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A series of ZrB 2 -ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B 4 C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB 2 : 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB 2 -ZrC-SiC composites with various SiC content from 0 to 20 vol%. [ABSTRACT FROM AUTHOR]

The residual stresses in ZrB 2 SiC ceramics with the addition of WC or ZrC were measured using Raman spectroscopy (RS) and X-ray diffraction (XRD). The temperatures at which the residual stresses began to accumulate were calculated. The resistance to high-temperature deformation of ZrB 2 SiC-based ceramics was evaluated via four-point flexure test under a static load (15, 25 MPa) for 5 h at 1600 °C and 1800 °C. The results indicate that a compressive residual stress existed in SiC particles and a corresponding tensile stress applied in ZrB 2 matrix. The values of thermal residual stresses increase in order of ZSZ (ZrB 2 SiC ZrC) < ZS (ZrB 2 SiC) < ZSW (ZrB 2 SiC WC). The temperature composite begin to accumulate the residual stresses also increases in the same order. The apparent different behaviors are resulted from their dissimilar grain boundary state and third phase nature in three ceramics. The phenomenon was explained by a proposed model. The results provide a path to design material compositions with an ideal mechanical properties and guidance role for applications. [ABSTRACT FROM AUTHOR]

Despite of good oxidation resistance and ablation resistance, challenge to densify and tendency to loss high temperature (H.T.) strength have limited its potential applications in aerospace for HfB 2 –SiC ceramic. In this work, dense HfB 2 –SiC ceramic with improved H.T. flexural strength was prepared using WC as sintering aid. Pure HfB 2 –SiC ceramic exhibited creep deformation at 1600 °C with measured strength value of 389 ± 82 MPa, whereas WC doped sample showed much higher flexural strength of 658 ± 69 MPa with linear elastic behavior prior to fracture. In addition to clean grain boundary, the addition of 5 vol.%WC into HfB 2 –SiC ceramic promoted the formation of fine (Hf,W)B 2 , (Hf,W)C and WB precipitates. This combinative effect is responsible for its excellent H.T. mechanical property of WC doped HfB 2 –SiC ceramic. [ABSTRACT FROM AUTHOR]

Abstract: Dense ZrB2–20vol% SiC ceramics (ZS) were fabricated by hot pressing using self-synthesized high purity ZrB2 and commercial SiC powders as raw materials. The high temperature flexural strength of ZS and its degradation mechanisms up to 1600°C in high purity argon were investigated. According to the fracture mode, crack origin and internal friction curve of ZS ceramics, its strength degradation above 1000°C is considered to result from a combination of phenomena such as grain boundary softening, grain sliding and the formation of cavitations and cracks around the SiC grains on the tensile side of the specimens. The ZS material at 1600°C remains 84% of its strength at room temperature, which is obviously higher than the values reported in literature. The benefit is mainly derived from the high purity of the ZrB2 powders. [Copyright &y& Elsevier]

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]

Abstract: Using WC as sintering aid, nearly full dense (∼99%) HfB2–20vol% SiC ceramics were sintered at 2200°C for 2h without external pressure. The densification mechanism, microstructure evolution, mechanical properties and oxidation resistance were investigated. The results indicated that complex chemical reactions of WC in HfB2–SiC system strongly related to the densification, microstructure and properties. The Young''s modulus, fracture toughness and 3-pt bending strength of HfB2–20vol% SiC with 10wt% WC were 511GPa, 4.85Mpam1/2 and 563MPa, respectively, which were comparable to some hot pressed HfB2–SiC ceramics in literature. The oxidation of HfB2–20vol% SiC with 10wt% WC at 1500°C in air exhibited parabolic kinetics. After oxidation at 1500°C for 10h, its weight gain and SiC-depleted layer thickness were 3.7mg/cm2 and 43μm, respectively, and its residual flexural strength was comparable to or even a little higher than the value before oxidation. [Copyright &y& Elsevier]

Abstract: This work compared phase composition, microstructures and mechanical properties of ZrB2—20vol.% SiC—5vol.%Yb2O3 prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength. [Copyright &y& Elsevier]

Coating refractory ceramics onto alloys has been considered to be a feasible way to enhance the mechanical properties along with the corrosion and radiation resistance of the cladding materials. Here, we report a room-temperature magnetron sputtering method to deposit SiC films onto Zircaloy-4 substrate. By optimizing variety of processing parameters including target-substrate distance, sputtering power and deposition pressure, we finally obtained a dense SiC film with high values of hardness (25.61 GPa), elastic modulus (214.9 GPa) and critical load (8.24 N). Structural characterization indicates nanocrystalline (∼10 nm) structures of the SiC films, which is expected to show better radiation resistance than bulk SiC. Effects of processing parameters especially the deposition pressure on film microstructures and mechanical properties have been investigated in depth, which involve difference in energy of sputtering particles, nucleation and growth of grains, and stress states of the deposited films. [ABSTRACT FROM AUTHOR]

Immersion corrosion tests of TiC 0.8 , TiC, TiC–20 vol% SiC, TiC–40 vol% SiC and SiC have been performed in molten FLiNaK salt at 800 °C for 25–200 h under argon cover gas. All of these five samples showed small mass loss and relatively good corrosion resistance in molten FLiNaK salt. The corrosion patterns of TiC 0.8 , TiC, TiC–20 vol% SiC and TiC–40 vol% SiC were inter-granular corrosion, which were attributed to the depletion of Ti along the grain boundaries. SiC exhibited a general corrosion process in which a carbon-rich layer formed on the surface, resulting from the depletion of Si. The carbon-rich layer protected SiC against further corrosion, hence lowering the corrosion rate. The corrosion results of TiC–20% SiC and TiC–40% SiC revealed the corrosion resistance of TiC could be improved by adding SiC. And the contribution of SiC to better corrosion resistance has been elucidated. [ABSTRACT FROM AUTHOR]

Monolithic ZrB 2 , ZrB 2 SiC, ZrB 2 ZrC and ZrB 2 SiC ZrC were prepared by hot pressing in ternary ZrB 2 SiC ZrC system. They exhibited different microstructural features. Grain sizes of each phase were smaller in the ternary composites. Contour maps of mechanical properties were also plotted in ZrB 2 SiC ZrC ternary diagram based on experiments and software fitting. They established the relationships between chemical composition and properties in a visual way. Young’s Modulus and Vickers hardness had linear relationships with composition, while the evolution of the strength and toughness was complicated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fine grained ZrB2–SiC–hBN composite were reactively densified from coarse precursors. [•] Employ the SHS ignited in ZrN–Si–B4C systems to improve microstructure homogeneity. [•] Strength reliability and strain tolerance of obtained ceramics were much improved. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the pressure loading cycle on spark plasma sintered ZrB2–SiC–Yb2O3 ceramics was investigated. Spark plasma sintering consists of three stages, i.e., a heating-up, isothermal and cooling stage. Applying the load during isothermal stage allowed evaporation of volatile species. Applying the load during the heating-up and isothermal stages resulted in the entrapment of volatile species and the formation of nanometer-sized spherical SiO2 particles. The presence of SiO2 particles led to a large thermal residual stress, and improved fracture toughness in the ZrB2–SiC–Yb2O3 ceramics. [Copyright &y& Elsevier]