Growth of cubic boron nitride films by ibad and triode sputtering: development of intrinsic stress

Two methods are employed to evidenced the stress behavior in c-BN films. On the one hand, in depth stress profile of c-BN film, deposited by ion beam assisted evaporation, was performed by recording infrared spectra and substrate curvature after reactive ion etching (RIE) steps. It shows a peak of stress up to −17 GPa in the h-BN basal layer and a stress relaxation when the cubic phase appears. On the other hand, dynamic stress profiles of c-BN films deposited by a triode sputtering system, are obtained by recording infrared spectra and substrate curvature after various c-BN deposition times, with the same experimental conditions. Likewise, a peak of stress of −12 GPa is unmistakably observed in the h-BN basal layer followed by a stress release during c-BN nucleation, where an average value of −12 GPa is observed in the c-BN film volume. These results provide a support for the stress model proposed by McKenzie even if along with a minimum stress a high level of densification of the layer is needed.     

Increase the hardness of polycrystalline cubic/wurtzite boron nitride composite through hybrid laser/waterjet heat (LWH) treatment

The material, boron nitride (BN), is chemically and thermally stable which is desirable for high-speed machining in demanding chemical and thermal environments. Although BN’s hardness is lower than that of single polycrystalline diamond, a novel laser/waterjet heat treatment process provides a new approach to increase the hardness of the dual-phase 50% cubic and 50% wurtzite (cBN/wBN) composite close to or as high as diamond’s hardness. Results indicate that experimentally measured hardness increase is dependent on the processing parameter such as laser fluence and overlap between heat treatment passes. Statistical analysis is carried out to identify the processing parameter that result in maximum hardness increase.     

Influence of boron impurities on the crystal structure of cubic boron nitride

The influence of boron impurities on the unit cell parameters of the boron nitride with a sphalerite structure is investigated. It is demonstrated that the presence of boron in amounts exceeding the stoichiometric composition leads to a distortion of the cubic lattice and reduces its symmetry. The revealed oriented distortion of the crystal lattice of the cubic boron nitride is explained by the incorporation of excess boron atoms not in a random manner but between paired corrugated hexagonal layers. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Deposition of cubic boron nitride films on diamond-coated WC:Co inserts

Cubic boron nitride (cBN) thin films were deposited on diamond-coated tungsten carbide (WC) cutting inserts using electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD). The effects of gas flow rate and substrate bias on the phase composition and structure of the BN films deposited on diamond surfaces were studied. It was revealed that both the cubic phase formation and the selective etching of hexagonal phase were controlled by modulating the hydrogen and boron trifluoride flow rate ratio. By the trial and error method the gas flow rate ratio and substrate bias voltage were optimized. Moreover the phase composition of the BN film was found to be affected by the thickness of diamond buffer layer and interrelated to the effective substrate bias. The hardness of the resulting cBN films reached the value of 70 GPa. In the synthesized coatings, the diamond beneath renders the best mechanical supporting capacity while the top cBN provides the superior chemical resistance and extreme hardness. The cBN/diamond bilayers deposited on WC inserts may serve as universal tool coatings for machining steels and other ferrous metals.     

X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products

Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO₂] and three calcium-bearing compounds: hannebachite [CaSO₃·1/2H₂O], gypsum [CaSO₄·2H₂O] and ettringite [(Ca₆(Al(OH)₆)₂(SO₄)₃·26H₂O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement.     

Local structure determination of aluminum in Y zeolite: application of low energy X-ray absorption fine structure spectroscopy

The local Al structure and electronic properties in Na-Y, NH₄-Y and H-Y zeolites have been determined by low energy AlXAFS. The Al-O bond distance of 1.700 Å in H-Y is longer than that in NH₄-Y and Na-Y, 1.636 and 1.620 Å, respectively. In addition, the white line intensity of the Alion in H-Y is higher than in NH₄-Y and Na-Y, indicating the electron density on the Alion is lower in H-Y than in the other two catalysts. The results of experimental Al-O bond distance and white line intensities are qualitatively in agreement with theoretically calculated Al-O bond distances in aluminosilicate clusters. However, further work is required in order to quantitatively evaluate the near edge structure of the X-ray absorption data.     

Recrystallization behaviour of cubic boron nitride under high pressure

The recrystallization behaviour of micron-sized cubic boron nitride (cBN) was studied by analysing the grain size and morphology of samples treated at 8−16 GPa/1500–2200 °C. The results show that the recrystallization temperature of cBN under a pressure of 8 GPa is approximately 1650 °C and increases by approximately 100 °C with every 2 GPa increase in pressure. Once grain recrystallization starts, the grains grow abnormally quickly as the temperature rises, and the strengthening effects of grain refinement and defect structure are greatly weakened. The recrystallization behaviour of cBN at high pressure is helpful to understand the sintering mechanism and control the microstructure and mechanical properties of sintered polycrystalline cBN compacts. In addition, the melting curve for cBN under high pressure is inferred according to the empirical relationship between recrystallization temperature and melting temperature, and the phase diagram for boron nitride is revised based on this new melting curve.     

Crystal-oriented mechanism of dynamic recrystallization nucleation in cubic boron nitride

Structural transformations occurring at 8 GPa and 2400 °C in twinned cubic boron nitride (cBN) with a triplex texture [112] have been explored by electron microscopy using the thin foil procedure. The following reconstructions have been established: destruction of the texture that occurs by rotation about the [220] axis, fragmentation of twinned grains, detwinning of large fragments by rotation about the [110] axis and formation of nuclei of a primary recrystallization from fragments with no twins.     

Development of matrix microstructure in polycrystalline cubic boron nitride ceramics

The development of matrix microstructure in polycrystalline cubic boron nitride cutting tool materials has been investigated by X-ray diffraction and scanning and transmission electron microscopy combined with energy dispersive X-ray spectrometry. The materials had a matrix based on Ti(C,O,N) and Al and were fabricated from powder mixtures milled with either WC-Co cemented carbide or (Ti,W)(C,N)-Co cermet milling media. The introduction of WC-Co debris resulted in the formation of an Al- and W-rich liquid phase during high pressure high temperature sintering. Crystalline intergranular phases rich in W partitioned from this liquid. Debris from the cermet milling media resulted in the formation of intergranular Co₂B; the W was retained in the (Ti,W)(C,N) structure during sintering. Al-rich reaction zones on cBN grain surfaces developed locally during high pressure high temperature sintering.     

Granularity-induced plastic deformation mechanism of pure polycrystalline cubic boron nitride

Combined X-ray diffraction(XRD) profile analysis and HRTEM observation, a method for exploration of plastic deformation in pure polycrystalline cubic boron nitride (PcBN) samples with sizes of primary cBN powders was developed. XRD profile results showed that the coarse-grained PcBN exhibited a larger micro-strain ε, a greater deformation stacking faults probability fD, which was an order of magnitude larger than that of fine-grained PcBN, but a smaller twin stacking faults probability fT. It was deduced that the plastic deformation of the coarse-grained PcBN was dominated by stacking faults and mechanical twins mode, which would result in strain strengthening and then recrystallization. While the manner of growth twins was the mainly modes of fine-grained PcBN by phase transform, especially utilized the curled SP2 structure as a basis for a cubic structure nucleation. Fundamental plastic deformation principles of the ultrafine polycrystalline cBN was crucial for the field of high-precision cutting tools.     

Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine

Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900?°C and total pressures of 50–1000?Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The as-deposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900?°C with relative low pressures of 50–200?Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100?°C with higher pressures above 250?Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d₍₀₀₂₎) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d_(002). The crystal growth mechanism of the three types of PBN was discussed.     

Synthesis of fine boron nitride powders by combining direct boron nitridation with carbothermic boron oxide reduction

Fine boron nitride powders were synthesized by combining direct nitridation of boron with carbothermic reduction of boron oxide. It has been established that the heat released in direct nitridation of boron can be utilized to carry out carbothermic reduction of boron oxide in a mode of self-sustained combustion. Intermediate gaseous products were found to play a key role the SHS reactions under study. The article is published in the original.     

Effects of silicon incorporation on composition,structure and electric conductivity of cubic boron nitride thin films

We have achieved in-situ Si incorporation into cubic boron nitride (c-BN) thin films during ion beam assisted deposition. The effects of silicon incorporation on the composition, structure and electric conductivity of c-BN thin films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and electrical measurements. The results suggest that the content of the cubic phase remains stable on the whole with the incorporation of Si up to a concentration of 3.3 at.%, and the higher Si concentrations lead to a gradual change from c-BN to hexagonal boron nitride. It is found that the introduced Si atoms only replace B atoms and combine with N atoms to form Si–N bonds, and no evidence of the existence of Si–B bonds is observed. The resistance of the Si-doped c-BN films gradually decreases with increasing Si concentration, and the resistivity of the c-BN film with 3.3 at.% Si is lowered by two orders of magnitude as compared to undoped samples.     

Characterization of coke deposited on catalysts by carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy

C K-edge NEXAFS spectra could distinguish between the structures of coke on the catalysts used in fluid catalytic cracking, hydrotreatment and dry reforming of methane from the characteristic ;π^* features. A particular advantage of NEXAFS spectroscopy was found in the characterization of highly aromatic coke that could not be analyzed by cross-polarization ¹³C-NMR due to the lack of magnetization transfer from ¹H to ¹³C. The NEXAFS results elucidated two different structures of coke on Ni catalysts used in dry reforming of methane; one with a graphitic structure formed at lower temperatures and the other with a non-graphitic structure formed at higher temperatures. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tribological behaviors of polycrystalline cubic boron nitride sliding against silicon nitride in air and vacuum conditions

In order to evaluate the friction and wear properties of polycrystalline cubic boron nitride (PCBN) based on the drilling tools cutting, the ball-on-disk tribological experiments of PCBN sliding against silicon nitride (Si₃N₄) were carried out in air and vacuum conditions. The tribological behaviors were investigated by Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and Nanomap-D three-dimensional White Light Interferometer. The results demonstrate that the coefficient of friction (CoF) is closely interrelated with the changing tendency of loads, where the CoFs gradually decrease with the growth of the load whether in air or in vacuum on account of a transfer film in air and a change from sliding friction to rolling friction in vacuum. Moreover, the CoF in vacuum condition is invariably greater than that in air under the similar load owing to friction heat. Furthermore, no observable abrasion appears on Si₃N₄ in air while severe abrasive wear is dominant on Si₃N₄ in vacuum. In addition, there is more intense adhesion on PCBN in vacuum than that in air. The reason is that the friction heat is gathered in vacuum condition with a confined environment.     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

Development of edge state on graphite surface induced by Ar+ irradiation studied using near-edge X-ray absorption fine structure spectroscopy

We have studied the electronic structure of the surface of graphite irradiated by Ar⁺ ions with various kinetic energies using in situ near-edge X-ray absorption fine structure spectroscopy in order to verify the appearance of an edge state over a macroscopic area and the variation of the edge state feature by severely damaging a sample. The disruption of the π bonding network of graphite is manifested by the reduction of the intensity of the π¹ peak upon Ar⁺ irradiation. The increase of the lower energy shoulder of the π¹ peak can be clearly attributed to the edge state of π-electron origin at vacancies. High energy Ar⁺ irradiation strongly induces an amorphous-like structure at the surface where carbon atoms have a random coordination. As a consequence, sp³-hybridized carbon atoms become involved in bonding, resulting in the mixing of π and σ states that then broadens the observed π¹ peak. In addition, the reduction in concentration of π electrons that screen core holes results in a blue shift of the central π¹ peak energy. Moreover, the edge state, which is the signature of π conjugation, survives even at the severely damaged Ar⁺ irradiated graphite surface having an amorphous-like structure.     

Small-scale mechanical properties of constitutive phases within a polycrystalline cubic boron nitride composite

Micromechanical properties of a polycrystalline cubic boron nitride (PcBN) composite have been assessed by statistical analysis of data gathered from experimental massive nanoindentation. The mechanical study was complemented with electron probe X-Ray microanalysis, aiming to correlate relative B/N ratio and local hardness for individual cBN particles. Best-fit of experimental and deconvoluted data is achieved by considering five mechanically different phases, defined on the basis of chemical nature, TiN/cBN interface presence, ratio between residual imprint dimension and microstructural length scale as well as phase stoichiometry. In-depth local micromechanical and chemical analysis permitted to propose and validate, for the first time, the existence of a correlation between intrinsic hardness and phase stoichiometry for cBN phase. Finally, based on experimental data measured by nanoindentation and analyzed in terms of plastic index, toughness for the PcBN composite studied is estimated to range between 4 and 6 MPa·√m.     

Synthesis of ultrafine nano-polycrystalline cubic boron nitride by direct transformation under ultrahigh pressure

Using a turbostratic pyrolytic boron nitride as a starting material, we synthesized a variety of ultrahard polycrystalline cubic boron nitride (PcBN) as a function of the heating duration changing from 1 to 60?min under a constant temperature and pressure conditions (1950?°C and 25?GPa) using a multi-anvil apparatus. When the heating duration was less than 13?min, ultrafine nano-polycrystalline cBN (U-NPcBN) with the mean grain size of <50?nm was produced. Among these U-NPcBNs those synthesized with 11–13?min were found to have a uniform texture composed purely of cBN (i.e. with no wurzite BN residue) and a Knoop hardness of >53?GPa, which is 20% higher than that of the hardest conventional binderless PcBN in practical use. Furthermore, the PcBNs synthesized with 18–20?min showed a unique nanocrystalline texture composed of relatively coarse grains dispersed in a fine grained matrix and even higher Knoop hardness (54.5–55.2?GPa).     

X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies of chloride in passive oxide films

X-ray absorption near edge structure (XANES), utilizing both electron yield and X-ray fluorescence detectors, and X-ray photoelectron spectroscopy (XPS) were used to follow chloride uptake by oxide-covered aluminum in 0.1 M NaCl solutions. The aluminum samples were polarized at selected potentials below (less positive than) the pitting potential. The electron yield XANES and XPS showed multiple peaks. The XPS chloride spectra showed two distinct sets of doublets. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. The XANES results also showed two peaks which are attributed to chloride on the surface and in the bulk of the oxide.