Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

Reduction of a NiO thin film deposited by PLD on a single crystal YSZ (111) substrate

The NiO/YSZ interface prepared by depositing NiO on a single crystal YSZ (111) substrate has been investigated by transmission electron microscopy. As deposited, a very thin nickel layer ascribing to the nonstoichiometry at the very beginning growth of NiO and an amorphous silica phase resulting from silicon segregation were present at the interface. The orientational relationship of NiO (1[`1] 1) (1\overline{1} 1) //Ni (1[`1] 1) (1\overline{1} 1) //YSZ (1[`1] 1) (1\overline{1} 1) with NiO [110]//Ni [110]//YSZ [110] was observed. The microstructural and chemical changes at the NiO/YSZ interface after being heated in vacuum and hydrogen indicated different reduction mechanisms. In vacuum, the reaction \textNiO ? \textNi + 1/ 2 \text O ₂ ( \textg ) {\text{NiO}} \to {\text{Ni}} + 1/ 2 {\text{ O}}_{ 2} \left( {\text{g}} \right) was prevailing at the interface between NiO and pre-existing Ni, which led to the thickening of nickel layer. In hydrogen, the reduction initiated on the NiO surface was dominant, following the chemical equation H₂ + O_O (NiO) → H₂O (g) + V_O ^.. (NiO) + 2e (Ni).     

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.     

Plasma assisted chemical vapour deposition of boron nitride coatings from using BCl3–N2–H2–Ar gas mixture

Boron Nitride coatings have been deposited by plasma-assisted chemical vapour deposition (PACVD) from BCl₃/N₂/H₂/Ar gas mixtures in a hot wall capacitively coupled radio-frequency (13.56 MHz) reactor. The nature of active species in the plasma during deposition was determined by Optical Emission Spectroscopy (OES) and Mass Spectrometry (MS). The plasma characterisation was performed as follows: first, an Ar/H₂ plasma was studied in order to understand the influence of molecular hydrogen in the discharge mixture. Then the two precursors N₂ and BCl₃ were added and the new gas mixture studied. Finally the deposition plasma was investigated. These characterisations were correlated to the microstructure and c-BN concentrations determined by Scanning Electron Microscopy (SEM) and Fourier Transformed Infrared Spectroscopy (FTIR).The study demonstrates the major role of atomic hydrogen on the possible mechanisms leading to BN deposition:—the introduction of hydrogen in Ar/N₂ controls the nature of the NH_x (from N to NH₃) species in the gas phase. These results are correlated to the relative amount of NH groups in the films,—by a modification of the excitation state of the plasma (n_e, T_e) the introduction of H₂ can increase the dissociation rate of the boron precursor BCl₃ and, reacting with chlorine, leads to the formation of HCl. This corresponds to an increase in the growth rate of the coatings.Finally, BN samples containing 5% of cubic phase were treated by Ar, Ar/H₂ and Ar/Cl₂ plasmas. These post treatments demonstrated that ion assisted preferential etching of h-BN by H or Cl atoms could be used to obtain large concentrations of c-BN coatings and possibly offer a new route for deposition of low stress cubic boron nitride.     

A new method of preparing Ni/YSZ cermet materials

Ceramic–metal composites (cermets) containing yttria-stabilized zirconia(YSZ) and Ni particles as anode materials in solid oxide fuel cells were prepared by a new method. The method encompasses nickel oxalate dihydrate precipitation on the nanometre YSZ powder, and decomposition at 633 K in inert atmosphere. The composite powders containing 30, 40, and 50% Ni manufactured by the oxalate method were compacted into disk pellets, and sintered in Ar containing 10% H₂. The structure of the cermet materials was characterized by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM), and mercury porosimetry. The thermal expansion coefficient (TEC) was determined by the dilatometric method. Electrochemical impedance spectroscopy (EIS) was used to determine electrical conductivity. The oxalate method leads to obtaining the Ni/YSZ anodes with the Ni content reduced to 40 wt% and well obeying based requirements for anode material in SOFCs.     

Hydrothermal-electrochemical codeposited hydoxyapatite/yttria-stabilized zirconia composite coating

Hydroxyapatite(HA)/yttria-stabilized zirconia(YSZ) composite coatings were deposited on titanium substrates using a hydrothermal electrochemical method in an electrolyte containing calcium, phosphate ions and YSZ particles. HA/YSZ composite coatings were prepared in different conditions with different electrolyte temperatures(100 ∼ 200°C), current densities(0.1 ∼ 10.0 mA/cm²), and particles content in bath(0 ∼ 100 g/L). The effect of YSZ additions on the phase composition, microstructure, thermal stability, corrosion behavior and the bonding strength of HA/YSZ composite coatings were studied. The results show that crystallinity of HA in HA/YSZ composite coatings increase continuously with the electrolyte temperature and close to stoichiometric HA. The n(Ca)/n(P) ratio at 200°C is about 1.67 according with stoichiometric HA. YSZ particles are imbedded uniformly between the HA crystals. The average HA crystal size are reduced owing to the additions of YSZ particles. After annealing at 1200°C, tetragonal phase YSZ tend to react with the released CaO to form cubic phase YSZ and CaZrO₃, which cause destabilization of HA to decompose into more α-TCP phase. The bonding strength between HA/YSZ composite coatings and titanium substrates increase with increasing volume content of YSZ in the composite coatings (V %). HA/YSZ composite coatings exhibit a better electrochemical behavior than pure HA coatings and uncoated Ti metals.     

Physicochemical properties of cubic Ni complex powders synthesized using urotropine chelating ligand for solid oxide fuel cells

New nanometer nickel-based solid oxide fuel cell (SOFC) anode material has been synthesized by a hydrothermal reaction with nickel (II) chloride as a metal-containing precursor and a tetra dentate chelating reagent, urotropine. SEM images reveal that the Ni complexes change from two-dimensional flower-like stacked sheets to truncated cubes with increasing mol concentration of urotropine chelating additive. The truncated cubes show sizes of 50.0–200.0 nm. The electrical conductivity of the truncated cubic Ni complex (60.0 wt.%)-loaded commercial YSZ (40.0 wt.%) for an application in SOFC anode is superior compared to that of commercial NiO/YSZ at the same composition. The zeta-potential value in aqueous solution determined by electrophoretic light scattering (ELS) show a positive surface charge in the range of pH 3.0–9.0, related to the surface stability, but it changes to a negative value at above pH > 9. The H₂-temperature-programmed reduction experiments confirm that the truncated cubic Ni complex-loaded commercial YSZ shows a better reducibility than the commercial NiO/YSZ.     

Fabrication of NiO/YSZ anode material for SOFC via mixed NiO precursors

NiO/YSZ anode material with 45 wt.% NiO for SOFC was synthesized by surface-modification of YSZ powder with mixed nickel oxide precursors. YSZ was treated with acidic nickel nitrate and then followed by basic nickel carbonate. Such prepared NiO/YSZ precursor showed nano-size homogeneous mixture upon calcination at 800 °C. The mixture was ground and subjected to calcination at 1200 °C to give rise to agglomerate-free NiO/YSZ composite powder with particle size of ∼ 0.4–0.7 μm. The composite powder was analyzed by XRD, zetapotential measurements, and SEM. The composite powder produced the Ni/YSZ cermet of homogeneous microstructure with a rigid YSZ skeleton, porosity of 33%, and an electrical conductivity of ∼ 430 S/cm at 800–1000 °C, which is much higher than that (∼ 180 S/cm) prepared by a conventional mixed oxide process.     

RF magnetron sputtered aluminium oxide coatings on iridium

The effects of process parameters on the microstructural morphology of aluminium oxide (Al₂O₃) coatings on Ir have been studied. Al₂O₃ coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200–600 W in an Ar, or Ar+1–10% O₂, sputtering gas atmosphere by RF magnetron sputtering. Al₂O₃ coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar+O₂, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a -Al₂O₃ phase, low Ar content and a relatively high hardness value of ca. 1050 H _v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al₂O₃ coatings on Ir-coated IG substrate. It was found that the interface between Al₂O₃ and Ir coatings was sharp and Al₂O₃ coatings remained intact with the Ir-coated IG substrate.     

Effect of oxygen partial pressure on the electrical and optical properties of highly (200) oriented p-type Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O films by DC sputtering

Thin films of NiO (bunsenite) with (200) preferential orientation were synthesized on glass substrates by direct current sputtering technique in Ar+O₂ atmosphere. Nanostructural properties of the NiO films were investigated by X-ray diffraction and also by atomic force microscopic (AFM) studies. Electrical and optical properties of the deposited films were investigated as a function of different partial pressure of oxygen in the sputtering gas mixture during deposition. The films showed p-type electrical conduction and the conductivity depends on the partial pressure of oxygen. The electrical conductivity (σ_RT) was found to be .0615 S cm⁻¹ for films deposited with 100% O₂ and its value sharply decreased with the decrease the partial pressure of O₂; for example σ_RT for 50% O₂ was 6.139 × 10⁻⁵ S cm^-1. The mechanism of the origin of p-type electrical conductivity in the NiO film is discussed from the viewpoint of nickel or oxygen vacancies, which generate holes and electrons respectively. X-ray photoelectron spectroscopic studies supported the above argument. Corresponding optical properties showed that the transparency decreases with increasing oxygen partial pressure and the bandgap also decreases.     

Microstructure of YSZ Coatings Deposited by PS-PVD Using 45 kW Shrouded Plasma Torch

In this study, a conventional 80 kW class plasma spraying system was used to produce yttria-stabilized-zirconia (YSZ) coatings by PS-PVD at a pressure of 100 Pa. A shroud was attached in the front of the plasma nozzle to restrain expansion of plasma jet. The torch was operated at an arc power of 45 kW and YSZ coatings were deposited at a powder feed rate of 0.2 g/min. Optical emission spectroscopy was used to diagnose the particle state in plasma jet. The surface morphology and cross-sectional morphology of coatings was characterized by field emission scanning electron microscope. It is found that the amount of YSZ evaporation is significantly enhanced through using a shroud. The coatings with a hybrid microstructure of splats and nanoclusters were deposited perpendicular to the coatings. The nanostructured clusters deposited out of the vapor are presented at splat interfaces. It is evident that using powders specially designed for PS-PVD and controlling heating of plasma jet to spray particles, PS-PVD deposition for a hybrid microstructure consisting of vapor phase deposit can be realized through conventional plasma spray system. Columnar grain structured YSZ can also be deposited by pure vapor phase at the side surface of the substrate.     

Structure, composition and electrical properties of YSZ films deposited by ultrasonic spray pyrolysis

Yttria-stabilized zirconia (YSZ) films with different yttria concentrations were prepared by ultrasonic spray pyrolysis on Si substrates at 525 °C, using solutions of zirconium and yttrium acetylacetonates in methanol. The chemical composition, structure and electrical properties of the films were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). XPS measurements show that the Y content in the films increases as the Y precursor in the solution increases. Carbon incorporation was also found in the films, although the concentration of this impurity was reduced as the incorporation of Y increased. XRD spectra show that the Zr_1−x Y _x O_2−x/2 polycrystalline films have the cubic phase of ZrO₂ and fully stabilized 8YSZ (8 at.% Y₂O₃ + 92 at.% ZrO₂), and that their lattice constant increases slightly as the Y content increases. The conductivity of all the as-deposited films as a function of temperature, showed an Arrhenius behavior, and with the exception of the film with the maximum Y content, the activation energies were in the range of 0.98–1.11 eV. The ionic conductivity of one of these films was similar to that measured for a pellet made of the 8YSZ standard powder.     

Preparation and structure development of NiO/YSZ nanocomposite powders by urea hydrolysis

NiO/YSZ composite powders, with various NiO contents, have been prepared by the urea hydrolysis method. The crystallization behavior and microstructure of composite powders has been studied in detail, using differential scanning calorimetry analysis, X-ray diffraction, and transmission electron microscope. The results indicated that the actual NiO content of the NiO/YSZ powders largely deviated from the nominal value, and finally reached a saturated value. The NiO addition would retard the crystallization of NiO/YSZ composite. When the calcination temperature was increased, the NiO crystallites first precipitated at around 500 °C, and then the YSZ phase presented at about 600 °C. The calcined powders consist of NiO/YSZ nanocomposite particles, which are comprised of nano-sized NiO and YSZ crystals. In addition, with the aid of H₂ plasma treatment, it is easier to distinguish the Ni and YSZ phases of Ni/YSZ cermets after sintering and subsequent reduction. This could reveal that such Ni/YSZ cermets exhibited a uniform microstructure that has fine Ni particles homogeneously dispersed within the YSZ matrix. As the NiO content was increased, the size and density number of the Ni phase within an YSZ matrix was increased.     

Phase stability and electrical property of NiO-doped yttria-stabilized zirconia

The effects of NiO solid solution into yttria fully stabilized cubic zirconia (YSZ) on the phase stability and electrical properties of YSZ were investigated. Time-dependent conductivity change of NiO-doped YSZ at 1000 °C was also investigated. Raman spectra showed that weak tetragonal peaks in the YSZ monolith disappeared when NiO was added. In addition, the time-dependent conductivity of YSZ monolith sharply decreased after the first 30 h of annealing at 1000 °C, whereas NiO-doped YSZ did not show such decline. Therefore, the solid solution of NiO enhanced YSZ stability and produced time-dependent conductivity change with moderate time.     

Entwicklung von Oxid – Keramik zur Anwendung als Wärmedämmschichten

Development of Oxide Ceramics for an Application as TBC The standard thermal barrier coating material yttria stabilised zirconia (YSZ) is limited in long term operation to a maximum temperature of about 1200°C. As a result further increase of the gas inlet temperature and hence the efficiency of gas turbines are hardly to achieve with YSZ coatings. In a screening procedure especially perowskite (ABO₃, A = Sr,Ba, B = Zr) and pyrochlore (A₂B₂O₇, e.g. A = La and other rare earth elements, B = e.g. Zr) materials have been identified as possible candidates for thermal barrier coatings. Basic physical properties (e.g. thermal expansion coefficient, thermal diffusivity and conductivity) of several candidates have been determined using sintered, dense samples. The possibility of optimization of the properties by using specific compositions will be discussed. From promising materials powders which are suitable for plasma‐spraying have been produced by spray‐drying. New TBC systems consisting of new materials (BaZrO₃, La₂Zr₂O₇) deposited by atmospheric plasma spraying and vacuum plasma sprayed MCrAlY bondcoats were tested in a gas burner facility. Especially La₂Zr₂O₇ coatings gave promising results. A further improvements could be achieved by the use of layered or graded coatings with a YSZ coating at the bondcoat interface and on top a layer of the new TBC material. First results of thermal cycling tests with 1250 and 1350°C surface temperature will be presented.     

Nickel and copper deposition on Al2O3 and SiC particulates by using the chemical vapour deposition–fluidized bed reactor technique

A chemical vapour deposition–fluidized bed reactor technique was developed to perform metal deposition on ceramic particulates. Experiments of nickel and copper deposition on Al₂O₃ and SiC particulates were conducted. Argon was used as the carrier gas to fluidize the ceramic particulates. The metal–H–Cl system was selected for the chemical vapour deposition. The volumetric ratios of the inlet gas were 3.5% HCl, 20.0% H₂, and 76.5% Ar. The deposition reactions were carried out at four different temperatures: 500, 600, 700 and 800 °C. Successful deposition of metallic nickel and copper on the ceramic particulates was observed. It was also noticed that the deposition rates varied with the types of substrates and deposited metals. This revised version was published online in November 2006 with corrections to the Cover Date.     

Comparison of the coatings deposited using Ti and B4C powder by reactive plasma spraying in air and low pressure

The coatings were deposited by reactive plasma spraying (RPS) in air and low-pressure plasma spraying (LPPS) based on the reaction between Ti and B₄C powder, respectively. The thermal spray powder of Ti and B₄C added with powder Cr (metallic binder) in air is compared with that without powder Cr addition in the low pressure. (Prior to deposition, the powder was screened and separated for RPS whereas spray drying, sintering and sieving were done for LPPS.) The phase composition and the microstructure of coatings were studied by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The anti-corrosion property of coatings was also investigated. It is found that the coating prepared by RPS, which is more densification, is composed of TiN, TiB₂, and a small phase fraction of titanium oxides. The composition of the coating deposited by reactive LPPS is TiB₂, Ti(C, N), Ti₄N_3−x and impurity phase of Ti₅Si₃. There is no appearance of titanium oxides in low pressure. The coatings have the typical lamellar structure and adhere to the bond coating well. The mean Vickers microhardness value of the coating deposited by RPS is higher than that of the coating deposited by LPPS. Furthermore, the corrosion resistance of the coating deposited by RPS is superior to that of the coating prepared by LPPS in near neutral 3.5 wt% NaCl electrolyte.     

Evaluation of elastic properties of reduced NiO-8YSZ anode-supported bi-layer SOFC structures at elevated temperatures in ambient air and reducing environments

Elastic properties of Ni-8YSZ anode-supported bi-layer SOFC structures were studied at elevated temperatures up to 1,000 °C in both ambient air and H₂ environments. The anode samples with desired porosity and microstructure were fabricated by reducing a NiO-8YSZ anode precursor structure in a gas mixture of 5% H₂–95% Ar at 800 °C for selected time periods up to 8 h. The development of the essential porous microstructure in forming the Ni-8YSZ cermet phase was analyzed with SEM. It was observed that the room temperature elastic moduli and hardness of the anode samples decrease significantly with increasing fraction of reduced NiO. Since the elastic properties of fully dense Ni, NiO, and 8YSZ are comparable to each other, the decrease in the magnitude in elastic moduli and hardness is evidently due to the colossal increase in porosity in the reduced Ni-8YSZ cermet anodes because of the reduction of NiO to Ni. At elevated temperatures, the Ni-8YSZ anodes show a complex profile of Young’s modulus as a function of temperature, which is significantly different from the unreduced NiO-8YSZ samples. When studied in ambient air, the Young’s modulus of the Ni-8YSZ samples decrease slowly up to ~250 °C, then more rapidly from 250 to 550 °C, and finally it increases monotonically with the increase in temperature. However, in reducing environment, the Young’s moduli values decrease continuously throughout the temperature range. Two sets of samples of different thicknesses were studied simultaneously to highlight the effects of the sample thickness on the elastic properties of the anodes.     

Microstructural and tribological characterization of air plasma sprayed nanostructured alumina–titania coatings deposited with nitrogen and argon as primary plasma gases

Air plasma sprayed nanostructured Al₂O₃–13wt%TiO₂ coatings were deposited as a function of critical plasma spray parameter (CPSP), defined as the ratio of arc power to primary gas flow rate, using nitrogen and argon as the primary plasma gases. Microstructural features including percentage of α-Al₂O₃ phase, percentage of partially melted/unmelted regions, microhardness, and wear characteristics were evaluated for the deposited coatings. Effect of CPSP on microstructural and wear characteristics of coatings deposited with nitrogen was found to be relatively small. In contrast, significant effect of CPSP on coating characteristics was found for coatings deposited with argon. In wear tests, while strong effect of normal load on weight loss was observed for coatings deposited with nitrogen, weight loss for coatings deposited with argon was nearly independent of applied normal load, at least for coatings deposited at the highest CPSP.     

Untersuchungen zur Beständigkeit thermisch gespritzter Schichten im Vergleich zu Hartchromschichten bei durch Abrasion dominierter tribologischer Beanspruchung

Investigations on thermal spray coatings resistance against abrasion dominated tribological load in comparison to hard chromium coatings HVOF iron and nickel based hard alloy as well as WC/Co(Cr) and Cr₃C₂/Ni20Cr coatings are compared to APS Al₂O₃/TiO₂ and Cr₂O₃, powder flame sprayed and fused composite coatings consisting of NiCrBSi and WC/Co and electrolytically deposited hard chromium coatings concerning their wear behavior for tribological load by lose abrasive particles (ASTM G65 and ASTM G75). Thereby the influence of newly developed HVOF torch combustion chambers with reduced critical diameter and divergent expansion nozzles that both permit increased combustion gas and therefore also particle velocities on microstructure and wear resistance of the produced coatings is studied. While there is no improvement of wear resistance for hard alloy coatings compared to mild steel substrates for the specific tribological boundary conditions of these tests, especially the carbide reinforced coatings permit improvement by more than one order of magnitude in ASTM G65 tests and even more than two orders of magnitude in ASTM G75 tests. Also, for both types of tribological load HVOF coatings with WC as reinforcing phase are clearly superior to electrolytically deposited hard chromium coatings. Both use of the combustion chamber with reduced critical diameter and the expansion nozzles with divergent contour result in improved wear resistance of the thereby produced coatings. The specific wear mechanisms are deduced based on SEM examination of worn specimen surfaces.