ZrB 2 matrix prepared via vacuum hot pressing sintering to improve the thermal shock resistance of ultra-high temperature ZrB 2 –SiC ceramics, namely, ZrB 2 -20 vol% nanoparticle SiC composites (ZrB 2 –20SiCnp). The thermal shock resistance of the material were determined at different temperatures via the water quenching-residual strength test method. The specific temperature of the water bath environment and the influence of different nanoparticle sizes on the thermal shock resistance of the material were also investigated. The results show that the critical thermal shock temperature difference (Δ Tc ) of the ZrB 2 –20SiCnp composite is 428 °C, which is 11.2% higher than 385 °C, Δ Tc of a SiC microparticle-strengthened ZrB 2 composite material. The results from the thermal shock analysis of the ZrB 2 –SiCnp composite indicate that Δ Tc is strongly dependent on the formation of unusual intragranular nanostructures. [ABSTRACT FROM AUTHOR]

Two ZrC-based composites, viz., ZrC+20vol.%SiC and ZrC+20vol.%SiC+20vol.%ZrB2, were prepared by hot pressing. The oxidation behavior of the composites was studied in the temperature range of 800°C-1300°C in air. From the mass gain tests, the surface and cross-sectional SEM observation and EDS analysis of the oxidized samples, it was found that the ZrC+20vol.%SiC composite seemed to have a high oxidation resistance below 1000°C, but the ZrC+20vol.%SiC+20vol.%ZrB2 composite appeared a more excellent oxidation resistance up to 1200°C. It is main reason that the oxidation rate of ZrC is higher than that of SiC below 1300°C, meaning that SiO2 from SiC is insufficient to form a dense tight protection film; however, the addition of ZrB2 particles promotes the formation of more borosilicate above 1000°C, which exerts more protective effect by self-healing mechanism. [ABSTRACT FROM AUTHOR]

To protect graphite matrix from oxidation, B modified ZrB 2 –SiC coatings were prepared on SiC coated graphite by a two-step pack cementation. The microstructure, thermal cycling and oxidation resistant properties of the coatings were studied. The results show that the as-prepared coatings were mainly composed of SiC, ZrB 2 and Si. ZS50-1B, ZS50-3B and ZS50-5B showed good thermal cycling resistance with 97.4%, 98.2% and 98.6% weight retention after 15 thermal cycles between 1500 °C and room temperature. ZS50-7B and ZS50-9B showed good oxidation resistance with 93.5% and 97.5% weight retention after 19 h oxidation at 1500 °C. [ABSTRACT FROM AUTHOR]

Abstract: Effect of the hot-pressing temperature (1750–1950°C) on the microstructure and mechanical properties of the hot-pressed ZrB2–SiC–ZrO2f ceramics was investigated in detail. It was indicated that when the hot-pressing temperature was above 1900°C, ZrO2 fibers were instable in ZrB2–SiC matrix which would degenerate into particles and the grain size of ZrO2 particles was bigger than the upper critical grain size so that the phase transformation of tetragonal to monoclinic phase would occur spontaneously during cooling process. The optimal hot-pressing temperature for this material system should be 1850°C to obtain high performance of the ZrB2–SiC–ZrO2f ceramics. The flexural strength and fracture toughness were enhanced to 1086±79MPa and 6.9±0.4MPam1/2, respectively. The toughness mechanism was mainly attributed to densification, phase transformation toughening, crack deflection, fibers fracture, fibers pull-out and fibers debonding. [Copyright &y& Elsevier]

ZrB2–SiC–ZrO2f ceramic was fabricated by hot-pressing at 1850°C for 1h under a uniaxial load of 30MPa in vacuum. The ZrB2–SiC–ZrO2f ceramic thus produced showed excellent fracture toughness due to the addition of ZrO2 fiber. The increase in toughness was attributed mainly to the addition of the ZrO2 fiber, which could enhance fiber pull-out, crack bridging and crack branching. In addition, the stress-induced transformation toughening was also considered to be main reason for the improvement in toughness. [ABSTRACT FROM AUTHOR]

Abstract: The reactive synthesis route, microstructure and properties of ZrB2–SiC–ZrC composites prepared from a mixture of Zr/B4C/Si powders via reactive hot pressing were investigated. The reactions of the Zr/B4C/Si system commenced at 800°C and completed after a 60min hold at 1700°C in argon atmosphere. The reacted products at 1700°C were composed of fine particles and the size was smaller than 1μm, which provided a large driving force for densification. The powders were sintered at 1750°C, 1800°C, 1850°C and 1900°C, respectively. The results showed that the relative density and average grain size of the composites increased with the increase of sintering temperature. The relationships between the microstructural features and mechanical properties of the composites were discussed. [Copyright &y& Elsevier]