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The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43 GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of > 9 μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294 N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances. [ABSTRACT FROM AUTHOR]

Polycrystalline cubic boron nitride (PcBN) compacts with high hardness, high thermal stability and high cutting performance have been prepared through method of high pressure infiltration of silicon melt. The scanning electron microscopy (SEM) and metallographic observations indicates that cBN–cBN bonding appeared extensively in the sintered samples. The hardness tests give an average Vickers hardness of 43 GPa for the well-sintered cBN–Si compacts, which is much higher than the conventional PcBN composites (~ 30 GPa). The onset temperature of oxidation can reach to 1300 °C, about 300 °C than that of the commercial PcBN (~ 1000 °C). According to the results of turning test, the cutting performance of our sintered samples is also superior to the commercial PcBN. [ABSTRACT FROM AUTHOR]

Abstract: Well-crystallized graphite and high energy ball-milled graphite were treated under 16GPa and temperatures up to 2500°C. Our experiments indicated that the ball-milled graphite can completely transform to nano-polycrystalline diamond (NPD) at 2100°C, while the well-crystallized graphite required 2500°C. The samples were characterized by X-ray diffraction, optical microscope, scanning electron microscope, and Vickers indenter hardness test. We found that high pressure not only made the conversion of graphite to NPD thermodynamically preferred, but also reduced the needed activation energy barrier. By analyzing thermodynamics and kinetics of the transformation, we suggested a low boundary for NPD forming region based on the phase diagram of carbon, and the formation mechanisms of Nano-polycrystalline diamond. [Copyright &y& Elsevier]

Abstract: The Vickers hardness of pressure synthesized WB compacts was measured using a diamond indentation method. Under the applied load of 4.9N, our test gave a maximum Vickers hardness of 30.1GPa for WB compacts. The hardness of WC-8wt.% Co composites was also measured under the same load for comparison, which was found to be much lower than that of WB. The Young''s modulus, bulk modulus, and shear modulus of WB and WC-8wt.% Co were further investigated by ultrasonic measurements. The results suggest that WB might be a promising hard alloy material for industrial applications. [Copyright &y& Elsevier]

Abstract: Internal residual metal phases (mainly cobalt) were effectively removed from polycrystalline diamond compacts (PDCs) by electrolysis to improve their high temperature wear resistance. Through turning granite (dry cutting), we have checked the wear resistance of PDCs with different residual metal removal depth (RMRD). The relationship between the measured wear rate and RMRD was obtained, and the results showed that PDCs treated by electrolysis have a significant improvement in wear resistance. X-ray diffraction (XRD) and scanning electron microscopy (SEM) observation of PDCs'' wear surface indicated that diamond–graphite phase transformation occurs for the samples with residual metal phase. The wear mechanism is discussed, and our study suggests that the graphitization is the main wear process of PDCs with internal residual metal phases. [Copyright &y& Elsevier]

Mechanical-performance of polycrystalline cubic boron nitride (PcBN) sintered using micron-sized powder particles might exceed that of samples produced from submicron crystals. However, no effective method has been found yet. In this study, commercial cubic boron nitride (cBN) powder (∼5 μm) was used to sinter high-performance PcBN without a binder. Two processes, isothermal compression (IC) and isothermal isobaric sintering (IIS), were used and compared. PcBN sintered with the former method possesses much more excellent mechanical performance, with the average Vickers hardness and indentation crack resistance of 73 GPa and 8.4 MPa·m1/2. The new temperature and pressure path adopted in the IC process can largely improve the hardness of PcBN sintered even with micro-sized powder. The quantitative analysis results of the defect structure in samples can be used as a reference for controlling the defect structure density to improve the material performance. • High hardness polycrystalline cubic boron nitride was manufactured. • New temperature and pressure path for preparing high performance superhard materials. • Positive effect of defect density on sample hardness is quantified. [ABSTRACT FROM AUTHOR]

Exploring the factors affecting the wear resistance of superhard materials is interesting and has practical industrial applications. Characterizing the hardness and grain size of superhard materials is more convenient and simple than measuring their wear resistance. Therefore, we expect to obtain a quantitative relationship between the hardness and grain size of superhard materials and their wear resistance. In this work, a series of polycrystalline cubic boron nitride (PcBN) compacts with different grain sizes and hardnesses were sintered under high temperature and pressure. Furthermore, turning tools were manufactured using these PcBN samples. The edge wear of the turning tool during the turning of granite was examined. Based on aforementioned measurements, the relationship between hardness and wear resistance is examined with same grain size (1–2 μm) and the influence of PcBN grain size on wear resistance is investigated under the same hardness (44.0 ± 1.2 GPa). The results show that the wear resistance of PcBN exhibits a regression parabolic and linearly increasing trends with the increase in hardness and grain size, respectively. Further investigation reveals that the compactness of PcBN materials, the bonding strength between the crystal grains, and the depth of the embedded crystal grains are critical factors affecting the wear resistance. These works can serve as a practical guide for the preparation as well as research and development (R&D) of PcBN and are expected to be extended to the R&D and application of other superhard materials and hard ceramics. [ABSTRACT FROM AUTHOR]

Multicomponent alloys prepared using different experimental methods under atmospheric pressure have been widely studied. However, research on multicomponent alloys involving methods using high pressures and high temperatures is still in its infancy. In the high-pressure CoCrFeNiMo x alloy, the combination of each element atom and the change in the sample crystal structure indicate a relationship between the evolution mechanism of the multi-element alloy microstructure and high pressure. We observe that molybdenum (Mo) atoms enter the medium-entropy alloy (MEA) lattice near its theoretical recrystallization temperature to form a high-entropy alloy (HEA). The diffusion of Mo atoms requires the energy provided by the temperature to overcome the low-entropy effect under high pressure, Mo atom reacts with MEA near its recrystallization temperature to form HEA means the formation of HEA under high pressure follows the specific steps. High pressure inhibits grain growth and generates numerous dislocations in the crystal, thereby preparing the high-pressure CoCrFeNiMo x alloy with a high hardness of 3.7 ± 0.12 GPa, which is harder than the CoCrFeNiMo x alloy prepared under atmospheric pressure. Additionally, the generalized high-pressure HEA constant is introduced, which is 1 ≥ P ≥ 0.17, proving that high-pressure promotes the synthesis of HEAs. This constant provides a new method for studying the thermodynamic behavior of HEAs under extreme conditions. • High-entropy allys are synthesized under extreme condition. • The formation of high-entropy allys follow the fixed step. • A generalized high-pressure high-entropy alloys constant is proposed. [ABSTRACT FROM AUTHOR]

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC 0.5 N 0.5 –15WC–10Mo 2 C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm 3 . Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m 1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we present a novel method to sinter tungsten bulk material under high pressure without any additives. The sintering temperatures were greatly reduced in our experiments. The sintered samples can achieve near-full densification (relative density of 99.5%), and have fracture toughness of about 50% higher than that from the traditional methods, without sacrificing hardness. Moreover, the grain sizes of the sintered samples maintained in the initio range. The sintering kinetics, microstructure evolutions, and relationships of mechanical properties versus pressure–temperature conditions were also discussed. [Copyright &y& Elsevier]

As a highly controversial candidate superhard material, tungsten tetraboride (WB 4) has attracted widespread attention due to its desirable mechanical properties. Here, single phase WB 4 ceramics and WB 4 -based materials containing tantalum (Ta) have been successfully synthesized by a high-pressure reaction at 5.0 GPa and 1400 °C - 3000 °C. We systematically investigated the effect of processing temperature on the phase stability of WB 4 and doping with different amounts of Ta on mechanical properties and thermal stability. The results confirm that increasing treatment temperature is beneficial to the synthesis of the WB 4 phase, and high pressure can effectively inhibit the decomposition of WB 4. Additionally, the hardness value of the WB 4 -based ceramic is found to decrease with the increase of the doped tantalum content. However, the hardness of WB 4 is less than W 0.9 Ta 0.1 B 4 (10 at. % Ta), indicating that a small amount of Ta doping is positive for the mechanical properties of WB 4. The results show that the W 0.9 Ta 0.1 B 4 ceramic synthesized at a temperature of 2800 °C and 5.0 GPa has excellent overall properties. Its Vickers hardness is 42 GPa (0.49 N), and its oxidation resistance temperature is 542 °C, which is much higher than the hardness and thermal stability of tungsten carbide (WC). This work can provide practical guide for the design of new transition metal boride superhard materials, and point out the visual direction for exploring the next generation of superhard materials. • We synthesized single phase WB 4 ceramics and WB 4 -based materials containing tantalum at 5.0 GPa and 1400 °C–3000 °C. • The study found that the decomposition temperature of WB 4 is increased to 3000 °C at 5.0 GPa. • When the amount of Ta added is 10 at. %, the hardness reaches 42 GPa and the thermal stability reaches 542 °C. [ABSTRACT FROM AUTHOR]