A study of diamond synthesis by hot filament chemical vapor deposition on Nc coatings

Deposition of diamond films onto various substrates can result in significant technological advantages in terms of functionality and improved life and performance of components. Diamond is hard, wear resistant, chemically inert, and biocompatible. It is considered to be the ideal material for surfaces of cutting tools and biomedical components. However, it is well known that diamond deposition onto technologically important substrates, such as co-cemented carbides and steels, is problematic due to carbon interaction with the substrate, low nucleation densities, and poor adhesion. Several papers previously published in the relevant literature have reported the application of interlayer materials such as metal nitrides and carbides to provide bonding between diamond and hostile substrates. In this study, the chemical vapor deposition (CVD) of polycrystalline diamond on TiN/SiN _x nc (nc) interlayers deposited at relatively low temperatures has been investigated for the first time. The nc layers were deposited at 70 or 400 °C on Si substrates using a dual ion beam deposition system. The results showed that a preliminary seeding pretreatment with diamond suspension was necessary to achieve large diamond nucleation densities and that diamond nucleation was larger on nc films than on bare sc-Si subjected to the same pretreatment and CVD process parameters. TiN/SiN _x layers synthesized at 70 or 400 °C underwent different nanostructure modifications during diamond CVD. The data also showed that TiN/SiN _x films obtained at 400 °C are preferable in so far as their use as interlayers between hostile substrates and CVD diamond is concerned. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

Raman spectroscopy investigations of TiBxCyNz coatings deposited by low pressure chemical vapor deposition

TiB_xC_yN_z coatings have been prepared applying LPCVD and characterized using SEM/EDX, XRD, and Raman micro-spectroscopy. It has been shown that first-order, defect-induced Raman spectra of good quality can be obtained from TiB_xC_yN_z coatings, even if buried within a multilayer stack. The Raman peak assignments fit well with previous work on TiC_1 − xN_x. Even small changes in the B:C:N ratio result in systematical shifts of the Raman peaks. With increasing nitrogen content, the acoustical phonons shift to lower frequencies. A high correlation of the Raman shifts with lattice constants derived from XRD has also been found. Additionally, intensity and FWHM of the Raman peaks also change going from carbon- to nitrogen-rich coatings. The sensitivity of the TA peak Raman shifts to changes of the investigated basic coating properties is largest for N-rich coatings. Looking at the full range of coatings the dependence of the Raman shifts is slightly nonlinear.The present work establishes Raman microscopy as a complementary non-destructive technique compared to XRD for studying coatings like TiB_xC_yN_z. Structural, optical and chemical properties can be determined with considerably higher spatial resolution.     

Structural characterization of HfN<Subscript>x</Subscript>-based films grown by metal organic chemical vapor deposition

In this paper, HfN_x-based films on SiO₂/Si stack were grown by metal organic chemical vapor deposition (MOCVD), and one of them was ex-situ annealed at elevated temperature. The structural parameters of HfN_x-based films for the as-grown and the post-growth annealing samples were characterized by Rutherford back-scattering spectrometry (RBS), Spectroscopic Ellipsometry (SE) and atomic force microscopy (AFM). The measurements of the post-growth annealing sample by RBS demonstrated that the N: Hf ratio of HfN_x-based films would decrease with depth increase. In addition, The SE results for the structure of HfN_x-based nitride films were in good agreement with those determined by RBS.     

Thermal conductivity of titanium dioxide films grown by metal-organic chemical vapor deposition

We studied the effect of the microstructures on the thermal conductivity of the titanium dioxide (TiO₂) films. TiO₂ films were grown by MOCVD, their morphologies were observed using a scanning electron microscope (SEM). The chemical composition was determined through Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) measurements. The thermal conductivity of the in-plane direction was measured using an alternating current calorimetric method (laser-heating Angstrom method) in the temperature range of 300 to 470 K. The authors fabricated a TiO₂ film with extremely low thermal conductivity (~ 0.5 Wm^− 1 K^− 1), in which a feather-like texture is regularly arranged in the direction perpendicular to the heat flow. The origins of the extremely low thermal conductivity were studied from a microstructural viewpoint.     

Controlled synthesis of TiO2 mesoporous microspheres via chemical vapor deposition

Anatase titanium dioxide (TiO₂) mesoporous microspheres with core-shell and hollow structure were successfully prepared on a large scale by a one-step template-free chemical vapor deposition method. The effects of various reaction conditions on the morphology, composition and structure of the products were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) technique and photoluminescence (PL) method. The results indicate that the product near the source was composed of core-shell structure TiO₂ microspheres with diameters from 3 to 5 μm. With increasing the distance between the source materials and the substrate, the hollow TiO₂ spheres with 1-2 μm dominant the products. A localized Ostwald ripening can be use to explain the formation of core-shell and hollow structures, and the size of the initial TiO₂ solid nanoparticles plays an important role in determining the evacuation manner of the solid in the ripening-induced hollowing process. The surface area of TiO₂ hollow microspheres determined by the adsorption isotherms was measured to be 74.67 m²/g. X-ray photoelectron spectroscopy (XPS) analysis revealed that the O-H peaks of hollow structures have a chemical shift compared with the core-shell structures. The optical property of the products was also discussed.     

Direct deposition of CeO2 films on Ni metal substrate by chemical vapor deposition

Cerium dioxide thin films have been grown in-situ directly on cube textured Ni substrate by metal-organic chemical vapor deposition (MOCVD). At a lower deposition temperature of 400°C, an amorphous film was formed. The texture of crystalline CeO₂ film was changed from (200) orientation to (111) orientation when the deposition temperature was increased from 450°C to 550°C. The growth rate was ~40 nm/min and the rms surface roughness was 50 nm for the CeO₂ film deposited at 450°C for 10 min. Surface roughness of the film was increased with the development of (111) orientation. Deposited CeO₂ film showed a mixed texture of (100)<001> and (100)<011> orientation. Depending on the deposition condition, the transition from (100)<001> texture to (100)<011> orientation was observed.     

Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

CO addition in low-pressure chemical vapour deposition of medium-temperature TiCxN1-x based hard coatings

TiC_xN_1-x base layers with Al₂O₃ top layers, both grown by chemical vapour deposition (CVD), are state-of-the-art in metal cutting with cemented carbide inserts. In order to influence the microstructure and properties of the base layer, five different TiC_xN_1-x coatings were grown by medium-temperature CVD with increasing CO fractions in the TiCl₄–CH₃CN–H₂–N₂–CO feed gas using an industrial-scale low-pressure CVD system. With the CO fraction in the feed gas rising up to 2.2 vol.%, oxygen contents of 2.2. at.% could be detected, which are distributed homogeneously over the coating thickness. The originally equiaxed TiC_xN_1-x grains with preferred {110} orientation obtained without CO addition change to randomly distributed plate-like grains, while the grain size decreases to the sub-micron range. Also the surface roughness decreases with rising CO addition. The incorporation of oxygen leads to a homogenous distribution of dislocations within the TiC_xN_1-x grains and to an increasing density of twin boundaries. The residual tensile stress shows a minimum of 200 ± 47 MPa, while the hardness increases to 29.1 ± 1.3 GPa and the elastic modulus reaches a maximum 569 ± 29 GPa for CO fractions of 1.5 vol.% in the feed gas.     

In situ IR pyrometric analysis during thermal treatment in air of TiOxNy coatings

IR pyrometry is an original diagnostic tool for in situ analysis of surface transformations of coatings or bulk materials during the thermal treatment under reactive atmosphere such as high temperature oxidation. Significant oscillations of the pyrometric signal were observed during annealing in air of TiO_1.5N_0.5 coatings in the temperature range 673-823 K. This is due to interferences resulting from multi-reflections at the interfaces of a transparent growing film. This reveals the formation of a TiO₂ thin film on the top of the TiO_1.5N_0.5 coating. Modeling of the time dependence of the IR pyrometric signal allows the determination of the oxide layer thickness, transformation rate and optical properties of the films under the growth conditions. The progressive oxidation of a compact amorphous TiO_1.5N_0.5 coating from the external surface to the substrate interface was supported by SIMS, XRD and reflectance analyses.     

SnO2 coated Ni particles prepared by fluidized bed chemical vapor deposition

A Fluidized Bed Metal–Organic Chemical Vapor Deposition (FB-MOCVD) process was developed for the growth of tin oxide thin films on large hollow Ni particles. Tetraethyl tin was used as tin source and dry air both as fluidization gas and oxidant reagent. The SnO₂ films were grown in a FBCVD reactor under reduced pressure (13.3 kPa) in the temperature range of 633–663 K. A series of specific experiments was carried out to optimize the design of the reactor and to determine the range of experimental parameters (flow rate, pressure, temperature) leading to good fluidization of the large hollow Ni particles used as base material. The SnO₂ films deposited on particles exhibited a dense nodular surface morphology similar to that previously observed on flat substrates. The relative thickness of the films was determined by EDS analyses and the real values were measured by SEM on cross-sections of particles. The SnO₂ films exhibit satisfactory thickness uniformity from one particle to another in the same batch and from run to run. XRD studies revealed that the films exhibited good crystallinity with the cassiterite structure. Traces of NiO were found at the SnO₂/Ni interface. Finally, the SnO₂ CVD coated-Ni particles were used as anodes in an electrochemical cell. The potential limit of oxygen evolution measured was that of the SnO₂ layer despite the initial porosity of the hollow Ni particles inherent to their preparation. This work is the first step towards the preparation of high specific surface area electrodes.     

Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.     

High temperature chemical vapor deposition of AlN/W1−xRex coatings on bulk SiC

The residual porosity of structural silicon carbide (SiC) composites limits their use in advanced nuclear systems. The use of thick coatings of high-Z materials like tungsten (W) or tungsten alloys (W_1−xRe_x) is a promising solution to overcome such problems. However, solid-state reactions occur between SiC and metals at high temperatures. An intermediate layer is therefore selected, based on thermodynamic computation. It is shown that aluminum nitride (AlN) could limit the interface reactivity at temperatures close to 1000 °C. Duplex AlN/W_1−xRe_x coatings were fabricated in two steps by chemical vapor deposition on bulk silicon carbide to verify experimentally the theoretical material solution approach. Electron probe micro-analyses showed that, at the micrometer level, there was no interface reaction during the growth process. It is the first time that such a material stack has been fabricated, and it seems promising for the high-temperature use of SiC with tungsten alloys.     

Mechanical properties and corrosion resistance of nanocrystalline ZrNxOy coatings on AISI 304 stainless steel by ion plating

Transition metal oxynitrides have become emerging decorative coating materials due to their adjustable coloration and high hardness and corrosion resistance. This research studied the effect of oxygen content on the coloration, mechanical properties and corrosion resistance of ZrN_xO_y thin films deposited on AISI 304 stainless steel using hollow cathode discharge ion plating (HCD-IP). The Zr/N/O ratios of the ZrN_xO_y films were determined using X-ray photoelectron spectroscopy (XPS). The color of the ZrN_xO_y thin film changed from golden yellow to blue and then slate blue with increasing oxygen content. X-ray diffraction (XRD) patterns revealed that phase separation of ZrN and m-ZrO₂ occurred as the oxygen content reached 31.2 at.%. ZrN(O) (ZrN with dissolving oxygen) is dominant at oxygen content less than 18.1 at.%, while m-ZrO₂ phase was prevailed at oxygen content above 40.3 at.%. Phase separation lowered the hardness of the ZrN_xO_y films as the fraction of ZrO₂ was less than 40%. The residual stresses in ZrN phase was higher than that in ZrO₂, and the residual stress decreased for the specimen containing 30 to 37% ZrO₂. For the samples containing more than 44% ZrO₂, the average residual stress was close to that in ZrO₂ phase. The corrosion resistance was evaluated by salt spray test and potentiodynamic scan in two solutions: 0.5MH₂SO₄ + 0.05 M KSCN and 5% NaCl solutions. The results showed consistent trend in the two solutions. From the results of potentiodynamic scan, corrosion resistance increased as the packing density of the film increased, whereas the film thickness was not a crucial factor on corrosion current; moreover, the electrical conductivity of the film may be one of the significant factors in corrosion resistance. Results of salt spray tests suggested that the corrosion of ZrN_xO_y in NaCl may play an important role in corrosion resistance.     

Characterization of TiO2 coatings prepared by a modified electric arc-physical vapour deposition system

TiO₂ thin coatings were prepared, on various substrates, through evaporation of metallic titanium in an oxidizing atmosphere by modified electric arc-physical vapor deposition (EA-PVD). The coatings were characterized chemically (by means of XPS and SIMS) and from the structural point of view (by means of XRD and Raman spectroscopy), in order to understand the factors which lead to homogeneous coatings with high anatase content. The type of substrate is the main parameter that influences the crystal structure of the coatings: when stainless steel is used as substrate the coatings consist essentially of rutile, while on glass substrates coatings containing mainly anatase are obtained. The photocatalytic activity of the samples upon UVA irradiation was tested by using phenol as the target molecule. Phenol in the solution can be photocatalytically and rapidly degraded through the EA-PVD anatase TiO₂ coatings.     

堇青石表面化学气相沉积二氧化钛的负载机理及负载动力学

以酸蚀改性堇青石为基体,利用化学气相沉积法（CVD）在基体上负载TiO₂,采用扫描电子显微镜、能谱仪、X射线衍射仪、BET比表面积法等对负载了TiO₂的堇青石进行表征,测定不同温度下的负载速度。结果表明：负载了TiO₂的堇青石主要由（211）及（200）取向的锐钛矿TiO₂组成,呈八面体和立方体形态,BET比表面积达78.80 m²·g^-1,平均孔径为9.80 nm,具有双峰分布特征。负载过程为TiCl₄及O₂向堇青石基体扩散吸附,TiCl₄分解为Ti⁴⁺并在高氧势下进入基体晶格形成TiO₂晶核,并经过择优取向和外延式生长,其负载沉积速率方程为,其中T为负载温度,为气相TiCl₄的分压。     

Production of MgB2 superconducting coatings by electrophoresis on metal substrates

Production of MgB₂ coatings on various metallic substrates was achieved by means of the direct electrophoretic deposition technique. An inexpensive simple heat treatment in evacuated quartz tubes was developed as an alternative to inert gas flow during the process. The films were characterized by XRD, SEM and SQUID. It resulted that the procedure led to the production of uniform, dense and well-adhesive superconducting films. Stainless steel proved to be the best substrate among the investigated metals.     

TiSiN nanocomposite coatings by chemical vapor deposition in a fluidized bed reactor at atmospheric pressure (AP/FBR-CVD)

TiSiN nanocomposite coatings were deposited on stainless steel by chemical vapor deposition in a fluidized bed reactor at atmospheric pressure (AP/FBR-CVD) by reaction of TiCl₄ and SiCl₄ with NH₃ at 850 °C. Coatings were characterized by means of GD-OES, XPS and XRD. TiSiN coatings with a Si content of 9 at.% showed a hardness of 28 GPa (the hardness of TiN and SiN_x coatings was around 21 GPa) and a lower oxidation rate under dry air at 600 °C. Our results show for the first time that AP/FBR-CVD can be tuned for the deposition of nanocomposite ceramic coatings.     

CVD deposition and characterization of coloured Al2O3/ZrO2 multilayers

Coloured Al₂O₃/ZrO₂ multilayers have been deposited onto WC-Co based inserts by a CVD process. Through physical as well as optical analysis of such multilayers, colour is believed to originate from interference. The coatings are obtained with good process reproducibility. It was found that the ZrO₂ process used in the multilayer, with ZrCl₄ as the only metal chloride precursor, results in a mixture of tetragonal and monoclinic ZrO₂ phases. However by adding a relatively small amount of AlCl₃ during such a process results in ZrO₂ layers being composed of predominantly tetragonal ZrO₂ phase. Corresponding multilayers seem to have a more fine grained and smoother morphology whereas multilayers containing monoclinic ZrO₂ phase seem to be less perfect with existence of larger grains of ZrO₂ which are believed to scatter light and alter the reflectance of such a multilayer. In addition to this, such multilayers were found to be free of or with greatly reduced amount of thermal cracks, normally present in pure CVD grown Al₂O₃ layers.It is believed that, in the studied Al₂O₃/ZrO₂ multilayers, the observed tetragonal ZrO₂ phase is the result of a size effect, where small enough ZrO₂ crystallites energetically favor the tetragonal phase. However as the ZrO₂ crystallite size distribution is shifted to larger sizes it is believed that a mixture of crystallites with both stable and metastable tetragonal phases as well as a stable monoclinic phase is obtained. The proposed metastable tetragonal ZrO₂ phase may in fact explain the absence of thermal cracks in such multilayers through a transformation toughening mechanism, well known in ZrO₂ based ceramics.