This work investigated the formation mechanism and the effects of temperature on the microstructure and shear strength of vacuum-brazed SiAlON ceramic/WC-Co cemented carbide joints using an Ag-Cu-Ti active filler as the interlayer. The diffusion of Ti elements toward substrates and their subsequent reaction during the brazing process led to the formation of TiN, TiC, and Ti5Si3 phases, where the featured reaction layers were established. A continuous reaction layer of TiN in the SiAlON substrate side and TiC in the WC substrate side were formed. The ceramic phases (Ti5Si3 TiN, and TiC) were distributed in a brazed intermediate layer. The highest shear strength of the brazed joints obtained at 850 degrees C was 325.28 +/- 20.27 MPa, demonstrating the feasibility of the Ag-Cu-Ti active filler in producing robust joints.

This research investigated the cutting performance of three different types of Si3N4-based ceramic tools (Si3N4, Si3N4-SiCw, and Si3N4-SiCw-HfB2) with distinct mechanical properties during turning of ductile cast iron. Si3N4 ceramic tool, characterized by high hardness and low toughness, showed minor micro chipping and the highest average flank wear rate during the initial wear stage, leading to a short cutting life of 1679 m. Si3N4-SiCw ceramic tool, with low hardness and high toughness, exhibited the highest average flank wear rate during the middle and final wear stages, resulting a short cutting life (1602 m). In contrast, the Si3N4-SiCw-HfB2 ceramic tool, featuring high hardness and high toughness, demonstrated the lower average flank wear rate during all the wear stages, leading to a longest cutting life (2636 m). The results indicated that the average flank wear rate during the initial wear stage was mainly influenced by toughness, while during the middle and final wear stages, the average flank wear rate was primarily dependent on hardness.

The (W,Mo,Ta,Nb,Ti)(C,N) high-entropy carbonitride powders were synthesized by the carbothermal reduction nitridation method, utilizing equimolar proportions of either graphite or carbon black. The subsequent utiliza-tion of these pre-synthesized powders yielded carbonitride ceramics after spark plasma sintering. The choice of carbon source played an important role in the microstructure, phase compositions and mechanical properties. By employing carbon black with its fine particle size and high reactivity, the synthesized powder maintained finer size, narrow powder size distribution (350 +/- 12 nm) and have less free-carbon (-0.032 wt%), which contributed to the high hardness (26.8 +/- 1.4 GPa) of the sintered ceramic. Conversely, utilizing graphite with its larger particle size and lower reactivity would increase the fraction of free-carbon, leading to the deterioration of the ceramic's performance.

Cutting performances of silicon nitride (Si3N4) ceramic cutting tools with and without boride additive (2.5 vol% ZrB2 or TiB2) prepared by hot-pressing at 1500 degrees C were investigated. Due to the alpha- to beta-Si3N4 phase transformation and low densification temperature, boride-containing Si3N4 ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si3N4-2.5 vol% TiB2 ceramic (similar to 2480 m) and Si3N4-2.5 vol% ZrB2 ceramic (similar to 2200 m) were higher than the monolithic Si3N4 ceramic (similar to 1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si3N4-based inserts were improved by adding 2.5 vol% ZrB2 or TiB2. The improved cutting performance indicated that the boride-containing Si3N4 ceramics are expected to be used in the field of ceramic cutting tools.

将平均粒径分别为150μm和50μm的SiC粉体与平均粒径分别为150μm和50μm的石墨粉体混合,得到粗粉末床(150μm SiC+150μm石墨)与细粉末床(50μm SiC+50μm石墨),粉末床中SiC与石墨的体积分数均为50％.采用粉末床技术对圆柱形Si3N4陶瓷预烧体进行放电等离子烧结,研究粗、细粉末床、预烧温度(1400℃和1500℃)和粉末床回收对等离子烧结Si3N4陶瓷圆柱体的变形程度、致密度、物相组成和显微结构的影响.结果表明,采用粗粒径粉末床、1500℃预烧温度和回收使用一次的粉末床制备的Si3N4陶瓷圆柱体,横截面形状保持度最高,达到93％,维氏硬度和断裂韧性分别为(18.73±0.24)GPa和(3.64±0.23)MPa·m1/2.Si3N4陶瓷的主相为α-Si3N4,晶粒形貌为等轴状.通过引入粉末床可克服放电等离子烧结制备Si3N4陶瓷制品的形状限制,有望实现高性能异形Si3N4陶瓷的制备.

In this study, a novel high-entropy carbide-based ceramic cutting tool was developed. The cutting performance of three kinds of high-entropy carbide-based ceramic tools with different mechanical properties for the ISO C45E4 steel were evaluated. Although the pure (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C-0.8 ceramic cutting tool exhibited the highest hardness of 25.06 +/- 0.32 GPa, the cutting performance was poor due to the chipping and catastrophic failure caused by the low toughness (2.25 +/- 0.27 MPa m(1/2)). The (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C-0.8-15 vol% cobalt cutting tool with highest fracture toughness (6.37 +/- 0.24 MPa m(1/2)) and lowest hardness (17.29 +/- 0.79 GPa) showed the medium cutting performance due to the low wear resistance caused by the low hardness. The (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C-0.8-7.7 vol% cobalt cutting tool showed the longest effective cutting life of similar to 67 min due to the high wear resistance and chipping resistance caused by the high hardness (21.05 +/- 0.72 GPa), high toughness (5.35 +/- 0.51 MPa m(1/2)), and fine grain size (0.60 +/- 0.15 mu m). The wear mechanisms of the cobalt-containing (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C-0.8 ceramic cutting tools included adhesive wear and abrasive wear and oxidative wear. This research indicated that the high-entropy carbide-based ceramics with high hardness and high toughness have potential use in the field of cutting tool application.

In this study, we investigated the cutting performance and wear mechanisms of Ti(C,N)-based cutting tools containing varying weight percentages (0%, 5%, 10%, and 15%) of high-entropy carbide (HEC) (V0.2Nb0.2 Mo0.2Ta0.2W0.2)C phase, when used for turning nodular cast iron. According to the turning test results, the cermet cutting tools containing 0 wt%, 5 wt%, 10 wt%, and 15 wt% HEC phases demonstrated effective cutting lives of 402, 720, 632, and 465 s, respectively. The tool with 5 wt% HEC phase showed the best cutting performance. When cutting nodular cast iron with cermet cutting tools, the main wear mechanisms observed were diffusion, oxidation, adhesion, and abrasion on the flank surface, along with diffusion, oxidation, and abrasion on the rake surface. The results of this study indicated that (V0.2Nb0.2Mo0.2Ta0.2W0.2)C could be adopted as an effective reinforced phase in the cermet cutting tools.

Using pre-synthesized high-entropy (Ta0.2W0.2Nb0.2Mo0.2V0.2)C carbide as the reinforcing phase, Ti(C0.7N0.3)based cermets were prepared by pressureless sintering at 1600 degrees C. The results revealed that due to the solid solution reaction between the mono-carbide and (Ta0.2W0.2Nb0.2Mo0.2V0.2)C, only one set of face-centered-cubic diffraction peaks in XRD was detected in the as-sintered cermets, alongside the typical core-rim structure. Compared to the Ti(C0.7N0.3)-based cermets without high-entropy reinforcing phase, the Vickers hardness was increased from 17.06 +/- 0.09 GPa to 18.42 +/- 0.33 GPa and the fracture toughness was increased from 9.21 +/- 0.31 MPa m1/2 to 12.56 +/- 0.23 MPa m1/2 by adding 10 wt% (Ta0.2W0.2Nb0.2Mo0.2V0.2)C. The wear resistance of the cermet was enhanced significantly with increasing (Ta0.2W0.2Nb0.2Mo0.2V0.2)C content. This work provided a potential that the high-entropy carbide can be applied as an effective reinforcing phase in the preparation of high-performance Ti(C0.7N0.3)-based cermets.

Topological superconductors have drawn significant interest from the scientific community due to the accompanying Majorana fermions. Here, the discovery of electronic structure and superconductivity (SC) in high-entropy ceramics Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 Cx (x = 1 and 0.8) combined with experiments and first-principles calculations is reported. The Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 Cx high-entropy ceramics show bulk type-II SC with Tc ≈ 4.00 K (x = 1) and 2.65 K (x = 0.8), respectively. The specific heat jump (∆C/γTc ) is equal to 1.45 (x = 1) and 1.52 (x = 0.8), close to the expected value of 1.43 for the BCS superconductor in the weak coupling limit. The high-pressure resistance measurements show a robust SC against high physical pressure in Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 C, with a slight Tc variation of 0.3 K within 82.5 GPa. Furthermore, the first-principles calculations indicate that the Dirac-like point exists in the electronic band structures of Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 C, which is potentially a topological superconductor. The Dirac-like point is mainly contributed by the d orbitals of transition metals M and the p orbitals of C. The high-entropy ceramics provide an excellent platform for the fabrication of novel quantum devices, and the study may spark significant future physics investigations in this intriguing material.

High hardness and high toughness (Ti,Nb,Ta,Mo,W)(C,N)-based ceramics were prepared using pre-synthesized high-entropy carbonitride (HECN) powder and high-entropy alloy (HEA) via spark plasma sintering. By HEA addition of 5 vol%, dense (Ti,Nb,Ta,Mo,W)(C,N)-based ceramics with fine microstructure was obtained at 1450 degrees C. Due to the presence of the precipitated (Mo,W)2C phase within the HECN matrix, which forms a solid solution with the added HEA, the ceramics exhibited the high Vickers hardness (24.5 +/- 1.3 GPa) and fracture toughness (6.1 +/- 0.4 MPa m1/2), arise from the fine grain structure of the ceramics and the pronounced effect of crack deflection. This study introduced a new strategy for the preparation of high-entropy carbonitride-based ceramics, which significantly improved the fracture toughness while maintained high hardness.

High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Material design and property tailoring possibilities emerge from this new class of materials. Here, we report the discovery of superconductivity around 2.35 K and topological properties in the (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C high-entropy carbide ceramic (HECC), which has not been observed before in any of the investigated HECC. Density functional theory calculations showed that six type-II Dirac points exist in (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C, which mainly contributed from the t2g orbitals of transition metals and the p orbitals of C. Due to the stability of the structure, we also observed robust superconductivity under pressure in this HEC superconductor. This study expands the physical properties of HECs, which may become a new material platform for superconductivity research, especially for studying the coupling between superconductivity and topological physics.

以Y2O3为烧结助剂,采用放电等离子烧结技术制备了以MoSi2为第二相的α-Sialon陶瓷,研究了MoSi2添加量(0～10％,质量分数)对陶瓷微观结构和性能的影响.结果表明:添加MoSi2后,陶瓷中α-Sialon晶粒从等轴状变为长棒状,且随着MoSi2添加量的增多,长棒状α-Sialon晶粒显著增多,长径比增大,当MoSi2质量分数为10％时,晶粒尺寸呈现显著的双峰分布;当MoSi2质量分数从0增加到10％时,陶瓷的相对密度由99.0％增加到99.7％,硬度由21.12 GPa降低到20.44 GPa,断裂韧度由4.80 MPa·m1/2增加到6.13 MPa·m1/2;在干切削镍基高温合金时,添加质量分数10％MoSi2的陶瓷刀具在达到磨损标准时的切削长度是未添加MoSi2陶瓷刀具的1.5倍,可见该刀具切削性能优异,其主要磨损形式为后刀面磨损和沟槽磨损,主要磨损机理为黏着磨损和磨粒磨损.

Silicon nitride (Si3N4) ceramics have a wide potential application in industrial processes, but high fracture toughness and hardness are always difficult to be handled, thus affecting the application of Si3N4 ceramics. In this paper, silicon nitride ceramics were fabricated via spark plasma sintering at 1600 ℃ with high-entropy boride phase (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 as sintering aids. The effect of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 on the phase assemblage, densification, microstructure and mechanical properties was investigated. Compared with (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2-free Si3N4 ceramic, the addition of only 1.0% (in volume fraction) (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 in the ceramic can increase the mass fraction of β-Si3N4 from 38% to 53% at 1600 ℃ , showing a bimodal microstructure. As a consequence, the fracture toughness of Si3N4-based ceramic increases from (5.4±0.3) MPa·m1/2 to (6.9±0.2) MPa·m1/2, and the hardness maintains (20.2±0.2) GPa. The relative density of Si3N4-based ceramics decreases and the phase transformation enhances with increasing the high-entropy boride fraction to 2.5% and 5.0%, thus leading to a decrease in the hardness.

Low-temperature hot pressed Si3N4 ceramics show a greater Vickers hardness and a lower fracture toughness, while high-temperature hot pressed Si3N4 ceramics show a lower Vickers hardness and a higher fracture toughness. To prepare Si3N4 ceramics with a great Vickers hardness and a high fracture toughness, a low-temperature hot pressed Si3N4 ceramic was prepared via sintering at 1500 ℃ with 20% SiC whisker (SiCw, in volume fraction) and 2.5% ZrB2. The phase composition, relative density, microstructure and mechanical properties of low-temperature hot pressed Si3N4 ceramic were investigated, compared with high-temperature hot pressed Si3N4 ceramics at 1800 ℃ . The results show that the addition of SiCw inhibits the densification of Si3N4 ceramics sintered at 1500 ℃ , and the relative density decreases from 97.9% to 92.9%, the Vickers hardness decreases from 20.5 GPa to 16.4 GPa, and the fracture toughness increases from 2.9 MPa·m1/2 to 3.4 MPa·m1/2. However, the addition of SiCw and ZrB2 promotes the phase transition from α-Si3N4 to β-Si3N4. The relative density of Si3N4–SiCw–ZrB2 reaches 97.5%, and the Vickers hardness and fracture toughness are 20.7 GPa and 5.1 MPa·m1/2, respectively. Compared with low-temperature hot pressed Si3N4 ceramic, the fracture toughness of Si3N4–SiCw–ZrB2 ceramic is improved without reducing the Vickers hardness. Compared with high-temperature hot pressed Si3N4 ceramic, the hardness is greatly improved. The Si3N4–based ceramic with high hardness and toughness prepared via a low-temperature hot pressing with SiCw and ZrB2 can have a promising application in ceramic structural parts.

通过气压烧结制备添加质量分数5％TiO2的Si3N4陶瓷并制成刀具,研究了TiO2对其显微组织、力学性能和切削性能的影响,并与未添加TiO2烧结Si3 N4陶瓷作对比.结果表明:添加TiO2烧结Si3 N4陶瓷主要由长棒状与等轴状的β-Si3 N4晶粒组成,并伴有均匀分布的TiN相,与未添加TiO2烧结Si3 N4陶瓷相比,晶粒得到细化,硬度上升而断裂韧度略有下降;在连续切削灰铸铁过程中,添加TiO2的Si3N4陶瓷刀具具有更长的切削寿命(有效切削长度为2410 m),并且保持了刃口的完整性,切削后黏着磨损碎片较小.

In order to prepare high toughness (Ti,Zr,Nb,Ta,Mo)C ceramics at low temperatures while maintaining high hardness, a liquid-phase sintering process combined with Co-based liquid-phase extrusion strategy was adopted in this study. The densification temperature can be lowered to 1350 °C, which is much lower than the solid-state sintering temperature (∼2000 °C) generally employed for high-entropy carbide ceramics. When sintered at 1550 °C and 30 MPa applied pressure, part of the Co-based liquid-phase was squeezed out of the graphite mold, such that only ∼3.21 vol% of Co remained in the high-entropy ceramic. Compared to the Co-free solid-state sintered (Ti,Zr,Nb,Ta,Mo)C ceramics, prepared at 2000 °C and 35 MPa, the hardness was slightly decreased from 25.06±0.32 to 24.11±0.75 GPa, but the toughness was increased from 2.25±0.22 to 4.07±0.13 MPa·m 1/2 . This work provides a new strategy for low-temperature densification of high-entropy carbides with both high hardness and high toughness.

High-entropy(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )C ceramics, with different contents（0, 5, 10, and 20 vol.%） of Si C whiskers（SiC w ）, were fabricated by spark plasma sintering using raw powders synthesized via carbothermal reduction. The application of a uniaxial compaction force led to texture development of the SiC w within the(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )C matrix. Fracture toughness increased with the increase in SiC w content, while Vickers hardness remains almost unchanged. The toughness of(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )C-20 vol.% SiC w ceramics reached 4.3 ± 0.3 MPa m 1/2 , which was approximately 43% higher than that of the monolithic(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )C ceramic(3.0 ± 0.2 MPa m 1/2 ). The main toughening mechanisms were attributed to crack deflection, whisker debonding, and whisker pullout.

High toughness and high hardness Si3N4 ceramics with fine and bimodal microstructure was prepared by combination of HfB2 additive and spark plasma sintering (SPS). HfB2-free SPSed Si3N4 ceramics at 1600 degrees C exhibited major alpha-Si3N4 phase, a fine and equiaxed grain microstructure, high Vickers hardness (21.3 +/- 0.2 GPa), and low fracture toughness (3.7 +/- 0.3 MPa m1/2). The introduction of 2.5 vol% HfB2 into Si3N4 ceramics at 1600 degrees C promoted alpha-to beta-Si3N4 phase transformation, produced a fine and bimodal microstructure, significantly improved the toughness (7.7 +/- 0.6 MPa m(1/2)) but slightly decreased the hardness (20.4 +/- 0.6 GPa) which was higher than that of traditionally hot-pressed or pressurless sintered Si3N4 ceramics (-14-18 GPa).

High-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics, with different carbon contents (x=0.55−1), were prepared by spark plasma sintering using powders synthesized via a carbothermal reduction approach. Single-phase, high-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics could be obtained when using a carbon content of x=0.70−0.85. Combined ZrO2 and Mo-rich carbide phases, or residual graphite, existed in the ceramics due to either a carbon deficiency or excess at x=0.55 and 1, respectively. With the carbon content increased from x=0.70 to x=0.85, the grain size decreased from 4.36 ± 1.55 μm to 2.00 ± 0.91 μm, while the hardness and toughness increased from 23.72 ± 0.26 GPa and 1.69 ± 0.21 MPa·m1/2 to 25.45 ± 0.59 GPa and 2.37 ± 0.17 MPa·m1/2, respectively. This study showed that the microstructure and mechanical properties of high-entropy carbide ceramics could be adjusted by the carbon content. High carbon content is conducive to improving hardness and toughness, as well as reducing grain size.