Residual Stresses In Diamond Films Research Articles

A novel method to mitigate residual stress in CVD diamond film on steel substrates with a single intermediate layer

Direct deposition of CVD diamond on steel produces useless diamond film which delaminates on cooling from deposition temperature [1–9]. The majority of current research efforts focus on single and multiple intermediate layers for CVD diamond deposition on steel substrate. There are many excellent diffusional barriers to transition metals migration from bulk to substrate surface to form graphitic sp2 bonds. However, there is no practical solution to the thermal expansion coefficient (TEC) mismatch between diamond and steels. In this work we use a vanadium carbide thermodiffused coating as the intermediate layer between diamond and AISI O1 steel substrate. It acts as a diffusional barrier and mitigates diamond residual stress after cooling from CVD temperature. Increasing vanadium carbide layer thickness creates a transition zone that attenuates diamond residual stress. Up to now, the vanadium carbide is the only single layer barrier able to inhibit graphitic phase formation and mitigate residual stress in diamond films to the level of 0.54 GPa.

Novel Surface Treatment Strategy to Improve the Binding Strength for Diamond Film on Ti Substrate

This work introduced a novel surface treatment strategy to improve the binding strength for diamond film on Ti substrate by etching the Ti substrate with alkali. The presence of diamond, titanium carbide (TiC) phase and the residual stress in diamond films (DFs) was checked by employing Raman spectrum and X-ray diffraction. It was found that the morphology of diamond grain was effected by the surface situation of Ti substrate. And the residual stress was calculated with an empirical equation. The adhesion between the diamond film and Ti substrate (Ti–DFs) was indirectly indicated by elasticity modulus calculated with Nanoindentation. The elasticity modulus was significantly increased to 13.81 GPa for the Ti surface activated by NaOH solution, compared with 1.7 GPa for the diamond film fabricated without surface activation process. As a result, the binding force between Ti–DFs was significantly increased because of the presence of the porous structure on the surface. The internal cross-linked channel structure on Ti surface can play a significant part to improve the binding force between Ti–DFs.

Interfacial stress evolution simulation on the graphite substrate/interlayer/diamond film during the cooling process

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films. The evolution of interfacial stress during the cooling process is critical for the fabrication of large-area crack-free diamond films, so it is very worthy of further study. Variable-temperature X-ray diffraction (XRD) tests and finite element simulation were performed to evaluate the stress evolution of a diamond film. The film (with diameter of 150mm) was prepared on a graphite substrate with Ti interlayer using DC arc plasma jet. Residual compressive stress in the diamond film is gradually increased during the cooling process. Moreover, the value and growth rate of the residual stress from the XRD tests are significantly lower than the finite element simulation results. All XRD peaks of diamond, graphite, and titanium carbide (TiC) generated on both sides of the Ti interlayer shift to a larger angle. By contrast, several XRD peaks of Ti deviate to a low angle. Elastic deformation of the Ti layer may offset the lattice contraction caused by the cooling effect. Elasticity modulus of TiC is significantly greater than that of Ti, which is more inclined to release the stress of the diamond film by self-fracturing. After the variable-temperature tests, residual compressive stress of the diamond film at room temperature is 1.1GPa, whereas that before the tests is 1.4GPa. These values are significantly lower than the finite element simulation results (3.05GPa).

Intrinsic stress evolution during different growth stages of diamond film

The CVD diamond films at different growth stages were prepared on the same substrate by DC arc plasma jet CVD. The surface morphologies and residual stress of diamond films were characterized by scanning electron microscope and Raman spectroscopy. The intrinsic stress of diamond films were obtained by subtracting thermal stress from residual stress. The results showed that the intrinsic compress stress was 0.42GPa for the precoalescence stage due to the sp2 carbon inclusions in diamond islands. With the diamond islands growing up and even snapping together to form grain boundary at the coalescence stage the intrinsic compress stress was 0.06GPa. Eventually, the diamond islands coalesced into a continuous polycrystalline film, the intrinsic compress stress vanished and converted to tensile stress, the intrinsic tensile stress reached 1.41GPa due to the number of grain boundaries increasing. The evolution of intrinsic stress during diamond film initial growth was mainly attributed to the surface free energy of diamond islands changed to the free energy of grain boundaries which was consistent with the Nix-Clemens model.

Sandblasting pretreatment for deposition of diamond films on WC-Co hard metal substrates

Sandblasting combined with acid etching was utilized as pretreatment of Co-cemented tungsten carbide substrates for the deposition of chemical vapor deposition (CVD) diamond films. Diamond films were deposited on the pretreated surface as well as on substrates treated by a two-step alkali-acid pretreatment as a contrast. The field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, Rockwell hardness tester and turning test were respectively conducted to characterize the pretreated substrates and deposited diamond films. Surface roughness of the treated surface was controlled by adjusting sandblasting parameters. An enhancement of surface roughness and removal of the binder phase were detected on the treated surface. The porous Co-depleted carbide layer caused by sandblasting and acid-etching pretreatment was obviously thinner compared to the two-step pretreatment. The nucleation stage of CVD process was investigated and the nucleation density was obviously enhanced by the sandblasting pretreatment. Indentation tests exhibited an improvement of adhesive strength compared to the two-step pretreatment. Moreover, the XRD patterns showed that the residual stress of diamond films was decreased. The turning tests showed that the diamond coated tools with sandblasting-acid pretreatment exhibited less area of flank wear and film shedding.

Structural evolution of Ti destroyable interlayer in large-size diamond film deposition by DC arc plasma jet

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films by self-fracturing and facilitate the harvest of a crack-free free-standing diamond film prepared by direct current (DC) arc plasma jet. In this study, the evolution of the Ti interlayer between large-area diamond film and substrate was studied and modeled in detail. The evolution of the interlayer was found to be relevant to the distribution of the DC arc plasma, which can be divided into three areas (arc center, arc main, and arc edge). The formation rate of titanium carbide (TiC) in the arc main was faster than in the other two areas and resulted in the preferred generation of crack in the diamond film in the arc main during cooling. Sandwich structures were formed along with the growth of TiC until the complete transformation of the Ti interlayer. The interlayer released stress via self-fracture. Avoiding uneven fragile regions that formed locally in the interlayer and achieving cooperatively released stress are crucial for the preparation of large crack-free diamond films.

The influence of deposition temperature on the adhesion of diamond films deposited on WC–Co substrates using a Cr–N interlayer

The possibility of combining the hardness of diamond with the toughness of WC–Co has attracted a great attention toward the deposition of diamond films on WC–Co substrates due to their potential industrial applications. However, the performance of diamond coated WC–Co machining tools is limited by the coating‐substrate adhesion strength. In this work, we report on the effect of deposition temperature on the adhesion of diamond films deposited on WC–10%Co substrates. Prior to diamond deposition, an interlayer of 1.5 μm thick Cr–N was deposited on the WC–Co substrates to minimize cobalt diffusion. Diamond films were deposited using hot filament chemical vapor deposition technique at different temperatures (500–750 °C). Phase purity and residual stress of diamond films were investigated by Raman spectroscopy. Adhesion was determined using Vickers hardness test with discrete loads of 20 kgf by examining the failure occurred at the weaker interface, which allows an easy propagation of indentation induced lateral cracks. The weaker interface was found at the diamond/Cr–N interlayer. Improved adhesion was found for diamond films deposited at 650 °C, compared to the films deposited at other temperatures. A comprehensive analysis about the optimum deposition temperature that leads to better adhesion is discussed.

Fabrication and Application of Si-Doped Diamond Coated Welding Dies

Si-doped diamond films are deposited on cobalt cemented tungsten carbide (WC-Co) welding dies using hot filament chemical vapor deposition (HFCVD) method, where tetraethoxysilane (C8H20O4Si) is introduced in the reactive chamber as silicon source by bubbling method. Filed emission scanning electron microscope (FESEM) and Raman spectroscopy are used to characterize the as-deposited diamond films. The results show that silicon doping can reduce the diamond crystal size and residual stress of diamond films, and also increase the FWHM of first order diamond Raman line. The polishing time of diamond coated welding dies also can be shortened by silicon doping. Si-doped diamond coated welding dies possesses comparable practical application performance with conventional diamond coated welding dies. Compared with the nylon and WC-Co welding dies, the working lifetime of diamond coated welding dies increases 200 and 10 times, respectively.

Analysis of residual stresses in diamond coatings deposited on cemented tungsten carbide substrates

Residual stresses in diamond films grown on WC–Co substrates have been investigated by Raman spectroscopy, x-ray diffraction (XRD), and curvature methods. Microcrystalline diamond films were deposited at 650–700 °C in a conventional hydrogen–methane environment by the microwave plasma-enhanced chemical vapor deposition technique. The film thickness, measured from cross-sectional micrographs taken by scanning electron microscopy, changed from 1.5 to 16.5 μm as the growth time increased from 1 to 12 h. The type and the magnitude of the total residual stress obtained from curvature and XRD measurements agreed very well in all of the samples and changed from tensile to compressive as film thickness increased. However, Raman spectroscopy results showed that all films exhibited compressive stress due to the domain size effect. Different methane fractions, varying from 1% to 3%, have been utilized for diamond growth, and the total residual stress increased as more methane was included.

Investigation of Technology and Analysis of Residual Stress in Large Area Diamond Films

We investigated the residual stress in diamond films grown on molybdenum substrates as a function of different places in the same large sample. The diamond film wafers of Ф60 mm diameter were deposited at 900°C by high power DC arc plasma jet CVD method using a gas mixture of methane (1.8% vol.) and hydrogen ( 90% vol.). The grain sizes, obtained from the top view scanning electron microscopy (SEM) images, were found to become larger from center to the border in the same sample, and the x-ray diffraction indicated that the intensity of characteristic spectroscopy in same diamond film was changed from (220) to (111) with the increases of (311). Profile curves presented the appreciable difference of surface texture from center to edge. The film had 4.3GPa of residual compressive stress. Examination of the Raman spectra of the film revealed that residual stress in the film of up to approximately 0.70GPa, and the Raman spectroscopy shifts from 1332.99cm-1 at the center to 1331.17cm-1 at the border, which means the stress mode changed from compressive to tensile. These demonstrated a significant inhomogeneity of stress in diamond films. The differences have been attributed partly to high temperature inhomogeneity arc jet during growth and morphological aspects of the film growth. The relationships between stress and methane concentration, and substrate temperature are discussed in detail.

Residual Stress Analysis of Microwave Plasma CVD Diamond Films

The residual stress and crystallographic texture of diamond films were investigated in the present work. The diamond films were synthesized on (100) silicon wafer by Microwave Plasma Chemical Vapor deposition (MPCVD). Then the residual stresses of the films were measured by X-ray diffractometer equipped with the two-dimensional detector. The residual stresses can be classified into two categories, i.e., the intrinsic stresses and the thermal stresses. It was shown that the thermal stresses were compressive in the temperature range studied and the intrinsic stresses were tensile. The crystallographic textures of the films were measured by X-ray diffractometer with the method of pole figure and orientation distribution function (ODF). The experimental results suggest that the crystallographic textures of the films depend upon the deposition temperature and methane flow rates, and the components and intensity of crystallographic textures have effect on the residual stresses in diamond films to a certain extent.

Analysis of residual stress in diamond films by x-ray diffraction and micro-Raman spectroscopy

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from −60° to +60°, and applying the sin2 ψ method. For the micro-Raman spectroscopy, we used the summation method, which consists in recording and adding a large number of spectra in different places of a selected area of the sample. All Raman spectra were fitted with Lorentzian lines to separate the contribution of the pure diamond and the other nondiamond (graphite) phases. This spectral analysis performed in each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which increases from 70% to 90% as the thickness goes from 3 to 42 μm. The type and magnitude of the residual stress obtained from x-ray and micro-Raman measurements agreed well for films thicker than 10 μm. For films thinner than this value, an opposite behavior between both results was observed. We attributed this discrepancy to the domain size characteristic of each technique.

Residual stresses in chemical vapour deposited diamond films

Abstract In this paper we report the determination of residual stresses in diamond films grown on Si(100) using a plate bending theory and a bi-metal theory combined with micro-Raman spectroscopy. Raman spectra show that with an increase in the film thickness, the characteristic diamond line shifts from higher wave numbers (>1332 cm −1 ) to lower ( −1 ), indicating a change of compressive to tensile bi-axial stress with increase in the film thickness. A plate bending theory and a bi-metal theory are used to determine the distribution of the stress induced by the thermal mismatch. The modelled results show that the bi-axial stress decreases linearly along the film growth direction and the stress at the film/substrate interface decreases when the film becomes thicker. The difference from the Raman results is attributed to intrinsic stress.

Evaluation of residual stresses in chemical-vapor-deposited diamond films

In this article we report the determination of residual stresses in diamond films grown on Si (100) by using a plate bending theory and a bimetal theory combined with micro-Raman spectroscopy. Raman spectra show that with increase in the film thickness the characteristic diamond line shifts from higher wave numbers (>1332 cm−1) to lower (<1332 cm−1), indicating a change in the nature of residual stress with the film growth. A plate bending theory and a bimetal theory are used to determine the distribution of residual stress induced by thermal mismatch. The modeled results show that the thermal stress decreases linearly along the film growth direction and the stress at the film/substrate interface decreases once the film becomes thicker. The difference from the Raman results is attributed to intrinsic stress, originating from lattice mismatch between the diamond and Si at the growth temperature and from a variation in microstructure of the film with its growth evolution.

Residual stress in diamond films: origins and modelling

The evolution of stress in diamond layers is investigated both experimentally and theoretically in order to develop a comprehensive view of the formation of residual stress. A compressive stress maximum associated with grain coalescence, a decreasing stress with increasing layer thickness and strong stress inhomogeneities at the level of the grain size are observed by in situ macro- and ex situ micro-Raman spectroscopy in diamond films grown on silicon (001) substrates. For most diamond deposits on silicon, neglecting wafer bending for the calculation of thermal stress turns out to be an inappropriate approximation, but even the exact modelling of the thermal stress by means of plate theory and finite element calculations only explains a minor part of the observed stress. Detailed finite element calculations reveal that the average thermal stress, and the thermal stress distribution, are largely modified by temperature gradients during deposition, and by film morphology. Tensile stresses can form due to temperature gradients and surface roughness relaxes an essential part of the thermal stress. The expected average stresses are calculated for common cases. Stress measurements using micro-Raman spectroscopy confirm these predictions obtained from modelling.The microstructure, in particular coherency strains, surface energy effects and disclinations, can contribute substantially to the observed compressive stress maximum at small layer thickness. During grain coalescence, the formation of disclinations can be energetically more favourable than small angle grain boundaries. The related stress fields are estimated to be of the order of several GPa. The formation of large local compressive stresses during grain coalescence is confirmed by micro-Raman spectroscopy. At small layer thicknesses, the evolution of stress is dominated by the microstructure and morphology, whereas at higher thicknesses the thermal stress, including bending effects and temperature inhomogeneities during deposition, is more important.

Elastic modulus of and residual stresses in diamond films

Diamond thin films were prepared using microwave-enhanced chemical vapor deposition techniques with different concentration ratios of CH4 to H2 at temperatures of 800 °C and 900 °C. The elastic modulus of the films varies non-linearly with the change in the ratio of CH4 to H2, and is not significantly affected by changes in the process temperature. Compressive and tensile residual stresses develop in the films prepared at 800 °C and 900 °C, respectively. The magnitude of the stresses is a function of the CH4H2 ratio. An analysis of Raman spectra reveals that the quality of the films is influenced by the CH4H2 ratio.

Finite element analysis of the effectiveness of interlayers in reducing thermal residual stresses in diamond films

The residual stresses resulting from cooling of diamond thin films deposited at elevated temperature have been investigated using finite element analysis. Interlayers of various properties and thicknesses between the diamond and substrate have been simulated to determine the potential for residual stress reduction. Interlayer materials of WC, Si 3N 4 and TiC with tungsten and WC-6%Co substrates were modeled. Multiple interlayer systems composed of Si 3N 4, Si and WC, with Si 3N 4 next to the diamond and WC next to the WC-6%Co substrate, were also modeled to determine if residual stress advantages could be obtained by a simplified gradient material approach. These studies were supplemented by parametric studies to determine the effects of interlayer thickness, elasticity, plasticity and thermal expansion coefficients. Although thermal expansivity is the most significant property in elastic interlayers, the yield strength is most critical in plastic interlayers.