Superhard Research Articles

(Invited) Pressure-Induced New Carbon Materials with New Properties

Design and synthesis of new carbon materials have been attracting intensive attention due to their application in various fields. The study of carbon nanostructures under high pressure provides us a very effective method to create new carbon material which is hardly discovered at ambient conditions because of the appearance of nanometer scale size effect and the novel high pressure behavior. In this presentation, several examples on high pressure induced novel structures in recently studied fullerene based molecular cocrystals will be outlined. Particularly, anomalous negative volume compressibility observed in the eutectic Sc3N@C80 -cubane crystal under high pressure will be introduced. The charge transfer from fullerene cage to cubane promotes the configuration change of cubane, which transforms into a “low density” configuration, resulting in an abnormal lattice expansion of the crystal. This is the first experimentally discovered crystal which has negative volume compressibility. We will also present the synthesis of nearly all sp3 amorphous carbon bulk material from high temperature and high pressure (HPHT)-treated C60 for the first time. This new sp3 amorphous carbon material shows an optical band gap of ~2.7eV and extremely high Vickers hardness of 102 GPa under 9.8N load, close to that of diamond, which is a multifunctional superhard amorphous carbon material with the highest thermal conductivity and hardness known in the amorphous materials up to now.

Theoretical Assessment of the Role of Bond Material during Grinding of Superhard Materials with Diamond Wheels

The grinding of superhard materials poses an important challenge to manufacturing industry, due to the increased wear and the high possibility of fracture of both the wheel and workpiece material. Various strategies have been proposed for effective grinding of these materials, but further research is still required, especially in cases in which the hardness of the wheel and the workpiece are almost equal. In this study, the role of the bond of a diamond grinding wheel during the processing of superhard materials, such as synthetic diamond, is investigated using theoretical models and FE simulation. Six different types of bonds are studied and the effect of their properties on the stress distribution of workpiece material is determined. Results indicate that even a slight increase in elastic modulus can affect considerably the stress state of the workpiece, directly affecting the critical embedding value of grains into the bond, something that can alter considerably the efficiency of grinding superhard materials. Thus, grinding wheels with bonds of high elastic modulus should be selected in order to increase grain retention, increase processing efficiency, and reduce specific consumption even at higher cross feeds and wheel speeds.

A triatomic carbon and derived pentacarbides with superstrong mechanical properties

SummaryDiamond has the largest hardness of any natural material with an experimental Vickers hardness value of 90–150 GPa. Here, we reported the stable triatomic carbon allotrope with giant hardness closing that of diamond and a family of pentacarbides with superstrong mechanical properties from the state-of-the-art theoretical calculations. The triatomic carbon allotrope can be transformed into a two-dimensional carbon monolayer at a high temperature. We predicted that the triatomic carbon allotrope holds a hardness of 113.3 GPa, showing the potential capability of cracking diamond. Substitution with Al, Fe, Ir, Os, B, N, Si, W, and O element resulted in strong pentacarbides with Young’s modulus of 400–800 GPa. SiC5, BC5, IrC5, and WC5 are superhard materials with Vickers hardness over 40 GPa, of which BC5 was successfully synthesized in previous experimental reports. Our results demonstrated the potential of the present strong triatomic carbon and pentacarbides as future high-performance materials.

Preparation of superhard nanometer material cBN reinforced Ni-W-P nanocomposite coating and investigation of its mechanical and anti-corrosion properties

In this work, superhard nanoparticle cubic boron nitride (cBN) was introduced into Ni-W-P bath, and Ni-W-P/cBN composite coatings with different cBN concentrations were successfully prepared by pulse electrodeposition technique. In addition, the effects of different cBN dosages on the wear resistance and corrosion resistance of Ni-W-P/cBN composite coatings were also studied. Vickers microhardness tester and multifunctional material surface testing machine was used to evaluate the hardness and wear resistance of composite coatings. The corrosion resistance properties of the coatings were analyzed and characterized by EIS and potentiodynamic polarization. The results show that when the cBN content in the bath is 2 g/L, the comprehensive properties of the Ni-W-P/cBN composite coating are improved most significantly. Benefiting from the mechanical enhancement effect of cBN, the microhardness of the Ni-W-P/cBN (2 g/L) composite coating is increased to 543.0 HV, and the friction coefficient is reduced to 0.39 (the original Ni-W-P coating has a lower Microhardness 343.4 HV and higher friction coefficient 0.68). Furthermore, it is found that the corrosion resistance of the composite coating is greatly improved by electrochemical technology. The corrosion potential of Ni-W-P/cBN (2 g/L) composite coating is − 0.46 V, corrosion current is 9.3 μA/cm2 and corrosion rate is 0.11 mm/ year. (The corrosion potential of Ni-W-P coating is −0.53 V, corrosion current is 30.4 μA/cm2 and corrosion rate is 0.36 mm/ year).

Prospects for the development of deep-feed grinding of gas turbine engine parts using highly porous cubic boron nitride wheels

The technologies of deep grinding of gas turbine engine parts are analyzed. The features of a grinding tool made of traditional abrasives and superhard materials are revealed. Ways to improve the effectiveness of this treatment using highly porous cubic boron nitride wheels are proposed. Keywords; deep-feed grinding, gas turbine engine, multi-coordinate grinding center, nickel heat-resistant alloy, highly porous cubic boron nitride wheel. d_i_volkov@rsatu.ru, boris_tsvetkov@mail.ru

Mechanical properties of pyrophyllite under the coupling of high temperature-pressure: A first-principle study

Pyrophyllite [Al2Si4O10(OH)2] is known to be the pressure transmission medium in the high temperature-pressure synthesis industry of super-hard materials, which is attributed to its exceptional support properties, heat resistance, and pressure transmission. Therefore, it is of utmost importance to characterize the mechanical behavior of pyrophyllite under the coupling action of high temperature-pressure (the combined effect of temperature-pressure is called T-P coupling in this work). In this work, the optimal structures, elastic constant, elastic modulus, and anisotropy index of pyrophyllite under T-P coupling are investigated systematically based on the first principle calculation for the first time. Several structure parameters, including lattice constants and layer spacing, indicate that the effect of T-P coupling on the structure mainly acts on internal layer spacing (dI). The elastic modulus (B: Bulk modulus, E: Young's modulus, G: Shear modulus) of pyrophyllite increase gradually with augmented pressure and decreases with the process of heating. Under T-P coupling, high pressure weakens the degree of influence caused by temperature on the elastic modulus of pyrophyllite. This phenomenon is mainly attributed to the structural compactness of pyrophyllite caused by pressure, which weakens the thermal expansion effect caused by heating. As a result, the impact of temperature on the stiffness of pyrophyllite was weakened with rising pressure. At the same time, the changes in crystal structure and atomic distribution density under different T-P conditions also change the toughness and elastic anisotropy of pyrophyllite. The 3D spatial distribution diagrams of anisotropy reflect that temperature increases the elastic mechanical anisotropy of pyrophyllite, in contrast to pressure which tends to decrease it. The sealed pressure transmission medium should not only have good mechanical and thermodynamic properties, but also have insulation. The change of electronic structure under T-P coupling shows that pyrophyllite always maintains good insulation characteristics. These results reveal new insight into the application of pyrophyllite in the synthesis of super-hard materials. And all of these studies are expected to be a guideline for others in their experimental investigations.

Investigation on orbital EDM for double-hole thin wall of integrated flexible joint

The thin neck structure of integrated flexible joint is the key factor to realize high-precision navigation in dynamically tuned gyroscope. The components of the thin neck structure include two adjacent circular holes and a thin-walled structure between the two holes. The thin wall is vulnerable to be deformed under the external loading which is generated by the tool when using traditional machining methods such as drilling and boring. Simultaneously, pronounced fabrication cost will occur as the cutting tools are also ready to be damaged for machining small holes based on superhard materials. To resolve above problems, a machining method is proposed for the structure of double-hole thin wall in the step-by-step orbital electrical discharge machining (EDM) by using a high rotation speed electrode. The procedure for EDM of double-hole flexible thin wall is designed, and the processing parameters of each step machining are optimized though orthogonal experiment and signal-to-noise ratio analysis. The machining experiments of double-hole thin wall of 3J33B material are proceed using the optimized parameters. The results show that the hole diameter of the double-hole flexible thin wall is 2 mm, the hole depth is 8 mm, and the average thickness of the thin wall is about 46.5 μm. The thickness range between the measured point and the average is 1.55 μm. Comparing with average thickness of 46.5 μm, the error is less than 3%, and the overall thickness is uniform relatively.

Monoclinic mC28 carbon: A sp2-sp3 hybridized carbon allotrope with superhard and metallic properties

• A novel carbon structure with mixed sp 2 and sp 3 bonds (named as mC 28 carbon) was theoretically proposed. • The mC 28 carbon is dynamically stable and has a lower enthalpy than graphite under ~ 29 GPa, which implied it could be a metastable phase of carbon. • It is also calculated to be a metal combined with high hardness (the calculated Vickers hardness value reached 85 GPa ) New carbon allotropes with mixed sp , sp 2 and sp 3 bonds are now the focus of theoretical and experimental researches due to their abnormal properties. We here theoretically proposed a novel three-dimensional carbon structure with mixed sp 2 and sp 3 bonds (named as mC 28 carbon) via particle swarm optimization method. mC 28 carbon is built by the ‘diamond-like’ sp 3 networks connected by sp 2 bonded carbon atoms. It has a lower enthalpy than graphite after ∼29 GPa and is dynamically stable at zero pressure. More interesting, mC 28 carbon is metallic, which can be attributed to the connected sp 2 bonding carbon atoms. They provide a channel for their extra delocalized valence electron that siting at p z orbital. mC 28 carbon is also a superhard material with theoretical Vickers hardness of 85 GPa.

Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness (Small 22/2022)

Superhard Nanocrystalline Cubic Silicon Carbide In article number 2201212, Guoying Gao, Bo Xu, and co-workers successfully synthesize nanocrystalline β-SiC bulks with asymptotic Vickers hardness over the superhard threshold of 40 GPa, broadening the family of superhard materials. The frontispiece emphasizes the superhardness of nanocrystalline SiC bulk by scratching the cBN single crystal.

Heterogeneous Diamond-cBN Composites with Superb Toughness and Hardness.

The traditional hardness-toughness tradeoff poses a substantial challenge for the development of superhard materials. Due to strong covalent bonds and intrinsic brittleness, the full advantage of microstructure engineering for enhanced mechanical properties requires further exploration in superhard materials. Here heterogeneous diamond-cBN composites were synthesized from a carefully prepared precursor (hBN microflakes uniformly wrapped by onion carbon nanoparticles) through phase transitions under high pressure and high temperature. The synthesized composites inherit the architecture of the precursors: cBN regions with an anisotropic profile that spans several micrometers laterally and several hundred nanometers in thickness are embedded in a nanograined diamond matrix with high-density nanotwins. A significantly high fracture toughness of 16.9 ± 0.8 MPa m1/2 is achieved, far beyond those of single-crystal diamond and cBN, without sacrificing hardness. A detailed TEM analysis revealed multiple toughening mechanisms closely related to the microstructure. This work sheds light on microstructure engineering in superhard materials for excellent mechanical properties.

Electron-Hole Excitation Induced Softening in Boron Carbide-Based Superhard Materials.

Photomechanical effect in semiconductors refers to a phenomenon that plastic deformation is influenced by light-induced electron-hole (e-h) excitation. To date, increasing amounts of theoretical and experimental studies have been performed to illustrate the physical origin of this phenomenon. In contrast, there has been little discussion about this effect in superhard materials. Here, we adopted constrained density functional theory simulations to assess how e-h excitation influences two boron-based superhard materials: boron carbide (B4C) and boron subphosphide (B12P2). We found that the ideal shear strengths of both systems decrease under e-h excited states. Under e-h excitation, the redistribution of electrons and holes contributes to the decreased strength, weakening the bonds initially broken under the shear deformation. The simulation results provide a fundamental explanation for the softening effects of superhard materials under e-h excitation. This study also provides a basis to tune the mechanical properties of superhard materials via light irradiation.

A novel superhard boron nitride polymorph with monoclinic symmetry

In this work, a new superhard material named Pm BN is proposed. The structural properties, stability, mechanical properties, mechanical anisotropy properties, and electronic properties of Pm BN are studied in this work. Pm BN is dynamically and mechanically stable, the relative enthalpy of Pm BN is greater than that of c-BN, and in this respect, and it is more favorable than that of T-B3N3, T-B7N7, tP24 BN, Imm2 BN, NiAs BN, and rocksalt BN. The Young’s modulus, bulk modulus, and shear modulus of Pm BN are 327 GPa, 331 GPa, and 738 GPa, respectively, and according to Chen’s model, Pm BN is a novel superhard material. Compared with its original structure, the mechanical anisotropy of Young’s modulus of Pm BN is larger than that of C14 carbon. Finally, the calculations of the electronic energy band structure show that Pm BN is a semiconductor material with not only a wide band gap but also an indirect band gap.

Superhard single‐phase (Ti,Cr)B2 ceramics

AbstractA nominally pure and dense (Ti0.9Cr0.1)B2 ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 μm and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB2 ceramic obtained under the same processing conditions, the (Ti0.9Cr0.1)B2 ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 μm), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB2 ceramics. This is the first report for dense and single‐phase (Ti,Cr)B2 ceramics as superhard materials.

First-principles study on the superconductivity of doped zirconium diborides

Recent experiments [Barbero et al. Phys. Rev. B 95, 094505 (2017)] have established that bulk superconductivity $({T}_{c}\ensuremath{\sim}8.3\text{--}8.7\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ can be induced in $\mathrm{Al}{\mathrm{B}}_{2}\text{\ensuremath{-}}\mathrm{type}\phantom{\rule{0.16em}{0ex}}\mathrm{Zr}{\mathrm{B}}_{2}$ and $\mathrm{Hf}{\mathrm{B}}_{2}$, highly covalent refractory ceramics, by vanadium (V) doping. These $\mathrm{Al}{\mathrm{B}}_{2}\text{\ensuremath{-}}\mathrm{structured}$ phases provide an alternative to earlier diamondlike or diamond-based superconducting and superhard materials. However, the underlying mechanism for doping-induced superconductivity in these materials is yet to be addressed. In this paper, we have used first-principles calculations to probe electronic structure, lattice dynamics, and electron-phonon coupling (EPC) in V-doped $\mathrm{Zr}{\mathrm{B}}_{2}$ and consequently examine the origin of the superconductivity. We find that, while doping-induced stress weakens the EPC, the concurrently induced charges strengthen it. The calculated critical transition temperature $({T}_{c})$ in electron (and V)-doped $\mathrm{Zr}{\mathrm{B}}_{2}$ is at least one order of magnitude lower than experiments, despite considering the weakest possible Coulomb repulsion between electrons in the Cooper pair, hinting a complex origin of superconductivity in it.

Effect of aluminum addition on the structure and electronic properties of boron nitride

The influence of the structure type on the boron nitride (BN) properties such as excellent thermal and chemical stability and super hardness is well known. The effect of aluminum addition on four different structure types of boron nitride (wurtzite, h-BN, sphalerite, and rock salt) and its electronic properties have been investigated in our study. Here we present an ab initio study of the B1-xAlxN solid solutions for (x ​= ​0; 0.125; 0.25; 0.375 and 0.5), and corresponding structure-property relationship. Novel structures have been discovered in new Al-rich boron nitride compounds and some of them indicate AlN-BN layer separation. Electronic properties have been studied in great detail using hybrid B3LYP approximation. Present results suggest great diversity of the electronic properties generated by adding aluminum to boron nitride and that by using these predicted modifications in various investigated AlN-BN compounds, one could obtain fine-tuning of the band gap in boron nitride electroceramics.

Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness.

Superhard materials other than diamond and cubic boron nitride have been actively pursued in the past two decades. Cubic silicon carbide, i.e., β-SiC, is a well-known hard material with typical hardness <30GPa. Although nanostructuring has been proven to be effective in enhancing materials' hardness by virtue of the Hall-Petch effect, it remains a significant challenge to improve hardness of β-SiC beyond the superhard threshold of 40GPa. Here, the fabrication of nanocrystalline β-SiC bulks is reported by sintering nanoparticles under high pressure and high temperature. These β-SiC bulks are densely sintered with average grain sizes down to 10nm depending on the sintering conditions, and the Vickers hardness increases with decreasing grain size following the Hall-Petch relation. Particularly, the bulk sintered under 25GPa and 1400°C shows an average grain size of 10nm and an asymptotic Vickers hardness of 41.5GPa. Boosting the hardness of β-SiC over the superhard threshold signifies an important progress in superhard materials research. A broader family of superhard materials is in sight through successful implementation of nanostructuring in other hard materials such as BP.

Interrelationship of bonding strength with structural stability of ternary oxide phases of MgSnO3: A first-principles study

We have studied the Ilmenite, Perovskite and LiNbO3 type crystal structures of ternary oxide MgSnO3 by first-principles methods using density functional theory (DFT) and beyond. We conclude that MgSnO3 in LiNbO3 type is both mechanically and dynamically stable, whereas Ilmenite and Perovskite crystal structures are mechanically stable but dynamically unstable. Vibrational stability in MgSnO3 warrants some distortion in the octahedra caused due to the strength of bonding between the atom pairs Sn–O in LiNbO3 type crystal structure. Similarly, Ilmenite and Perovskite crystal structures require a higher number of Mg–Sn and (Mg–Mg, Sn–Sn, O–O and Mg–O) bonds respectively. Ilmenite and LiNbO3 type crystal structures can be used as window layers featuring a large bandgap of 5.22 eV and 3.88 eV respectively, along with a lower absorption coefficient and reflectivity. Likewise, Perovskite crystal structure with a bandgap of 2.55 eV can be deployed as an absorber layer that traps green light of the solar irradiation in tandem solar cells. Perovskite crystal structure has the lowest charge carrier effective masses among all the structures in MgSnO3. LiNbO3 type crystal structure shows a high hardness of 56.5 GPa. It should be tested experimentally in applications requiring super-hard materials.

Effect of Particle Size Distribution, pH, and Na+ Concentration on the Chemical Mechanical Polishing of Sapphire and 4H-SiC (0001)

As two typical representatives of super-hard materials, sapphire and silicon carbide, their processing has always been a hot spot. Chemical mechanical polishing (CMP) technology is the only way to achieve global planarization, and it has also become one of the most important processes for precision machining of sapphire and silicon carbide. This paper introduced the relationship between the removal rate and surface roughness of sapphire and 4H-SiC (0001) and the size distribution and pH of alumina slurry. More importantly, this paper explored the negative effect of Na+ on the removal rate of alumina-based sapphire polishing slurry, and the more negative effect of Na+ on the removal rate of alumina-based silicon carbide polishing slurry and the surface state of SiC(0001). At the end of the article, the polishing mechanism of sapphire with alumina as a brasive was given.

Tailoring the microstructure, mechanical and tribocorrosion performance of (CrNbTiAlV)N high-entropy nitride films by controlling nitrogen flow

• The (CrNbTiAlV)N x high-entropy nitride films were fabricated by magnetron sputtering. • The films transformed from amorphous to nanocrystalline structures with the increment of nitrogen. • The tribocorrosion mechanism of the high-entropy nitride films was discussed. • The film deposited at nitrogen flow of 38 sccm exhibits the best comprehensive mechanical and tribocorrosion performance. The (CrNbTiAlV)N x high-entropy nitride films were fabricated by adjusting nitrogen flow via magnetron sputtering. The microstructure, mechanical, electrochemical and tribocorrosion performances of the films were studied. The results show that the films transform from amorphous to nanocrystalline structure as nitrogen flow increased. The nanocrystalline films show super hardness (> 40 GPa) and adhesion strength (> 50 N). The amorphous film has a pretty anti-corrosion in static corrosion, while not in tribocorrosion condition. The film deposited at nitrogen flow of 38 sccm exhibits the optimal tribocorrosion performance in artificial seawater, with the highest open circuit potential (∼ − 0.1 V vs. Ag/AgCl), the lowest friction coefficient (∼ 0.162) and wear rate (∼ 7.48 × 10 −7 mm 3 N − 1 m − 1 ).

Mechanical response of nanoindentation and material strengthening mechanism of nt-cBN superhard materials based on molecular dynamics

In this paper, the mechanical response of nanoindentation process of nano-twinned cubic Boron Nitride (nt-cBN) materials is investigated by molecular dynamics method. The introduction of nano-twinned structure in cBN materials can lead to superior mechanical properties than those of single-crystal cBN (sc-cBN) materials. The mechanism of material strengthening is derived by studying the dislocation propagation during the indentation process. The limitation of dislocation propagation by nano-twinned layers leads to the accumulation of dislocations in the interlayer to form immovable dislocation multi-junctions, which leads to the increase of intracrystalline back stress and raises the yield stress of the material. In the high-pressure environment, dislocations can react with twin boundary layers and split into multiple dislocations, increasing the dislocation density and the number of dislocation multi-junctions in the material. The reaction between dislocations and twin-crystal plays an important role in twin-crystal strengthening.