Synthesis and characterization of nano-crystalline CVD diamond film on pure titanium using Ar/CH4/H2 gas mixture

Titanium and Ti alloys have poor tribological properties and deposition of a well adherent diamond coating is a promising way to solve this problem. But diamond film deposition on pure titanium and Ti alloys is always difficult due to the high diffusion coefficient of carbon in Ti, the large mismatch in their thermal expansion coefficients, the complex nature of the interlayer formed during diamond deposition, and the difficulty of achieving very high nucleation density. A nano-crystalline diamond (NCD) film can resolve Ti and Ti alloys weak tribological performance due to its smooth surface. A well-adhered NCD film was successfully deposited on pure Ti substrate by using a microwave plasma assisted chemical vapor deposition (MWPCVD) system in the environment of Ar, CH₄ and H₂ gases at a moderate temperature. Detailed experimental results on the preparation, characterization and successful deposition of the NCD film on pure Ti are discussed.     

Two-step growth of high-quality nano-diamond films using CH4/H2 gas mixture

Diamond films with fine grain size and good quality were successfully deposited on pure titanium substrate using a novel two-step growth technique in microwave plasma-assisted chemical vapor deposition (MWPCVD) system. The films were grown with varying the methane (CH₄) concentration at the stage of bias-enhanced nucleation (BEN) and nano-diamond film deposition. It was found that nano-diamond nuclei were formed at a relatively high methane concentration, causing a secondary nucleation at the accompanying growth step. Nano-diamond film deposition on pure titanium was always very hard due to the high diffusion coefficient of carbon in Ti, the big difference between thermal expansion coefficients of diamond and Ti, the complex nature of the interlayer created during diamond deposition, and the difficulty in achieving very high nucleation density. A smooth and well-adhered nano-diamond film was successfully obtained on pure Ti substrate. Detailed experimental results on the synthesis, characterization and successful deposition of the nano-diamond film on pure Ti are discussed.     

Characteristics of a well-adherent nanocrystalline diamond thin film grown on titanium in Ar/CH4 microwave CVD plasma

The fabrication of a well-adherent diamond film on titanium and its alloys is always problematical due to the different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density. In this work, well-adherent and smooth nanocrystalline diamond (NCD) thin film is successfully deposited on pure titanium substrate by microwave plasma-assisted chemical vapor deposition (MWPCVD) method in Ar/CH₄ environment. It is found that the average grain size is less than 20 nm with a surface roughness value as low as 12 nm. Morphology, surface roughness, diamond crystal orientation and quality are obtained by characterizing the sample with field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy, respectively. Detailed experimental results and mechanisms for NCD film deposition are discussed.     

Nanocrystalline diamond growth on different substrates

Nanocomposite films consisting of diamond nanoparticles of 3-5 nm diameter embedded in an amorphous carbon matrix have been deposited by means of microwave plasma chemical vapour deposition (MWCVD) from CH₄/N₂ gas mixtures. Si wafers, Si coated with TiN, polycrystalline diamond (PCD) and cubic boron nitride films, and Ti-6Al-4V alloy have been used as substrates. Some of the substrates have been pretreated ultrasonically with diamond powder in order to enhance the nucleation density n_nuc. It turned out that n_nuc depends critically on the chemical nature of the substrate, its smoothness and the pretreatment applied. No differences to the nucleation behaviour of CVD PCD films were observed. On the other hand, the growth process seems to be not affected by the substrate material. The crystallinity (studied by X-ray diffraction) and the bonding environment (investigated by Raman spectroscopy) show no significant differences for the various substrates. The mechanical and tribological properties, finally, reflect again the influence of the substrate material: on TiN, a lower hardness was measured as compared to Si, PCD and c-BN, whereas the adhesion of c-BN/nanocrystalline diamond (NCD) system was determined by that of the c-BN film on the underlying Si substrate.     

Effect of argon and substrate bias on diamond thin film surface morphology

Nanocrystalline materials are of high interest, because mechanical and physical properties of such materials are different from those or coarse-grained type. Continuous and smooth nanocrystalline diamond (NCD) thin films were successfully grown on mirror polished silicon substrates, using double bias plasma-enhanced hot filament chemical vapour deposition technique. A gas mixture of Ar:CH₄:H₂ and CH₄:H₂ was used as the precursor gas. The effect of the gas composition, flow rate and substrate bias during deposition on diamond crystallite size was investigated. Changing the growth parameters facilitates control of grain size of polycrystalline diamond thin films from microcrystalline to nanocrystalline. The structure of fine-grained NCD films has been studied with scanning electron microscopy and Raman spectroscopy.     

Effect of rapid thermal annealing treatment on the field-emission characteristics of nanocrystalline diamonds grown on various metal/silicon substrates

In this study, nanocrystalline diamond (NCD) films were deposited on various metal/silicon substrates using a microwave plasma chemical vapor deposition system. Metal layers used are chromium, titanium, aluminum and were used as the electron source for field emitters. These NCD/metal/silicon structures were subsequently annealed at 500 °C in a rapid thermal annealing (RTA) furnace. After RTA treatment, the surface of NCD films becomes flat and the grain boundaries can no longer be clearly seen. The intensity of graphitic peak is substantially decreased and the sp³ content of NCD films is increased. The chemical composition of NCD film remains unchanged after RTA treatment, but the sp³/sp² ratio in C 1s has been increased. It is found that the field-emission characteristics of diamond emitter not only can be effectively controlled by the metal used in the metal/NCD/Si structure, but also can be further enhanced by the improved microstructure of the NCD film obtained after RTA treatment.     

Low-temperature growth of nanostructured diamond films

Nanostructured diamond films are grown on a titanium alloy substrate using a two-step deposition process. The first step is performed at elevated temperature (820 degrees C) for 30 min using a H2/CH4/N2 gas mixture to grow a thin (approximately 600 nm) nanostructured diamond layer and to improve film adhesion. The remainder of the deposition involves growth at low temperature (< 600 degrees C) in a H2/CH4/O2 gas mixture. The continuation of the smooth nanostructured diamond film growth during low-temperature deposition is confirmed by in situ laser reflectance interferometry, atomic force microscopy, micro-Raman spectroscopy, and surface profilometry. Similar experiments performed without the initial nanostructured diamond layer resulted in poorly adhered films with a more crystalline appearance and a higher surface roughness. This low-temperature deposition of nanostructured diamond films on metals offers advantages in cases where high residual thermal stress leads to delamination at high temperatures.     

Deposition of diamond coating on pure titanium using micro-wave plasma assisted chemical vapor deposition

The nucleation and growth of diamond coatings on pure Ti substrate were investigated using microwave plasma assisted chemical vapor deposition (MW-PACVD) method. The effects of hydrogen plasma, plasma power, gas pressure and gas ratio of CH₄ and H₂ on the microstructure and mechanical properties of the deposited diamond coatings were evaluated. Results indicated that the nucleation and growth of diamond crystals on Ti substrate could be separated into different stages: (1) surface etching by hydrogen plasma and the formation of hydride; (2) competition between the formation of carbide, diffusion of carbon atoms and diamond nucleation; (3) growth of diamond crystals and coatings on TiC layer. During the deposition of diamond coatings, hydrogen diffused into Ti substrate forming titanium hydride and led to a profound microstructure change and a severe loss in impact strength. Results also showed that pre-etching of titanium substrate with hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Plasma power had a significant effect on the surface morphology and the mechanical properties of the deposited diamond coatings. The effects of gas pressure and gas ratio of CH₄ and H₂ on the nucleation, growth and properties of diamond coatings were also studied. A higher ratio of CH₄ during deposition increased the nuclei density of diamond crystals but resulted in a poor and cauliflower coating morphology. A lower ratio of CH₄ in the gas mixture produced a high quality diamond crystals, however, the nuclei density and the growth rate decreased dramatically.     

Effect of carbon concentration on the optical properties of nanocrystalline diamond films deposited by hot-filament chemical vapor deposition method

With reducing diamond grain size to nano-grade, the increase of grain boundaries and non-diamond phase will result in the change of the optical properties of chemical vapor deposition (CVD) diamond films. In this paper, the structure, morphology and optical properties of nanocrystalline diamond (NCD) films, deposited by hot-filament chemical vapor deposition (HFCVD) method under different carbon concentration, are investigated by SEM, Raman scattering spectroscopy, as well as optical transmission spectra and spectroscopic ellipsometry. With increasing the carbon concentration during the film deposition, the diamond grain size is reduced and thus a smooth diamond film can be obtained. According to the data on the absorption coefficient in the wavelength range from 200 to 1100 nm, the optical gap of the NCD films decreases from 4.3 eV to 3.2 eV with increasing the carbon concentration from 2.0% to 3.0%. From the fitting results on the spectroscopic ellipsometric data with a four-layer model in the photon energy range of 0.75-1.5 eV, we can find the diamond film has a lower refractive index (n) and a higher extinction coefficient (k) when the carbon concentration increases.     

Heating of polymer substrate by discharge plasma in radiofrequency magnetron sputtering deposition

The substrate used for the thin film deposition in a radiofrequency magnetron sputtering deposition system is heated by the deposition plasma. This may change drastically the surface properties of the polymer substrates. Deposition of titanium dioxide thin films on polymethyl methacrylate and polycarbonate substrates resulted in buckling of the substrate surfaces. This effect was evaluated by analysis of atomic force microscopy topography images of the deposited films. The amount of energy received by the substrate surface during the film deposition was determined by a thermal probe. Then, the results of the thermal probe measurements were used to compute the surface temperature of the polymer substrate. The computation revealed that the substrate surface temperature depends on the substrate thickness, discharge power and substrate holder temperature. For the case of the TiO₂ film depositions in the radiofrequency magnetron plasma, the computation indicated substrate surface temperature values under the polymer melting temperature. Therefore, the buckling of polymer substrate surface in the deposition plasma may not be regarded as a temperature driven surface instability, but more as an effect of argon ion bombardment.     

Nanocluster deposition for oxide thin film growth at near room temperature

Metal-organic chemical vapor deposition (MOCVD) at near room temperature would not only enable integration of oxide films on polymers but would provide the capability of conformal coating of high-aspect ratio features required for fabrication of many micro-and nanoelectronic devices. The concept of near room temperature MOCVD (nanocluster deposition: NCD) consists of the production of a single phase with nanosized crystalline nuclei by a chemical vapor reaction at the showerhead maintained above the decomposition temperature of the precursors and consequently deposition of the nanosized crystalline films on unheated substrates. Deposition of the nanosized crystalline nuclei on unheated substrates was performed by controlling both the showerhead temperature and the working pressure. The Bi(3)NbO(7) (BNO) films deposited without substrate heating (real temperature of substrate surface: 50?°C) exhibit a crystalline single phase with smooth and dense morphologies, a dielectric constant of 30, a leakage current density of ～10(-6)?A?cm(-2) at 0.3?MV?cm(-1) and a step coverage of approximately 93% for films deposited at 100?°C on high-aspect ratio features. An NCD provides a new platform for near room temperature deposition of oxide thin films, opening the way for film deposition on polymer substrates to enable a flexible electronic device technology.     

Nanocrystalline diamond piezoresistive sensor

The design, fabrication and test of piezoresistive sensors based on nanocrystalline diamond (NCD) films are reported. The CoventorWare FEM calculations of the mechanical stress and geometrical deformations of a 3-D structure are used for a proper localization of the piezoresistor on the carrying substrate. The boron-doped piezoresistive sensing element was realized using a directed patterned growth of NCD film on SiO₂/Si by microwave plasma-enhanced chemical vapour deposition (CVD). The gauge factor of boron-doped NCD films was investigated in the range from room temperature up to 200 °C and from 0 to 5 N of the applied force. These NCD piezoresistive sensor elements are compared with a Silicon-on-Insulator (SOI) based piezoresistive sensor and their high-temperature applications are discussed.     

Nanocrystalline diamond coating on carbon fibers produced at different temperatures: Morphological, structural and electrochemical study

In this paper, the morphological, structural and electrochemical properties of nanocrystalline diamond (NCD) films grown on carbon fibers (CF) were investigated. The CF substrates were produced at three different heat treatment temperatures (HTT):1000, 1500 and 2000 °C. The HTT variation promoted different organization indexes on the CF structures. Consequently, the NCD coating formation was strongly affected by the substrate HTT. The changes in the properties of the diamond films were discussed as a function of the film morphology evolution using CH₄ flow rate of 0.25, 0.5 and 1.0 sccm in the feed gas. The X-ray diffraction measurements for the CF and NCD/CF composites were determinant to characterize the crystallinity of the NCD films as a function of the CF HTT and of the CH₄ addition. Based on the diffractograms, the Scherrer's equation was applied to the (111) NCD peak, resulting in grain size values varying from 11.0 to 5.0 nm depending on the CH₄ flow rate and on the CF HTT. The scanning electron microscopy images confirmed the deposition of a continuous NCD coating with high nucleation rate covering the whole CF, while their quality was analyzed by Raman spectroscopy measurements. The NCD grain agglomerates increased as a function of the increase in the CH₄ flow rate from 0.25 to 1.0 sccm, showing similar film morphology to that of the unfaceted diamond balls obtained by chemical vapor deposition. This behavior confirmed the expected tendency by decreasing the diamond quality with the CH₄ addition, especially for the films grown on CF treated at 1500 and 2000 °C. This performance was also corroborated by the cyclic voltammetry measurements concerning the electrode potential window and their responses in a redox couple.     

XPS study of the a-C:H/Ti and a-C:H/a-Si interfaces

In this study ultrathin hydrogenated amorphous carbon (a-C:H) films have been grown onto the titanium and amorphous silicon (a-Si) overlayers by direct ion beam deposition using acetylene gas as a hydrocarbon source. X-ray photoelectron spectroscopy (XPS) was used for study of the DLC-Ti and DLC-Si interfaces. It was revealed that a-Si is a good interlayer for improvement of adhesion in the case of diamond-like carbon film deposition onto the steel substrate at room temperature. a-C:H film growth without substantial intermixing occurred on the a-Si. On the other hand, adhesion between the Ti interlayer and the diamond like carbon film was very sensitive to the deposition conditions (presence of the pump oil) as well as structure and stress level of the Ti film. It was explained by strong intermixing between the growing carbon film and Ti. Bad adhesion between the growing DLC film and Ti interlayer was observed despite formation of the TiC. At the same time, formation of the TiO_x was not an obstacle for good adhesion. It is shown that composition of the used hydrocarbon gas, structure of the Ti thin film and mechanical stress in it had greater influence on adhesion with a-C:H film than elemental composition of the Ti interlayer surface.     

YSZ thin films deposited by e-beam technique

YSZ thin films were grown evaporating cubic and tetragonal phase ZrO₂ stabilized by 8 wt.% of Y₂O₃ (8% of YSZ) ceramic powders by using e-beam deposition technique. Operating technical parameters that influence thin film properties were studied. The influence of substrate crystalline structure on growth of deposited YSZ thin film was analyzed there. The YSZ thin films (1.5-2 μm of thickness) were deposited on three different types of substrates: Al₂O₃, optical quartz (SiO₂), and Alloy 600 (Fe-Ni-Cr). The dependence of substrate temperature, electron gun power, and phase of ceramic powder on thin film structure and surface morphology was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The substrate temperature was changed in the range of 20-600° C (during the YSZ thin film deposition) and its influence on the crystallinity of deposited YSZ thin films was analyzed. It was found that electron gun power and substrate temperature has the influence on the crystallite size, and texture of YSZ thin films. Also, the substrate has no influence on the crystal orientation. The crystallite size varied between 20 and 40 nm and increased linearly changing the substrate temperature. The crystal phase of evaporated YSZ powder has the influence on the structure of the deposited YSZ thin films.     

Interfacial structure, residual stress and adhesion of diamond coatings deposited on titanium

The interfacial structures of diamond coatings deposited on pure titanium substrate were analyzed using scanning electron microscopy and grazing incidence X-ray diffraction. Results showed that beneath the diamond coating, there was one titanium carbide and hydride interlayer, followed by a heat-affected and carbon/hydrogen diffused Ti layer. Residual stress in the diamond coating and TiC interlayer under different process parameters were measured using Raman and X-ray diffraction (XRD) methods. Diamond coatings showed large compressive stress on the order of a few giga Pascal. XRD analysis also showed the presence of compressive stress in the TiC interlayer and tensile stress in the Ti substrate. With increasing deposition duration, or decreasing plasma power and concentration of CH₄ in gas mixture, the compressive residual stress in the diamond coating decreased. The large residual stress in the diamond coating resulted in poor adhesion of the coatings to substrate, but adhesion was also related to other factors, such as the thickness and nature of the TiC interlayer, etc. A graded interlayer design was proposed to lower the thermal stress, modify the interfacial structure and improve the adhesion strength.     

Characterization of Ti-containing amorphous carbon films prepared on titanium substrates

Amorphous carbon (a:C) films prepared on pure titanium (Ti) substrates exhibit relatively high intrinsic compressive stress. In order to obtain low stress films with varied electrical and mechanical properties, metal (Ti) ions are incorporated into the plasma. This is done with the help of metal containing carbon targets. Amorphous carbon films with varied percentage of Ti were deposited on polished pure Ti substrates using Filtered Cathodic Vacuum Arc (FCVA) technique together with substrate pulse biasing. Characterizations of the films were carried out using various equipments including Raman Spectroscopy, X-ray diffractometer, Atomic Force Microscopy (AFM), Pin-on-Disk Tribometer and Micro-Scratch Tester; and properties such as microstructure, crystallography, film stress, morphology, frictional coefficient and critical load were investigated as a function of Ti content in the target. The results suggest that the film prepared with 5 at.% Ti-containing carbon target, under 7 kV substrate pulse bias voltage, displays almost zero stress. However such films are inferior in its Tribological properties compared to that of pure a:C films.     

Titanium oxy-nitride films deposited on stainless steel by an ion beam assisted deposition technique

Surfaces of stainless steel SUS304 were coated with titanium oxy-nitride (TiON) films at temperatures of 400–770°C using an ion-beam assisted deposition technique constructed from an electron beam evaporator for Ti evaporation and a microwave ion source for ionizing nitrogen gas. The N ions were accelerated at energies of 0.5–2.0 keV. Most of the deposited TiON films consisted of (60–80)% TiN and (40–20)% TiO₂, and the fraction of TiO₂ increased with increasing substrate temperature. Hardness of the TiNO films varied in the range from 160 GPa to 260 GPa with increasing substrate temperature. The titanium oxy-nitride film could be deposited on stainless steel without a significant deterioration surface layer at 600°C. However, when TiNO films were deposited at temperatures higher than 700°C, the thickness of the TiNO films were significantly thinner and a thick layer containing nitride such as Cr₂N, CrFe, Fe₂N and Fe₄N was formed in a near surface region of stainless steel because more nitrogen diffused into stainless steel.     

Nanocrystalline diamond films deposited using a new growth regime

Abstract

Time modulated chemical vapour deposition (TMCVD), a new method for depositing nanocrystalline diamond (NCD) coatings, is reported. The key feature of the process is that it utilises modulated methane flow to promote secondary nucleation of nanoscale diamond crystallites. The growth modes of films deposited using both TMCVD and conventional hot filament CVD methods are described. Moreover, a pictorial model showing the key stages of film growth during NCD deposition using TMCVD is presented. The ability of this new process to promote secondary diamond crystallites has been demonstrated.     

The microstructure and wettability of the TiOx films synthesized by reactive DC magnetron sputtering

Different chemical state of titanium oxide films were deposited on commercially pure Ti (CP Ti) by reactive DC magnetron sputtering under different oxygen flow rates to examine a possibility of their applications to endovascular stents. The chemical composition and crystal structure of the obtained films were analyzed by XPS and XRD, respectively. In dependence on the deposition parameters employed, the obtained films demonstrated different mixture of anatase TiO₂, Ti₂O₃, TiO and Ti. The wettability of the films was measured by the water contact angle variation. By formation of titanium oxide film on CP Ti, contact angle was decreased. In order to modify and control the surface wettability, the resultant TiO_x films were etched subsequently by different plasma. The wettability was influenced by etched process according to the decreased contact angle values of etched TiO_x film. Furthermore, TiO_x films became highly hydrophilic by ultraviolet (UV) irradiation, and returned to the initial relatively hydrophobic state by visible-light (VIS) irradiation. The wettability of the TiO_x film was enabled to convert between hydrophilic and hydrophobic reversibly by alternative UV and VIS irradiation. By adjusting deposition parameter and further modification process, the wettability of the TiO_x films can be changed freely in the range of 0–90°.