Heterogeneous diamond phases in compressed graphite studied by electron energy-loss spectroscopy

Chemical mapping imaged by electron energy-loss spectroscopy based on scanning transmission electron microscopy was conducted on a compressed graphite specimen containing different carbon allotropes (hexagonal diamond, cubic diamond, and graphite phases). This imaging process allows visualization of the complex spatial distribution of different diamond phases, and their coexistence was confirmed using dark field (DF) imaging. The chemical mapping images showed spatial distribution of local bonding state for hexagonal and cubic diamond phases in the whole specimen, while the DF images showed only a part of crystalline segments with long-range order. Thus, the chemical mapping method has an advantage for the purpose of observing locally the existence of individual carbon allotropes in the whole specimen. The size distribution of the hexagonal diamond phase is approximately 10–100 nm. These findings indicate that the compressing method can potentially synthesize ~ 100 nm large diamond phases.     

Observation of electron field emission patterns from B-doped diamond films

To develop electron beam sources of carbon materials, field emission patterns were observed in three different setups. The first was a diode-type, in which a carbon specimen was facing to a positively biased fluorescence plate. The second was a triode-type, in which a positively biased grid was placed between them. In the third setup, a commercial electron gun was modified so that it could accommodate a carbon specimen and a grid. A fluorescence plate was placed in a vacuum chamber outside the gun. As the carbon specimen for electron emission source, B-doped diamond films, a single crystal diamond with a B-doped layer, an undoped diamond film and a glass-like carbon both with a fibrous structure at the surface, and a sponge carbon were used. It was found that electron emission from edges was dominant for 1×1 cm diamond films and carbon specimens in the diode-type setup. In the triode-type setup, the edges of the specimens were masked with a Kapton® tape. The electron emission occurred only from some spots on the specimen. In the electron gun setup, it was confirmed that an electron beam was generated, and a fairly uniform circle was seen on the fluorescence plate under defocused situation, while the circle became smaller by adjusting the current of the focusing lens. Although more uniform emission from the electron source materials seemed to be necessary for practical applications, it was demonstrated that an electron beam could be generated even in such a simple setup.     

Estimation of air void and aggregate spatial distributions in concrete under uniaxial compression using computer tomography scanning

Normal- and high-strength concrete cylinders (designed compressive strengths of 30 and 90 MPa at 28 days) were loaded uniaxially. Computer tomography (CT) scanning technique was used to examine the evolution of air voids inside the specimens at various loading states up to 85% of the ultimate compressive strength. The normal-strength concrete yielded a very different behaviour in changes of internal microstructure as compared to the high-strength concrete. There were significant instances of nucleation and growth in air voids in the normal-strength concrete specimen, while the increase in air voids in the high-strength concrete specimen was insignificant. In addition, CT images were used for mapping the aggregate spatial distributions within the specimens. No intrinsic anisotropy was detected from the fabric analysis.     

Accurate measurement of fracture toughness of free standing diamond films by three-point bending tests with sharp pre-cracked specimens

The three-point bending test was used for the first time for evaluating plane strain fracture toughness, K_IC, of free-standing pre-cracked diamond film specimens. A novel approach to introduce sharp cracks into diamond films was presented. This approach, with a success rate of approximately 80%, allowed the initiation and growth of sharp cracks at the top of the notch in diamond films being observed and controlled in situ under a scanning electron microscope (SEM). This made a more accurate and reliable measurement of K_IC in diamond films possible. Fracture toughness values obtained with the sharp pre-cracked specimens were compared with those of simply laser-notched ones. Results showed that with pre-cracked specimens, the measured fracture toughness would be approximately 15% lower than that of laser-notched specimens with notch widths of 0.1–0.12 mm. The method provided for accurate determination of plane strain fracture toughness which may help to clarify the discrepancies and to understand the mechanical properties of CVD diamond films.     

Preparing SiC/diamond coatings via chemical vapor deposition of SiC on diamond-coated graphite and their frictional properties

SiC/diamond coatings with excellent frictional properties were successfully prepared using graphite as substrate. Diamond particles with size of 25–38 μm were firstly bonded on graphite substrate through PVA glue, followed by chemical vapor deposition (CVD) of SiC with varied MTS flow on the diamond-coated graphite substrate to enhance the adhesion of diamond particles. The influence of the MTS flow on the SiC coatings was investigated. The results showed that polycrystalline SiC coating with good crystallinity has been obtained. With MTS flow increasing, the SiC grains feature increased surface roughness and greater sizes of the SiC crystallite resulting from the co-deposition of SiC and carbon with increased carbon containing species. Reciprocating sliding wear tests were conducted to investigate the coefficient of friction. With increasing applied load, while the low-flow specimens showed a remarkable increase in the friction coefficient resulting from degradation of the SiC coatings, the high-flow specimens maintained a relatively low friction coefficient during wear tests indicating strong holding force to diamond particles of the SiC coatings. The reason for low friction coefficient of the high-flow specimens was that GCr15 steel ball was wearing by the SiC/diamond coatings with good affinity to the substrate resulting in a flat–flat contact on the contact area.     

Fretting wear of thin diamond films deposited on steel substrates

The dominant wear mechanism of thin diamond films deposited onto steel substrates and the effect of film thickness on their lifetime under fretting conditions were studied by analyzing the running-in and the main period of the coatings wear life. Steel plate and steel ball specimens for the present study were both coated with diamond by chemical vapor deposition (CVD). The wear tracks resulting from the fretting tests were investigated by various surface analysis methods. The results showed that the dominant wear mechanism of the diamond coatings, when both surfaces are coated, is an abrasive form of fretting wear. Under these conditions, the lifetime of the diamond films increased with increasing film thickness. It was found that the wear rate during the main period is independent on the initial thickness of the diamond film and therefore its life depends on the residual thickness at the end of the running-in period.     

The application of photoluminescence and Raman spectroscopy of synthetic diamond

The properties of the photoluminescent centres H3, 1.945 eV and their associated zero phonon lines (ZPLs) have been used to study various types of synthetic diamond in relation to their growth history. The shapes and widths of the 1.945 eV ZPL have been related to the presence of line and point defects in the different specimens using the theory of Stoneham and correlated with the crystal morphology. Micro-Raman spectroscopy has been used to study the spatial variation of internal stress in synthetic diamond, including single crystal and polycrystalline specimens. Micro-Raman mapping, in which the peak position and width of the allowed Raman peak in diamond are monitored, provides direct information on the state of stress and has been successfully employed to examine the stress distribution around cracks produced by indentation in the diamond.     

Some methodological features of determination of crack resistance in ceramic materials

The known methods for forming stress concentrators (cracks and notches) in ceramic specimens in order to determine their crack resistance are described. A method for forming a notch with a tip curvature radius of at most 10 µm is suggested. The notch is first formed in the process of pressing the specimen in a specially designed mold and then the green specimen is cut additionally from the tip of the notch by a steel blade with a thickness of 0.1 mm and a grinding angle of 14°. After sintering, this specimen does not contain induced defects that are possible when sintered specimens are notched by a diamond disk by the conventional method. It is shown for a ZrO₂ ceramics partially stabilized by 12 mol.% CeO₂ and an Al₂O₃ ceramics with 0.5 wt.% MgO possessing a layered granular structure that an incorrect choice of the tip curvature radius can result in an erroneous evaluation of the optimality of the structure of the material and an incorrect choice of the technological parameters for its production. Notching by the suggested method made it possible to establish the discrete nature of fracture of the layered granular structure of the ceramics from strain diagrams and the mechanisms of crack propagation causing this kind of fracture.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 9, pp. 26 – 30, September, 1996.     

TEM Studies of Nanocarbons and Nanodiamonds (ND): Mechanical milling of ND and Cu

This paper describes the nano-structures of Cu nanocomposites containing 20 and 30 at.% diamond nanoparticles. The Cu nanocomposite powders were fabricated by mechanical milling method and were consolidated by hot extrusion. The solidified sample were thinned to prepare the specimens by focused ion beam technique and the specimens were examined by TEM and EDS. The micro-structural observation showed that the homogeneous distribution of diamond nanoparticles in the Cu nanocomposite could be achieved in 20 at.% diamond content, and that the nanocomposite with homogeneous distribution of diamond nanoparticles has ultra-fine grain structure of approximately 50 nm in diameter. TEM observation revealed the existence of single-digit nano-sized flakes of Fe/Cr in ND aggregates for the first time. The flakes were assigned as contamination from the stainless-steel balls during MM procedure. This finding revealed a possibility of ultra-fine abrasion by the single-digit nano-diamond.     

Effect of different surface treatments on the repair bond strength of resin composites with titanium

The purpose of this study was to evaluate the effect of different surface treatment combinations on the bonding of composite resins to NiCr and titanium alloys after thermal cycling. Square-shaped specimens (10?mm x 10?mm x 2?mm) were made from NiCr and titanium alloys. The specimens were divided into 6 pretreatment groups (n?=?11): (1) machined titanium (control, no treatment); (2) CoJet sand application; (3) grinding with a diamond bur; (4) metal primer application; (5) CoJet sand?+?metal primer application; and (6) grinding with a diamond bur?+?metal primer application. The surface roughness of the mechanically treated specimens (control, grinding, CoJet sand) was evaluated. The surface morphology of both metals and elemental composition were examined with SEM and EDS. The composite resin was applied to the specimens. Shear bond strength (SBS) was tested after thermal cycling (5000 cycles, 5?°C to 55?°C). Failure modes were determined. The data were analyzed using the Shapiro-Wilk test, two-way ANOVA and post hoc Fisher’s LSD test (p?=?.05). For titanium specimens, the grinding?+?metal primer exhibited higher values than the other groups, and all groups showed higher SBS values than the control group. Combined use of CoJet sand, grinding with a diamond bur, and metal primer application would be useful for enhancing the bond strength of composite resin to titanium. The grinding of the NiCr surface with a diamond bur is the only method that could improve the bond strength of a composite resin compared to the other methods.     

Grain Growth in Superplastically Deformed Zinc Sulfide/Diamond Composites

Grain growth in superplastically deformed ZnS/diamond composites was much greater than in undeformed specimens exposed to the same time and temperature. The extent of grain growth depended on the volume fraction of diamond particles. Grain growth was suppressed by the addition of 10 wt% diamond particles in superplastically deformed specimens. In static annealing, however, only 1 wt% diamond particles were enough to supress grain growth, which is a factor of 10 smaller than estimated by the Zener condition.     

Model of precision surface treatment of quartz glass with a bound abrasive

A model of precision surface treatment of silica glass with a grinding tool on a pitch binder was developed. The relationships between the properties of the materials (fused silica, diamond abrasive grains) and the controlled parameters of the process were obtained. Conditions for the efficient removal of the subsurface damaged layer from the specimen under brittle fracture conditions are presented.Translated from Steklo i Keramika, No. 9, pp. 26–29, September, 1995.     

Influence of sample thickness and measurement set-up on the experimental evaluation of permeability of metallic foams

The pressure drop measured when a fluid flow through a porous medium is presumed a linear function of the medium thickness along the flow direction. Recent works have, however, reported that such linear dependence is not valid for highly porous foams. The deviations observed were attributed to the relative contribution of entrance and exit effects on the total pressure drop. In this work, the effect of the measurement set-up and of the specimen thickness on the pressure drop was investigated. Permeability measurements were carried out with nickel-chromium foams. Two different set-ups for holding the samples were tested. Pressure drop vs. velocity curves were obtained with water at room temperature. The results obtained showed that the normalized pressure drop was affected by the specimen thickness and the set-ups used. The size of the annular section supporting the specimens has a clear influence on the magnitude of the deviation from the expected linear behavior between the pressure drop and the medium thickness. This effect has been attributed to the fluid diffusion and flow expansion into the annular region covered by the specimen support. The magnitude of this effect is more important as the thickness of the specimens increased. No significant entrance or exit effect was observed when the supporting annular section was minimized. The dispersion of permeability constants caused by this effect was determined and discussed.     

Microstructural features and thermal stability of alumina-bonded nano-polycrystalline diamond synthesized by detonation sintering

In this work, alumina-bonded nano-polycrystalline diamond (NPD) was synthesized by detonation sintering in the temperature range of 3000–3500 K and pressure range of 15–25 GPa. The microstructures and thermal stability of the NPD detonation sintered at 3255.05 K and 24.51 GPa were studied, and are described herein. Transmission electron microscopy and electron diffraction revealed that the polycrystalline diamond has a unique formation process and no graphitization. Scanning electron microscopy indicated that the size of polycrystalline particles increased in samples 2, 3, and 4. And thermogravimetric analysis indicated that the thermal stability of the diamond particles was enhanced. The 18% mass loss of specimen corresponded to the oxidation and decomposition of the amorphous carbon and carbon-containing compounds synthesized by detonation. Finally, graphitization calculations showed that the graphitization probability of polycrystalline diamond produced at the temperature of 3255.05 K and pressure of 24.51 GPa was 15.04%.     

Zero-biased and visible-blind UV photodetectors based on nitrogen-doped ultrananocrystalline diamond nanowires

Taking advantage of the superflat surface of ultrananocrystalline diamond (UNCD), highly precise UNCD nanowire (NW) arrays were fabricated to develop high-performance UV photodetectors. The large surface-to-volume ratio of a nanowire significantly increases the number of surface trap states, and the reduced dimensionality effectively confines the active area of the charge carrier and shortens the transit time, which results in an enhanced photoconductivity and response speed. In this paper, the zero-biased UV photodetectors based on nitrogen-incorporated ultrananocrystalline diamond nanowire arrays have been demonstrated and characterized. The estimated responsivity was 2.0 A/W for 350 nm incident light when the device operated at room temperature. The UVA and UVB photocurrent signals from this visible blind photodetector were well defined with a rise and decay time of less than 1 s.     

Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: Effects of Substrate Pretreatments

The nucleation and growth of diamond films on Nicemented carbide is investigated. Substrates made of WC with 6 wt% of Ni were submitted to grinding, and then to different pretreatments (scratching, etching, and/or decarburization) before diamond deposition. Diamond synthesis was carried out by hot-filament chemical vapor deposition (HFCVD) using a mixture of CH₄ (1% v/v) and H₂. Depositions were performed for different lengths of time with the substrates at various temperatures. The specimens were analyzed before and after deposition by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffractometry (XRD). Raman spectra showed that the phase purity of the diamond films was not affected by the presence of nickel on the substrate surface. After wet etching pretreatments, the nucleation of diamond was enhanced, mainly at the WC grain boundaries. Continuous films were obtained on scratched and etched substrates. The decarburizing treatment led to the formation of metallic tungsten and of brittle nicke–tungsten carbide phases. These phases reacted in the early stages of diamond film formation with gaseous carbon species with a parallel process which competes with stable diamond nucleus formation. The diamond film formed after long-term deposition on these samples was not continuous.     

Microstructure and performance of brazed diamonds with multilayer graphene-modified Cu–Sn–Ti solder alloys

This study aims to improve the hardness of solidified Cu–Sn–Ti solder alloys and reduce the erosion of diamonds caused by these solder alloys during brazing. To achieve this aim, a new type of multilayer graphene-modified Cu–Sn–Ti composite solder alloy was proposed for brazing diamonds. The brazed diamond specimens were subjected to morphological observation, characterization of the interfacial microstructures. The static compressive strength and impact toughness of brazed diamond grits were measured. The Vickers microhardness of the solidified solder alloy was quantified, and the microstructure of the solidified solder alloy was also analysed. The results show that brazed diamond specimens fabricated with the No. 2 composite alloy containing 1 wt% multilayer graphene exhibited the best morphology. Addition of excess multilayer graphene reduced the flow properties of the molten Cu–Sn–Ti composite solder alloy. The dominant phases in the solidified Cu–Sn–Ti solder alloys were α-(Cu), Sn₃Ti₅, and CuSn₃Ti₅. Cu, Sn, and Ti were adsorbed by the multilayer graphene, forming C-rich and TiC-dominant phases. Consequently, erosion of the diamonds was reduced during brazing, and TiC was formed in the solidified solder alloy. Thus, increasing the content of multilayer graphene enhanced the static compressive strength and impact toughness of the brazed diamond grits, and increased the hardness of the solidified Cu–Sn–Ti solder alloy.     

Thermal shock resistance of zircon/SiC composite

Abstract

In the present work, thermal shock tests of zircon/silicon carbide (30 wt-%) composite specimens were evaluated up to 1000°C and compared with pure zircon specimens at nearly the same porosity content. Results confirmed that the fracture strength of the quenched specimens was not affected with the increase in quenching temperatures by incorporating SiC particles, indicating resistance to crack propagation. On the other hand, the critical temperature difference ΔT, below which material maintains its original strength, was found higher in composite rather than pure zircon specimens. X-ray diffraction investigations showed that zircon on the surface layer of composite specimens decomposes and produces a specimen comprising core and shell.     

Mechanism of photoconductivity gain and persistent photoconductivity for diamond photodetector

Photoconductivity gain (PG) and persistent photoconductivity (PPC) properties observed in diamond photodetector are theoretically explained by solving rate equations which describe capture and emission processes of photo-generated hole and electron through a boron acceptor and a hole trap in a diamond epilayer and a nitrogen donor in a diamond substrate. Formation of one-sided pn-junction between the epilayer and substrate and slow hole capture rate of acceptor and hole trap levels provide accumulation of photo-generated hole in the epilayer, which produces the PG larger than the ideal responsivity. The PPC current after turning off the deep ultraviolet light is interpreted as due to the slow hole capture rate of the acceptor and trap levels.     

Change of hydraulic conductivity during compression of undisturbed and remolded clays

Eight series of falling head hydraulic conductivity tests were performed on both undisturbed and remolded specimens of four clays from China using a modified oedometer to investigate the change of hydraulic conductivity during compression. Similar to their compression behavior, the undisturbed specimens showed the curves of hydraulic conductivity versus effective vertical stress lying above those of the remolded specimens. Past studies showed that the compression curves of remolded clays could be unified into a unique line using the void index. However, such unification between the void index and the hydraulic conductivity for remolded clays was not found in this study. The relationship between the void index (or void ratio) and the hydraulic conductivity for undisturbed specimen was almost identical to that of the remolded specimen. The relationship between the void ratio and the hydraulic conductivity on a logarithm scale was nearly linear with a slope of 0.5 times the initial void ratio at strains < 20%, and became concave upwards within the full range of strains during compression. Similar to the oedometer test data, the relationship between the specific volume and the hydraulic conductivity could be expressed by a straight line in a bilogarithmic plot within the full range of strains during compression.