Pulsed laser processing of nano-polycrystalline diamond: A comparative study with single crystal diamond

We have conducted laser processing of ultrahard nano-polycrystalline and single crystalline diamonds (NPD, SCD, respectively) using nano-pulsed near-ultraviolet laser, and the machining properties were compared through microstructural examinations by SEM, TEM and Raman spectroscopy. The cut depth of the laser-cut grooves was observed to be deeper for the NPD than for the SCD. This is probably due to the lower thermal conductivity feature of NPD, which provides higher absorption efficiency of the laser energy and decreases the laser ablation threshold. TEM cross-section observation showed that the processed grooves in the both types of diamonds are covered with identical laser-modified layers (~ 1 µm thick) composed of roughly oriented nanocrystalline graphite. A marked difference was observed between the laser-processed surfaces of NPD and SCD: in the former the diamond–graphite interface is almost linear and undamaged, whereas in the latter the boundary is slightly folded and significantly distorted. These textural features suggest that different laser-machining processes are involved between NPD and SCD in the microscopic scale. Our results demonstrate that pulsed laser can be used even more effectively for the fabrication of nano-polycrystalline diamond than the case for single crystal diamond.     

Mechanical properties of synthetic type IIa diamond crystal

The mechanical behavior of synthetic type IIa diamond has been investigated by the Knoop hardness measurement and observation of the cleavage surfaces. It was clarified that the Knoop hardness in (100)100 of synthetic diamonds increases with decreasing of the nitrogen impurities concentration, and that the synthetic type IIa diamond, having few nitrogen impurities, has the highest hardness of synthetic diamonds. In addition, it was found that the Knoop hardness in (100)110 of synthetic type IIa diamond is extremely high, and the anisotropy in the hardness of the diamond is different from those of natural diamond and synthetic type Ib diamond. The cleavage surfaces of the synthetic type IIa diamonds were very smooth and showed remarkably regular cleavage patterns. These results indicate that there are very few impurities and crystal defects in the synthetic type IIa diamond, and also suggest that the diamond has high resistance to plastic flow.     

Morphological evolution of polished single crystal (100) diamond surface exposed to microwave hydrogen plasma

It is shown that exposure of polished single crystal diamond surfaces to microwave hydrogen plasma at optimized conditions may be applied as a very efficient method for the smoothing out of single crystal diamond surfaces. The effect of microwave (MW) hydrogen plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing scratches or lines. However adverse effects on the surface morphology through the formation of pits were found to occur upon long exposure times and high plasma power. A systematic study of the influence of hydrogen microwave plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub-nano-metric resolution by atomic force microscopy and scanning electron spectroscopy.     

Synthesis of large single crystal diamond using combustion-flame method

A combustion flame method is used to synthesize large single crystal diamond in ambient atmosphere. The basic of this technique was originally described by Hirose and Kondo in 1988 [Hirose H, Komaki K. Eur Pat Appl 1988:EP324538]. The advantage of this method is the high growth rate of diamond films, which is about 60 μm/h [Alers P, Hanni W, Hintermann HE. A comparative study of laminar and turbulent oxygen-acetylene flames for diamond deposition. Diam Relat Mat 1992;2:393-6]. The diamond can grow on itself to achieve large single-crystal. Negative substrate-bias effects on diamond growth have been investigated. Diamonds films were characterized by scanning electron microscopy, Raman spectroscopy, and atomic force microscopy in tapping mode. For given conditions, diamond coatings with highly oriented {1 0 0} crystal facets were produced. Large singles crystals diamonds were obtained. The sizes of these crystals vary between 80 and 90 μm. These results are discussed with respect to the competing events occurring during the heteroepitaxial growth of diamond.     

High pressure–high temperature growth of diamond crystals using split sphere apparatus

An overview of the application of crystal growth fundamentals in the high pressure–high temperature production of diamond by solvent/catalyst technique is presented. The process, also called temperature gradient process, makes use of a molten catalyst to dissolve carbon from a source (graphite or diamond powder) and transport the dissolved carbon to a growth site where they precipitate on a diamond seed. The pressure and temperature requirements for the process are generally around 5.0–6.5 GPa and 1300–1700 °C, depending on the chemistry of the solvent used and the desired crystal geometry. In spite of major progress in the science and technology of diamond growth, large scale commercial production of diamonds single crystals for jewelry or electronic applications has not been feasible until recently. This has been mainly due to the substantial cost associated with the presses needed, and the difficulties in controlling the growth parameters and catalyst chemistry. The recent developments in the commercial production of diamond single crystals utilizing the Split Sphere pressurization apparatus are discussed.     

Effects of graphitization degree of crucible on SiC single crystal growth process

Effects of graphite crucible on mass transport and crystal growth process has been investigated in the fabrication of SiC single crystal by the seeded sublimation growth method. Different graphitization degrees of the crucibles were obtained by heat treatment at various temperatures between 2100 and 2300 °C. The crucibles were subjected to SEM and XRD in which the graphitization degree was determined quantitatively. The experimental results indicate that the graphite crucible plays an important role in the SiC crystal growth by providing carbon. High crystal growth is obtained by using the untreated crucibles (corresponding to low graphitization degree), which contributes to the reaction activity between Si and graphite of the crucible. Increasing the graphitization degree results in degradation in crystal growth, even in the graphitization of the SiC seed crystal.     

Multiple twinning and nitrogen defect center in chemical vapor deposited homoepitaxial diamond

Homoepitaxial diamond films were grown on polished {100} faces of single crystal type IIa diamond substrates using microwave plasma assisted chemical vapor deposition system. 14 homoepitaxial diamond films were grown under a variety of substrate temperatures (1000–2000°C), methane concentration (1–6% in hydrogen gas) and processing pressure (60–200 Torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is incorporated as a singly substitutional impurity (P1-defect center) and the nitrogen concentration is in the range 10–100 parts per million (ppm). The substitutional nitrogen concentration in homoepitaxial diamond was observed to decrease with increasing substrate temperature. Multitwin percentages of all grown diamonds derived from EPR spectra are correlated with the growth parameter α, which is simply the growth velocity along the 〈100〉 direction divided by the growth velocity along the 〈111〉 direction. With the aid of multitwin morphology and multitwin percentages derived from EPR, we describe conditions under which a twin-free and low defect single crystal diamond can be grown from the vapor phase on the {100} oriented substrates.     

Structure and properties of Dalan detonation diamonds

This paper presents the results of an investigation of the fine crystal structure of Dalan diamonds synthesized from graphite and carbon black by detonation. The phase composition of the diamond powders was found to vary widely; the structural and structural-morphological states of diamond particles were studied. The main types and subtypes of detonation diamonds are characterized according to transmission electron microscopy data. Assumptions are made concerning diamond formation during detonation synthesis. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 108–116, January–February, 2009.     

Rapid sintering of nano-diamond compacts

Diamond compacts were sintered from nano-size diamond crystals at high pressure, 8 GPa, and temperature above 1500 °C for very short times ranging from 5 to 11 s. Structure and mechanical properties of the compacts have been characterized. Although we have not completely avoided graphitization of diamonds, the amount of graphite produced was low, less than 2%, and despite relatively high porosity, the compacts were characterized by high hardness, bulk and Young moduli.     

Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology,CCL-SEM and CL spectroscopy studies

Colorless octahedral diamonds up to 150 μm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na₂Mg(CO₃)₂–graphite and NaKMg(CO₃)₂–graphite systems at pressures of 8–10 GPa and temperatures of 1700–1800 °C. Seeded growth of carbonate–carbon (CC) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic “metal–carbon” (MC) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa.     

Rocking curve imaging for diamond radiator crystal selection

The combination of low atomic number, high crystal packing density, and very high Debye temperature makes diamond the best material for use as a bremsstrahlung radiator in the coherent bremsstrahlung (CB) process, a process that is uniquely suited for generating highly polarized high-energy photon beams for photonuclear experiments. The crystal quality of the diamond radiator has a vital effect on the polarization and other properties of the photon beam and the best large-area diamond monocrystals currently available, both natural and synthetic, contain many defects that can degrade their performance as CB radiators. The diamonds used for this study were synthetic type Ib samples produced through the HPHT process by the firm Element Six. They were examined using the double crystal rocking curve imaging method in a synchrotron X-ray beam. Dislocation densities were calculated from the measured rocking curve peak position maps in the way proposed by Ferrari et al. [1]. It is shown that dislocation is one major defect that affects the rocking curve width in local regions. The most significant contribution to the whole-crystal rocking curve width for thin crystals is the systematic variation of the peak position across its surface. This is interpreted in terms of a large-scale bending of the entire crystal. Data supporting this interpretation are presented, and possible explanations for the bending and methods for its mitigation are discussed.     

人造六方金刚石的工艺及其在表面技术中的应用——人造六方金刚石的工艺过程

介绍了一种较实用的定向飞片爆炸法人造六方金刚石的装置及工艺过程.爆炸致飞片以高压30～100GPa冲击含石墨的样品使其中的六方晶石墨变为六方金刚石.爆炸后样品经回收及提纯,除去其它金属和非金属杂物,剩下石墨和金刚石混合物,再用物理法和化学法去除石墨,便得到了纯净的金刚石微粉.用X射线衍射法测定其晶体结构并确定六方和立方金刚石的含量及纯度.以SEM照片显示爆炸后的铸铁样品中金刚石的形貌及分布特征.     

Nanoindentation of diamond,graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C₆₀ molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C₆₀ films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.     

Interfacial zone surrounding the diamond in reaction bonded diamond/SiC composites: Interphase structure and formation mechanism

Diamond/SiC composites have attracted considerable research interests due to their outstanding properties sought for a wide range of applications. Among a few techniques used for the fabrication of diamond/SiC composites, molten Si infiltration is an approach highly favored due to its cost-effectiveness and process flexibility. This study critically evaluated the interfacial zone surrounding the diamond in a reaction bonded (RB) diamond/SiC composite. XRD suggests that the composite consists of diamond, α-SiC, β-SiC, Si, and graphite. TEM reveals that a thin layer of graphite surrounds the diamond grain and it appears to form through a process of diamond graphitization and amorphous carbon transformation during the fabrication. In addition, a carbon dissolution and saturation process is proposed as a predominant mechanism for the formation of nano-crystalline SiC near the interface as well as the defects inside the SiC grits. A minor Al₄C₃ phase is occasionally detected near the interface region.     

Thermal analysis for graphitization and ablation depths of diamond films

In the present study, a thick diamond film deposited on a 6″ silicon wafer was planarized by an ArF excimer laser. In order to predict the graphitization thickness formed in the diamond film by a laser beam moving with a sliding velocity as well as a fluence, the general solution of the three-dimensional temperature distribution is successfully developed to be a function of the fluence and pulse duration of the laser beam. A reported model developed for the graphitization probability of diamond film is adopted in the present study to determine the graphitization thickness if the temperature solutions are substituted into this model. The ablation thickness in the graphitization layer can be determined so long as the local temperature in the specimen is higher than the ablation temperature of the graphitization layer. Then, the thickness of graphite residual on the working surface after ablation can be obtained by the subtraction of the ablation thickness from the graphitization thickness. This graphite thickness is proved to be very close to the value obtained from scratching tests. This implies that the temperature solutions predicted by the present model are trustworthy. The effect of the scanning velocity is insignificant to the surface temperature if the pulse duration time is much shorter than the time period between two adjacent laser pulses.     

Experimental design methodology applied to study a diamond purification process

The synthesis of graphite at higher pressure and temperature conditions in the presence of a catalyst–solvent metallic alloy is a commonly applied process for industrial diamond production. The product obtained after this synthesis is an agglomerate composed by diamonds crystals, metallic particles, non-transformed graphite and other compounds. The diamond extraction from the agglomerates is known as purification. In this work the diamond purification process by alkaline melt was studied by statistical methods. It was shown that the alkaline melt could be a very efficient process and environmentally correct procedure for diamond purification.     

Formation of diamond and graphite at high pressure using glassy carbon source

Experimental study on diamond and graphite formation with presence of metal catalysts under 6 GPa and 1300–1600 °C was carried out using pyrolyzed furfuryl alcohol resin (glassy carbon) or graphite and Mg(OH)₂ mixture. Diamond was formed from glassy carbon pyrolyzed higher than 1500 °C in vacuum. Graphite crystals were dominantly formed when glassy carbons pyrolyzed below 900 °C or graphite containing Mg(OH)₂ higher than 3 wt.% were used as starting carbons even by long reaction time (28 h) or diamond seeding experiments. Degree of graphitization of glassy carbon with catalyst metal was increased markedly under diamond-forming pressure and temperature condition. It was apparent that the lower degree of graphitization of starting carbon is not an essential factor to prevent diamond formation. The results revealed that graphite crystals were grown when starting carbons contained approximately higher than 1000 ppm of hydrogen. It was suggested that if the metal carbon system contains a higher amount of C–O–H fluid than that of threshold, diamond nucleation was prevented and graphite was dominantly precipitated.     

Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion

Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10–30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2–6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P≧15 GPa and T≧2300 °C have extremely high hardness (120–145 GPa), which is equivalent to that in the (001)〈100〉 of the synthetic high-purity (type IIa) diamond crystal (116–130 GPa).     

The influence of recess depth and crystallographic orientation of seed sides on homoepitaxial growth of CVD single crystal diamonds

Microwave plasma chemical vapor deposition (MPCVD) has gained increasing attention as a feasible and effective way to produce large, high-quality, single-crystal diamonds. However, the growth of polycrystalline diamond on the periphery of the seed crystal and the cracking generated by the internal stress during the growing process lead to significant decline of the quality and integrity of the CVD diamond, thus increasing the difficulty of synthesizing large diamond layers. Although optimized growth parameters and refined substrate holders have been employed by some researchers to improve the periphery quality of CVD diamond layers, more research needs to be done in this area. In this study, we used a specially designed substrate holder with a circular recess, in which the seed crystal was placed. By designing substrate holders with different recess depths and a seed crystal with different side-surface crystallographic orientations, we aimed to determine the influence of the recess depths and the crystallographic orientation of seed sides on the growth quality according to polarizing microscope, laser Raman spectroscopy, UV fluorescence imaging, and photoluminescence (PL) mapping measurements. The results demonstrate that as the recess depth increases, the amount of polycrystalline diamonds and the internal stress on the periphery are controlled effectively. The crystalline quality is improved, and the growth rate is decreased. In addition, compared to the periphery with {100} seed sides, the periphery with {110} seed sides displays better crystalline quality, lower internal stress, and fewer polycrystalline diamonds after growth, which is probably due to the intrinsic nature of the growth steps propagating on the {100} diamond surface and the effect of nitrogen atoms on the growing process in the diamond lattice.     

CVD金刚石薄膜技术发展现状及展望(下)

简要描述了CVD金刚石薄膜技术的发展历程。介绍了纳米特别是超纳米金刚石膜、CVD金刚石大单晶的技术特点及其应用。超纳米金刚石膜在MEMS(微机电系统)、电化学和生物医学上的应用和CVD金刚石大单晶是当前的研究热点。简言之,金刚石的发展向着更大或者更小的方向深入进行,即"非大即小"。