Influence of SPS temperature on the properties of TiC–SiCw composites

Titanium carbide (TiC) composites containing 10 vol% silicon carbide whisker (SiC_w) were spark plasma sintered at different temperatures of 1800, 1900, and 2000 °C under a pressure of 40 MPa and a holding time of 7 min. At the sintering temperature of 1900 °C, the relative density, Vickers hardness, and flexural strength of the sintered samples hit their maximum values of 98.7%, 24.4 GPa, and 511 MPa, respectively. The microstructural characteristics of the sintered samples were assessed by optical and field emission scanning electron microscopy (FESEM) and XRD. The results revealed that at 1900 °C, the dispersion of SiC_w in the TiC matrix was homogenous, no chemical reaction took place between the reinforcement and the matrix, and produced a fine-grained microstructure. It was found that the thermal conductivity of SPSed samples did not have the same trend with relative density and mechanical properties. A maximum value of 32.3 W/mK was measured for the thermal conductivity of the composite sintered at 2000 °C.     

多元稀土钨线材轧制致密化及退火分析

通过使用二辊轧机,采用一火八道次的热轧制工艺进行多元稀土钨合金线材开坯,对开坯后的多元钨合金线材进行退火处理。利用扫描电子显微镜(SEM)、能谱、光学显微镜、显微硬度计等表征分析轧制致密化过程,轧制、退火对钨线材组织性能的影响。研究结果表明:轧制过程中烧结孔发生变形、闭合,最终在剪切力的作用下消失,多元稀土钨线材密度增加,在轧制温度1 650℃,保温时间30min,轧制速度170m/s下进行轧制,孔隙度由6.50%减少至4.61%,钨线材密度由17.84g/cm3增加到18.12g/cm3;轧制组织中的晶粒在轧制方向上伸长,形成相互挤压、重叠的加工态组织,热轧过程主要发生动态回复过程,在晶粒内部形成胞状亚结构;在1 100℃至1 300℃退火保温时仅发生回复并未再结晶,在1 400℃保温60min发生再结晶,晶粒未完全长大,在1 600℃退火保温60min时,再结晶过程完成。     

SLA 3D printing of high quality spine shaped β-TCP bioceramics for the hard tissue repair applications

β-TCP has excellent biodegradability and bioabsorption properties, and is regarded as an ideal hard tissue repair material. In the present study, 3D printing β-TCP green bodies was realized using the stereo lithography apparatus (SLA) technology. The effects of the KH-560 dispersant and solid loading on the slurry properties were investigated systematically. The optimized KH-560 addition amount and the solid loading of the slurry were 2.0 wt% and 48 wt%, respectively, and the corresponding slurry for the subsequent SLA 3D printing exhibited good fluidity, uniform dispersion and good stability. The sintering schedule of the printed β-TCP green bodies was optimized by the DSC-TG analysis. By sintering the green bodies at 1050 °C for 8 h, high quality β-TCP bioceramics without crack or deformation were fabricated. It was found that increasing the solid loading of the slurry would decrease the porosity while reducing the shrinkage degree of the β-TCP ceramics. However, the slurry could hardly be printed when its solid loading was higher than 50 wt%.     

Simulation of microscopic shrinkage behaviour in sintering of powder compact

A method for simulating microscopic shrinkage behaviour of powder particles in sintering of a compact is proposed on the basis of the granular element method. In this method, the powder particles are modelled as many circular elements undergoing viscoplastic deformation due to surface tension during the sintering, and the microscopic shrinkage is calculated by equilibrating forces acting on the elements. The variation in shape of necks between the elements during the sintering is taken into consideration. Plane-strain shrinkage in sintering is calculated under regular and irregular dispositions of powder particles. In the regular disposition of powders having the same diameter, the obtained shrinkage behaviour is compared with the experimental one using glass rods and the calculated one by the viscoplastic finite-element simulation respectively. It is shown from the simulation of irregular disposition that the densification due to the sintering is accelerated by mixing powders having different diameters.     

单向碳/碳复合材料的化学气相渗透特征

等温化学气相渗透（ＩＣＶＩ）是制备高性能碳／碳复合材料最为重要、应用最为广泛的技术手段。本文通过研究ＩＣＶＩ制备单向碳／碳复合材料的致密化过程,分析了单向碳／碳复合材料致密化的特点,并对致密化均匀性问题进行了初步探索,为优化工艺、提高致密化效率,提供一定的借鉴。     

超重力熔铸铁基金属陶瓷材料多相运动学特征及其致密化机制

采用Fe_3O_4/Al/铁基硬质合金的铝热体系,超重力熔铸铁基金属陶瓷梯度材料,分析了超重力场中多相(硬质合金颗粒、Al_2O_3陶瓷相及气体相)在高热高温铁熔体内的运动学特征及其致密化机制。结果表明,根据Stocks定律,弥散相在高温熔体内的运动速度影响因素主要有颗粒与基体熔体密度差、超重力系数、颗粒半径及温度等,在一定的超重力系数下,材料的致密度与气泡尺寸、铁熔体存在时间有关。V-Fe和Cr-Fe 2种颗粒主要集中于金属陶瓷材料的顶端,沿超重力方向粒子尺寸逐渐减小,而Mo-Fe颗粒主要集中在金属陶瓷材料与45钢分界面处,不同颗粒在材料内的分布状况有较大差异,这是金属陶瓷材料硬度呈梯度变化的主要原因。     

A mass-spectroscopical study of the decomposition of Ti(NMe2)4 in a mixed Ar–H2–N2 pulsed d.c. plasma

The favourable gas-phase conditions for deposition of TiN have been determined by a mass-spectroscopic investigation of the gaseous species in an ambient of tetrakis(dimethyl-amino)titanium (TDMAT) molecules during pulsed d.c. plasma-enhanced deposition processes. The gas-phase composition was varied, at a pressure of 0.4 Torr, a temperature of 350 °C and a bias voltage of 500 V, based on an Ar, H₂, N₂ ternary diagram. The results reveal that hydrogen plays a key role in the cleavage of –NMe₂ from the central Ti atom. The addition of N₂ to the hydrogen plasma opens up the possibility for transamination reactions by NH_x formation (1<x<3), known by thermal CVD using NH₃. This addition also leads to powder formation, which seems to reach a maximum within a 100% N₂ plasma. In a nitrogen plasma, only relatively small amounts of gaseous species, like HCN, NH₂CN, and CH₃CN, are detected, which indicates that residual hydrocarbon fragments of TDMAT must be incorporated into the powder and coating. Even small amounts of Ar addition to a hydrogen plasma convert TDMAT to powder particles, which is the opposite of the densification purpose of Ar bombarment. No gaseous species, apart from small amounts of HCN, are detected, suggesting hydrocarbon-containing coatings. If Ar:H₂:N₂=1:1:1, no specific mechanism is dominant under the conditions used here. Decreasing the deposition temperature and pressure and increasing the bias voltage seem to favour the cleavage of –NMe₂ ligands.     

Densification behavior,microstructure, mechanical and optical properties of Mg-doped sialon with fluoride additives

Mg-doped sialon ceramics with the composition of M_0.4Si_10.2Al_1.8O₁N₁₅ were fully densified by hot pressing at 1850 °C for 1 h, using 0.5 wt.% MgF₂ or CaF₂ as a sintering additive. Densification behavior, phase assemblage, microstructure, and mechanical and optical properties were investigated in detail. The addition of fluorides, especially MgF₂, not only resulted in more high-temperature liquid by promoting the dissolution of more N and other constituents but also reduced the viscosity of liquid due to the terminal effect of fluorine. Consequently, the densification was effectively improved. Additionally, the fluoride addition facilitated the formation of a small amount of β-sialon. Both the samples possessed high hardness (∼20 GPa) and fracture toughness (∼4.2 MPa m^1/2). The CaF₂-added sample exhibited higher infrared transmittance than its counterpart due to less residual glass phase. The present work implies that fluorides are also very effective sintering additives for densifying α-sialon.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.