Bilge Saruhan

6 Concluding Remarks

Oxide CMCs are attractive materials because they are inherently resistant to oxidation. Efforts to develop the oxide CMCs were intensified in the last decade, relying onto these and similar high temperature properties. Previous to oxide CMCs, the non-oxide CMCs were successfully developed, mainly bearing in mind the structural applications. Recently, demanding requirements for the conditions in air were set bringing the need for better materials withstanding even higher temperatures and longer exposure times (i.e. ≥ 1200°C and >10.000 hours) than those initially given. In general, space applications entail higher temperatures (i.e. 1500°C) than those anticipated for gas turbine engines, however, hereby much shorter exposure times are required (< 5 hours) making it bearable for non-oxide CMCs. Thus, the non-oxide CMCs having relatively high ultimate fracture strengths (e.g. 300–600 MPa) found an appropriate use in space applications.

Chemical vapor deposition of ZrO2 and C/ZrO2 on mullite fibers for interfaces in mullite/aluminosilicate fiber-reinforced composites

For the realization of crack deflection and fiber pull-out in aluminosilicate fiber-reinforced dense mullite-matrix composites, suitable fiber/matrix-interfaces are an important requirement in order to obtain sufficiently weak bondings between fibers and matrices. Two types of chemical vapor deposited (CVD) fiber/matrix-interfaces have been studied in the present work porous ZrO 2 and C/ZrO 2 -double layers. In the latter case, carbon was burned out to form a gap during the processing of composites (fugitive coating). Porous ZrO 2 coatings were produced by an optimized CVD-process with Zr-acetylacetonate as a precursor. The constancy of the layer thickness depended on the deposition temperature. It was found that at a temperature of approximately 300°C and a pressure of 5 hPa, suitably uniform layers with thickness ranging between 100 and 300 nm were achieved. The coatings contained approximately 15 wt% carbon which produced, after annealing in air, a porous structure. The deposition kinetics can be described by a first order reaction. The carbon layer in C/ZrO 2 -double layers was produced by using propane. The thickness of carbon layer was 10 and 100 nm, respectively. Aluminosilicate fiber/mullite matrix composite prepegs were fabricated by infiltration of coated and unidirectionally oriented fiber (0°) with a slurry, containing a pre-mullite powder, calcined at 1100°C. Uniaxial hot-pressing of dried prepegs was carried out at < 1250°C for 15 min, at 20 MPa. Prepegs with ZrO 2 fiber/matrix-interfaces were hot-pressed in air, while the samples with C/ZrO 2 -interfaces were processed in flowing argon. After hot-pressing, samples with C/ZrO 2 -interfaces were heat-treated in air (1000°C) in order to burn out the C-layer (fugitive coating). These composites yielded a controlled fracture with a high deflection rate and a favorable fracture strength of about 200 MPa, due to crack-deflection and fiber pull-out. Composites with ZrO 2 -interfaces, on the contrary yielded no crack deflection or pull-out. Therefore, they are less damage tolerant than those having C/ZrO 2 double layer systems.

Damage tolerant oxide/oxide fiber laminate composites

Abstract Oxide-fiber/oxide-matrix composites were developed using non-infiltrated woven fiber layers between matrix-infiltrated fiber layers in order to achieve damage tolerant behavior. A fiber interface coating was not used. This technique enables damage tolerance in materials with strong fiber-matrix bonding and under oxidizing conditions. Fabrication of composites was carried out through a slurry infiltration technique. Slurries for fiber (Nextel™ 720, 3M) infiltration were prepared using a submicron α-Al 2 O 3 powder coated with an amorphous SiO 2 -layer through a sol–gel process. Hot-pressing was used to densify and bond the laminate layers together, followed by pressureless heat-treatment to allow mullite to form. Room temperature three-point bending tests were performed on as-received samples and on samples which underwent long-term annealing at high temperatures (1200–1300°C) in air. Subsequent examination revealed that due to the lack of a fiber interface coating, matrix-infiltrated fiber layers behaved in a quasi-monolithic manner with little or no crack deflection. Layers of non-infiltrated fibers, however, provided damage tolerance by deflecting cracks in the plane of the laminate and by serving as a mechanical bond between matrix-infiltrated layers. The laminate composites demonstrate reasonable room-temperature fracture strength both in the as-received state (88 MPa) and after exposure to 1300°C air for 200 h (72 MPa) along with extensive fracture deflection through the layers of non-infiltrated fiber. Composite properties, specifically fracture strength and damage tolerance, can be tailored by varying lay-up and processing parameters such as fiber-matrix ratio and type of fiber weave.

Preliminary results on a novel fabrication route for α-Al2O3 single crystal monofilament-reinforced reaction-bonded mullite (RBM)

Abstract Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al 2 O 3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density). Single crystal α-Al 2 O 3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO 2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO 2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.

Developments in Processing of Ceramic Top Coats of EB-PVD Thermal Barrier Coatings

Partially Yttria Stabilized Zirconia (PYSZ) based Thermal Barrier Coatings (TBC) manufactured by EB-PVD process are a crucial part of a system, which protects the turbine blades situated at the high pressure sector of aero engines and stationary gas turbines under severe service conditions. These materials show a high strain tolerance relying on their unique coating morphology, which is represented by weakly bonded columns. The porosity present in ceramic top coats affects the thermal conductivity by reducing the cross sectional area through which the heat flows. The increase in thermal conductivity after heat-treatment relates to the alteration of the shape of the pores rather than the reduction of their surface-area at the cross section. The studies carried out by the authors demonstrate that the variation of the parameters during the EB-PVD processing of PYSZ based top-coats alters the columnar morphology of the coatings. Consequently, these morphological changes affect primarily the thermal conductivity and eventually the Young’ Modulus which are the key physical properties of this material group. New ceramic compositions covering zirconia coatings stabilized with alternative oxides, pyrochlores and hexaluminates are addressed. Failures occurring in ceramic top coats mark the lifetime of TBC system and therefore, it is necessary that their performance should go beyond that of the-state-of-the-art materials. This context summarizes the research and developments devoted to future generation ceramic top coats of EB-PVD TBCs.

Planar, Impedance-Metric NOX Sensor with NiO Sensing Electrode for High Temperature Applications

In this study, an innovative type of sensor configuration is used which contains an electrolyte constructed of quasi-single crystalline columns, a porous NiO sensing electrode (SE) and a conductive Pt reference electrode (RE) deposited on the backside. As electrolyte EB-PVD manufactured discs of FYSZ and PYSZ composition were used. Gas sensing characterization of the sensors was carried with typical flue-gas mixtures in various concentrations. Impedance and potential analysis were done at a frequency range of 100 kHz and 0.005 Hz. Sensors with FYSZ electrolyte could be operated at higher temperatures compared to those with PYSZ electrolytes. The highest achieved sensing temperature in this study was 600degC. NO was successfully detected under the presence of 5%vol. O2 at the maximum operating temperature.

Pseudocapacitive and hierarchically ordered porous electrode materials supercapacitors

Commercially available double layer capacitors store energy in an electrostatic field. This forms in the form of a double layer by charged particles arranged on two electrodes consisting mostly of active carbon. Such double layer capacitors exhibit a low energy density, so that components with large capacity according to large electrode areas are required. Our research focuses on the development of new electrode materials to realize the production of electrical energy storage systems with high energy density and high power density. Metal oxide based electrodes increase the energy density and the capacitance by addition of pseudo capacitance to the static capacitance present by the double layer super-capacitor electrodes. The so-called hybrid asymmetric cell capacitors combine both types of energy storage in a single component. In this work, the production routes followed in our laboratories for synthesis of nano-porous and aligned metal oxide electrodes using the electrochemical and sputter deposition as well as anodization methods will be described. Our characterisation studies concentrate on electrodes having redox metal-oxides (e.g. MnOx and WOx) and hierarchically aligned nano-porous Li-doped TiO2-NTs. The material specific and electrochemical properties achieved with these electrodes will be presented.

Al-doped TiO2 semiconductor gas sensor for NO2-detection at elevated temperatures

Al-doped TiO2 semiconductor layers were investigated as gas sensor for NOx detection at temperatures up to 800°C. Thin sensor layers were deposited by reactive magnetron sputtering technique from the metallic targets of Ti and Al under addition of oxygen onto the alumina substrates which consisted of screen-printed interdigited Pt-electrodes on the front and no heater at the backside. Al-content was adjusted to 6 atm. % in TiO2. The layers were characterized by XRD and SEM for phase and microstructural constituents. The phase content is consisted of anatase phase on deposition and was converted to rutile at temperatures exceeding 600°C. After annealing at 800°C for 3 hours in air, the sensor response is measured towards NO2 concentrations up to 200 ppm under dry and humid conditions (5 % H2O) in argon as the carrier gas. The sensors sensitivity and crosssensitivity towards CO was also investigated. Al-doped TiO2 sensor layers exhibited very promising results for sensing NO2 selectively at temperatures exceeding 500°C.

Studying of PYSZ and FYSZ turbine blade coatings by small-angle neutron scattering

We present here results of small-angle neutron scattering (SANS) investigation of partially and fully yttria stabilized zirconia (PYSZ and FYSZ, respectively) turbine blade coatings (TBC). The samples have been prepared by electron beam physical vapour deposition method (EB-PVD) with various setup parameters. Structure parameters of porosity have been studied in-situ with high-temperature furnace. Temperature dependence of specific surface and anisotropy parameter of porosity has been obtained. Differences in character of microstructural morphology changing with in-situ thermal treatment as well as open and closed porosity ratio in PYSZ and FYSZ has been described and discussed.

Sintering Inhibition in Nanostructured EB-PVD-TBCs by applying Liquid-Phase-Infiltration

Abstract. The new generation gas turbines are to generate higher gas temperatures which may seriously affect the thermal stability of the state-of-the-art TBCs (e.g. Partially Yttria Stabilized Zirconia, PYSZ). The reasons for that is the occurrence of considerable phase transformation and/or sintering-induced volume changes which drastically degrade the columnar structure of EB-PVD coatings and raise the modulus of elasticity, and as a result the internal stresses, thus restricting the favorable strain tolerance of TBCs. There are two suggested strategies for the improvement of TBCs to be used in the new generation of gas turbine engines: (1) infiltrate coatings with another oxide and hence reduce the diffusion rate at the nanostructured feather-like features and avoid intracolumnar pore coalescence, (2) use new TBC-compositions with alternative crystal structures. This context deals mainly with the first strategy, i.e. infiltration of these structures with a liquid-phase sol based on titania to inhibit the sintering. The applied infiltration method is dip-coating via sol-gel-process. As-coated and infiltrated EB-PVD-TBCs are heat-treated at 1000° and 1100°C and characterized by SEM/EDX. A substantial improvement in the thermal stability of morphology was observed by infiltration of EB-PVD PYSZ TBCs with titania.