Strength and plastic deformation of polycrystalline diamond composites

ABSTRACT

The use of nanopolycrystalline diamond has allowed a systematic study on deformation of polycrystalline diamond composites (PCDCs). Bulk PCDCs samples containing either Co or SiC as a binding agent were deformed under high pressure and temperature to strains up to 18% at strain rates ～10^?5?s^?1. All samples exhibit strong work hardening. The strength of PCDCs depends on the amount and type of binding agents and is consistently weaker than that of diamond single crystals. The weakening may be due to the binder materials, which play an important role in affecting grain boundary structures. In SiC-based PCDC, significant grain fragmentation occurs. Nearly all grain boundaries are wetted by SiC after large deformation, resulting in lower strength. In Co-based PCDC, the microstructure is dominated by dislocations, deformation twins, and separated grain boundaries. The density of deformation twins increases significantly with strain, with the twin domain width reaching as low as 10–20?nm at 14% strain.     

Photothermal and micro-thermal characterization of metal coated diamond crystallites

Photoacoustic effect and scanning near field microscopy have been applied to characterise the thermal properties of diamond crystallites which were coated by a Cu film, a Cr-film and a Cr-Cu film. The Cu-coated diamond exhibits a considerable thermal barrier at the Cu-diamond interface which had disappeared for the crystallites with a thin bond Cr-layer between diamond and Cu. The local inspection of the thermal conductivity with a thermal nano-probe operated in the 3ω-mode reveals slight local variations of the thermal conductivity of the Cr-coated crystallite.     

Effects of large anisotropic elastic stresses on the resonant ferromagnetic absorption in a polycrystalline ferrite

Uniform compression along the field direction is considered in relation to the additional broadening. It is found that an additional absorption peak can also occur if the compressive force is large enough.Read at the Third All-Union Conference on Ferrites, Leningrad, 25 October 1963.     

具有立方晶系结构的多晶体材料的弹性常数——Y弹性常数

提出了一个新的物理参量“Ｙ弹性常数”,并阐述了其物理含义.并将其应用于具有立方晶系结构的多晶体材料,推导了立方晶系结构的多晶体材料的Ｙ弹性常数,通过算例与具有立方晶系结构的多晶体材料的X射线弹性常数进行了比较.运用这个Ｙ弹性常数进一步推导出的多晶体材料整体的机械弹性常数的表达式与Krner的研究结果完全符合. 关键词： Y弹性常数 立方晶系 多晶体材料     

Reactions and stresses in polycrystalline diamond-metal and diamond-carbide compacts

Abstract

The kind of bonding phase has a significant influence on the microstructure and mechanical properties of diamond compacts. Microstructural studies of diamond with 5% wt. Ti and 5%wt. Tic (and also 30%wt. Tic) were carried out with a Transmission Electron Microscope. The TEM microstructural observations show differences between the metal and metal carbide bonding phase in diamond compacts. The mismatch of thermal expansion coefficients between diamond and the bonding metal or the compound induces significant internal stresses and may generate micro-cracks in polycrystalline diamond compacts. Twins and dislocations are the important details of microstructures in diamond crystals after HPHT sintering. They can appear as a result of residual stress relaxation. Results of measurements of residual stresses on a diamond compact surface by means of the “sin²ψ X-ray diffraction method are reported.     

Photoacoustic spectroscopy of diamond powders and polycrystalline films

Photoacoustic spectroscopy is used to study optical absorption in diamond powders and polycrystalline films. The photoacoustic spectra of diamond powders with crystallite sizes in the range from ∼100 μm to 4 nm and diamond films grown by chemical vapor deposition (CVD) had a number of general characteristic features corresponding to the fundamental absorption edge for light with photon energies exceeding the width of the diamond band gap (∼5.4 eV) and to absorption in the visible and infrared by crystal-structure defects and the presence of non-diamond carbon. For samples of thin (∼10 μm) diamond films on silicon, the photoacoustic spectra revealed peculiarities associated with absorption in the silicon substrate of light transmitted by the diamond film. The shape of the spectral dependence of the amplitude of the photoacoustic signal in the ultraviolet indicates considerable scattering of light specularly reflected from the randomly distributed faces of the diamond crystallites both in the polycrystalline films and in the powders. The dependence of the shape of the photoacoustic spectra on the light modulation frequency allows one to estimate the thermal conductivity of the diamond films, which turns out to be significantly lower than the thermal conductivity of single-crystal diamond. Fiz. Tverd. Tela (St. Petersburg) 39, 1787–1791 (October 1997)     

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Finite-element modeling of stresses and strains in a diamond anvil cell device: case of a diamond-coated rhenium gasket

The stresses and strains in a diamond anvil cell device were investigated using a finite-element code NIKE2D for the case of an ultra-hard composite gasket material. The pressure distribution in a diamond-coated rhenium gasket was measured by the energy dispersive diffraction technique to 213 GPa and compared with the finite-element modeling results. We examine various models for the mechanical properties of diamond-coated rhenium gasket as well as for diamond failure for shear stresses exceeding 100 GPa. The elastic and plastic properties of gasket were varied such that a good agreement between the experimentally measured pressure distribution and the computational pressure profiles were obtained. As a result, we obtained the effective Young’s modulus, Poisson’s ratio, yield stress for indented gasket, linear hardening modulus, and hardening parameter value for this layered ultra-hard composite gasket material. Future diamond design strategies for attainment of extreme high pressures using ultra-hard gasket materials are also discussed.     

MD simulation of plastic deformation nucleation in stressed crystallites under irradiation

The investigation of plastic deformation nucleation in metals and alloys under irradiation and mechanical loading is one of the topical issues of materials science. Specific features of nucleation and evolution of the defect system in stressed and irradiated iron, vanadium, and copper crystallites were studied by molecular dynamics simulation. Mechanical loading was performed in such a way that the modeled crystallite volume remained unchanged. The energy of the primary knock-on atom initiating a cascade of atomic displacements in a stressed crystallite was varied from 0.05 to 50 keV. It was found that atomic displacement cascades might cause global structural transformations in a region far larger than the radiation-damaged area. These changes are similar to the ones occurring in the process of mechanical loading of samples. They are implemented by twinning (in iron and vanadium) or through the formation of partial dislocation loops (in copper).     

Effect of elastic stresses on the thermoelectric properties of Pb1−x GexTe epitaxial layers

The temperature dependence of the thermopower of Pb_1?x Ge_xTe (x = 0.01?0.05) epitaxial films grown on BaF₂(111) substrates was measured. The temperature of the ferroelectric phase transition was found to disagree with that obtained for bulk single crystals of the same composition. This disagreement was explained as resulting from the effect of elastic stresses arising in the crystal lattice of the Pb_1?x Ge_xTe compound due to the difference between the thermal expansion coefficients of the film and the substrate during cooling of the layers from the growth temperature to the temperature of measurement.     

Ultrasonic measurement of the elastic properties of benzoyl glycine single crystals

Certain organic crystals are found to possess high non-linear optical coefficients, often one to two orders of magnitude higher than those of the well-known inorganic non-linear optical materials. Benzoyl glycine is one such crystal whose optical second-harmonic generation efficiency is much higher than that of potassium dihydrogen phosphate. Single crystals of benzoyl glycine are grown by solvent evaporation technique usingN, N-dimethyl formamide as the solvent. All the nine second-order elastic stiffness constants of this orthorhombic crystal are determined from ultrasonic wave velocity measurements employing the pulse echo overlap technique. The anisotropy of elastic wave propagation in this crystal is demonstrated by plotting the phase velocity, slowness, Young’s modulus and linear compressibility surfaces along symmetry planes. The volume compressibility, bulk modulus and relevant Poisson’s ratios are also determined. Variation of the diagonal elastic stiffness constants with temperature over a limited range are measured and reported.     

Electromigration in polycrystalline and single crystal magnesium

The electromigration of magnesium single and polycrystal samples was measured under the influence of a d.c. current density of approximately $\text{6×10}{}^3\text{A}\text{cm}{}^2$ using the vacancy flux technique. The single crystal data were taken on samples with the c-axis of the metal making angles of 22° and 63° respectively with the cylindrical axis of the specimen. Accurate measurements of the longitudinal and transverse motion were made over a period of several weeks and used to calculate a value for the atom drift velocity, V_a. The atom drift velocity is used to calculate Z¹, the effective eiectromigration charge on the ion. All of the observed motion was directed toward the anode. The polycrystal runs yielded an average value of $\text{Z}{}^1\text{?}\text{= ?2.03 ±0.33}$. The single crystal runs resulted in an anisotropic drift velocity ratio of $\text{V}{}_{\mathrm{a\perp }}\text{V}{}_{\mathrm{a\parallel }}\text{= 1.4}$. However, the product of diffusivity times resistivity is anisotropic by the same ratio as the atom drift velocity and the correlation factor f is nearly isotropic for magnesium. As a result Z¹ is isotropic and turns out to be ?1.6.     

Pressure indicator in elastic and plastic media

The behaviour of small pressure indicator in solid media is considered theoretically. The difference between elastic modules of the indicator and the media results in essential difference between corresponding pressures. It became smaller if effects of plastical flow are taken into account. If the plastic threshold increases with pressure the correction will not vanish even for infinite pressure.     

Tensor of internal stresses in polycrystalline grains with complex bending

Processes that occur upon the straining of a polycrystal are studied. Components of the internal stress tensor were determined. Changes in the bending and torsion stresses in the crystal lattice of a strained polycrystal are analyzed. A correlation between changes in the internal stress tensor components and the fraction of material with complex bending is established.     

Thermal conductivity of polycrystalline CVD diamond: Experiment and theory

The temperature dependences of thermal conductivity κ of polycrystalline CVD diamond are measured in the temperature range from 5 to 410 K. The diamond sample is annealed at temperatures sequentially increasing from 1550 to 1690°C to modify the properties of the intercrystallite contacts in it. As a result of annealing, the thermal conductivity decreases strongly at temperatures below 45 K, and its temperature dependence changes from approximately quadratic to cubic. At T > 45 K, the thermal conductivity remains almost unchanged upon annealing at temperatures up to 1650°C and decreases substantially at higher annealing temperatures. The experimental data are analyzed in terms of the Callaway theory of thermal conductivity [9], which takes into account the specific role of normal phonon-phonon scattering processes. The thermal conductivity is calculated with allowance for three-phonon scattering processes, the diffuse scattering by sample boundaries, the scattering by point and extended defects, the specular scattering by crystallite boundaries, and the scattering by intercrystallite contacts. A model that reproduces the main specific features of the thermal conductivity of CVD diamond is proposed. The phonon scattering by intercrystallite contacts plays a key role in this model.     

On intensities in high pressure neutron powder diffraction using single and polycrystalline diamond anvils: small versus large sample volumes

We report a quantitative comparison of measured intensities of neutron powder diffraction data collected in a single-crystal diamond anvil cell and in large-volume sintered diamond anvils. As expected from the difference in sample volumes, the latter provides 1–2 orders of magnitude higher intensities, depending on the anvil material. The remaining differences are due to effects of absorption and angular aperture.     

Connection between hardness and stresses in the plastic region

In studying plastic deformation by measurement of hardness [1] it is necessary to use a hardness-stress calibrating graph plotted from the results of tests under the simplest stress conditions. For the use of these graphs in plastic deformation in general to be well-founded, it is essential to have experimental data confirming that the hardnessstress curve is common to various states of stress. However, [1–3] give this data from tensile, compressional and torsional tests only. Clarification of this problem is particularly important, because all the stress and deformation components for a number of cases of practical importance can be determined by reference to the stress intensities at various points in a body [4].The present work gives the results of experimental work on the connection between stresses and hardness under conditions of two- and three-dimensional stress.