WC Substrate Research Articles

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 105 cm−2, whereas over 1010 cm−2 after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.

Effect of temperature grain size and cobalt content on hardness of tungsten carbide-cobalt: T.V.Milman et al. (Inst. for Problems in Materials Science, Kiev, Ukraine.) Int. J. Refractory Metals Hard Mater., vol 17, no 1–3, 1999, 39–44

Ni doped Tungsten Carbide thin films (WC+Ni) were deposited onto stainless steel, WC–Co and glass substrates by non-reactive d.c. magnetron sputtering process using a sintered WC–6% Ni target. The WC+Ni hard coating films had thicknesses between 0.6 and 1.5 μm and were deposited at temperatures between 300°C and 550°C. The structure of the films was characterized using X-ray Diffraction (XRD), Secondary Ion Mass Spectroscopy (SIMS) and Reflection Absorption Infrared Spectroscopy (RAIRS) techniques. A morphological analysis was carried out using Atomic Force Microscopy (AFM) and SEM, while the microhardness was measured by nanoindentation. Reflection Absorption Infrared Spectroscopy results confirm that the WCx formation was dependant on substrate temperature. AFM and SEM analysis indicated that the surface morphology is formed by rounded grains. Hardness values deduced from microindentation curves by statistical average on each sample is 18 GPa for all the samples, the same value that has been found from the microindentation measurements on the bare WC substrate.

Growth and erosive wear performance of diamond coatings on WC substrates

Abstract Adherent diamond coatings up to ∼30 μm thick have been grown by microwave plasma CVD on WC substrates and their erosive wear properties investigated under high velocity air–sand erosion testing. Coating properties, particularly residual stress and adhesion, are sensitive functions of surface pre-treatment and deposition procedures. Adherent coatings offer significantly better erosion resistance compared to uncoated substrates, with the erosion rate being lower by up to a factor of ∼5 based on the results obtained to date. Also films with low residual stress appear to exhibit longer lifetimes under testing. Failure tends to occur catastrophically along the coating–substrate interface. The grain boundaries do not appear to represent an easy route for crack propagation. Their strength appears to be comparable to that of the grains themselves.

Adhesion properties of CVD diamond film on binder-less sintered tungsten carbide prepared by the spark sintering process

Abstract This paper describes the adhesion property of chemical-vapor-deposited (CVD) diamond film on tungsten carbide (WC) bodies prepared by spark sintering without a binder. WC bodies ranging from 70 to 95% of density ratio have been sintered under different sintering conditions, and their mechanical properties, such as hardness and bending strength, have been measured. High-quality diamond films with a higher nucleation density compared to that on WC–Co have been deposited on this WC substrate using the microwave plasma CVD method. A comparison study using indentation tests shows that the adhesive strength of diamond films on this binder-less sintered WC is remarkably superior to that on WC–Co, which is believed to result from the increase in diamond nucleation density, the enhanced mechanical bonding between the substrate and the diamond film and the well-matching of thermal expansion coefficients caused by the absence of the fatal obstacle of cobalt. Moreover, an increase in adhesive strength has occurred on the binder-less sintered WC with lower density ratios.

Improvement in adhesion of diamond films on cemented WC substrate with Ti–Si interlayers

Abstract Diamond films were deposited on the cemented WC+(3–5)% Co substrates by a microwave plasma chemical vapor deposition system. The substrates were pretreated with various processing steps before diamond deposition, including: polishing, etching for Co removal, Ti coating by DC sputter, and amorphous Si coating by E-gun. The residual stress of the films was determined by both Raman shift and low incident beam angle X-ray diffraction (LIBAD) methods. The adhesion of the films was evaluated by indentation adhesion testing. The film morphology and film–substrate interface structure were examined by SEM and Auger electron spectroscopy, respectively. The results show that Ti–Si can be a good interlayer to improve film adhesion and inhibit diffusion of Co to the substrate surface on diamond nucleation. This is due to the formation of strong TiC and SiC bonding to enhance film adhesion; Si acts as a promoter for diamond nucleation, and the residual stress with application of interlayer is much lower than that interlayer-free. The results also show the existence of an optimum Ti thickness for the best film adhesion.

Hardness and morphological characterization of tungsten carbide thin films

Ni doped Tungsten Carbide thin films (WC+Ni) were deposited onto stainless steel, WC–Co and glass substrates by non-reactive d.c. magnetron sputtering process using a sintered WC–6% Ni target. The WC+Ni hard coating films had thicknesses between 0.6 and 1.5 μm and were deposited at temperatures between 300°C and 550°C. The structure of the films was characterized using X-ray Diffraction (XRD), Secondary Ion Mass Spectroscopy (SIMS) and Reflection Absorption Infrared Spectroscopy (RAIRS) techniques. A morphological analysis was carried out using Atomic Force Microscopy (AFM) and SEM, while the microhardness was measured by nanoindentation. Reflection Absorption Infrared Spectroscopy results confirm that the WCx formation was dependant on substrate temperature. AFM and SEM analysis indicated that the surface morphology is formed by rounded grains. Hardness values deduced from microindentation curves by statistical average on each sample is 18 GPa for all the samples, the same value that has been found from the microindentation measurements on the bare WC substrate.

Microhardness and adhesion measurements of reactively sputtered TiN/AlN multilayer coatings deposited as function of mass-flow of nitrogen

Multilayer coatings of (Ti, Al)N x have been deposited by reactive sputtering from Ti and Al targets in a side-by-side configuration on WC and stainless steel substrates. The rotation of the substrate holder varied from 2 to 14 r.p.m. corresponding to a bilayer thickness of 0.8–8 nm. The acoustic emission scratch technique for adhesion measurements was used for studying coating performance, and critical load values for the coatings on WC substrate up to 150 N were obtained. The Vickers microhardness in the load range 0.003–2 N was measured, and in order to obtain true hardness values, an optimal range of indentation depth and coating thickness was determined. Depending on the nitrogen flow rate and bilayer thickness, hardness values of 18–23 GPa were obtained at the film surface.

Transmission electron microscope observation of diamond/WC interface

Abstract The growth of polycrystalline diamond films on WC substrate has been investigated by transmission electron microscopy. Preferential growth of columnar subgrains of 10–50 nm in diameter results in the formation of large grains with 4 μm spacing. The interfacial stress between diamond and WC is accommodated by twins and stacking faults. Graphite layers of 10–40 nm in thickness are observed locally, but mostly the diamond is deposited directly on WC grains.

Growth of diamond films on SiC, WC and cubic BN substrates

The growth morphology of diamond films grown on single crystals of SiC and on sintered WC and cubic BN (CBN) substrates by hot filament assisted chemical vapour deposition was examined using transmission electron microscopy and scanning electron microscopy. Diamond was found to have the form of particles on the substrates of SiC and WC in the initial stage of film growth. Both an amorphous layer and a directly bonded area were seen at the interface. Several orientation relationships, different from the cube/cube relation, were observed in these systems. On the other hand, in the case of diamond films on CBN substrates, the growth morphology of diamond was affected by the surface condition of the substrates. When CBN substrates were polished with a diamond paste before deposition, diamond grew in the form of particles. The growth morphology was changed by ion sputtering of the surface of the substrate from particle growth to uniform film growth. These results are discussed on the basis of lattice mismatch at the interface.

The nature of stresses in hot-filament chemical vapour deposited diamond thin films on WC substrates

The nature of film stresses in hot-filament chemical vapour deposited (HFCVD) diamond thin films on tungsten carbide substrates, is reported. Commercial WC substrates were subjected to various surface treatments. Subsequently, they were coated with a diamond film and examined for stresses using X-ray diffraction. All but one of the stress measurements indicated various levels of compressive stresses in the film and at the film–substrate interface. These stresses are compared with those obtained by other researchers. Intrinsic film stresses were also computed for diamond films and found to be tensile. WC drills, of 0.125 in. diameter, were also diamond coated and the stress levels measured along drill flanks and flutes. Significant variations were found in these stresses, and the results were analysed from a film–substrate adhesion perspective.

Origins of the residual stress in CVD diamond films

Diamond films were deposited on (100) Si wafer, WC (5%Co) and quartz substrate materials by a microwave plasma chemical vapor deposition (CVD) system. The effects of deposition and substrate conditions on residual stress of the films were systematically investigated. The films were characterized by scanning electron microscopy, X-ray diffraction, Raman and indentation adhesion testing. The film structure including its non-diamond carbon content, crystal size, texture coefficient, film thickness and surface roughness were examined. The results show that the residual stress of the films is a function of the surface pretreatment, in addition to the substrate material and deposition conditions. The origins of the residual stress are mainly the thermal stress and the intrinsic stress. The intrinsic stress is mainly from the effect of the non-diamond carbon content in the diamond crystals, not at the crystal boundaries. A greater non-diamond carbon content in diamond crystals results in a greater residual stress. The texture of the films has no significant effect on the residual stress. A low compressive residual stress on Si wafer is beneficial to the adhesion of the film.

The modification of mechanical properties and adhesion of boron carbide sputtered films by ion implantation

Mechanical and tribological properties of boron carbide films have been modified by nitrogen ion implantation in the near surface region. For nanocrystalline boron carbide films, hardness, friction coefficient and linear wear intensity were reduced from 3000 to 2800, from 0.18 to 0.09 and from 5.0 × 10 −8 to 2.7 × 10 −8, respectively. For amorphous boron carbide films the hardness and the linear wear intensity increased while the friction coefficient decreased. These modifications were attributed to the creation of defects and to the formation of boron nitride phases. The interface between boron carbide films and Si or WC substrates has also been modified by ion beam mixing and MeV-ion irradiation. Adhesion of B 4C films to the substrate was assessed by measurements of critical loads in scratch tests. For films on Si-substrates the critical load increased from 10 N to 14 N after ion beam mixing and from 10 N to 20 N after MeV-ion irradiation.

Transmission electron microscope observation of diamond / WC interface

Commercial diamond coated WC-Co cutting tool has been investigated by transmission electron microscope (TEM) to understand the nucleation and growth of diamond on WC substrate. Cross-sectional specimens have been prepared by mechanical polishing followed by ion milling. TEM observations have been performed using analytical TEM, JEM 4000FX (JEOL Ltd.).The surface of the coated diamond is composed of grains of 2 - 5 μm in diameter. Each main grain is formed by columnar subgrains of 10 - 50 nm in diameter with similar orientations. Cobalt, a binder in cemented carbide, within 5 μm in depth from the carbide surface has been found to be removed to increase the adhesion of diamond.One of diamond subgrains, which has been grown large in [001] orientation on WC grains is shown in Fig. 1. High density of twins are found on (111) plane starting from WC grain boundaries. It is thought that the interfacial stress between diamond layer and WC grains is accommodated by these twins.

Amorphous diamond film by enhanced arc deposition

A new type of nonhydrogenated diamond-like carbon film defined as amorphous diamond (a-D) can be deposited from Enhanced Arc carbon source on various substrate materials such as Si, high speed steel (HSS) and WC at room temperature. The role of evaporation and condensation of highly ionized carbon plasma has been investigated in this study. Experimental results show that significant relationships exist between the energy distribution of the flux species, the particle surface interactions and the microstructure and mechanical properties of the a-D films. Films deposited in optimum conditions at growth rate of 6 μm h −1 exhibit high hardness and Young's modulus, with peak values measured by the nanoindentation technique of around 95 GPa and 1150 GPa respectively, approaching those of natural diamond. Furthermore, the specific possibility of the modified arc technique compared with other physical vapor deposition processes leads to excellent adhesion on various substrates. Scratch test measurements reveal values of 50 N–80 N on HSS and WC substrates respectively.

Characterization of diamond films on binderless WMo composite carbide

Diamond films have been grown on binderless WMo composite carbide substrates by microwave-plasma-enhanced CVD from a hydrogen-methane gas mixture. The influences of substrate pre-treatment and deposition temperature on the nucleation, crystallinity and morphology of the resultant diamond films were investigated. Indentation tests were conducted to compare adhesion of the various films to the substrates. The results of this study indicate that diamond powder scratching of substrate surfaces before film deposition is required to increase nucleation density in order to grow continuous films. For a 0.5% methane concentration, temperature variation in the range 895–985°C appears to have little influence on the film morphology and crystallinity. When deposited under the same conditions, the adhesion of diamond films on binderless W-Mo composite carbide substrates was shown to be better than that of diamond films on pure W and comparable with that on 6 wt.% Co-bonded WC substrates.

Effect of ion implantation on ion-plated diamond-like carbon films

A Raman spectroscopic study has been carried out on the effects of ion implantation of argon or nitrogen on the structures of ion-plated diamond-like carbon (DLC) films. The carbon films were deposited on WC substrates at room temperature, 200, 250, 300 and 350 °C. Implantation of 150 keV Ar + or N + into the films was carried out with a dose of 1 × 10 16 ion cm −2 at room temperature. The structure of the carbon films was estimated by laser Raman spectroscopy. It was found that the Raman spectra for non-implanted and implanted DLC films consisted of four main peaks: two well-known peaks belonging to graphite and long-ranged-disordered graphite structures, and two other new peaks which can be considered as follows. One peak corresponds to an amorphous-like structure with C=C bonding, and the other peak corresponds to a kind of hydrogenated-like carbon with chains of alternate C=C and C-C bonds. The as-deposited DLC films had various structures, depending on the ion plating conditions. Ion implantation in DLC specimens changed their structures, making them the same. It is expected that an equilibrium of mutual transfer (amorphous graphite) was achieved during ion implantation.

Modeling of Thermal Stresses in Composite Diamond Coatings and Mechanisms of Improvement of Adhesion

AbstractThermal stress influences adhesion of many coatings applied on tool substrates. Management of thermal stresses is important for improvement of coatings and tool life. We have shown that stresses can be controlled by developing a composite layer of diamond with carbides and nitrides such as TiC or TiN. We have modeled the thermal stresses in these composite diamond coatings using finite element analysis. The composite diamond coatings consist of a discontinuous layer of diamond with an embedded layer of TiC or TiN, and a top layer of continuous diamond. For comparison, a single layer of diamond coating has also been used. The thermal stresses in these coatings on WC(Co) and Si3N4 tool substrates were calculated. Results show that the thermal stresses at the interface between the coatings and the substrate are relaxed after introducing the composite layers. This stress relaxation is responsible for the improvement of the adhesion of composite coatings.

Tribological behaviour of chemical vapour deposition diamond films on various cutting tools

Diamond films were deposited using a hot-filament-assisted chemical vapour deposition system on various cutting tool materials ( i.e. cemented WC, sintered SiC, Si-Al-O-N, etc.). The friction coefficients of the films, the adhesion strengths of the films on the substrates, and the surface roughness of the substrates and the films were measured and their correlations analysed. The results indicate that the cutting performance of the diamond-coated inserts is strongly related to the adhesion strength and the friction coefficient of the film. Furthermore, the adhesion strength is also dependent upon the surface Co content and the surface roughness of the pretreated substrate, and the friction coefficient of the film is a function of the surface roughness and the quality and size of the diamond microcrystals. There exists an appropriate range of the substrate roughness values for an optimum adhesion strength. For the cemented WC substrates, the pretreatments of the substrate to remove the surface Co and to microscratch the surface to obtain an optimum surface roughness are essential to the growth of films with a lower friction coefficient and a better cutting performance. The cutting performance of the films on the cemented WC substrates is better than that of those on the sintered SiC or the Si-Al-O-N substrates. This is because there is a lower friction coefficient and a much higher adhesion strength of the film on the cemented WC substrates.

The role of cemented WC substrate morphology on the diamond film growth and cracking resistance

Chemical vapor deposition (CVD) synthetic diamond film is well known and widely studied recently for its potential applications in many engineering fields. Due to its extreme strength and hardness, good abrasive resistance, and low friction coefficient, the synthetic film can be used in tribological situations such as machining or wear resistant coatings. In these cases, the film can be grown on cutting tools, dies or sleeves directly. It is expected that the grown film must possess good properties such as film uniformity, smoothness, and especially good adhesion strength (cracking resistance). And these properties are believed to relate to the substrate morphology intimately. In this paper, the authors report on an investigation of the influence of cemented tungsten carbide substrate morphology on the diamond film growth and cracking resistance.

Diamond film growth on cemented tungsten carbides studied by SEM, AES and XPS

Diamond films were deposited on cemented WC substrates using a hot filament chemical vapor deposition (CVD) system with a mixture of CH 4 and H 2 as the source gases. The effect of the surface pretreatments of scratching the surface and etching to remove the surface Co from the substrate on diamond nucleation and growth was studied. The effect of CH 4 concentration on diamond growth was also examined. Scanning electron microscopy, Auger electron, X-ray photoelectron and energy dispersive spectroscopies were used to examine the film morphology, quality and compositions. The results show that a good diamond-crystal quality can be obtained by reducing the surface Co content in the substrate to below 1% Co in the surface layer of the substrate inhibits both diamond nucleation and growth. Polishing and etching treatments to scratch the surface and to remove the surface Co of the substrates are essential to maintain the surface roughness in an appropriate range to obtain diamond films with a good quality and adhesion. A change in CH 4 concentration in the source gases from 0.5 to 0.15% can change the preferred orientation of the diamond films from the {100} to the {111} plane.