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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]

The structure of ZrB2 under high-pressure was predicted by Particle Swarm Optimization method (CALYPSO). We investigated the structure stability, phonon dispersion curve, elasticity, electronic structure and thermodynamic properties of ZrB2 under high-pressure and high-temperature via first-principles calculations. It maintained the hexagonal structure when the pressure lowers below 600 GPa at 0 K, which is confirmed by the calculated phonon dispersion curve. Studies indicate that the elastic modulus and Poisson’s ratio increase monotonically with pressure, as supported by some theoretical and experimental evidences. Calculated anisotropic factors demonstrate that compression and shear isotropy of ZrB2 weakens as the pressure increases. Using the quasi-harmonic approximation Debye model, the Debye temperature, sound velocity, expansion coefficient, thermal capacity under the high-temperature and pressure were also predicted. [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]

High temperature flexural strength of ZrB2-20 vol% SiC ceramics ( ZS) up to 1600°C in high purity argon atmosphere was significantly improved by adding 5 vol% WC, but degraded when 5 vol% ZrC was added. ZrB2-20 SiC-5 WC ceramic ( ZSW) has a very high strength (mean ± SD) of 675 ± 33 MPa at 1600°C, and also an elastic and transgranular fracture mode was observed. According to the analysis of the fracture modes and crack origins in ZSW ceramics, the improvement in strength above 1000°C was attributed to the removal of the oxide impurities from grain boundaries. [ABSTRACT FROM AUTHOR]

Monolithic ZrB2 ceramic and ZrB2–20vol.% SiC composite were joined to themselves using pure nickel powder. The interfacial phenomena were observed and analyzed. The joining effects were evaluated by shear strength. Resulting from the special interfacial microstructure, the shear strength of ZrB2/Ni/ZrB2 system was as high as 59.7±5.3MPa. In the ZrB2–SiC/Ni/ZrB2–SiC system, the reaction between Ni and SiC resulted in a homogeneous joint composition, which was beneficial to the joining effects. [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]

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]

Surface oxygen impurities have been well proved to accelerate the ceramic particle coarsening during sintering, therefore impede the densification of non-oxide ceramics. However, the characteristic of these impurities is still poorly understood. In this work, several state-of-the-art technologies on microscopy and microanalysis techniques were combined to investigate the bonding character, morphology and composition of oxygen contamination on ZrB 2 powders. The assemblage of light elements, i.e. O, C, N, B and Si on the surface and in the lattice of the ZrB 2 powders was thoroughly analyzed and compared before and after its removal upon heating. [ABSTRACT FROM AUTHOR]

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]

Abstract: Oxidation behavior of hot forged textured ZrB2–20vol% MoSi2 ceramics with platelet ZrB2 grains was investigated at 1500°C for exposure time from 0.5 to 12h. Compared to untextured ceramics, the textured ceramics showed obvious anisotropic oxidation behavior and the surface normal to the hot forging pressure demonstrated better oxidation resistance. Such improvement in the oxidation resistance is primarily considered as a higher intrinsic ZrB2 atomic density on the orientated {00 l} planes in the textured ceramics. It is expectable that the anisotropic textured ZrB2–MoSi2 ceramics can offer better oxidation resistance when a certain surface with higher oxidation resistance is exposed to air at elevated temperature. [Copyright &y& Elsevier]

Abstract: ZrC ceramics with additions of MC (M=V, Nb, and Ta) were prepared by hot pressing, and the effect of MC additions on densification was analyzed in terms of the solubility limit and kinetics of formation of MC solid solutions with ZrC. VC additions of 2.5–10vol% (3.5–13.5mol%), which is higher than its solid solubility limit of 1.3vol% (1.8mol%), effectively promoted the densification process and nearly fully dense ZrC ceramics were obtained by hot-pressing at 1900°C. In contrast, both NbC and TaC additions, which can form unlimited solid solutions with ZrC, have no obvious contribution to ZrC densification. This is because formation of the solid solution of NbC and TaC with ZrC matrix requires higher temperature and longer time due to their stronger bonding energy, compared to VC. SEM observation demonstrated that the VC addition resulted in larger grains, compared to ZrC ceramics with NbC and TaC additions. [Copyright &y& Elsevier]

Textured and platelet-reinforced ZrB2-based ultra-high-temperature ceramics (UHTCs) are prepared via reactive hot pressing and successive hot forging. The textured materials display significantly anisotropic mechanical properties, and the flexural strength has been improved by 52% owing to the orderly arrangement of platelet ZrB2 grains. The texturing of ZrB2-based platelet composites should open up an effective approach to optimizing the properties of ZrB2-based UHTCs. [Copyright &y& Elsevier]