Small amount TiB2 addition into B4C through sputter deposition and hot pressing

Small amount of TiB₂ (<5 wt%) was added into B₄C through a novel method that combines the use of sputter deposition and hot pressing. Sputter deposition provided more uniform dispersion of TiB₂ grains with smaller grain sizes as compared to the conventional particulate mixing. Small amount TiB₂ addition demonstrated to be an effective way for improving the fracture behavior and toughness of B₄C while not sacrificing its outstanding lightweight property to a large extent: 2.3 wt% TiB₂ addition brought 15% improvement in indentation fracture toughness while resulting in less than 2% increase in density. The improvement can be attributed to the combination of crack impeding by TiB₂ grains and crack deflection at the B₄C–TiB₂ interfaces. TiB₂ also played as grain growth inhibitor resulting in a slight increase (2%) in Vickers hardness. Another intention of employing sputter deposition was to modify the grain boundary of B₄C; however, neither formation of Ti-containing phase nor Ti segregation has been observed at grain boundaries likely due to the poor wettability of B₄C.     

In-situ elongated aluminium borate grains toughening WC- 1.87 wt % Al2O3- 4.13 wt % ZrO2 composite via spark plasma sintering

In this work, a novel way was developed to facilitate the sintering of a binderless cemented carbide with Al₂O₃ contain while improving its toughness. WC- 1.87 wt % Al₂O₃- 4.13 wt %ZrO₂ cemented carbides with 1 wt % B₂O₃ as additives were consolidated by high energy ball-milling and spark plasma sintering the as-milled composite powders. The effects of 1 wt % B₂O₃ content on the sintering behaviour, microstructure and mechanical properties of the obtained cemented carbides were investigated. The presence of B₂O₃ significantly lowers the sintering temperature by forming liquid phase and react with Al₂O₃ which contributed to obtaining fully dense specimens at 1350 °C and maintains fine grain sizes of WC until the temperature exceeding 1450 °C. The sintering temperature of the specimens with optimum mechanical properties has been also reduced comparing that of the original WC- 1.87 wt % Al₂O₃- 4.13 wt %ZrO₂ cemented carbides. Furthermore, the addition of B₂O₃ triggered the reaction between B₂O₃ and Al₂O₃ resulting in forming in-situ elongated aluminium borate grains (A₄B₂O₉ and A₁₈B₄O₃₃ whiskers), which promoted the toughness. The specimens sintered at 1450 °C exhibited optimal mechanical properties: the Vickers hardness and fracture toughness were 19.26 GPa and 11.49 MPa m^1/2, respectively.     

Effect of nanorelief structure formed in situ on tribological properties of ceramics in dry sliding

Nanorelief structure formed on the surface of B₄C–SiC ceramics by an in-situ method is reported. The nanorelief structure formed by the in-situ method, which differs from the relief structure produced by laser treatment, results from preferential wearing of SiC particles, which have lower hardness, by free diamond particles and mixed wear particles of B₄C and SiC during polishing and sliding, respectively. The effect of the nanorelief structure formed by the in-situ method on the sliding dry friction against a SiC ball is examined using a pin-on-disk tribometer. A comparison of dry sliding of the B₄C–SiC composite ceramics with nanorelief structure to that of SiC ceramics without nanorelief structure reveals that B₄C–SiC composite ceramics have a lower coefficient of friction and a significantly lower specific wear rate than the SiC ceramics. Worn surface analysis indicates that the nanorelief structure does not disappear after a dry sliding test. The depth of the nanorelief structure formed by the in-situ method ranges from 10 to 40 nm. The friction and wear results are associated with the nanorelief structure formed by the in-situ method.     

Preparation of submicron boron carbide powder through gas-solid reaction method

Boron carbide submicron powder was synthesized with boron oxide and graphene as starting materials by gas-solid reaction method using two different apparatuses. The effects of calcination temperature and holding time, apparatus type and B₂O₃/C ratio of the starting materials on the phase composition and morphology of the synthesized powders were evaluated. A newly formed residual carbon morphology distinct from original graphene were present in samples synthesized at a higher B₂O₃/C ratio or temperature. The synthesis temperature of ∼1500 °C was found to be more suitable to obtain boron carbide powder without the existence of residual carbon. The new type of apparatus enabled the synthesis of boron carbide phase at a relatively lower temperature, due to its more efficient use of B₂O₃ vapor.     

Ni-Cr合金保护气氛钎焊金刚石的分析

采用Ni-Cr合金在保护气氛炉中进行了钎焊单晶金刚石磨粒的试验,使用SEM对Ni-Cr合金钎焊金刚石的碳化物形貌和钎料组织进行了观察分析。结果表明：在保护气氛炉中,Ni-Cr合金可以实现金刚石的高强度连接,焊后金刚石棱角清晰、形貌完好,并在金刚石的表面生成排列较整齐的一层Cr3C2,Cr3C2的生长方向与金刚石的外露（111）和（100）晶面有一定的位向关系,在（111）晶面的碳化物形成了互成60°的关系,在（100）晶面的碳化物主要是互成90°和45°的关系。钎料组织主要是γ-Ni基体上分布有Cr7C3,其显微硬度最高达到900HV。     

特粗硬质合金制备工艺的开发

通过控制球磨时间,球料比及保持WC粉原有碳含量,采用结晶完整性好的WC粉开发出性能优异的特粗晶硬质合金。结果表明,采用结晶完整性好,费氏粒度为30μm的碳化钨粉末为原料制备合金,可以制备出WC晶粒度达到8μm,物理性能优异的WC-Co硬质合金。     

CVD纳米金刚石涂层工具研究进展

概述了CVD纳米金刚石涂层工具的研究开发现状、存在的主要问题,重点介绍了硬质合金基体表面预处理方法及纳米金刚石生长工艺参数对CVD纳米金刚石涂层工具结构和性能的影响。其中,介绍了硬质合金基体表面预处理方法主要有酸液浸蚀去钴、施加中间过渡层、机械或等离子体处理、负偏压等;纳米金刚石生长工艺参数则主要从碳源浓度、基体温度、反应气压三方面进行了介绍;最后,对CVD纳米金刚石涂层工具发展趋势和应用前景作出了展望。     

Densification and joining of a (HfTaZrNbTi)C high-entropy ceramic by hot pressing

A bulk (Hf_0.2Ta_0.2Zr_0.2Nb_0.2Ti_0.2)C high-entropy ceramic (HEC) with a high density was prepared by hot pressing (HP), and through a robust joining technique, large-sized piece was fabricated. A hot-pressed carbide HEC with a single-phase and homogeneous composition was obtained at the sintering temperatures from 1800 to 1950 °C for 30 min under a pressure of 30 MPa. The influence of sintering temperature on the mechanical properties of the HEC was investigated, and the flexural and compressive strengths were reported. Additionally, the feasibility of active brazing of this HEC was studied and solid joints with high shear strength were obtained by atomic diffusion and chemical reaction at the interface, providing a key approach to fabricate complex components of HECs.     

Applications of analytical electron microscopy to guide the design of boron carbide

Compositional analysis of boron carbide on nanometer length scales to examine or interpret atomic mechanisms, for example, solid-state amorphization or grain-boundary segregation, is challenging. This work reviews advancements in high-resolution microanalysis to characterize multiple generations of boron carbide. First, ζ-factor microanalysis will be introduced as a powerful (scanning) transmission electron microscopy ((S)TEM) analytical framework to accurately characterize boron carbide. Three case studies involving the application of ζ-factor microanalysis will then be presented: (1) accurate stoichiometry determination of B-doped boron carbide using ζ-factor microanalysis and electron energy loss spectroscopy, (2) normalized quantification of silicon grain-boundary segregation in Si-doped boron carbide, and (3) calibration of a scanning electron microscope X-ray energy-dispersive spectroscopy (XEDS) system to measure compositional homogeneity differences of B/Si-doped arc-melted boron carbides in the as-melted and annealed conditions. Overall, the improvement and application of advanced analytical tools have helped better understand processing–microstructure–property relationships and successfully manufacture high-performance ceramics.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.