Superconductivity in heavily B-doped diamond layers deposited on highly oriented diamond films

We deposited a [100]-oriented B-doped diamond layer by three methods to clarify the effects of film morphology on the transition from metallic to superconducting diamond. Heavily B-doped [100]-oriented diamond layers were deposited on [first method] undoped polycrystalline diamond films with [111] faces, [second method] highly oriented undoped diamond (HOD) thin films with a pyramidal surface, and [third method] thick undoped HOD films with a pyramidal surface. We confirmed that the B-doped layer in the third method was oriented in the [100] direction by scanning electron microscopy (SEM). The highest transition temperatures were T_c(onset) = 5.0 K and T_c(zero) = 3.1 K for the B-doped layer deposited on a thick HOD film with a pyramidal surface under a zero magnetic field. By contrast, T_c(onset) was 4.1 K for a heavily B-doped diamond layer deposited on a thin HOD film with a pyramidal surface, and was 3.9 K for a heavily B-doped diamond layer deposited on an undoped polycrystalline diamond film. These differences in T_c for our samples are affected by disorder and effective hole-carrier doping in each sample. Using the third method, we successfully deposited a high-quality B-doped [100] layer in three steps: (first step) depositing a [100] HOD film on a Si [100] substrate, (second step) depositing an HOD film with a pyramidal surface, and (third step) depositing a [100]-oriented B-doped layer. The change in the electronic states due to the B-doping of diamond films and the film morphology were investigated by x-ray photoelectron spectroscopy (XPS) measurements and band calculations.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Effect of WC grain growth inhibitors on the adhesion of chemical vapor deposition diamond films on WC–Co cemented carbide

Two sets of Co-cemented tungsten carbide (WC–Co) cutting inserts were sintered using WC powders having different average sized particles (1 and 6 μm). Fine grained WC–Co inserts contained 5.8 wt.% Co and were doped by 0.2 wt.% VC and 0.2 wt.% TaC, which acted as grain growth inhibitors in the liquid-phase sintering. Coarse grained substrates contained 6 wt.% Co and no dopants. Prior to deposition, the inserts were etched using Murakami reagent and then with an acid solution of hydrogen peroxide. The substrates were coated by 31–33-μm diamond films using hot filament chemical vapor deposition (HFCVD) in an atmosphere of 1.5% methane in hydrogen for 14 h, at a substrate temperature of 950 °C. Upon cooling from CVD temperature, only films deposited onto coarse grained inserts were adherent, while films grown on fine grained substrates underwent spontaneous delamination. This fact was due to the presence of a layer of graphitic carbon at the interface between the diamond film and fine grained substrates only. The formation of this sp²-carbon layer correlated well with the observed huge segregation of grain growth inhibitors at the interface between diamond and fine grained substrates.     

A study of diamond film deposition on WC–Co inserts for graphite machining: Effectiveness of SiC interlayers prepared by HFCVD

Thin silicon carbide (SiC) films were deposited from tetramethylsilane/hydrogen gas mixture on Co-cemented tungsten carbide (WC–Co) inserts by using Hot-Filament Chemical Vapour Deposition (HFCVD) technique. Grazing incidence X-Ray Diffraction (XRD) confirmed that the films were composed of cubic silicon carbide (β-SiC) and that small amounts of dicobalt silicide (Co₂Si) were formed. These films were used as interlayers for subsequent CVD of diamond films. XRD and combined Scanning and Transmission Electron Microscopies showed that the binder phase reacted during CVD to form cobalt silicides. However, these intermetallic compounds did not have bad effects on diamond adhesion. Dry turning of graphite was chosen to check the multilayer (SiC + diamond) film performance. For the sake of comparison, machining tests were also carried out under identical conditions using commercial sintered diamond (PCD) inserts and WC–Co diamond coated inserts with no interlayer. The wear mechanism of the tools has been identified and correlated with the criterion used to evaluate the tool life. The results showed that multilayer (SiC + diamond) coatings exhibited the longest tool lives. Therefore, thin SiC interlayers proved to be a new viable alternative and a suitable option for adherent diamond coatings on cemented carbide components and cutting tools.     

Optimization of diamond coated microdrills in aluminum alloy 7075 machining: A case study

Aluminum alloy 7075 is widely used for producing micro-scale heat sinks, micro-fluidic devices, micro-propellers and so on. This paper deals with optimizing microstructure and thickness of diamond coatings on microdrills used in 7075 aluminum alloy machining. Firstly, the friction tests between microcrystalline diamond (MCD), nanocrystalline diamond (NCD) films and aluminum alloy reveal that the stable coefficient of friction (COF) of MCD–aluminum alloy working pair is 0.240, much higher than that of NCD–aluminum alloy working pair (0.072). The decrease of COF is mainly attributed to the lower roughness of NCD films and the presence of more graphite or the non-diamond phases in NCD coatings. Afterwards, comparative cutting tests involving MCD, NCD, diamond-like coating (DLC) and TiAlN coated microdrills show that after drilling 200 holes, NCD coated microdrills exhibit the best cutting performance. Furthermore, NCD coated microdrills with coating thicknesses of 1 μm, 2 μm, 4.5 μm and 7 μm are fabricated and their cutting performance is studied in aluminum alloy machining. The cutting experiments show that the NCD coated microdrill with coating thickness of 4.5 μm shows the best cutting performance, exhibiting not only lowest flank wear and no tool tipping or chipping on the main cutting edges but also the highest quality of drilled holes because of the outstanding adhesive strength and wear resistance of the NCD coating.     

High speed continuous and interrupted dry turning of A390 Aluminum/Silicon Alloy using nanostructured diamond coated WC–6 wt.% cobalt tool inserts by MPCVD

Nanostructured diamond films were grown to a thickness of approximately 35 µm by a 30 kW, 915 MHz, microwave plasma-assisted chemical vapor deposition (MPCVD) on chemically treated WC–6 wt.% Co tool inserts. Rockwell indentation tests were performed to evaluate the adhesion of the films and compared to that of traditional microcrystalline diamond. A series of high speed dry turning tests on high-silicon (18 wt.% Si) aluminum alloy A390 under continuous and interrupted modes were performed and comparisons were carried out to investigate the wear behavior on tool inserts that were uncoated, coated with nanostructured diamond, and commercial PCD (polycrystalline diamond cutter) ones. The tests showed that nanostructured diamond coatings demonstrated excellent durability against the highly abrasive A390 aluminum–silicon alloys in high speed dry turning. Ultra fine grain structure of this coating produces workpiece surface finish comparable or even better than PCD tools in the range we studied. Excellent coating adhesion of nanostructured diamond on WC–6% Co substrates leads to reliable wear behavior. For the first time, we evaluated the performance of nanostructured diamond film coated insert under high speed interrupted turning mode. A “self-cleaning” mechanism was observed which can significantly improve the performance of nanostructured diamond films. Micro-Raman spectra were taken on tested tools to study the wear mechanism of the coating.     

Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers

Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si₃N₄) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si₃N₄ as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10^− 7 mm³/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si₃N₄ inserts may have great potentials for advanced materials machining.     

Cutting performance of time-modulated chemical vapour deposited diamond coated tool inserts during machining graphite

In this investigation, two types of diamond coatings were deposited onto commercially available cemented tungsten carbide (WC-Co) tool inserts using two distinctive processes: i) conventional hot filament chemical vapour deposition (HFCVD) and ii) our recently developed time-modulated chemical vapour deposition process (TMCVD). The TMCVD process enabled the production of smaller sized diamond grains by the promotion of secondary nucleation processes occurring during the larger flow methane modulations. With the conventional HFCVD process, the methane concentration in the gas phase was kept constant during the entire diamond chemical vapour deposition (CVD) process and we labelled the resulting coating as “conventional diamond”, whereas the coating was referred to as “TMCVD diamond” when the methane content in the gas phase was time-modulated by changing its flow rate during CVD. Inserts coated employing both conventional HFCVD and TMCVD deposition process were characterized by scanning electron microscopy (SEM) and Raman spectroscopy and then were tested for turning performance using graphite as working material. For sake of comparison, polycrystalline diamond (PCD) and bare WC-Co inserts were also used for graphite turning. The different wear mechanisms have been compared and discussed in terms of diamond coating microstructure. Repeated turning tests showed that the TMCVD diamond coated inserts exhibit a better wear resistance with respect to inserts coated with conventional diamond coatings, PCD and bare WC-Co inserts.     

Adhesion strength of diamond films on heat-treated WC–Co cutting tools

The adhesion strength and deposition behavior of diamond films with different grain size onto heat-treated WC–Co cutting tool inserts were investigated. The diamond film was deposited on WC–6%Co cutting tool inserts by the hot-filament chemical vapor deposition method, with H₂/3% CH₄ mixed gas. The N₂ gas was incorporated in the mixed gas to refine the grain size of the deposited diamond film (nanocrystalline diamond: NCD).Pores were observed in the interface region between the micrometer-size diamond film (MCD) and the WC–Co cutting tool insert. This suggested that the growth of diamond grains on top of elongated WC grains, which was induced by heat treatment to improve the adhesion strength of the deposited film, hindered the deposition of diamond in the valley area between the elongated WC grains. By contrast, in the case of the NCD film with a grain size of less than 50 nm obtained by addition of N₂ gas, no pores were observed, due to the fact that the refined diamond grains filled the interface region regardless of the existence of the elongated WC grains. The adhesion strength of the NCD film was likely to be greater than that of the MCD film on the heat-treated WC–Co cutting tool insert, which was explained by the full coverage with small diamond grains at the rough interface region.     

Tribological properties of DLC films with different hydrogen contents in water environment

Diamond-like carbon (DLC) films were deposited on silicon wafers by thermal electron excited chemical vapor deposition (CVD). To change the hydrogen content in film, we used three types of carbon source gas (C₇H₈, CH₄, and a CH₄+H₂) and two substrate bias voltages. The hydrogen content in DLC films was analyzed using elastic recoil detection analysis (ERDA). Tribological tests were conducted using a ball-on-plate reciprocating friction tester. The friction surface morphology of DLC films and mating balls was observed using optical microscopy and laser Raman spectroscopy.Hydrogen content in DLC films ranged from 25 to 45 at.%. In a water environment, the friction coefficient and specific wear rate of DLC films were 0.07 and in the range of 10⁻⁸–10⁻⁹ mm³/Nm, respectively. The friction coefficient and specific wear rate of DLC film in water were hardly affected by hydrogen content. The specific wear rate of DLC film with higher hardness was lower than that of film with low hardness. Mating ball wear was negligible and the friction surface features on the mating ball differed clearly between water and air environments, i.e., the friction surface on mating balls in water was covered with more transferred material than that in air.     

Hot filament chemical vapour deposition and wear resistance of diamond films on WC-Co substrates coated using PVD-arc deposition technique

Different Cr- and Ti-base films were deposited using PVD-arc deposition onto WC-Co substrates, and multilayered coatings were obtained from the superimposition of diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The behaviour of PVD-arc deposited CrN and CrC interlayers between diamond and WC-Co substrates was studied and compared to TiN, TiC, and Ti(C,N) interlayers. Tribological tests with alternative sliding motion were carried out to check the multilayer (PVD + diamond) film adhesion on WC-Co substrate. Multilayer films obtained using PVD arc, characterised by large surface droplets, demonstrated good wear resistance, while diamond deposited on smooth PVD TiN films was not adherent. Multilayered Ti(C,N) + diamond film samples generally showed poor wear resistance.Diamond adhesion on Cr-based PVD coatings deposited on WC-Co substrate was good. In particular, CrN interlayers improved diamond film properties and 6 μm-thick diamond films deposited on CrN showed excellent wear behaviour characterised by the absence of measurable wear volume after sling tests. Good diamond adhesion on Cr-based PVD films has been attributed to chromium carbide formation on PVD film surfaces during the CVD process.     

Tribological behavior of GNPs doped Al2O3 coating fabricated by SHVOF thermal spraying on cemented carbide

This paper aims to experimentally investigate the effect of graphene nanoplatelets (GNPs) doped Al₂O₃ coating deposited on the surface of cemented carbide substrate using suspension high velocity oxy fuel (SHVOF) thermal spraying technique. Scanning electron microscopy was applied to characterize GNPs doped Al₂O₃ feedstock, the surface morphologies of cemented carbide before and after spraying, and the wear track morphology of cemented carbide after wear tests. The phases of GNPs doped Al₂O₃ feedstock, uncoated and coated cemented carbide were analyzed by X-ray diffraction. The existence of GNPs was analyzed by Raman spectroscopy. A mixture of un-molten and molten splats formed on the surface of cemented carbide substrate after SHVOF thermal spray. The average coefficient of friction (CoF) of coated samples was slightly lower than that of uncoated samples, which might be due to the friction-reduction effect of GNPs. The wear rate of the samples was one order of magnitude higher than that of the alumina ball, showing that the wear of samples was the main wear between the friction couples. The wear mechanism of uncoated sample was mainly fatigue spalling, and that of cemented carbide substrate coated with GNPs doped Al₂O₃ coating was mainly plowing and abrasive wear.     

Machining behaviour of silicon nitride tools coated with micro-, submicro- and nanometric HFCVD diamond crystallite sizes

Very smooth CVD diamond films are used as direct coatings on Si₃N₄ tool substrates. By adjusting deposition parameters, namely Ar/H₂ and CH₄/H₂ gas ratios, and substrate temperature, nano- (27 nm) and submicrometric (43 nm) crystallite sized grades were produced in a hot filament reactor. Also, a conventional 5 and 12 μm micrometric grain size types were produced for comparison. Normalized coated inserts were tested for dry turning of WC–25 wt.% Co hardmetal. All the CVD diamond grades endured the hardmetal turning showing slight cratering, having the flank wear as the main wear mode. Their turning performance was distinct, as a consequence of morphology and surface roughness characteristics. Among all the tested tools, the more even surface and the submicrometric grade presented the best behaviour regarding cutting forces, tool wear and workpiece surface finishing. For this coating, the depth-of-cut force attained the lowest value, 150 N, the best combination of wear types (KM = 30 μm, KT = 2 μm and VB = 110 μm) and workpiece surface finishing (Ra = 0.2 μm).     

Sol–gel derived undoped and boron-doped ZnO semiconductor thin films: Preparation and characterization

We report the influence of boron doping concentration on the microstructure, electrical and optical properties of solution-processed zinc oxide (ZnO) thin films. The B doping concentration in the resultant solutions was varied from 0 to 5 at%, and the pH value of each synthetic solution was adjusted to 7.0. XRD measurements, SEM observations, and SPM examinations revealed that boron doping produced ZnO thin films consisting of a fine grain structure with a flat surface morphology. Moreover, ZnO thin films doped with B raised the texture coefficient along the (002) plane. All B-doped ZnO (ZnO:B) thin films exhibited higher transparency than that of the undoped ZnO thin film in the wavelengths between 350 and 650 nm. The optical band gap and Urbach energy of the ZnO:B thin films were higher than those of the undoped thin film. According to electrical transport characteristics, the 1% B-doped ZnO thin film exhibited the highest Hall mobility of 17.9 cm²/V s, the highest electron concentration of 1.2×10¹⁵ cm⁻³, and the lowest electrical resistivity of 2.2×10² Ω cm among all of the ZnO:B thin films.     

Synthesis and characterization of Ru doped CuO thin films for supercapacitor based on Bronsted acidic ionic liquid

The synthesis of nanostructured ruthenium (Ru) doped copper oxide (CuO) thin films by colloidal solution method and ionic liquid are presented. The prepared colloidal solution was spin coated on the stainless steel substrates. The coated films were used to measure the specific capacitance in the task specific Bronsted acidic that is in 3-carboxymethyl-1-methylimidazolium bisulfate [CMIM] [HSO₄] ionic liquid (IL). Further, the films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Fourier transform Raman spectroscopy (FT-Raman) and cyclic voltammetry (CV). The Ru doped CuO films exhibit higher specific capacitance, C_sp (C_sp = ratio of average current in CV and a product of scan rate and mass deposited on the film) with the larger potential window as compared to undoped CuO film. The highest C_sp of 406 F g⁻¹ was observed for 15 volume percent of Ru doping concentration. This is the first successful step towards development of ecofriendly CuO based supercapacitors in task specific IL synthesized by green technology.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Electron field emission from the 2D hole gas in hydrogen terminated,polycrystalline diamond

The results of field emission measurements from hydrogen and non-hydrogen terminated polycrystalline, B-doped and undoped diamond surfaces are reported. The field emission from the undoped polycrystalline diamond show different current density vs. electric field characteristics than field emission from hydrogen terminated polycrystalline B-doped diamond and from hydrogen terminated single crystal undoped diamond. These differences are attributed to differences in the conduction mechanisms, being bulk conductivity in B-doped diamond and surface conductivity in undoped hydrogen terminated single crystalline and polycrystalline diamond. In contrast to the hydrogen terminated single crystal undoped diamond case, where discrete jumps in the current density vs. electric field curves are observed [L. Gan, E. Baskin, C. Saguy and R. Kalish, Phys. Rev. Lett. 96, 196808 (2006).] the results for the undoped polycrystalline case follow the Fowler Nordheim formalism, reflecting the presence of an ensemble of 2D surface energy states due to the various angles between the crystallite surfaces and the electric field. The parameters extracted from the Fowler Nordheim representation of the data for hydrogen terminated B-doped and undoped polycrystalline diamond, yield a Fermi energy level near the surface region for of 0.34 eV below the valance band maximum for the undoped sample.     

Deposition and characterization of diamond coatings on WC-Co cutting tools with W/Al interlayer

A new double interlayer W/Al was developed for chemical vapor deposition (CVD) of diamond coatings on cemented WC-Co cutting tools to enhance diamond nucleation and adhesion. A thin layer of Al directly deposited on WC-Co is used to suppress the interfacial graphitization induced by Co and an additional thin layer of W is used to enhance diamond nucleation. The microstructure and adhesion of diamond coatings grown on the W/Al/WC-Co and, for comparison, on W/WC-Co as well as bare WC-Co were investigated. The results demonstrate that diamond coatings grown on W/Al are continuous and well adhesive. The advantage of the interlayer includes that nano-crystalline diamond can be achieved even under typical microcrystalline diamond growth conditions. In addition, the W/Al interlayer of overall 50–65 nm thickness would cause marginal lost of cutting edge sharpness and mechanical integrity of coated cutting tools.     

Enhanced sealing performance with CVD nanocrystalline diamond films in self-mated mechanical seals

Nanocrystalline diamond (NCD) films were evaluated as protective tribo-coatings on silicon nitride mechanical seal rings. The NCD films were deposited by microwave plasma assisted chemical vapour deposition (MPCVD) method from a CH₄/H₂/N₂ gas mixture. The sealing performance and friction behaviour of self-mated NCD films were assessed using the ring-on-ring tribological test in planar configuration varying the rotating speed and the applied load. Water sealing conditions were obtained in the P · V (P, the effective pressure and V, the linear speed) range of 0.5–4.8 MPa ms^− 1. The high hardness and smoothness of the NCD films resulted in a very low and stable friction coefficient value of 0.01, without any measurable wear.     

The annealing effect on the electrical property of the Al/undoped diamond film

Polycrystalline diamond films were deposited using a methane–hydrogen gas mixture in a microwave plasma assisted chemical vapor deposition system. Prior to deposition the silicon substrate was seeded, by photoresist, with 0.1 μm diamond powder. Then, the polycrystalline diamond films were annealed at 800°C under N₂ gas flow for 1 h. From the ESCA analyses, it was observed that the oxygen signal increases substantially, the carbon signal became very weak and the silicon (Si) signal showed up appreciably after the 800°C annealing process. In this study, a conduction mechanism was successfully established for the Al/annealed undoped diamond structure in the temperature ranges of 30–300°C. It was considered that a Schottky contact was also formed in the diamond grain boundaries. The modified equivalent circuit for the Al/annealed undoped diamond structure is an ideal Schottky diode in series with the bulk resistance for the bulk diamond crystallites, which is also in parallel with an opposite pole of the ideal Schottky diode in series with the grain boundary resistance for the diamond grain boundaries. It was observed that the electrical characteristics of the Al/annealed undoped diamond structure showed more ohmic behavior at high temperatures. It was suggested that the oxidation layer in the Al/annealed diamond interface was degraded after the high temperature measuring process. It was found that the field-activated transport mechanism, in which the series resistance can be represented by the modified Frenkel–Poole equation, the effective oxidation layer thickness and the electromigration effects both being included, gave a better fit to the experimental data.