Kee Sung Lee

Oxygen-permeating property of LaSrBFeO3 (B=Co, Ga) perovskite membrane surface-modified by LaSrCoO3

Abstract Oxygen permeation fluxes have been investigated as a function of temperature for a mixed ionic–electronic conducting La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and La0.7Sr0.3Ga0.6Fe0.4O3−δ (LSGF) membranes. Ga-doped composition, which is known for its chemical and structural stability, shows limited oxygen permeation flux as compared with a Co-containing system. However, modification of both surfaces with catalytically surface-reactive La0.6Sr0.4CoO3−δ (LSC) makes Ga-doped perovskite an excellent oxygen-permeable membrane. It has been demonstrated that the effective area of reactive free surface is an important factor in determining the effectiveness of surface modification by La0.6Sr0.4CoO3−δ. On the contrary, the oxygen permeation flux of La0.6Sr0.4Co0.2Fe0.8O3−δ is not affected by surface modification. The rate-determining process is discussed in conjunction with the apparent activation energy for oxygen permeation. As cationic substitution occurs in the vicinity of the interface between the coating layer and substrate membrane at elevated temperatures, the intermediate composition, that is, (La1−xSrx)(Coy′GayʺFe1−y′−yʺ)O3−δ, has been formed in La0.7Sr0.3Ga0.6Fe0.4O3−δ membrane surface-modified by La0.6Sr0.4CoO3−δ.

Damage-Resistant Brittle Coatings

Laminate structures consisting of hard, brittle coatings andsoft, tough substrates are important in a wide variety of engi-neering applications (cutting tools, electronic multilayers, la-minated windscreens), biological structures (teeth and dentalcrowns, shells, bones), and traditional pottery (ceramicglazes). A hard outerlayer variously offers increased load-bearing capacity, wear resistance, thermal and corrosion pro-tection, electrical insulation, and aesthetics; a compliant un-derlayer offers stress redistribution and damage tolerance.But hard layers are susceptible to cracking, especially in sur-face-concentrated loads from static or cyclic contacts. In nat-ural or restorative tooth structures, for instance, forces in ex-cess of 100 N operate at contacts between opposing cusps ofcharacteristic radii 2–4 mm over 10

Cracking of brittle coatings adhesively bonded to substrates of unlike modulus

The role of elastic mismatch in determining critical conditions for indentation fracture in brittle coatings on substrates of unlike modulus was investigated. A model transparent trilayer system, consisting of a glass coating layer bonded to a thick substrate of different glass or polymer by a thin layer of epoxy adhesive, facilitated in situ observations of crack initiation and propagation. A tungsten carbide sphere was used to load the layer system. Abrasion flaws were introduced into the top and bottom glass coating surfaces to control the flaw populations and to predetermine the origins of fracture: cone cracks occurred at abraded top surfaces, radial cracks at abraded bottom surfaces. Analytical relations for the critical loads are presented for each crack system in terms of elastic modulus mismatch, indenter and coating dimensions, and material fracture parameters. Implications concerning materials selection for resistance to crack initiation are considered.

Enhancement of oxygen permeation by La0.6Sr0.4CoO3−δ coating in La0.7Sr0.3Ga0.6Fe0.4O3−δ membrane

Abstract A La0.7Sr0.3Ga0.6Fe0.4O3−δ membrane with perovskite structure is fabricated by a conventional solid-state reaction. As the oxygen permeation flux of the La0.7Sr0.3Ga0.6Fe0.4O3−δ membrane was lower than commercial gas separation membranes, we coated a La0.6Sr0.4CoO3−δ layer to enhance the oxygen permeation flux. The La0.6Sr0.4CoO3−δ coating was very effective for increasing oxygen flux, as the flux was as much as 2 to 6 times higher than that of an uncoated La0.7Sr0.3Ga0.6Fe0.4O3−δ membrane. Moreover, the oxygen permeation flux was strongly influenced by the porosity of the coating layer. XRD analysis after the oxygen permeation confirmed that Ga-based membranes have a phase stability.

Effect of Microstructure on Dielectric Properties of Si3N4 at Microwave Frequency

Silicon nitride (Si3N4) has been researched intensively because of superior mechanical properties up to high temperature. The mechanical properties of Si3N4 are strongly related to microstructure. The microstructure control of silicon nitride is well known to be a key issue for tailoring the mechanical properties of structural ceramics. This work was performed to reveal the effect of microstructure on dielectric properties at microwave frequency. Three starting powders were used fine, course a-Si3N4 and b-Si3N4. Sintering additives, 5 wt.% Y2O3, 2 wt.% Al2O3 and 1 wt.% MgO were mixed with each starting powder. Si3N4 ceramic with different b/a phase specimen were obtained by hot pressing. The post-resonator method was used for the measurement of dielectric properties, dielectric constant (e′) and dielectric loss (tand), at microwave frequency range. Silicon nitride ceramics show dielectric constant of 8.1 – 8.6 and dielectric loss 1.1 x 10-3 – 5.6 x 10-3. The effect of grain size and the role of phase on microwave dielectric properties are discussed.

Influence of Subsurface Layer on the Indentation Damage Behavior of YSZ Thermal Barrier Coating Layers Deposited by Electron Beam Physical Vapor Deposition

The thermal barrier coating must withstand erosion when subjected to flowing gas and should also maintain good stability and mechanical properties while it must also protect the turbine component from high temperature, hot corrosion, creep, and oxidation during operation. In this study we investigated the influence of subsurface layer, Al₂O₃ or NiCrCoAlY bond coat layer, on the indentation damage behavior of YSZ thermal barrier coating layers deposited by electron beam physical vapor deposition (EB-PVD). The bond coat is deposited using different process such as air plasma spray (APS) or spray of high velocity oxygen fuel (HVOF) and the thickness is varied. Hertzian indentation technique is used to induce micro damages on the coated layer. The stress-strain behaviors are characterized by results of the indentation tests.

Crack Healing, Reopening and Thermal Expansion Behavior of Al2TiO5 Ceramics at High Temperature

The low thermal expansion (α25-1100oC = 0.05 ~ 1.6 × 10–6/K) of Al2TiO5 ceramics are apparently due to a combination of grain boundary micro cracking caused by the large thermal expansion anisotropy of the crystal axes of the Al2TiO5 phase. During the reheating run, the individual crystallites expanded at low temperature; thus, the solid volume of the specimen expanded into the micro cracks, where as the macroscopic dimensions remained almost unchanged. As a result, the material expanded very little. The micro cracks closed at higher temperatures. This result is closely related to relatively steeper thermal expansion curves.

Effect of Grain Boundary Phase on Contact Damage Resistance of Silicon Nitride Ceramics

Contact damage resistances of silicon nitride ceramics with various grain boundary phases are investigated in this study. The grain boundary phases are controlled by the addition of different types of sintering additives, or the crystallization of intergranular phase in a silicon nitride. We control the microstructures of materials to have similar grain sizes and the same phases to each other. Contact testing with spherical indenters is used to characterize the damage response. The implication is that the grain boundary phase can be another controllable factor against contact damage and strength degradation even though it is not critical relative to the effect of grain morphology.

Static and Dynamic Indentation Damage in SiC and Si3N4

Hertzian and explosive indentations were used to determine the damage behavior of SiC and Si3N4 ceramics. Specimens were selected with different microstructures. In order to observe the subsurface damaged zone, the bonded interface technique was adopted. It was found that the damage response depends strongly on the microstructure of ceramics. Examination of subsurface damage reveals a competition between brittle and quasiplastic damage mode: brittle fracture mode is dominant in fine grain microstructure; quasiplastic deformation occurs in coarse grain. Dynamic indentation induces subsurface yield zone which contains extensive micro-cracks. The role of microstructure on static and dynamic damage behavior are discussed in terms of the weakness of grain boundary and grain size.

STRENGTH DEGRADATIONS FROM HERTZIAN CONTACT DAMAGES IN NITRIDED PRESSURELESS SINTERED SILICON NITRIDE CERAMICS

Strength degradations in silicon nitride ceramics subject to damage from contact with hard spheres are investigated. Strengths against indentation load, number of cycles in contact, or stress-rate parameter are reported and compared with theoretical models. Silicon nitride ceramics are prepared by nitride pressureless sintering (NPS) process, which process is the continuous process of nitridation reaction of Si metal combined with subsequent pressureless sintering. Microstructure characterizations reveal silicon nitride fabricated by NPS process exhibits a quasi-plastic mode, with continuous strength loss beyond a load above the onset of yield, and falloff at high number of cycles, > 105 at contact load, P = 950 N, using WC sphere r = 1.98mm. The strength degradation is substantially faster by dynamic fatigue. Failures originated from contact damages, quasi-plastic microcrack zones, with developing radial cracks during strength test. The implication is that quasi-plastic damage of NPS silicon nitride itself can preserve benefits from the inherent higher damage tolerance at lower number of cycles of contacts, but fatigue susceptibility at multicycle contacts and lower stressing rate.