Colossal shear-strength enhancement of low-density cubic BC2N by nanoindentation

Recently synthesized low-density cubic BC2N exhibits surprisingly high shear strength inferred by nanoindentation in stark contrast to its relatively low elastic moduli. We show by first-principles calculation that this intriguing phenomenon can be ascribed to a novel structural hardening mechanism due to the compressive stress beneath the indenter. It significantly strengthens the weak bonds connecting the shear planes, yielding a colossal enhancement in shear strength. The resulting biaxial stress state produces atomistic fracture modes qualitatively different from those under pure shear stress. These results provide the first consistent explanation for a variety of experiments on the low-density cubic BC2N phase across a large range of strain.     

Superhard Pseudocubic BC2N superlattices

It is currently under debate whether diamondlike BC2N may be harder than cubic BN (c-BN). Using the bond counting rule, we have performed an unconstrained search and identified a series of short period (111) superlattices that have much lower total energy than previously proposed structures. By examining the ideal strength of these pseudocubic boron-carbonitrides, we show that they are harder than c-BN. Our results are consistent with experimental findings, but in contrast with a recent theoretical study, which claimed that the BC2N is less hard than c-BN.     

Superhard cubic BC2N compared to diamond

Recent experiments claimed successful synthesis of cubic boron-carbonitride compounds BC2N with an extreme hardness second only to diamond. In the present Letter, we examine the ideal strength of cubic BC2N using first-principles calculations. Our results reveal that, despite the large elastic parameters, compositional anisotropy and strain dependent bonding character impose limitation on their strength. Consequently, the hardness of the optimal BC2N structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of BC2N nanocomposites is most likely due to the nanocrystalline size effect and the bonding to the surrounding amorphous carbon matrix. This may prove to be a general rule useful in the quest for new superhard covalent materials.     

Superdense tI12 carbon: Unexpectedly high elastic moduli but low ideal strength

The basic physical and chemical properties of new carbon allotropies are important to explore their further technique and industrial applications. Here, a systematic theoretical investigation on the electronic, dynamical, and elastic properties for the superdense carbon (tI12) are performed, especially the ideal tensile and shear strength and the corresponding bond-breaking modes are explored to uncover its intrinsic mechanical nature and the corresponding bond-breaking modes. Our results show that the bulk, shear and Young's modulus of tI12 carbon are ultrahigh, close to those of diamond, reflecting its excellent performance of the substance's resistance to be deformed elastically at small strains. However, the calculated tensile and pure shear strengthes are remarkably lower than that of diamond, which is attributed to its original structural anisotropy by analyzing the atomic structural deformation under different strains. The current results highlighted the need to carefully examine the stress response at large strains, which provide crucial insights for the bond-breaking modes and deformation mechanisms that may lead to conclusions different from those obtained from equilibrium structures.     

Fluctuations of tensile strength and hardness of c-BC?N crystals induced by difference in atomic configuration

At the atomistic level, the physical properties of a material are determined by its structure such as atomic arrangements. Here first-principles calculations were performed to investigate the effect of atomic configuration on the tensile strength and Vickers hardness of cubic-BC?N (c-BC?N) crystals. Depending on the degree of mixture between diamond and c-BN, the tensile strength of c-BC2N crystals can vary drastically from 27 to 77 GPa. The magnitude of the Vickers hardness fluctuations (~10 GPa) is also comparable to the experimental difference (~14 GPa). Thus, atomic-scale characterization of c-BC?N crystal structures may unveil the discrepancy of the measured Vickers hardness in experiments, and uncover the obvious differences of tensile strength described in theoretical calculations.     

Revealing the Pressure-Induced Softening/Weakening Mechanism in Representative Covalent Materials

Diamond, cubic boron nitride(c-BN), silicon(Si), and germanium(Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated.It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond(c-BN), and weakening in Si(Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.     

衬底材料对制备立方氮化硼薄膜的影响

较系统地研究了不同衬底材料对制备氮化硼薄膜的影响。用热丝增强射频等离子体CVD法,以NH3,B2H6和H2为反应气体,在Si,Ni,Co和不锈钢等衬底材料上,成功生长出高质量的立方氮化硼薄膜,还用13.56MHz的射频溅射系统将c-BN薄膜沉积在Si衬底上,靶材为h-BN(纯度为99.99％）,溅射气体为氩气和氮气的混合气体,所得到的氮化硼薄膜中立方相含量高于90%,用X射线衍射谱和傅里叶变换红谱对样品进行了分析表明,衬底材料与c-BN的晶格匹配情况,对于CVD生长立方氮化硼薄膜影响很大,而对溅射生长立方氮化硼薄膜影响不大。     

高质量宽带隙立方氮化硼薄膜的研究进展

文章着重介绍了最近研制出的高质量宽带隙立方氮化硼薄膜的三种制备方法和结构特性 :(1)用射频溅射法在Si衬底上制备出立方相含量在 90 %以上 ,Eg>6 .0eV的c-BN薄膜 ;(2 )用离子束辅助的化学气相沉积法(CVD) ,在金刚石上外延生长出立方含量达 10 0 %的单晶c -BN薄膜 ;(3)用微波电子回旋共振CVD法 (MW -ECR-CVD)在金刚石上外延生长出高纯c-BN薄膜 .这些高纯c -BN薄膜 ,可应用于制作各种半导体 (主要是高温、高频大功率 )电子器件 .     

Heteroepitaxial growth of cubic boron nitride films on single-crystalline (001) diamond substrates

Heteroepitaxial cubic boron nitride (c-BN) films of significantly improved crystalline quality have successfully been deposited on (001) diamond single crystals using an ion beam assisted preparation method. The results of various characterization techniques prove that films containing 100% pure c-BN phase were nucleated directly on top of diamond without any intermediate hexagonal BN layer. Epitaxially grown, 500-nm-thick c-BN films are mechanically stable even under ambient conditions, though they still exhibit a compressive stress of 5 GPa. Their rocking angles of 0.2°, as observed by X-ray diffraction, point to a hitherto unprecedented quality of the films. Their surface smoothness, the magnitude of their Youngs modulus as well as their ultrahardness corroborate the outstanding quality of these epitaxially grown c-BN films on single-crystalline diamond. PACS 68.55.Jk; 81.15.Jj; 62.20.Qp; 81.05.Je     

Novel technique for hetero-epitaxial diamond film growth on cubic boron nitride from iron carbide at high temperature and high pressure

Cubic boron nitride (c-BN) crystals about 0.1–0.3 mmin dimension were treated with iron carbide powders (high purity 99%) with size of 80–100 mesh at a high temperature of 1620 K and a high pressure of 5.2 GPa. It was found that hetero-epitaxial diamond films have been grown on the c-BN from iron carbide. The formation of dia-mond films on the cubic boron nitride can be confirmed by laser Raman spectra, face scan of elements and reflective high-energy electron diffraction. It was suggested that diamond films could be epitaxially formed on the c-BN through decomposition of iron carbide. This approach provides a possible and very effective way to realize hetero-epitaxial growth of homogeneous and large-area diamond films on c-BN, which is different from the conventional technique using a chemical vapor deposition method. Received: 20 December 2000 / Accepted: 9 January 2001 / Published online: 28 February 2001     

Surface properties of cubic boron nitride thin films

Studying the surface properties of cubic boron nitride (c-BN) thin films is very important to making it clear that its formation mechanism and application. In this paper, c-BN thin films were deposited on Si substrates by radio frequency sputter. The influence of working gas pressure on the formation of cBN thin film was studied. The surface of c-BN films was analyzed by X-ray photoelectron spectroscopy (XPS), and the results showed that the surface of c-BN thin films contained C and O elements besides B and N. Value of N/B of c-BN thin films that contained cubic phase of boron nitride was very close to 1. The calculation based on XPS showed that the thickness of hexagonal boron nitride (h-BN) on the surface of c-BN films is approximately 0.8 nm.     

超硬立方BC2N材料与金刚石的比较

摘要最近有实验称成功合成超硬立方BC2N材料，其硬度仅次于金刚石．文章采用第一性原理计算方法，研究立方BC2N晶体材料的理想强度．计算结果显示，虽然立方BC2N晶体具有很大的弹性系数，但材料中化学成分的各向异性和原子键特性随外加应力变化的非线性性质限制了立方BC2N晶体的强度．最硬的立方BC2N晶体结构的硬度应低于立方BN，后者为目前已知的第二硬材料．实验中观测到的立方BC2N材料的超硬特性应源自材料中的纳米颗粒效应．制备立方BC2N纳米复合材料将是合成新型超硬材料的新方法．     

First-principles calculation and molecular dynamics simulation of fracture behavior of VN layers under uniaxial tension

We develop the second nearest-neighbor modified embedded atom method (2NN MEAM) potential for vanadium nitride (VN) in terms of the individual vanadium and nitrogen. The potential parameters are determined by fitting the cohesive energy, lattice parameter, and elastic constants of the VN with the NaCl-type structure, which are obtained by first-principles calculations. We find that the developed potentials can be used to describe the fundamental physical properties of the V–N system with other lattice structures. The calculated tensile stress–strain curves of the VN layers by first principles agree with those obtained by molecular dynamic simulations, validating the use of the developed potential. The bond breaking and its growth and coalescence are found to play an important role in the formation of fracture. We also find that temperature influences markedly the breaking of bonds, which can be attributed to the deviation of atoms from their equilibrium positions due to the thermal activated vibration, or to the superposition of the thermal energy to the deformation energy. Moreover, no dislocations and slips are found throughout the deformation process.     

Phase transformation in BN films by nitrogen-protected annealing at atmospheric pressure

Two groups of RF-sputtered BN films (pure hexagonal phase and approximately 27.5% cubic content, respectively) were annealed at 600 to 1000 °C under nitrogen at atmospheric pressure after deposition. FTIR spectroscopy indicates a reversible transformation from hexagonal phase to cubic phase, and again hexagonal phase. The most effective temperature for h-BN converting to cubic zincblende (c-BN) is 900 °C. Further, the indirectly stepwise transformation from hexagonal (h-BN) to explosive BN (E-BN) and then to c-BN employing metastable E-BN as an intermedium was observed. In addition, we tentatively put forward that the existence of defective h-BN and the N defects plays a key role on h-BN to c-BN transformation.     

六方氮化硼的结晶度对合成立方氮化硼的影响

 将3种不同初始结晶度的六方氮化硼（h-BN）分别与Li₃N按一定比例混合,在低于立方氮化硼（c-BN）的合成温度和压力条件下,采用高温、高压预处理方式,使3种h-BN的结晶度发生改变。对不同预处理条件下得到的h-BN进行X射线衍射分析,确定了结晶度的变化程度。用不同结晶度的h-BN作为初始原料合成c-BN,观察并分析了h-BN向c-BN转化时微观结构的变化以及h-BN的结晶度对合成c-BN的影响。实验结果表明,结晶度低的h-BN更容易合成c-BN。     

Raman signatures of strong and weak hydrogen bonds in binary mixtures of phenol with acetonitrile,benzene and orthodichlorobenzene

The present study aims at investigating the effect of hydrogen bonds of phenol in binary mixtures of phenol with three solvents viz. acetonitrile, orthodichlorobenzene and benzene respectively in order of decreasing hydrogen bond strength. Raman spectroscopy in correlation with density functional theory (DFT) calculations has led to a profound understanding of changes in structure, energy, dipole moment and other physical and chemical properties of phenol pertaining to hydrogen bond formation in solution. The spectral variation in wavenumber and linewidth of ring deformation, ring stretching, C≡N stretching and C―H stretching modes have been analyzed in detail. The breaking of self association of phenol in solution and formation of strong or weak hydrogen bonds depending on the nature of the solvent has been discussed by comparing the Raman and DFT results for three different solvents. Copyright © 2016 John Wiley & Sons, Ltd.     

Preparation of super-hard coatings by pulsed laser deposition

Amorphous diamond-like carbon (DLC) films and nanocrystalline cubic boron nitride (c-BN) films were prepared by pulsed laser deposition. DLC films with 80 to 85% sp³ bonds prepared at a laser fluence above 6 J/cm² and a substrate temperature below 100 °C show high compressive stresses in the range of 8 to 10 GPa. Those stresses can be completely removed by means of pulsed laser annealing, allowing the preparation of DLC films with several-micrometre thickness. c-BN films were prepared with additional ion-beam bombardment at a substrate temperature of 250 °C. The properties of DLC and c-BN films deposited at high growth rates up to 100 nm/min are presented . PACS 81.15.Fg; 68.60.Bs: 62.40.+i     

A novel evaluation method for thin films mechanical characterizations using cutting system

In this study, we suggest a nano-cutting system to determine the shear strength of the thin films using fracture mechanics analysis of the diamond blade. Based on Merchant's cutting model, we analyze the thin films cutting process with regard to shear angle and resistant forces as initiation of the yield in the chip to establish a direct correlation between the cutting forces and the shear strength. Validating the proposed method was conducted using homogenous polycarbonate disk showing similar shear strengths between different cutting directions. Next, we examined a thin copper electroplated film used in traces of printed circuit board. A thin copper film was examined and found the intrinsic shear strength (307.5 MPa) and adhesion force (44 N/m) between the film and substrate. The result was comparable with tensile strength values reported in the literatures. Finally, we used SEM to visually verify the feasibility of nano-cutting technique to determine thin film properties.     

Al晶界的第一性原理拉伸试验

基于密度泛函理论和局域密度近似的第一性原理方法，进行了Al晶界的第一性原理拉伸试验.得到Al晶界的理论拉伸强度为9.5 GPa，对应的应变为16%.根据价电荷密度、键长和原子构型随应变的变化，我们证实断裂发生在晶界面，其特征是所有界面键的断裂.同时还发现在周围原子键的数目减少的情况下，界面重构的Al-Al原子键具有共价键的性质.因此Al晶界依然保持着较高的界面强度. 关键词： Al晶界 第一性原理拉伸试验 理论拉伸强度     

First-principles investigation on ideal strength of B2 NiAl and NiTi alloys

For B2 NiAl and NiTi intermetallic compounds, the ideal stress–strain image is lack from the perspective of elastic constants. We use first-principles calculation to investigate the ideal strength and elastic behavior under the tensile and shear loads. The relation between the ideal strength and elastic constants is found. The uniaxial tension of NiAl and NiTi along <001> crystal direction leads to the change from tetragonal path to orthogonal path, which is driven by the vanishing of the shear constant C(66). The shear failure under {110}{111} shear deformation occurring in process of tension may result in a small ideal tensile strength(~ 2 GPa) for NiTi. The unlikeness in the ideal strength of Ni Al and Ni Ti alloys is discussed based on the charge density difference.