Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering

Fine MoSi₂ powders containing a small amount of Mo₅Si₃ have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500°C and 30 MPa. Dense MoSi₂ materials, in which the grain size is ∼7.5 μm, have been fabricated at 1300°C. They exhibit excellent mechanical properties: Vicker’s hardness H_v (10.6 GPa), fracture toughness K_IC (4.5 MPa m^1/2), and bending strength σ_b (560 MPa). The strength of 325 MPa can be retained up to 1000°C.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Pressureless sintering ZrB2-SiC ceramics at low temperatures

ZrB₂-SiC ceramics are prepared by pressureless sintering using ZrB₂ powders and liquid polycarbosilane (LPCS) precursors. The LPCS can effectively reduce the sintering temperature. The phases of the sintered ceramics are characterized by X-ray diffraction, and their morphologies are observed by scanning electron microscopy. From these results, it is learned that LPCS can provide free carbon and silicon at high temperatures. Therefore, the oxides on the ZrB₂ surface can be removed by free carbon, and the densification process can be promoted by silicon. These coupled effects make it possible to pressureless sinter the ZB₂-SiC ceramics at low temperatures.     

Preparation of nanostructured TiO2 ceramics by spark plasma sintering

The effect of spark plasma sintering (SPS) on the densification of TiO₂ ceramics was investigated using a nanocrystalline TiO₂ powder. A fully-dense TiO₂ specimen with an average grain size of ∼200 nm was obtained by SPS at 700 °C for 1 h. In contrast, a theoretical density specimen could only be obtained using conventional sintering above 900 °C for 1 h with an average grain size of 1-2 μm.     

Microstructure and mechanical properties of hot pressed ZrB2-SiCp-ZrO2 composites

The present paper investigated the microstructure and mechanical properties of ZrB₂-10 vol.%SiC_p-10 vol.%ZrO₂ composites hot pressed at three temperatures. Phase transformability from t-ZrO₂ to m-ZrO₂ during fracture was analyzed through calculating the volume fractions of m-ZrO₂ and t-ZrO₂ on polished and fracture surfaces. The densification temperature was found to have a significant effect on the microstructure, phase transformation and the properties of the composites. When the composite was hot pressed at 1950 °C, the average grain size was 9.5 µm, and the fracture toughness was 4.5 MPa·m^1/2. Comparatively, when the composite was hot pressed at 1750 °C, the average grain size was 3.4 µm, and the fracture toughness increased by ~ 50% to 6.8 MPa·m^1/2.     

反应法制备硼化锆耐超高温涂层

为了提高C/SiC复合材料的超高温抗烧蚀性能,以锆粉、硼粉和酚醛树脂为原料,通过泥浆涂刷后高温烧结的方法在C/SiC表面制备了ZrB2涂层,研究了涂层的烧结反应过程,并对其组成、结构和抗烧蚀性能进行了表征.结果表明:1200℃前Zr先与碳反应生成ZrC,然后在1400～1600℃时ZrC与B反应生成ZrB2.浆料配比为n(Zr):n(B):n(C)=1.0:1.5:1.0时,1600℃制备的涂层由ZrB2、少量的ZrC及ZrO2组成.氧乙炔焰烧蚀60s后,由于ZrB2氧化形成了ZrO2熔融层,涂层后的C/SiC复合材料的线烧蚀率几乎为零,而未涂层的C/SiC复合材料的线烧蚀率为0.064mm/s.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Effect of graphite flake on the mechanical properties of hot pressed ZrB2-SiC ceramics

A ZrB₂ ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB₂-SiC-G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB₂-SiC-G composite was 6.1 ± 0.3 MPa·m^1/2, which was much higher than that of monolithic ZrB₂, ZrB₂-SiC composite and similar ZrB₂-SiC-C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB₂-based ceramics.     

Consolidation and mechanical properties of nanostructured MoSi2-SiC-Si3N4 from mechanically activated powder by high frequency induction heated sintering

A dense nanostructured MoSi₂-SiC-Si₃N₄ composite was sintered by the high frequency induction heating method within 2 min from mechanically activated powder of Mo₂N, Mo₂C and Si. Highly dense MoSi₂-SiC-Si₃N₄ composite was produced under simultaneous application of a 80 MPa pressure and the induced current. Mechanical properties and grain size of the composite were investigated. The average hardness and fracture toughness values obtained were 1420 kg/mm² and 4.5 MPa m^1/2, respectively.     

Fabrication of ZrB2-Zr cermet using laser sintering technique

A combination of laser sintering/melting and induction heating techniques is used to fabricate ZrB₂-Zr ceramic-metal composite (cermet) 3-D structures using layer by layer deposition. It is shown that the interface between layers comprises an ~ 20 µm thick Zr-rich phase near the remelted zone. The starting powder mixtures consist of 30-50 wt.% Zr, which is used to assist binding of ZrB₂ particles. Sintered objects have densities > 95% of theoretical values and have microhardness values up to 16.0 GPa. ZrB₂-Zr cermet may reduce the likelihood of mechanical failures by increasing toughness and impact resistance contributed from the continuous Zr matrix, yet preserve good abrasive characteristics provided by ZrB₂ grains. The proposed method allows fabricating complex-shaped structures, and joining or repairing objects made from analogous ultra-high temperature materials.     

Preparation and dielectric properties of BaCu(B2O5)-doped SrTiO3-based ceramics for energy storage

BaCu(B₂O₅) (BCB) was used as sintering aids to lower the sintering temperature of multi-ions doped SrTiO₃ ceramics effectively from 1300 °C to 1075 °C by conventional solid state method. The effect of BCB content on crystalline structures, microstructures and properties of the ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and dielectric measurements, respectively. The addition of BCB enhanced the breakdown strength (BDS) while did not sacrifice the dielectric constant. The enhancement of BDS should be due to the modification of microstructures, i.e., smaller and more homogeneous grain sizes after BCB addition. The dielectric constant of BCB-doped ceramics maintained a stable value with 1.0 mol% BCB, which was dominated by the combination of two opposite effects caused by the presence of second phases and the incorporation of Cu²⁺ and Ba²⁺, while further increase was owing to the increase of dissolved Ba²⁺ ions when the content of BCB is more than 2.0 mol%. The multi-ions doped SrTiO₃ ceramics with 1.0 mol% BCB addition showed optimal dielectric properties as follows: dielectric constant of 311.37, average breakdown strength of 28.78 kV/mm, discharged energy density of 1.05 J/cm³ and energy efficiency of 98.83%.     

Al2O3基多孔隔热材料表面Al2O3/MoSi2涂层的制备及其性能

采用刷涂法在Al₂O₃基多孔隔热材料表面制备Al₂O₃/MoSi₂涂层,涂层以硅溶胶作为粘结剂,纳米Al₂O₃与Al₂O₃纤维作为耐高温组分,MoSi₂为高发射率组分。通过SEM、XRD对Al₂O₃/MoSi₂涂层微观表面结构、物相组成进行分析。研究纳米Al₂O₃与Al₂O₃纤维的质量比和MoSi₂含量对Al₂O₃/MoSi₂涂层耐温性能的影响,并对Al₂O₃/MoSi₂涂层的抗热震性能、发射率进行表征。结果表明,当纳米Al₂O₃与Al₂O₃纤维的质量比小于1∶1时,热考核后Al₂O₃/MoSi₂涂层表面无裂纹产生;当纳米Al₂O₃与Al₂O₃纤维的质量比在1∶2~1∶4之间时,Al₂O₃/MoSi₂涂层中的纤维网络较完整。MoSi₂的含量为20%时,Al₂O₃/MoSi₂涂层抗热震实验循环25次后表面保持完好,热考核后在2.5~25 μm波段的平均发射率在0.85左右,具有较高的发射率。      

Oxidation mechanism of ZrB2/SiC ceramics based on phase-field model

In this paper, the phase-field oxidation mechanism of ultra-high temperature ZrB₂/SiC ceramics is investigated theoretically. Firstly, a phase-field model is developed to analyze the oxidation behaviors of multiphase materials. Secondly, the evolutions of the porosity and the oxidation stress for the oxidized ZrB₂/SiC ceramics with different temperatures and different oxygen partial pressures are predicted, and the influences of the mechanical factors on the oxidation behaviors of ZrB₂/SiC ceramics are discussed. Finally, two-dimensional oxidation behaviors of ZrB₂/SiC ceramics are simulated and analyzed.     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.     

Effect of the La addition content on valence electron structure and properties of ZrB2 ceramics

ZrB₂ is a combined bonding compound composed of strong covalent bonds which make it difficult to sinter and densify. Thus, rare earth or other metal elements are usually used to be sintering additives to improve its sintering properties. To forecast properties of ZrB₂ solid solutions with addition of lanthanum, their valence electron structure (VES) was calculated by using the empirical electron theory (EET) of solids and molecules, and the effect of lanthanum with various proportion on the VES and properties of ZrB₂ ceramics has been studied. The results show that with the increase of the lanthanum addition content, the hybridization steps of Zr and B atoms of ZrB₂ solid solutions are still A16 and 5, respectively. The hybridization step of lanthanum is always A1. The covalent electron numbers and bonding energy of the strongest bonds of the ZrB₂ matrix decrease with the lanthanum addition content increase. These suggest that the addition of lanthanum will improve the fracture toughness and decrease the hardness, crystal cohesive energy and melting point of ZrB₂. In a word, its sintering properties are improved, and its densities are increased.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Joint between MoSi2 and 316L stainless steel surfaces and its interfacial mechanical properties

In this study, we proposed a new method using the spark plasma sintering technique to join ceramics to alloys. MoSi₂ and 316L stainless steel were chosen as sample materials and can be welded well with graded interlayers. We found that dense uniformed joints were achieved because of the comparable coefficient of thermal expansion of the interlayers. Furthermore, such a compatibility between the graded interlayers prevented MoSi₂ with low toughness from the occurrence of microcracks resulted from the residual stresses formed during cooling of the joint.     

Pressure effects on elastic and thermodynamic properties of ZrB2

A theoretical formalism to calculate the single crystal elastic constants for hexagonal crystals from first principle calculations is described. The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. The calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline ZrB₂ are also calculated and compared with corresponding experimental values. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential. It is found that the elastic constants and the Debye temperature of ZrB₂ increase monotonically and the anisotropies weaken with pressure. The thermal properties including the equation of state, linear compressibility, ductility, and the heat capacity at various pressures and temperatures are estimated.