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The preparation of dense single-phase boride ceramics has always been a challenge. In this paper, single-phase W 2 B 5 ceramic with excellent performance was successfully prepared using W 2 B 5 powder and SPS-HF technology. Under the sintering condition of 1800 °C and 50 MPa, a nearly dense W 2 B 5 ceramic material with excellent mechanical properties was obtained. The relative density of the as-sintered W 2 B 5 ceramic achieved 96.72 % without any sintering additive. Additionally, the Vickers hardness, fracture toughness as well as bending strength were 30 GPa (9.8 N), 5.39 MPa·m1/2 and 522.7 MPa, respectively. The outstanding hardness and bending strength of W 2 B 5 ceramic could be attributed to its monophase W 2 B 5 structure and high densification degree. The primary fracture mode of W 2 B 5 ceramic was transgranular fracture, and the toughening mechanisms included crack deflection, crack bridging, and crack branching. [ABSTRACT FROM AUTHOR]

This is the first comprehensive review on inherent anisotropic features of transition metal diboride (MB 2 ) and their implementation for tailoring the microstructure and properties of MB 2 -based Ultra-high temperature Ceramics (UHTCs). The emphasis is on the processing approaches, microstructures, and properties of self-reinforced and/or textured MB 2 -based composites with elongated MB 2 grains. The crystal structure characteristics and grain growth behaviour of MB 2 are also critically reviewed. Benefiting from the tailored microstructure, the MB 2 -based ceramics exhibit some improved properties. Considering the success of Si 3 N 4 ceramics in the field of structural ceramics, it is expected that the potential MB 2 -based ceramic composites with abundant elongated MB 2 grains, textured structures, and controlled grain boundaries would possess improved fracture toughness, thermal shock resistance, and reliable high-temperature properties, which are desired for their practical applications. Accordingly, microstructure designing and tailoring provide an important perspective for the future development of UHTCs. [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]

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]

ZnTaO ceramics with various amount of AlO additive were synthesized by a conventional mixed-oxide route. The grain growth of ZnTaO ceramics was accelerated with AlO additive. However, excessive addition (>1.0 wt%) of AlO leaded to abnormal grain growth. With AlO addition, the AlO additive did not solubilized into ZnTaO structure but resulted in forming the second phase. The AlO addition resulted in the lower sintering temperature of ZnTaO ceramics and improved microwave dielectric properties. The dielectric constant (ε) of the samples did not change much and ranged from 32.41 to 34.33 with different amount of AlO addition. The optimized quality factor (Q × f) was higher than 70,000 GHz as a result of the denser ceramics. The temperature coefficient of resonant frequency (τ) of the doped ZnTaO ceramics could be optimized to near-zero. [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]

High-strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC1− x, Zr3 Al3 C5 and Zr- Al compound such as AlZr3 and Al- C- Zr. The intermediate product AlZr3 below 1600°C and remained Al- C- Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C-1900°C) could be attributed to the formation of nonstoichiometric ZrC1− x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC0.85- Zr3 Al3 C5 and ZrC0.80- Zr3 Al3 C5 based ceramics had 3-point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2-18.3 GPa and 3.3-3.5 MPa m1/2, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Zirconium carbide and hafnium carbide ceramics were prepared by reactive spark plasma sintering, using oxides and carbon black as the raw materials. The phase and microstructure evolution during the sintering process was investigated. It was found that ZrC and HfC powder intermediates were formed at 1750 and 1800°C, respectively. Due to the high sinterability of the in situ formed carbide particles, samples with a fine microstructure could be densified at a relatively low temperature. [Copyright &y& Elsevier]

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: 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: A pressureless sintering process, using a small amount of boron carbide (≤2wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100°C to 2350°C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300°C) to liquid phase sintering (LPS, above 2300°C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B4C on the microstructure development and mechanical properties of the sintered ceramics were investigated. [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: High quality ceramics in Zn(Ta1−x Sb x )2O6 (0.05≤x≤0.20) compounds were synthesized by the conventional solid-state reaction method. The effects of Sb5+ ion substitution on microstructures, phase composition and microwave dielectric properties were investigated. A phase change or crossover from ZnTa2O6 to ZnSb2O6 occurred at x=0.1. Well-developed microstructures of ceramics can be achieved at 1250°C and the distribution of grain size was optimized due to the existence of ZnSb2O6 phase. With Sb5+ ion substitution, the quality factors Qf values were enhanced effectively and ranged from 72,400 to 119,600GHz owing to the meliorative grain size distribution. The dielectric constant and the temperature coefficient of resonant frequency (τ f ) were strongly dependent upon the Sb5+ ion concentration and the presence of the second phase of ZnSb2O6. [Copyright &y& Elsevier]

zirconium oxycarbide (ZrC x O y ) ceramics were hot pressed using ZrC-5ZrO 2 and ZrC-5ZrO 2 -5Zr (number in mol%) mixtures as starting powders, and the solid solution and densification processes were investigated. When 5 mol% ZrO 2 were added to ZrC matrix, ZrC x O y and CO were formed during the reaction between ZrC and ZrO 2 occurred above 1600 °C. And the sintered ZrC x O y ceramic had numerous pores. However, the co-doping of ZrO 2 and Zr could reduce the reaction temperatures and CO amounts in ZrC-5ZrO 2 -5Zr system, resulting in a near fully dense ZrC x O y ceramic. The chemical composition of obtained ZrC x O y ceramics were also calculated according to oxygen content and reaction processes. [ABSTRACT FROM AUTHOR]

Dense TiC-based ceramics were obtained at 1500 °C by hot pressing, benefiting from Ti and Si additives in the TiC matrix. The in-situ formed Ti 3 SiC 2 platelet grains refined TiC grains and reinforced the obtained ceramics. The TiC–20 vol.% Ti 3 SiC 2 ceramic exhibits a much higher flexural strength of 1003 MPa, compared with 450 MPa of the pure TiC ceramic. Owing to the formation of continuous heat-conducting paths by the bridged Ti 3 SiC 2 platelet, a high thermal conductivity over 33 W/m · k was measured in TiC–40 vol.% Ti 3 SiC 2 at 500 °C, which is nearly 1.5 times than that in the pure TiC ceramics with a similar grain size. [ABSTRACT FROM AUTHOR]

Molybdenum or tungsten borides are excellent high-temperature ceramics or electrocatalysis materials. In this paper, Mo or W borides were prepared by boronizing Mo or W powder in Al melt with B 4 C as boron source. By thermodynamic calculation, the feasibility of preparing various borides and the stability of borides in Al melt were evaluated. Through high temperature reaction at 1523 K and the following acidic leaching, single phase MoAlB, W 2 B 5 , WB, and W 2 B 5 powders were successfully synthesized. Owing to the decarburizing function of Al melt, C contents of various boride products were below 1 wt%. In addition, the phase transition processes of various products in the synthesis stage were revealed. • Pure phases MoAlB, Mo 2 B 5 , WB and W 2 B 5 powder are prepared in Al melt. • Al melt has a good decarburization effect. • Due to the effect of Al melt, MoAlB, Mo 2 B 5 , and W 2 B 5 particles reach 10 μm. [ABSTRACT FROM AUTHOR]

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]

The influence of the spark plasma sintering (SPS) heating rate on densification, microstructure and strength of ZrB2–20vol.% SiC composites with or without 5vol.% Yb2O3 addition was assessed. A low heating rate promoted densification, induced grain growth and slightly increased the strength in the Yb2O3-free ceramics, whereas the low heating rate inhibited the densification, maintained the grain size and greatly decreased the strength in the Yb2O3-doped composites. [Copyright &y& Elsevier]

Abstract: Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si3N4/BN) ceramic composites were fabricated at a relatively low temperature of 1450°C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si3N4/BN ceramic composites to be intergranular. The flexural strength and Young''s modulus decreased with the increasing BN content. The reaction-bonded Si3N4/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition. [Copyright &y& Elsevier]

Three typical ceramic processing were respectively used to synthesize (Ti 0.2 Zr 0.2 Nb 0.2 Ta 0.2 W 0.2)C high-entropy carbide (HEC) ceramics by spark plasma sintering. Although single-phase composition characterized by X-ray diffraction were obtained by the three processes, the microstructures and elemental distributions are different. The reasons for the formation of these features are preliminarily discussed. The results demonstrate that the particle sizes of the starting metallic powders was a key factor for obtaining a homogeneous distribution of each elements in the HEC. Carbide process with relatively finer starting carbide powders compared to the above metallic starting powders resulted in an HEC with homogeneous distribution of elements, but the obtained ceramics showed the lowest relative density. For oxide process, it is considered that the obviously higher reaction temperature between ZrO 2 and graphite resulted in a two-phase structure of an HEC and a zirconium-rich phase, but the obtained HEC showed the highest relative density. [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]

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]

Titanium carbide ceramics with different contents of boron or B 4 C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB 2 platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B 4 C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m 1/2 ) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B 4 C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB 2 platelets. [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]

Due to the extremely harsh reaction conditions, it is very difficult to control the particle morphology during the synthesis of ZrC, a well-known ultra high temperature ceramic material. In this paper, carbon encapsulated zirconia microspheres (ZrO 2 @C) are synthesized via two consecutive hydrothermal reactions in one pot. After carbothermal reduction, ZrC particles with controlled spherical morphology can be prepared. The spherical morphology suggests that carbon diffuses into zirconia during carbothermal reduction. The influences of reactant concentrations, reaction time and temperature on the microencapsulation process are also investigated. Meanwhile, as a comparison, zirconia coated carbon microspheres (C@ZrO 2 ) are also prepared, which results in ZrC particles with layered structure. [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: Using nano-sized starting powders, TiN ceramics were hot pressed under different sintering atmospheres, which were nitrogen, argon and vacuum (5Pa). The effects of sintering atmosphere on the densification process and microstructures of TiN ceramics were investigated. At 1500°C, ceramics densified under vacuum demonstrated higher density compared with nitrogen and argon, owing to the high nitrogen vacancy concentration which came from the dissociation of TiN under vacuum circumstance. At 1600°C, however, the densification process under vacuum will be hindered because of the decomposition of TiN. Combining a lower sintering temperature, smaller grain size and higher grain boundary concentration, the argon atmosphere is more suitable for fabrication of TiN ceramics as nuclear materials. [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: 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]

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]