Continuous Diamond Films Research Articles

Macroporous diamond foam: A novel design of 3D interconnected heat conduction network for thermal management

Diamond with a 3D porous macrostructure has a combination of outstanding intrinsic physicochemical properties and unique structural advantages, which is highly promising for wide-scale practical applications. In this paper, a novel macroscopic porous structure of diamond foam (DF) prepared by hot filament chemical vapor deposition (HFCVD) is proposed for thermal management. The continuous diamond film coated on macroporous Cu foam (CF) substrate forms a monolith of 3D interconnected diamond network, which can act as an effective conductive highway for heat transfer, leading to an exceptional thermal transport property significantly superior to that of CF. Then, DF was further infiltrated with epoxy resin as a thermal conducting filler through the vacuum impregnation process. The thermal conductivity of DF/epoxy composite significantly increased up to 10-fold from 0.23 (neat epoxy) to 2.28 W/m K even with a fairly low diamond loading of 1.2 wt%. The further finite element analysis indicates that the formation of 3D interconnected heat conduction network is advantageous to dissipate heat efficiently in practical application. This work opens up new opportunities toward realizing the application of macroporous DFs and their composites for the use in heat dissipation and the future development of high-performance thermal management systems.

Simulation of Substrate Temperature Distribution in Diamond Films Growth on WC-Co Tools Using HFCVD Method

The substrate temperature is an important factor for diamond films fabricated by hot filament chemical vapor deposition (HFCVD), which affects the grain size and quality of diamond films. In order to deposit polycrystalline diamond films with good quality on the cutting tool, the substrate temperature distribution needs to be further studied. Thus three-dimensional finite element simulations are used to investigate the temperature field with different arrangements of filaments which have profound influences on substrate temperature distribution. Based on the simulation results, the optimum parameters of distance away from drill points and gap between filaments are founded. Subsequently, experiments of depositing diamond films on WC-Co drills are conducted with the optimum values gained from the simulation results. Then, the as-fabricated diamond films are investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results indicate that the surface of drill are covered with a layer of continuous diamond films, which validate that the simulated deposition parameters are conducive and provide a new method to adjust the substrate temperature distribution in the CVD reactor for depositing diamond films.

Low-stress diamond films deposited on stainless steel by a two-step dropped power process in chemical vapor deposition

A continuous low-stress diamond film has been prepared on stainless steel by a novel two-step dropped power process using a Cr/CrN/CrTiAlN film as an interlayer. After the interlayer was deposited on the stainless steel, isolated diamond particles were firstly prepared under high power of 1800W for 10min in CVD, which has low stress, releasing most of thermal stress. Then, the low power of 1500W was adopted to form continuous diamond films. The stress is obviously lower than that of the continuous diamond film deposited only in the first CVD process, indicating that the two-step dropped power process surely decreases the internal stress of the film. The blocky microstructure on sandblasted stainless steel is beneficial to releases the stress. And the existence of very small amount of pores in the diamond film also benefits the stress releasing.

Templated diamond growth on porous carbon foam decorated with polyvinyl alcohol-nanodiamond composite

Porous and self-standing structures are highly promising as membranes, electrodes, high sensitivity gas sensors, and supercapacitors due to their high surface area and tuneable surface properties. Composition of such materials with a polycrystalline diamond film may provide new functionalities for biological and electrochemical applications. However, the diamond growth on foreign substrates as a pinhole-free continuous thin film is still a not a trivial task. Here, we present a successful templated deposition process of polycrystalline diamond film on reticulated vitreous carbon foam (C-foam). Samples of polyvinyl alcohol (PVA) polymer-nanodiamond (ND) mixtures with various ND concentrations were prepared and characterized by means of optical spectroscopy, Raman spectroscopy, dynamic light scattering, zeta potential and pH measurements. The ND/PVA mixtures are employed for the preparation of diamond film by microwave plasma enhanced chemical vapour deposition process, and the effect of their composition (NDs/PVA ratio) on the nucleation (seed density, clustering) and diamond growth (substrate etching, polymer transformation) is analysed. The nucleation, early-stage of diamond growth and the resulting diamond-coated C-foams are characterized by scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The obtained results underline the importance to tailor the NDs/PVA ratio for a well-defined three-dimensional C-foam coverage with the continuous diamond film.

Fabrication of Low Dislocation Density, Single-Crystalline Diamond via Two-Step Epitaxial Lateral Overgrowth

Continuous diamond films with low dislocation density were obtained by two-step epitaxial lateral overgrowth (ELO). Grooves were fabricated by inductively coupled plasma etching. Mo/Pd stripes sputtered in the grooves were used to inhibit the propagation of dislocations originating from the diamond substrate. Coalescent diamond films were achieved by ELO via microwave plasma-enhanced chemical vapor deposition. Etch-pits were formed intentionally to characterize the quality of the epitaxial films and distinguish different growth areas, as dislocations served as preferential sites for etching. In the window regions, a high density of dislocations, displayed as dense etch-pits, was generated. By contrast, the etch-pit density was clearly lower in the overgrowth regions. After the second ELO step, the dislocation density was further decreased. Raman spectroscopy analysis suggested that the lateral overgrowth of diamond is a promising method for achieving low dislocation density films.

The dynamic sublayers for improving the adhesion of CVD diamond films on copper

Polycrystalline diamond films were fabricated on copper substrates by a multi-step process comprised of physical vapor deposition of Al-based composite interlayer on Cu substrate and depositing continuous diamond film on composite interlayer by plasma assisted chemical vapor deposition (PACVD). The interface characteristics and adhesion strength were investigated by Auger electron spectroscopy, Raman analysis, calotest and indentation test. The results show that the continuous film without cracks is successfully obtained. The improved adhesion between diamond film and substrate results from the low residual stress in the film due to their compensation by Al interlayer in the sample cooling process.

Overgrowth of diamond layers on diamond microneedles: New concept for freestanding diamond substrate by heteroepitaxy

A new concept for producing freestanding diamond substrate by heteroepitaxy is proposed. Thick diamond growth by heteroepitaxy is often prevented by heteroepitaxial-strain-related substrate bowing as it leads to substrate cracking. The possibility of using diamond microneedles as a mechanism to neglect and/or exploit substrate bowing is discussed in this new concept; the self-breaking effect of the microneedles is proposed as an application to prevent the main bulk diamond layers from cracking. With an aim toward the realization of this concept, the present study shows the first two important experimental verifications through a homoepitaxial experiment with a high-pressure high-temperature diamond substrate: (1) fabrication of high-aspect-ratio diamond microneedles (2 and 100μm in diameter and length, respectively) by a thermo-chemical etching reaction and (2) overgrowth of diamond on the microneedles with air gaps remaining between microneedles after continuous diamond film overgrowth. We also detail a growth scheme that illustrates how continuous diamond films are created from the microneedles. The strong feasibility for applying the concept in actual heteroepitaxy is suggested through the present study.

High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h− 1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520 W m− 1 k− 1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.

Investigation of residual stress in structured diamond films grown on silicon

Thin diamond strips on Si with the thickness of approx. 0.5 and 1μm and two different widths (100 and 200μm) were fabricated in two different ways: i) selective ion etching of the continuous diamond films and ii) selective area diamond growth. The stress induced in the films was measured by Raman spectroscopy. The measured values were in the range from −0.7 to −0.1GPa. It was found that the stress was compressive and independent of the film thickness. In the films deposited at 950K, more compressive stress than at 1100K was measured. The thermal part of the stress as a consequence of heterostructure cooling from high deposition temperature down to room temperature was calculated by Finite Element Method (FEM) simulations and compared with the measurement. It was found that the calculated thermal stress of the continuous film was rather close to the stress values measured for the etched films. The qualitative distribution of the stress measured from the diamond strip center to its edge coincided very well with the FEM simulations. It was shown that relevant stress studies can be made with a relatively low spatial resolution of the Raman measurement in combination with qualitative results of FEM simulations.

Metal dusting, carburization and diamond deposition on Fe–Cr alloys in CH4–H2 plasma atmospheres

Fe–Cr model alloys (with 20–80wt.% Cr) were exposed to a CH4–H2 mixture in a microwave plasma enhanced chemical vapor deposition (CVD) atmosphere. Severe metal dusting and carburization are observed on the low-Cr alloys, meanwhile, diamond cannot nucleate until a graphite intermediate layer has formed on the substrate, which takes a long incubation time. Increasing Cr concentrations in the Fe–Cr alloys promotes the formation of a Cr carbide buffer layer, which inhibits metal dusting and the formation of graphite interlayer. Consequently, diamond nucleation and growth are greatly enhanced, and continuous diamond films with enhanced adhesion are deposited on the Fe–80Cr alloys.

Enhanced diamond deposition on Kovar alloy substrate with Al interlayer

During chemical vapour deposition of diamond coatings on transition metal (Fe, Co, Ni) substrates, porous graphitic carbon preferentially forms on the substrate surfaces and it induces a spontaneous spallation of the diamond films. In this study, an Al surface barrier film of 80nm thick has been used on a Kovar alloy substrate to enhance the interfacial adhesion of diamond films. Characterisation of the top deposit, interlayer and the underlying substrate was performed by Raman spectroscopy, energy dispersive X-ray analysis, scanning electron microscopy, synchrotron NEXAFS, and X-ray diffraction. The Al interlayer has effectively inhibited the formation of intermediate graphitic carbon and markedly enhanced the adhesion of diamond films. Continuous and adherent diamond films were achieved on the alloy substrate. This investigation also indicates the importance of the continuity of the Al interlayer in maintaining an integrity of the adherent diamond films.

Transformation of polymer composite nanofibers to diamond fibers and films by microwave plasma-enhanced CVD process

In this work, polyvinyl alcohol (PVA) fibers were used as a polymer matrix containing ultra-dispersed diamond (UDD) nanoparticles. Growth of diamond fiber-like structures and films by microwave plasma-enhanced chemical vapor deposition was studied as a function of UDD concentration in the PVA matrix. The influence of surface tension (fibers radii) for nucleation/seeding is discussed. Using a high UDD concentration in the polymer matrix lead to the formation of fiber-like structures. The composite PVA polymer nanofibers with the highest concentration of UDD nanoparticles resulted in the growth of nearly continuous diamond film at low thickness of 250 nm.

Diamond growth on copper rods from polymer composite nanofibres

The potential uses of diamond films can be found in a diverse range of industrial applications. However, deposition of diamond films onto some foreign materials is still not a simple task. Here we present the growth of adherent diamond films on copper rods with the focus on substrate pre-treatment by polyvinyl alcohol composite nanofibres. The primary role of the polymer fibres substantially act as a carbon source which enhances the diamond nucleation and accelerates a homogenous CVD growth. Diamond growth was carried out in pulsed linear antenna microwave chemical vapour deposition system, which is characterized by cold plasma due to larger distance of hot plasma region from the substrate, at various gas compositions. The large distance between plasma source and the substrate holder also allows the uniform deposition of diamond on a large number of substrates with complex geometry (3D objects) as well as for the vertically positioned substrates. Moreover, the inhomogeneity in diamond film thickness deposited on vertically positioned substrates was suppressed by using polyvinyl alcohol nanofibre textile. Combination of PVA polymer fibres use together with this unique deposition system leads to a successful overcoating of the copper rods by continuous diamond film without the film cracking or delamination. We propose that the sequence of plasma-chemical reactions enhances the transformation of certain number of carbon atoms into the sp3-bonded form which further are stabilized by atomic hydrogen coming from plasma.

Preparation and characterization of diamond film on Cu substrate using Cu-diamond composite interlayer

Large area diamond films were fabricated on copper substrates by a multi-step process comprised of electroplating Cu–diamond composite layer on Cu substrate, plating a Cu layer to fix the protruding diamond particles, and depositing continuous diamond film on composite interlayer by hot-filament chemical vapor deposition (HFCVD). The interface characteristics, internal stress and adhesion strength were investigated by scanning electron microscopy, Raman analysis and indentation test. The results show that the continuous film without cracks is successfully obtained. The microstructure of the film is a mixture of large cubo-octahedron grains grown from homo-epitaxial growth and small grains with (111) apparent facets grown from lateral second nuclei. The improved adhesion between diamond film and substrate results from the deep anchoring of the diamond particles in the Cu matrix and the low residual stress in the film.

Nanocrystalline Diamond Matrix Deposited on Copper Substrate by Radical Species Restricted Diffusion

Nanocrystalline diamond matrix (or patterned nanocrystalline diamond) have been grown by hot filament chemical vapor deposition (HFCVD) on copper substrates, which were masked by a copper template filled with through-holes. The influence of the mixing ratio for CH4/H2 source gases, total gas pressure and the aspect ratio (the ratio of hole depth to its diameter) on the morphology, grain size and quality of diamond films were investigated. Continuous diamond films were obtained under 2.0 kpa. When increasing the aspect ratio from 0.67 to 2.0, a gradual reduction of diamond grain size from micrometer to nanometers scale was observed. The formation of nanocrystalline diamond (NCD) matrix can be attributed to the restricted diffusion of radical species and the diamond nucleation kinetics on copper substrates. By through-holes of templates on copper substrates to restrict the diffusion and transport of radical species, NCD matrix was successfully deposited on copper substrates.

Direct Coating Adherent Diamond Films on Fe-Based Alloy Substrate: The Roles of Al, Cr in Enhancing Interfacial Adhesion and Promoting Diamond Growth

Direct CVD deposition of dense, continuous, and adherent diamond films on conventional Fe-based alloys has long been considered impossible. The current study demonstrates that such a deposition can be realized on Al, Cr-modified Fe-based alloy substrate (FeAl or FeCrAl). To clarify the fundamental mechanism of Al, Cr in promoting diamond growth and enhancing interfacial adhesion, fine structure and chemical analysis around the diamond film-substrate interface have been comprehensively characterized by transmission electron microscopy. An intermediate graphite layer forms on those Al-free substrates such as pure Fe and FeCr, which significantly deteriorates the interfacial adhesion of diamond. In contrast, such a graphite layer is absent on the FeAl and FeCrAl substrates, whereas a very thin Al-rich amorphous oxide sublayer is always identified between the diamond film and substrate interface. These comparative results indicate that the Al-rich interfacial oxide layer acts as an effective barrier to prevent the formation of graphite phase and consequently enhance diamond growth and adhesion. The adhesion of diamond film formed on FeCrAl is especially superior to that formed on FeAl substrate. This can be further attributed to a synergetic effect including the reduced fraction of Al and the decreased substrate thermal-expansion coefficient on FeCrAl in comparison with FeAl, and a mechanical interlocking effect due to the formation of interfacial chromium carbides. Accordingly, a mechanism model is proposed to account for the different interfacial adhesion of diamond grown on the various Fe-based substrates.

Enhanced diamond nucleation on copper substrates by employing an electrostatic self-assembly seeding process with modified nanodiamond particles

Nanodiamond seeding is a well-established approach to enhancing the nucleation density in chemical vapor deposition (CVD) diamond growth. However, the effects of nanodiamond seeding are highly dependent upon the dispersion properties of nanodiamond particles, the solvent and the interaction between nanoparticles and substrate surfaces. Surface modification and fractionalization were employed to improve the dispersion of nanodiamond particles and separate those particles into a more narrow range of particle size. Mono-dispersed nanodiamonds with a ζ-potential and average particle size of −41.5mV and ∼25.3nm, respectively, were then obtained. They can be charged on copper substrate without any contaminations. Two-dimensional self-assemblies of nanodiamond seeding were actualized. The density and homogeneity of nanodiamond particles which act as pre-existing sp3 seeds shorten the incubation time of diamond nucleation to less than 30min. High quality of 750nm thick continuous diamond film was deposited on copper substrate in 60min. Furthermore, we calculated electrostatic interaction energy between nanodiamond particle and copper substrate by using the nonlinear Poisson–Boltzmann theory, and discussed interaction energy of nanodiamond-Cu substrate and nanodiamond–nanodiamond in the seeding process.

Enhanced diamond CVD synthesis on steel substrates modified by ion beam implantation and sputtering of Al

Graphite carbon preferentially forms on steel substrates during CVD diamond synthesis process, resulting in poor interface adhesion along with severe carburization-induced substrate damage. In this study, an ion implantation or a sputtering deposition of Al has been carried out to modify steel substrates to enhance diamond deposition. The Al surface modification effectively inhibited formation of intermediate graphite layer, therefore markedly enhances the adhesion of diamond films produced. For the Al-ion implanted steels, the level of substrate carburization damage is dose-dependent and improved significantly with an increasing Al implantation dose from 5×1016ions/cm2 to 2×1017ions/cm2. Continuous and adherent diamond films are formed on the steel substrate modified by an ion-beam sputtered Al film. The fundamental growth mechanism is discussed.

Improved interfacial adhesion between diamond film and copper substrate using a Cu(Cr)–diamond composite interlayer

Large area diamond films were successfully fabricated on copper (Cu) substrates by first electroplating a Cu–diamond composite layer with a tiny amount (0.05–0.1wt.%) of chromium (Cr) in Cu and then growing the continuous diamond film on the composite interlayer by hot-filament chemical vapor deposition (HFCVD). The interfacial characteristics were investigated in detail by scanning electron microscopy and indentation test. The results show that using this new approach, the diamond coating could be firmly adhered to the substrate during rapid cooling after HFCVD. Doping Cr into the Cu–diamond composite interlayer and anchoring the diamond particles deep in the Cu(Cr) matrix are responsible for the much improved interface adhesion between coating and substrate.

A narrow biasing window for high density diamond nucleation on Ir/YSZ/Si(100) using microwave plasma chemical vapor deposition

We present a comprehensive study of the parameter space for biasing the Ir(100) surface to determine the optimal conditions for high density heteroepitaxial diamond nucleation. The nucleation density was determined in a bias voltage range from 0 to 225V, and methane concentration range from 0 to 10% using scanning electron microscopy imaging. These data show that high density nucleation exceeding 1011cm−2 occurs only in a narrow bias voltage range from 125 to 175V and a narrow methane concentration range from 1.5 to 3%. At bias voltages and methane concentrations outside of these windows epitaxial diamond nucleation densities fall abruptly to near zero. Using the conditions for high density epitaxial diamond nucleation 80nm thick featureless continuous diamond films grow already after 20min. In several microns thick films the misfit dislocations are confined within a narrow band of 1μm near the diamond–Ir interface showing low residual stress according to x-ray diffraction measurements.