Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: II, Effects of Deposition Conditions

Diamond films were deposited by hot-filament chemical vapor deposition (HFCVD) on substrates made of WC sintered with 6 wt% of Ni. The as-ground substrates were scratched with diamond powder (S samples) or scratched and wet-etched (SE samples). Diamond synthesis was carried out at substrate temperatures ranging between 600° and 1050°C, and using 1.0% or 2.0% CH₄ in H₂. The diamond nucleation density, as measured by scanning electron microscopy (SEM) and automatic image analysis (AIA), did not significantly change in the 600°-900°C temperature range, while at substrate temperatures higher than 900°C a steep decrease of the density of nuclei was observed and attributed to the thermal annealing of nucleation sites. The activation energy of the growth process was measured and found to be 21 ± 2 kcal/mol. Neither nucleation density nor growth rate were affected by an increase of CH₄ concentration in the feed gas, while a lack of crystallinity was observed at the higher methane concentration. Raman analysis showed that phase purity of the films was affected mainly by the substrate temperature: the lower the temperature, the better the film quality. The presence of Ni on the substrate surface did not induce the preferential formation of non-diamond carbon phases, as confirmed by comparing the Raman spectra obtained from both S and SE substrates. As a comparison, continuous films were deposited on scratched WC-5 wt% Co substrates under the same experimental conditions. The results indicated that the use of Ni as a binder is preferable to Co.     

Surface Pretreatments of Silicon Nitride for CVD Diamond Deposition

The efficiency of different surface pretreatments (four standard chemical etchings and four diamond powder abrasive procedures) on silicon nitride (Si₃N₄) substrates for chemical vapor deposition (CVD) of diamond has been systematically investigated. Blank Si₃N₄ samples were polished with colloidal silica (∼0.25 μm). Diamond nucleation and growth runs were conducted in a microwave plasma chemical vapor deposition apparatus for 10 min and 6 h, respectively. Superior results concerning nucleation density ( N _d∼ 10¹⁰ cm⁻² after 10 min), film uniformity, and grain size (below 2 μm after 6 h) were obtained for the mechanically microflawed samples, revealing that chemical etchings (hot and cold strong acids, molten base or CF₄ plasma) are not crucial for good CVD diamond quality on Si₃N₄.     

Low-Temperature Synthesis of Diamond-like Carbon Films on Steel Substrates by Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition

Using microwave electron-cyclotron-resonance plasma-enhanced chemical vapor deposition, diamond-like carbon films were directly grown at low temperatures (lessthan equal to400°C) on Fe-based alloy substrates without diamond seeding or use of a template layer. A single, broad line in the Raman spectra was observed in the region of 1328-1335 cm^-1 for films grown in gas mixtures with a ratio of CH₄:H₂ greaterthan equal to 2%. In contrast, disordered carbon and graphite phases appeared in the spectra for film grown with a concentration of 20% CH₄ in hydrogen. Diamond nucleation with an amorphous carbon layer was observed in the initial growth stage, while many diamond particles with irregular morphological features were observed on the surface of thicker films. These growth features are a consequence of the catalytic nature of the Fe-based substrate.     

Diamond Growth on a Si Substrate With Ceramic Interlayers

Deposition of diamond films on Si substrates precoated with a series of ceramic intermediate layers was examined. The interlayers containing SiC, SiN _x , SiCN, TiSiN, and TiAlSiN were prepared by a liquid injection plasma-enhanced chemical vapor deposition (PECVD) method using alkoxide solution precursors. The subsequent diamond synthesis on these coatings was carried out by microwave plasma-assisted CVD (MPCVD) using a H₂–1%CH₄ mixture. A higher nucleation density of diamond was obtained on these intermediate layers than on the as-polished Si wafer, along with a nonuniform surface distribution of diamond. Diamond powder scratching pretreatment of these interlayers enhanced the nucleation density and promoted the formation of fully uniform diamond films. Particularly, nanocrystalline diamond films were directly generated on TiSiN and TiAlSiN layers under an identical deposition condition that had favored the formation of microcrystalline diamond films on Si wafers and the Si(C,N) interlayers. The mechanism for this difference is attributed primarily to a higher amount of residual amorphous carbon in TiSiN and TiAlSiN layers than that inside Si(C,N) layers.     

Plasma Synthesis of Tungsten Carbide Nanopowder from Ammonium Paratungstate

A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC_{1− x} ) powder, which sometimes contained a small amount of W₂C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H₂) on the product composition and particle size were investigated. The produced tungsten carbide (WC_{1− x} ) powder was <20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC_{1− x} and W₂C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size <100 nm was obtained.     

Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Columnar and highly oriented (100) BaTiO₃ and SrTiO₃ thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO₃ and SrTiO₃ thin films were fabricated on LaAlO₃ by a seeded growth process that appeared to follow a standard "two-step" growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO₃ throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO₃ and PbTiO₃ seed layers were effective in promoting the growth of highly oriented (100) BaTiO₃ films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO₃ and TiO₂ appeared to dictate thin film microstructural evolution.     

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.     

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

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.     

Effect of WC-Co substrates pre-treatment and microstructure on the adhesive toughness of CVD diamond

Well separated diamond particles were nucleated and grown by hot filament chemical vapor deposition (HFCVD) onto Co-cemented tungsten carbide (WC-Co). Two carbide grades were prepared. The former one was a ISO grade K10 carbide having a 1-μm average WC grain size, 5.8 wt.% Co and 0.2 wt.% VC. The latter one had a 6-μm average WC grain size, with 6 wt.% Co. Prior to deposition the substrates were submitted to two different pretreatments. The adhesive strength of deposited diamond crystallites was quantitatively determined in terms of interface toughness by directly applying an external load to the CVD diamond particles in the scanning electron microscope (SEM). The adhesive toughness was determined from the measurement of the maximum load required to scratch off the diamond crystallites. The variation of adhesive toughness was correlated to both the microstructure and the pretreatments of the substrate.     

Early Stages of Diamond-Film Formation on Cobalt-Cemented Tungsten Carbide

The surface composition of cemented tungsten carbide (WC-5.8 wt% Co) was studied by X-ray photoelectron spectroscopy (XPS), during the early stages of diamond-film deposition, by hot-filament chemical vapor deposition (HFCVD). The nucleated diamond films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and automatic image analysis (AIA). The evolution of the surface composition of cemented tungsten carbide during the early stages of diamond-film deposition was strongly dependent on the substrate temperature. At relatively low temperature (750°C), cobalt-rich particles started to segregate at the substrate surface after a few minutes of diamond deposition. The conspicuous segregation of the binder partly inhibited the formation of stable diamond nuclei, through intense carbon dissolution or carbon segregation at the binder surface, but did not affect nucleic growth. At higher temperatures (940°C), no cobalt-rich particles formed at the substrate surface, even after 2 h of deposition. However, XPS results demonstrated the presence of cobalt in a surface layer, although in a lower amount than at 750°C. Nevertheless, the nucleation density of diamond at 940°C was much lower than at 750°C. Gaps between WC grains formed within 10 mins. Therefore, intergranular cobalt was removed at 940°C, a finding attributed to the etching performed by monohydrogen, rather than to binder evaporation. The time evolution of the substrate area fraction covered by diamond islands, S ( t ), was well described by Avrami kinetics for two-dimensional phase transformations, suggesting that diamond formation took place via a heterogeneous nucleation process. The S ( t ) functions exhibited a similar trend at 750° and 940°C, because the higher growth rate of diamond crystallites at higher temperature counteracted the slower nucleation rate at the higher temperature.     

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.     

Deposition of cubic boron nitride films on diamond-coated WC:Co inserts

Cubic boron nitride (cBN) thin films were deposited on diamond-coated tungsten carbide (WC) cutting inserts using electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD). The effects of gas flow rate and substrate bias on the phase composition and structure of the BN films deposited on diamond surfaces were studied. It was revealed that both the cubic phase formation and the selective etching of hexagonal phase were controlled by modulating the hydrogen and boron trifluoride flow rate ratio. By the trial and error method the gas flow rate ratio and substrate bias voltage were optimized. Moreover the phase composition of the BN film was found to be affected by the thickness of diamond buffer layer and interrelated to the effective substrate bias. The hardness of the resulting cBN films reached the value of 70 GPa. In the synthesized coatings, the diamond beneath renders the best mechanical supporting capacity while the top cBN provides the superior chemical resistance and extreme hardness. The cBN/diamond bilayers deposited on WC inserts may serve as universal tool coatings for machining steels and other ferrous metals.     

Deposition of Pb(Zr,Ti)O3 Thin Films by Metal-Organic Chemical Vapor Deposition Using β-diketonate Precursors at Low Temperatures

Lead zirconate titanate (PZT) thin films were deposited by metal-organic chemical vapor deposition (MOCVD) using β-diketonate precursors and 0₂ at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO₂/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO₂/Si substrates resulted in a very inhomogeneous deposition. A TiO₂ buffer layer deposited on Pt/SiO₂/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO₂-buffered Pt/SiO₂/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO₃-buffered Pt/SiO₂/Si substrates had good crystallinity and a- and c-axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000-å PbTiO₃,-thin-film-buffered Pt/SiO₂/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single-phase perovskite structure with a composition near the morphotropic boundary composition.     

Diamond deposition on chromium, cobalt and nickel substrates by microwave plasma chemical vapour deposition

Only after a relatively long incubation time (which is necessary to saturate the substrate and its surface with carbon by diffusion or formation of an intermediate layer) did diamond nucleation and deposition occur on Cr, Co and Ni, Also, prior to the onset of the diamond formation, non-diamond carbon layers can be formed with too high a concentration of CH₄. However, most of the experimental facts observed during the diamond depositions on Cr, Co and Ni surfaces can be explained by interactions occurring between the reaction gases and the substrates.

Chromium substrates form an intermediate carbide layer prior to diamond deposition. Diamond nucleation did not occur readily. Cobalt has only a low solubility for C. At low CH₄ concentrations, diamond was deposited on pure Co. No deposition of amorphous carbon was observed. Nickel has a certain C solubility. Diamond nucleation occurred only after the substrate and its surface had been carbon saturated. The length of the interval until saturation was reached depended on the substrate thickness.

During the time needed to cover the substrate fully with a diamond layer, the metal vapour from substrate interacted with the diamond growth. Large growth steps developed on the diamond crystal facets. Also refractory metal substrates placed near to the Cr, Co or Ni substrates were contaminated and their diamond coatings exhibited the same growth step features.     

Room-Temperature Thermal Conductivity of Cemented Transition-Metal Carbides

Measurements are reported of the room-temperature thermal conductivity of cemented multicarbides (WC-TiC _x -NbC _x -TaC _x /Co) and straight tungsten carbide (WC/Co), which are widely used tool materials. The thermal conductivity of cemented titanium carbide was found to be lower than that of cemented tungsten carbide. The difference is attributed to strong phonon and electron scattering from carbon atom vacancies in the nonstoichiometric cubic carbide TiC _x ; these defects are absent in stoichiometric hexagonal WC. Higher binder contents in tungsten carbide samples lowered the overall thermal conductivity. Scattering of electrons and phonons by C and W atoms in solid solution in the binder phase presumably reduces its thermal conductivity. No dependence on grain size was detected.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Structure Evolution of Highly Crystallized BaWO4 Film Prepared by an Electrochemical Method at Room Temperature

Highly crystallized BaWO₄ films have been prepared on a tungsten substrate in an alkaline solution containing barium ions by an electrochemical method with a constant direct current density of 1 mA/cm² at room temperature (25°C). The average grain size was about 13 μm, and the thickness about 9 μm after a treatment time of 35 min. The dependence of cell voltage on deposition time was divided into three steps: conduction, anodic oxidation, and breakdown steps. The BaWO₄ film formed during the first step. Electrochemical dissolution of metal tungsten occurred with an accompanying positive change of overpotential in the first step. The crystallization of BaWO4 was characterized by three-dimensional nucleation. In the second step, an amorphous tungsten oxide film formed, thereby increasing the potential. An electrical breakdown occurred in the third step, and the breakdown voltage (about 90 V) was practically the same as those of anodic tungsten oxide films.     

Monolayer-Mediated Deposition of Tantalum(V) Oxide Thin Film Structures from Solution Precursors

Integration of oxide thin films with semiconductor substrates is a critical technology for a variety of microelectronic memory and circuit applications. Patterned oxide thin film devices are typically formed by uniform deposition followed by postdeposition ion-beam or chemical etching in a controlled environment. This paper reports details of an ambient atmosphere technique which allows selective deposition of dielectric oxide thin layers without postdeposition etching. In this method, substrate surfaces are selectively functionalized with hydrophobic self-assembled monolayers of octadecyltrichlorosilane by microcontact printing (μ-CP). Sol-gel deposition of ceramic oxides on these functionalized substrates, followed by mild, nonabrasive polishing, yields high-quality, patterned oxide thin layers only on the unfunctionalized regions. A variety of micrometer-scale dielectric oxide devices have been fabricated by this process, with lateral resolutions as fine as 4 μm. In this paper, we describe the solution chemistry, evolution of microstructure, and electrical properties of Ta₂O₅ thin films, as well as the stress-related mechanism which enables selective de-adhesion and resultant patterning. Selectively deposited, 80-120 nm thick Ta₂O₅ thin film capacitors were crystallized on platinized silicon at 700-800°C, and had dielectric constants of 18-25 depending upon the processing conditions, with 1 V leakage current densities as low as 2 × 10⁻⁸ A/cm². The ability to selectively deposit Ta₂O₅ and other electrical ceramics (such as LiNbO₃ and PbTiO₃) on a variety of technologically important substrate materials suggests broad potential for integrated circuit and hybrid microelectronics applications.     

Microstructure of Molybdenum Disilicide-Silicon Carbide Nanocomposite Thin Films

Composite thin films of molybdenum disilicide-silicon carbide (MoSi₂-SiC) have been deposited via rf magnetron sputtering onto molybdenum substrates. An intermediate layer was deposited in the presence of nitrogen gas and evaluated as a potential diffusion barrier layer. The composite films have been characterized using X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and Auger electron spectroscopy. The as-deposited films were amorphous but crystallized into nanometer-sized grains after annealing under vacuum at 1000°C for 30 min. There was a significant amount of interdiffusion between the film and substrate, which resulted in the formation of subsilicides such as Mo₅Si₃ and MoSi₃, as well as Mo₂C. The films that were deposited via reactive sputtering in a nitrogen ambient were amorphous in both the as-deposited and annealed conditions. Significantly fewer second phases were detected with the presence of the intermediate layer, which suggests the potential use of the nitrided (MoSi _x N _y C _z ) layer as a high-temperature diffusion barrier layer for the silicon and carbon.     

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

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.