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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]

ZrN/TiZrN multilayers are deposited by using the cathodic vacuum arc method with different substrate bias (from 0 to -800 V), using Ti and Zr plasma flows in residual N2 atmosphere, combined with ion bombardment of sample surfaces. The effect of pulsed bias on the structure and properties of films is investigated. Microstructure of the coating is analyzed by X-ray diffraction (XRD), and scanning electron microscopy (SEM). In addition, nanohardness, Young's modulus, and scratch tests are performed. The experimental results show that the films exhibit a nanoscale multilayer structure consisting of TiZrN and ZrN phases. Solid solutions are formed for component TiZrN films. The dominant preferred orientation of TiZrN films is (111) and (220). At a pulsed bias of -200 V, the nanohardness and the adhesion strength of the ZrN/TiZrN multilayer reach a maximum of 38 GPa, and 78 N, respectively. The ZrN/TiZrN multilayer demonstrates an enhanced nanohardness compared with binary TiN and ZrN films deposited under equivalent conditions. [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]

Abstract: The isothermal oxidation of ZrB2 powder was carried out in the range of 650–800°C in a flowing air using thermogravimetric analysis (TGA). The evolution of the phase characterization and morphology of ZrB2 powder oxidized at 700°C for varying durations was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The excellent fit of TG curves by multiple-law model suggests that the oxidation of ZrB2 powder in air follows para-linear kinetics, and based on the fitted results, oxidation mechanisms can also be obtained. The reaction product of ZrB2 powder with oxygen is metastable tetragonal ZrO2 at 700°C, and the tetragonal phase transforms to the monoclinic phase with oxidation. The oxidation of ZrB2 powder is associated with surface microcrack formation, which is attributed to volume expansion resulting from oxidation of ZrB2 to tetragonal ZrO2 and tetragonal-to-monoclinic phase transformation of ZrO2. In the last stage of oxidation, each ZrB2 particle breaks into fragments. [Copyright &y& Elsevier]

A ZrB2–SiC–ZrC composite was prepared from a mixture of zirconium, silicon, and B4C via reactive hot pressing at a relatively low temperature (1800°C) for 60 min under 20 MPa in an argon atmosphere. The relative density was 96.8%, the micro-hardness (Hv10) was 16.7 GPa, and the fracture toughness was 5.1 MPa·m1/2. The presence of ZrC was helpful for the densification process and improved the mechanical properties of the composite. A model of the microstructure development of the composite was proposed to explain the phase distribution. [ABSTRACT FROM AUTHOR]

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: The microstructural evolution of ZrB2–20vol% MoSi2 composites during heat treatment with and without pressure at 2000°C for 1h in argon was investigated. Results showed the materials exhibited a multiphase layered structure after heat treatment without pressure. From surface to center, the layered structure consisted of (1) a Mo layer, (2) a Si layer, (3) a ZrB2–MoSi2 layer and (4) a partially MoSi2-depleted ZrB2 layer. The formation mechanism of layered structure was analyzed. After heat treatment with pressure, the materials with high toughness mainly contained ZrB2 phase. [Copyright &y& Elsevier]

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]

B 4 C–ZrB 2 ceramics were reactively hot pressed by three-step process using B and C with Zr (C–B–Zr mixtures) or ZrO 2 (C–B–ZrO 2 mixtures) as raw materials. Based on thermodynamic analysis and experimental results, the C–B–Zr or C–B–ZrO 2 mixtures were firstly held at 1000 °C, then held at 1650 °C, and finally held at 2000 °C under 30 MPa. B 4 C–ZrB 2 ceramics were obtained by using the three-step process and controlling the reaction sequence. The ZrB 2 grains in B 4 C–ZrB 2 ceramics from C–B–Zr mixtures had a bimodal grain size distribution including fine grains with size of 1–2 μm and large agglomeration-like grains with size of 10–30 μm, while that in the ceramics from C–B–ZrO 2 mixtures showed homogeneous distribution with small grain size of 1–2 μm. The B 4 C–ZrB 2 ceramics from C–B–ZrO 2 mixtures showed higher relative density, more homogeneous and fine-grained microstructure, higher hardness (33.0 ± 1.3 GPa), and comparable toughness (4.1 ± 0.2 MPam 1/2 ), compared with the ceramics from C–B–Zr mixtures. [ABSTRACT FROM AUTHOR]

ZrB 2 (ZB) and ZrB 2 –5 vol.% WC (ZW) were prepared by hot-pressing at 2100 °C. Combining experimental and theoretical methods, the tungsten (W) element in solid solution with ZrB 2 and ZrC was investigated. Experimentally, solid solution formation in ZW samples was supported by XRD and EDS. Theoretically; atomic scale computer simulations based on density functional calculations were used to interpret the different amount of solid solution and the changes in lattice parameters. The flexural strength of the samples was measured from room temperature to 1600 °C. The flexural strength of ZB and ZW changed differently as testing temperature increased. Then, the strength of ZB increased from 344 ± 12 MPa to 1281 ± 39 MPa, which is about 3.7 times of room temperature strength. Further temperature increasing led to strength decreasing. While for ZW, the strength increased continuously from room temperature to 1600 °C. SEM and TEM techniques were employed to clarify the different mechanism of fracture surfaces and grain boundaries of ZB and ZW. [ABSTRACT FROM AUTHOR]

Abstract: A solid state reaction of ZrH2, Si3N4 and B4C has been proposed to synthesize machinable ZrB2–SiC–BN composites. Dense ZrB2–SiC–BN composites with fine grain size and homogeneous microstructure were fabricated via spark plasma sintering at 1900°C in vacuum. The kinetics of the reaction process was studied with XRD results. SEM and TEM results showed that the in-situ formed BN phase was composed of micro-sized intergranular and nano-sized intragranular h-BN particles. The influences of the BN content on the mechanical properties as well as the thermal conductivities of the composites were studied. The composites that contained >20vol% BN exhibited excellent machinability and high strength. [Copyright &y& Elsevier]