Search

Starting from ZrN and amorphous boron, dense ZrB 2 ceramics with 37 vol% hexagonal BN were consolidated by spark plasma sintering. Benefiting from the moderate exothermic reaction between ZrN and B and the resultant fine powder generated, ZrB 2 -BN ceramics with relative density of 94% could be reached at 1100 °C, further improved to 97% by 1550 °C. The effects of sintering temperature and holding time on the densification behavior, microstructural evolution and mechanical properties of ZrB 2 -BN ceramics were investigated. ZrB 2 -37 vol%BN ceramics densified at 1700 °C exhibited attractive mechanical performance: a three-point bending strength of 353 MPa, a Vicker’s hardness of 6.7 GPa and a Young’s modulus of 197.5 GPa. Note that its strength dropped sharply to 191 MPa measured at 1300 °C. The combination of low sintering temperature (1100–1550 °C), low Young’s modulus (180–200 GPa) and relatively high strength (200–350 MPa) make reactively sintered ZrB 2 -BN composites as promising matrix for continuous fiber reinforced composites. [ABSTRACT FROM AUTHOR]

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]

Dense, highly textured Zr B2 and Zr B2- Mo Si2 ceramics were fabricated via a strong magnetic field alignment method followed by spark plasma sintering. Unlike with the previous studies, which only focused on the alignment of single-phase particles, both Zr B2 and Mo Si2, which exhibited a magnetic anisotropy, have been aligned in this study. The alignment of Mo Si2 in the same direction of Zr B2 enhanced the degree of orientation of Zr B2, decreased the grain size, but increased the aspect ratio of the platelet Zr B2 grains. The microstructure and anisotropic mechanical properties as well as the oxidation resistance in different directions were discussed. [ABSTRACT FROM AUTHOR]

Abstract: Fully dense ZrB2–20vol% SiC (ZS) ceramics were fabricated by self-made high purity ZrB2 and SiC powders hot-pressing at 1900°C/30MPa. Their properties were compared with other ZS ceramics with the same nominal composition, but prepared by pressureless sintering, using low purity commercially available powders. The hot-pressed composites show superior hardness (17.82GPa), elastic modulus (504GPa), flexure strength (462MPa at 25°C and 463MPa at 1300°C in air), electrical conductivity (7.68×10−6 Ω−1 m−1 at 25°C), thermal conductivity (93.7W/(mK) at 25°C, 77.2W/(mK) at 1200°C) and a lower average thermal expansion coefficient (6.66×10−6 K−1 from 25 to 2000°C) compared to the pressureless sintered materials. The improvement in these properties could be primarily explained by the decrease of impurities and second phase levels in hot pressed samples. [Copyright &y& Elsevier]

Abstract: Four-point flexure creep deformation of ZrB2–30vol% SiC ceramics in argon atmosphere under a static load of 19MPa for 0–100h at 1500 and 1600°C was investigated. The strain rate at 1600°C was 3.7 times higher than that at 1500°C. Microstructural evolution during creep consisted of nucleation and growth of triple-point cavitations which were always associated with SiC particles. Due to the low stress, only isolated cavitations were nucleated, and no microcracks were formed. For up to 100h at 1500 and 1600°C, the grains maintained their size and shape. The cavitations in both size and number showed no obvious difference from 26 to 100h at 1500°C, whereas that showed a significant increase from 26 to 100h at 1600°C. Present study suggested that ZrB2–30vol% SiC exhibited relatively good microstructural stability and creep resistance at 1500°C in argon atmosphere. [Copyright &y& Elsevier]

Abstract: A facile method based on an in situ reaction, called nitride boronizing, was first developed to directly coat ZrB2 particles with a nanoscale hexagonal boron nitride (h-BN) film. The crystallinity of the BN coating and the diameters of the ZrB2 particles could be tailored by changing the reaction conditions. A mechanism was proposed from thermodynamic calculations to clarify the formation of a core–shell nanostructure, i.e. the shock heat generated by the exothermal reaction. The core–shell ZrB2 powders showed excellent sinterability and the advantage in producing dense h-BN texture-free composites. Even when sintered at 2000°C for 20min and under a uniaxial pressure of 60MPa, no texture created by h-BN was observed. The isotropic bending strength (∼300MPa) of ZrB2–37vol.% BN ceramics from core–shell precursor powders were investigated and compared with ceramics of the same nominal composition, but densified from commercial powders containing highly aligned h-BN grains. The nitride boronizing approach could be explored to synthesize other h-BN-coated IVB group transition metal borides. [Copyright &y& Elsevier]

ZrB2-20vol% SiC composites added with 3vol% Yb2O3 were hot-pressed at 1900°C, and then heat-treated at 2000°C in argon. The effect of heat treatment on phase composition, microstructure, and mechanical properties of the composites was investigated. Results showed that the high temperature heat treatment led to the depletion of Yb-containing phases by decomposition or evaporation, the formation of some pores at multiple-grain junctions, significant microstructural coarsening with the increase of ZrB2 grain size from ∼1.9μm to ∼4.8μm, and the change of fracture mode from mixed inter/transgranular to fully transgranular. In addition, Vickers' hardness, fracture toughness and flexural strength were severely degraded after heat treatment. [ABSTRACT FROM AUTHOR]

Refractory metal diboride is a kind of excellent ultra-high temperature ceramics and could be applied as thermal shielding material for supersonic aircraft. In this paper, a two-step reaction process for the synthesis of refractory metal diboride powders was proposed using refractory metal oxide, carbon black (C), B 4 C, and calcium Ca as raw materials. Firstly, the carbothermal reduction reaction between oxides and C was taken place at 1773 K; and then, the reduction product was reacted with B 4 C and Ca for the purposes of boronization and decarburization. After acidic leaching, pure diboride was prepared. The experimental results indicated that diborides of Ti, Zr, Hf, V, Nb and Ta with low residual carbon content can be prepared by this method. In addition, the inheritance relationship of particle size between raw oxide and finally prepared diboride was revealed. By this method, ultra-fine TiB 2 (123 nm) and ZrB 2 (76 nm) powders were also successfully prepared, with the raw materials of nano TiO 2 and nano ZrO 2 , respectively. [ABSTRACT FROM AUTHOR]

The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m1/2 and 5.24 ± 0.41 MPa·m1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles. [ABSTRACT FROM AUTHOR]

Single-phase titanium-zirconium oxycarbonitride ceramics, prototypically Ti 1/2 Zr 1/2 (C 1/3 N 1/3 O 1/3), was obtained by spark plasma sintering of the corresponding Ti and Zr- oxide, carbide and nitride. It was revealed that a single-phase Ti 1/2 Zr 1/2 (C 1/3 N 1/3 O 1/3) solid solution with rock-salt structure formed at 2000°C, below which the secondary phase of ZrO 2 was evident. The as-sintered ceramics was chemically homogeneous from nanoscale to microscale and the carbon, nitrogen, and oxygen contents in Ti 1/2 Zr 1/2 (C 1/3 N 1/3 O 1/3) ceramics were determined to be 5.39wt%, 5.44wt% and 6.00wt%, corresponding to an empirical formula of Ti 0.51 Zr 0.49 (C 0.37 N 0.32 O 0.31). The product possesses a relative density of 95.2% and Vickers hardness of 21.6 ± 2.4 GPa. Image, graphical abstract [ABSTRACT FROM AUTHOR]

High entropy ceramics have attracted much attention in recent years, however, few open literature on the composites in which high entropy phases were used as matrix can be found. In this work, high entropy diboride powder was firstly synthesized, and then monolithic high entropy diboride ceramics of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 (sample HEB) and high entropy particulate composites of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2)B 2 -20 vol% SiC (sample HEB-SiC) were prepared by SPS at different temperatures. As-synthesized high entropy diboride powders demonstrated good sinterability in the temperature range from 1600 °C to 1900 °C. The grain growth rate of the high entropy phase was comparable to that in the ZrB 2 system, but the addition of SiC second phase particles inhibited the grain growth of the high entropy diboride phase. Due to the grain growth inhibition by the introduced SiC particles, the hardness for the particulate composites increased. Moreover, the SiC particles improved the fracture toughness through crack deflection and branching. [ABSTRACT FROM AUTHOR]

Abstract Gas burst has been commonly observed during reactive sintering, especially when combustion was initialized therein. In this study, a model system (Zr + 2B = ZrB 2) was chosen to investigate this phenomenon by a combination of thermo-analysis, mass spectrum investigation and electron microscopy studies. Results showed the combustion lead to a formation of various gaseous byproducts such as CO, CO 2 , N 2 , BO, BO 2 and B 2 O 3 etc. Especially for BO(g), its generation consumed boron in the raw materials, resulting in the residual Zr-containing phase that accelerated the formation of interlocked ZrB 2 grains in the sintered ceramics, which show relatively high strength (516 ± 42 MPa). Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

A large amount of multi-walled carbon nanotubes (MWCNTs) (10 vol%) were uniformly and intimately dispersed within ZrB 2 ceramic matrix using an aqueous hetero-coagulation approach followed by spark plasma sintering (SPS) at 1700 °C. Addition of MWCNTs inhibited grain growth, increased the relative density from 89% to 97%, and resulted in a composite with a fine, rather pore-free and uniform microstructure. Structural evolution of MWCNTs in the ZrB 2 matrix was further studied after SPS sintering at 1900 °C by XRD, Raman, scanning electron microscopy and TEM. When sintered at 1700 °C, the MWCNTs remained intact with their tubular one-dimensional structure though experiencing an expected radial elastic compression/buckling due to thermal misfit residual stresses. When sintered at 1900 °C, their unzipping into graphite nanoribbons was observed. For practical applications, MWCNTs content should however be lowered to prevent their agglomeration during grain growth. [ABSTRACT FROM AUTHOR]

Chemical reactivity of hot-pressed Si 3 N 4 –ZrB 2 ceramics at 1500–1700 °C under N 2 and Ar atmosphere is investigated by X-ray diffraction, thermodynamic calculation, and phase diagram. Results show that there are chemical reactions in Si 3 N 4 –ZrB 2 ceramics. The chemical reactions accelerate the decomposition and phase transformation of Si 3 N 4 . The reaction products include ZrN, BN, ZrSi 2 , SiC or Si, which strongly depend on the atmosphere or temperature. The source of N 2 for ZrN and BN products is both atmosphere and decomposition of Si 3 N 4 at 1600 °C or above, whereas that is only atmosphere at 1500 °C. There is no reaction in Si 3 N 4 –ZrB 2 ceramics at 1500 °C under Ar. The MgO–Lu 2 O 3 sintering adds lead to the significant increase of relative density of Si 3 N 4 –ZrB 2 from 48% to 94% at 1500 °C in Ar. Therefore, it is possible that dense Si 3 N 4 –ZrB 2 ceramics with no reaction could be achieved at 1500 °C or below in Ar. [ABSTRACT FROM AUTHOR]

A pressureless sintering process using different amounts of boron carbide (B 4 C) as sintering aid was investigated for the densification of zirconium diboride (ZrB 2 ). The sintering mechanism was converted from solid state sintering to liquid phase sintering as sintering temperature increased from 2150 to 2250 °C. B 4 C addition improved densification by removing the oxide impurities during solid state sintering. Besides, it could restrain the growth of ZrB 2 grains. At 2250 °C, ZrB 2 and B 4 C formed liquid phase which was well wetting ZrB 2 grains during liquid phase sintering process. The different roles of B 4 C on the microstructure development, mechanical properties and oxidation resistance of the sintered ZrB 2 -based ceramics were investigated. [ABSTRACT FROM AUTHOR]

Oxidation behavior of ZrB 2 –SiC ceramics containing 0, 10 and 20 vol% ZrC was studied at 1600 °C in stagnant air. The structures of oxide scales depended on ZrC content. The thickness of the SiO 2 -rich glass layer on the outermost surface decreased steeply with the addition of ZrC. One “ZrC-corroded layer”, containing ZrC x O y , SiC and ZrB 2 , was observed in ZrB 2 –SiC–ZrC. The oxidation kinetics was converted from parabolic to linear for the ceramics with 20 vol% ZrC. [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: AlB2 is used as a sintering additive to densify ZrB2 ceramics. The AlB2 phase plays a positive role in the densification process of ZrB2 ceramics by removing the oxygen impurity and liquid phase sintering. With 5vol% AlB2 addition, both mechanical properties and oxidation resistance can be improved compared with monolithic ZrB2 ceramics. [Copyright &y& Elsevier]

Highlights: [•] ZrB2 nanofibers are prepared by electrospinning via carbothermal reduction. [•] The obtained ZrB2 nanofibers are consist of nano-sized ZrB2 single crystals. [•] The obtained ZrB2 is nearly stoichiometric. [ABSTRACT FROM AUTHOR]

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process. [Copyright &y& Elsevier]

Abstract: Using elemental Zr, B, Mo and Si as raw materials, partially textured ZrB2–MoSi2 composites with in situ platelet ZrB2 grains were fabricated by reactive hot pressing. Synthesized by a combustion reaction, the in situ formed ZrB2 phase had unique characteristics to grow up to platelet grains, on the surroundings of Mo–Si–B ternary liquid phase at high temperature. Mechanical properties were dependent on grain size and aspect ratio of the ZrB2 platelets. The rotation and realignment of the platelet ZrB2 grains under hot pressing led to a partially textured microstructure, which showed anisotropic mechanical properties on different directions of the as-sintered samples. A roadmap of the reaction process, microstructure evolution and texture formation was given to describe the preparation of partially textured ZrB2–MoSi2 composites by reactive hot pressing. [Copyright &y& Elsevier]

Abstract: ZrB2 powders with different particle size or different morphologies have been synthesized by boro/carbothermal reduction (BCTR) method using ZrO2, B4C and graphite or carbon black as starting materials in this work. Results revealed that the carbon black with fine particles decreased the synthesis temperature and resulted in a finer ZrB2 powder compared with graphite in vacuum. However, the synthesis temperature was almost the same for both the carbon black and graphite cases if the reaction took place in argon atmosphere. Furthermore, vacuum atmosphere led to the formation of rod-like ZrB2 powder, whereas argon atmosphere resulted in quasi-spherical ZrB2 powder. Thermodynamic calculation was conducted and possible mechanisms were discussed for understanding the above phenomena. [Copyright &y& Elsevier]