HPHT growth and characterization of diamond from a copper-carbon system

Diamond crystallization in the Cu-C system is studied at high pressure high temperature conditions of 7.0 GPa and 1500–1800 °C. The minimum temperature of diamond synthesis by spontaneous nucleation is found to be 1800 °C. Diamond layers of optical quality are grown on the seed crystals via the chemical-potential difference method. From the infrared absorption measurements it is found that the diamond layers grown on the (100) faces of the seed crystals contain nitrogen impurities in the form of A centers with concentration from 700 to 1100 ppm and show strong hydrogen-related absorption peak at 3107 cm^− 1. A number of specific optical centers are found in photoluminescence spectra recorded for diamond layers grown on both (100) and (111) faces of the seed crystals. The newly observed 1.748 eV center is tentatively assigned to defects involving Cu impurities. The 1.787 eV center is suggested to appear preferentially in diamonds with high contents of nitrogen and hydrogen impurities.     

Crystal growth of diamond from a phosphorus solvent under high pressure–high temperature conditions

Crystal growth of diamond in a phosphorus solvent was studied on a seed of natural octahedral diamond at high pressures and high temperatures of 6.2–6.5 GPa and 1600–1700°C for 5–23 h. Although new growth layers of diamond up to 30 μm in thickness were observed on {111} faces of a seed, growth almost stopped within the first 5 h caused by the following occurrence of spontaneous nucleation. The growth layer was usually gray or bluish gray in color and the surface became undulated with dense triangular growth hillocks and growth steps of 〈110〉 directions. On the grown surface were observed two other characteristic features: one is a circular depression with various size particularly observed in the beginning of the growth and the other is a peculiar growth step with the direction indexed as 〈321〉.     

Diamond formation from graphite in the presence of anhydrous and hydrous magnesium sulfate at high pressures and high temperatures

The diamond forming regions in the anhydrous MgSO₄ and hydrous MgSO₄·H₂O–graphite systems, as well as the morphology of diamond synthesized from both of systems were investigated under the conditions of 6.5–7.7 GPa and 1600–2000 °C. The region was basically the same in both systems, although the melting point of MgSO₄ is at least 600 °C higher than that of MgSO₄·H₂O at 7.7 GPa. In the anhydrous system, the appearance of {111} faces is favored at higher temperatures and that of {100} faces at lower temperatures, while in the hydrous system, {111} faces are predominant at all examined conditions.     

Layer-by-Layer assembly and redox properties of undoped HPHT diamond particles

The surface properties of undoped diamond particles are investigated by a combination of zeta potential measurements in solution and electrochemical studies in thin layer assemblies. High-Pressure High-Temperature (HPHT) 500 nm diamond particles exhibit positive and negative zeta potentials depending on pH. The estimated point of zero zeta potential (pzzp) was 6.6, while mobility measurements provided an average charge per particle of ?(843 ± 31)e at high pH. The charge indicates that approximately 50 ppm of surface atoms involves ionisable impurities. The positive charge measured at low pH is of similar magnitude and could be related to nitrogen impurities. The surface charge in basic solutions allows the electrostatic adsorption of diamond particles on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO₂ electrodes (ITO). The particle number density shows a strong dependence on pH, with a maximum value of (1.7 ± 0.3) × 10⁸ cm^?2. Electrochemical studies carried out in the absence of redox species in solution revealed signals associated with sp² type surface states. Analysis of electrochemical responses concluded that 1 × 10⁴ redox centres per particle are involved in a single electron transfer process. We demonstrate that this simple yet versatile approach is rather sensitive to the extent of sp² hybridisation at the surface of diamond powders.     

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.     

Substrate effect on wear resistant transition metal nitride hard coatings: Microstructure and tribo-mechanical properties

Four types of different hard transition metal nitrides (TMN:ZrN, CrN, WN and TiN) coatings were deposited on Si (100) and 316LN stainless steel substrates using DC magnetron sputtering. A comprehensive study of microstructure and substrate dependent tribo-mechanical properties of TMN coatings was carried out. Higher hardness (H) and elastic modulus (E) were obtained for WN (H=40 GPa and E=440 GPa) and TiN (H=30 GPa and E=399 GPa) coatings. This is related to the formation of (100) and (111) preferred orientations in WN and TiN coatings, respectively. However, the less hardness and elastic modulus were obtained for ZrN and CrN coatings where (200) orientation is preferred. Remarkably, low friction coefficient (0.06–0.57) and higher wear resistance in the coatings deposited on steel substrates are directly associated with the higher resistance to plastic deformation (H³/E²) and the presence of intrinsic compressive stress. Three body wear modes enhanced the friction coefficient (0.15–0.62) and the wear rate in the coatings deposited on Si substrates. This is primarily associated with low fracture toughness of brittle single crystalline Si (100) substrates. Steel-on-steel contact was dominated in ZrN/steel sliding system. This occurs due to the severe adhesive wear mode of steel ball, whereas, the abrasive wear modes were attained for the CrN, WN and TiN coatings sliding against steel balls.     

Growth of diamond with Zr-containing molten metal solvents and metal elements as incorporated impurities

Single crystals of diamond were grown using high pressure–high temperature conditions in alloy solvents with Zr added as a nitrogen getter. Problems in the growth process caused by the Zr addition were observed. With a reaction container made of SiO₂ or SiO₂-based material, its nitrogen-gettering ability was markedly decreased. MgO and Al₂O₃ were found to be suitable materials for the container to avoid this problem. Diamonds grown from Zr-containing solvents always had highly developed {111} facets. Zr of 3.8 wt.% in the solvent is sufficient to grow type 2-a diamond. Concentrations of Zr as impurity in single crystals of diamond grown in the Zr-containing solvents have been evaluated as of the order of 0.1 ppm.     

Characterization of crystallinity of a large self-standing homoepitaxial diamond film

We have investigated the crystallinity of a self-standing homoepitaxial diamond film grown at a high rate using a polarized optical microscope, X-ray rocking curves and X-ray topographs. The self-standing film was made by removing a synthetic type-Ib(001) substrate after the film growth using a “lift-off process”. The cross-nicol optical microscope image demonstrated that there exists strain spreading at a relatively large area in the film and the strain inherits over that in the substrate. From cross-sectional X-ray topographs, the dominant defects, observed as dotted patterns in plan-view topographs and micrographs, are dislocations extending along the film growth direction 〈001〉 and generating at the interface between the film and the substrate. The density of such defects counted from these measurements was ~ 1 × 10³ cm^? 2. The dotted patterns observed in X-ray topographs of (220), (? 220) and (400) diffraction planes are almost the same with each other, suggesting that dominant line defects along 〈001〉 are mixed dislocations with a burgers vector directing between 〈001〉 and in-plane of the sample. The high resolution X-ray rocking curve showed that the peak is very sharp with full width at half maximum of 7.56″, which indicates that imperfection of our single-crystal diamond plate is almost the same as that of high-quality high-pressure high-temperature, synthetic type-Ib diamond.     

Net shape forming of green alumina via CNC machining using diamond embedded tool

Diamond embedded tools were fabricated through nickel electroplating of mild steel conical shaped shank for machining of green alumina compacts using Computer Numerical Control (CNC) machine. Flat and pointed end conical tools embedded with different grain sizes viz. ～117 μm (120 mesh) and ～20 μm (625 mesh) of diamond particles were used for green machining of alumina. The hardness values of 625 and 120 mesh tools were measured to be 13.79±3 GPa and 11.84±6 GPa, respectively. Diamond embedded tools were successfully used for net shape fabrication of symmetrical and unsymmetrical objects such as cylinder, dental crown, 3D pattern via CNC machining with submicron range surface roughness. Analysis of mechanical properties and Weibull modulus of the green and sintered alumina samples after green state machining revealed that net shape forming via green machining of alumina using diamond embedded tool is viable.     

Preparation of highly oriented β-SiC bulks by halide laser chemical vapor deposition

Highly oriented β-SiC bulks with high hardness were fabricated by halide laser chemical vapor deposition (HLCVD) using SiCl₄, CH₄ and H₂ as precursors. The effects of total pressure (P_tot) and deposition temperature (T_dep) on the preferred orientation, microstructure, deposition rate (R_dep) and micro-hardness were investigated. The 〈110〉-oriented β-SiC bulks were obtained at low P_tot (2–4 kPa), non-oriented β-SiC bulks were obtained at mediate P_tot (6 kPa), and 〈111〉-oriented β-SiC bulks were obtained at high P_tot (10–40 kPa), exhibiting faceted, cauliflower-like and six-fold pyramid-like microstructure, respectively. The maximum R_dep of 〈111〉- and 〈110〉-oriented β-SiC bulks were 3600 and 1300 μm/h at, respectively. The activation energy obtained by the plot of lgR_dep-T_dep⁻¹ is 170 to 280 kJ mol⁻¹, showing an exponential relation with P_Si. The Vickers micro-hardness of β-SiC bulks increased with increasing P_tot and showed the highest value of 35 GPa at P_tot = 40 kPa with a complete 〈111〉 orientation.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

Effect of diamond surface orientation on the thermal boundary conductance between diamond and aluminum

[100] and [111] oriented diamond substrates were treated using Ar:H and Ar:O plasma treatments, and 1:1 HNO₃:H₂SO₄ heated at 200 °C. Subsequent to these treatments, an aluminum layer was either evaporated or sputteredon the substrates. The thermal boundary conductance (TBC) as well as the interfacial acoustical reflection coefficient between this layer and the diamond substrate was then measured using a Time Domain ThermoReflectance (TDTR) experiment. For the Ar:H plasma treated surfaces the [111] oriented faces exhibited conductances 40% lower than the [100] oriented ones with the lowest measured TBC at 32 ± 5 MWm^− 2 K^− 1. The treatments that led to oxygen-terminated diamond surfaces (extiti.e. acid or Ar:O plasma treatments) showed no TBC anisotropy and the highest measured value was 230 ± 25 MWm^− 2 K^− 1 for samples treated with Ar:O plasma with a sputtered Al layer on top. Sputtered layers on oxygen-terminated surfaces showed systematically higher TBC than their evaporated counterparts. The interfacial acoustic reflection coefficient correlated qualitatively with TBC when comparing samples with the same type of surface terminations (O or H) but this correlation failed when comparing H and O terminated interfaces with each other.     

Mechanical and electrical properties of micron-thick nitrogen-doped tetrahedral amorphous carbon coatings

The effect of nitrogen doping on the mechanical and electrical performance of single-layer tetrahedral amorphous carbon (ta-C:N) coatings of up to 1 μm in thickness was investigated using a custom-made filtered cathode vacuum arc (FCVA). The results obtained revealed that the hardness and electrical resistance of the coatings decreased from 65 ± 4.8 GPa (3 kΩ/square) to 25 ± 2.4 GPa (10 Ω/square) with increasing nitrogen gas ratio, which indicates that nitrogen doping occurs through substitution in the sp² phase. Subsequent AES analysis showed that the N/C ratio in the ta-C:N thick-film coatings ranged from 0.03 to 0.29 and increased with the nitrogen flow rate. Variation in the G-peak positions and I(D)/I(G) ratio exhibit a similar trend. It is concluded from these results that micron-thick ta-C:N films have the potential to be used in a wide range of functional coating applications in electronics.     

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.     

Influence of diamond particles content on the critical load for crack initiation and fracture toughness of SiOC glass–diamond composites

The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m^1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ～ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.     

Diamond wear properties in cold plasma jet

Cold plasma jet is applied to control the contact interface and restrain the diamond wear when cutting ferrous metals. A new generation device was developed to generate stable cold plasma jet under atmospheric pressure. Friction & wear tests and thermal analysis experiments were conducted on a mirror steel NAK80/diamond friction pair in atmospheres of air, nitrogen, and nitrogen cold plasma jet (NPJ), respectively. The friction and diamond graphitization rules were determined through the experiments. Results proved that the friction pair exhibited the best friction and wear properties in NPJ atmosphere. When the rotation speed was ≥ 300 r/min, the nitrogen cold plasma atmosphere was found to reduce the friction coefficient of the friction pair, and the friction coefficient tended to be stable when the load was ≥ 50 N. Moreover, the graphitization temperature of diamond increased from 885 °C to 1185 °C after NPJ treatment.     

Reactive hot pressing of SiC-ZrC composites at low temperature

SiC-ZrC composites with relative density in excess of 99% were prepared by reactive hot pressing (RHP) of SiC and ZrH₂ at 1800 °C for 1 h. The reaction between SiC and ZrH₂ resulted in the formation of ZrC_1-x. The formation process and densification behavior during RHP process were investigated. Low temperature densification of SiC-ZrC composites is attributed to the formed nonstoichiometric ZrC_1-x and the removal of SiO₂ impurity on the surface of SiC particles. As reinforced phase, ZrC_1-x has inhibiting effect on the abnormal grain growth of SiC, resulting in homogeneous microstructure of fine SiC grains. Adding 10 wt% ZrH₂ to SiC, the formed SiC-4.62 vol% ZrC composite exhibited better mechanical properties (Vickers hardness of 27.6 ± 0.7 GPa, flexure strength of 448 ± 38 MPa, fracture toughness of 6.0± 0.3 MPa·m^1/2, respectively) than monolithic SiC ceramic.     

Experimental study on the stability of graphitic C3N4 under high pressure and high temperature

The stability and decomposition of graphitic C₃N₄ (g-C₃N₄) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C₃N₄ was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C₃N₄ has a positive slope (~ 0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic CN framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C₃N₄ was found in the studied P–T range.     

Nanoindentation and tribology of VC,NbC and ZrC refractory carbides

Deformation, damage and wear characteristics of spark plasma sintered VC, NbC and ZrC refractory carbides have been investigated using nanoindentation, tribology and micro/macro − indentation tests. Fractography, using SEM, AFM and confocal microscopy, was used for the characterization of deformation and damage mechanisms. Considerable indentation load − size effect was found in all systems with hardness values from 30 to 36 GPa to 13–17 GPa corresponding to the applied loads of 1 mN and 100 N, respectively. During nanoindentation, characteristic stress-drops were observed on hardness-displacement profiles of NbC and ZrC at depth region of 15–30 nm while this was not typical in VC. The highest coefficient of friction was measured for NbC with a value of 0.45 and the lowest for ZrC with an average value of 0.3. The wear rate of the NbC, and VC was similar, approximately 3 × 10⁻⁶ mm³/Nm only the wear rate of ZrC was larger, approximately 2 × 10⁻⁵ mm³/Nm.