Journal of Superhard Materials最新文献

The aluminum–boron–nitrogen phase diagram is calculated at a pressure from 0.1 MPa to 9 GPa by the models of phenomenological thermodynamics in the Thermo-Calc software package with interaction parameters from literature data and equation of states for competing phases. The comparison of our calculation results with the experimental studies of the phase composition of polycrystalline boron nitride samples sintered ay high pressures and temperatures shows their agreement.

Abstract—For structurally homotypical specimens different in the ZrO₂ content from the (94WC–6Co) + ZrO₂ matrix material used in diamond-containing С_diamond–(WC–Co) composites formed by spark plasma sintering, the dependences of the relative density ρ_rel, the ultimate strength under compression R_cm and bending R_bm, the microhardness Н_V, and the fracture toughness K_Iс on the zirconia content have been established. The addition of 6 wt % zirconia to the WC–6Co composite leads to an increase in the relative density from 0.948 to 0.990, the ultimate compression strength R_cm from 4950 ± 110 to 5600 ± 120 MPa, the ultimate bending strength R_bm from 1935 ± 80 to 2660 ± 115 MPa, and the fracture toughness K_Iс from 13.8 ± 0.71 to 16.9 ± 0.76 MPa m^0.5 at a slight decrease in hardness (from 15.9 ± 0.72 to 15.1 ± 0.33 GPa). Such values are caused by the decrease of main WC phase grains in size with tetragonal t-ZrO₂ phase transformation and, correspondingly, by the growing role of transformation strengthening mechanism and the active action of inner mechanical compressive microstresses. When the ZrO₂ additive to the WC–6Co composite is increased to 10%, the parameters ρ_rel, R_cm, R_bm, and K_Iс are gradually decreased. At the same time, the material at the indentor imprint edge begins to destruct, and crack propagate in a chaotic way. It has been revealed that the properties ρ_rel, R_cm, R_bm, and K_Ic are worsened at a zirconia nanopowder content above 6 wt % in the WC–Co composite due to the formation of agglomerates during the mixing of components, their separation under sintering, and the formation of micropores and microcracks.

The diamond/metallic film interface has been studied using transmission electron microscopy and electron backscatter diffraction. This interface is significant in the graphite/diamond transition procedure, where a metallic film from the Fe–Ni–C system covers the diamond during its growth at high pressure and high temperature. It is reveals that the metallic film interface consists of γ-(Fe,Ni) and orthorhombic Fe₃C, with γ-(Fe,Ni) present in tetragonal shapes whose exposed surfaces are likely to be (001) crystal surfaces. Furthermore, the valence electron structures of Fe₃C, γ-(Fe,Ni), and diamond were calculated, and the relative electron density differences of diamond growth interfaces were analyzed using the empirical electron theory of solid and molecules. It is found that the relative electron density differences of Fe₃C/diamond interfaces are continuous at the first order of approximation, indicating that the carbon atoms decomposing from Fe₃C can be transformed into diamond structure. Additionally, the relative electron density differences of γ-(Fe,Ni)/Fe₃C interfaces were found to be continuous. Therefore, it is suggested that carbon atoms for diamond growth may come from the decomposition of Fe₃C, while γ-(Fe,Ni) serves as a catalytic phase to promote the decomposition of Fe₃C.

A new method of coating diamond particles was proposed in this paper, that is, using the high temperature generated by the thermal explosion reaction to induce the sublimation of volatile substances in the raw material and then deposit them on the surface of diamond particles to form one coating. The thermal explosion reaction of Mo/Al/B₂O₃ system was selected as an example. Results showed the high temperature generated by the thermal explosion reaction may induce a certain amount of Al volatilization. Al was deposited on the surface of diamond particles and reacted with trace oxygen in the environment to form Al₂O₃. Thus, Al–Al₂O₃ composite was coated on the surface of diamond particles.

For cast tungsten carbide, the conditions of preparation in an arc furnace with a consumable electrode under different pressures (up to 9 MPa) in the working chamber have been studied. It has been shown that tungsten carbide decomposes under melting to form a product composed of WC and W₂C phases and free carbon. The results of studying the integral microhardness, grain crush strength, wear resistance, and abrasivity, which grow for carbides synthesized at a higher gas pressure, are presented. It has been shown that the composition of the product and, therefore, its properties can be controlled by varying the tungsten carbide melting conditions. Based on the behavior of refractory compounds at supermelting temperatures, an explanation is given to the obtained results, and a mechanism is proposed for the formation of the structure and properties of tungsten monocarbide melted in a low-temperature plasma.

The investigation into the mechanism of polymethylmethacrylate (PMMA) polishing using dispersed systems of micro- and nanoparticles of polishing powders revealed that the formation of slurry particles in the processed material is driven by Förster resonance energy transfer between the energy levels of the polishing powder particles and the processed material. This transfer occurs within the open microcavity created by the surfaces of the processed material and the polishing powder particles, in a multimode regime. The material removal rate during PMMA polishing is determined by the combined coefficients of volumetric wear and the total lifetime of the excited states of clusters on the processed surface, along with the resulting quality factor of the resonator at all allowed frequencies of the discrete spectrum. The results of the theoretical calculation of the material removal rate during PMMA polishing are in good agreement with the data from the experimental determination of polishing performance, with a deviation of 1–3%.

Drawing from our research and an extensive review of publications, we have compiled data from goniometric studies concerning the morphology of mineral inclusions in diamonds sourced from both kimberlites and placers. Specifically, we have examined the morphology of olivine, garnet, and chrome-spinelide inclusions. In the majority of instances, the crystalline shape of these mineral inclusions has been instrumentally confirmed to mirror that of diamonds. The formation of such shapes is attributed to the influence of the negative morphology of diamond crystals, the host mineral, on mineral inclusions. The shape of these inclusions reflects various features of diamond morphology, encompassing simple forms, habits, outlines, and surface topography of faces. Compelling evidence supporting the syngenesis of mineral inclusions with diamond-shaped crystals and the diamonds within which they are found lies in indicators of their simultaneous growth. These indicators include the presence of induction pseudo-edges, pseudo-faces, and pseudo-vertices on inclusion crystals.

The results of studying the thermal conductivity of hot-pressed AlB₁₂–AlN ceramic composites with different AlN concentrations were presented. The thermal conductivity coefficient was measured for composite specimens at room temperature and approximated for AlB₁₂.

In the framework of a crystallochemical approach, new hexagonal (P6₃/mc) sp³-bonded BN polytypes (4H, 6H and 8H) and ternary BC₂N were proposed by rationalized substitutions of C for B and N in hexagonal carbon allotrope C₈ (4C carbon) with cfc topology, and density functional theory calculations of their ground states were performed. All new phases were found to be cohesive and stable mechanically (elastic constants) and dynamically (phonon band structures). According to modern models of hardness, the new phases were recognized as superhard with Vickers hardness above 50 GPa. Their electronic band structures exhibit insulating behavior with large band gaps.

Research on grinding high-speed steel with cubic boron nitride wheels has revealed challenges in evaluating the energy efficiency of grinding with diamond abrasive wheels made of superhard materials (SHMs) for hard-to-machine tool materials. These challenges arise due to the specific energy consumption index of the grinding process, which determines the ratio of effective grinding power. In addition to considering specific energy consumption and the energy efficiency coefficient of the process corresponding to the processing process, it is imperative to account for the wear of diamond abrasive wheels through the index of relative consumption of SHM grains in the working layer of the wheel during grinding. A novel relationship for calculating the energy efficiency coefficient (EEC) for diamond abrasive processing with SHM wheels has been proposed. It has been demonstrated that reducing the temperature in the grinding zone enhances the energy EEC. To achieve this temperature reduction, it is advisable to avoid metallic coating on the grains of SHMs and instead utilize an increased concentration of SHM grains in the working layer of the wheel. This adjustment results in an augmentation of the energy EEC, as elucidated by the proposed equation for calculating the EEC.