Browsing by Author "Knapp, James"
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Item Density Functional Theory Study of Gas Adsorption on Lanthanum Zirconate Nanostructured Coating Surface(Office of the Vice Chancellor for Research, 2015-04-17) Guo, Xingye; Zhang, Jing; Jung, Yeon-Gil; Li, Li; Knapp, JamesLanthanum zirconate (La2Zr2O7) is a typical pyrochlore ceramic material, which can be used as thermal barrier coating (TBC). However, it may deteriorate by oxidizing and corrosive gases, such as CO2, O2, SO2 and CH4 during its operation process at elevated temperatures. This work investigates CO2, O2, SO2 and CH4 gas adsorption mechanism on La2Zr2O7 nanostructured coating surfaces using the density functional theory (DFT) calculations. La2Zr2O7 surface energies on (001), (011) and (111) planes were calculated. Results show the most thermodynamically stable surfaces of La2Zr2O7 are (011) and (111) planes, due to their low surface energies. Adsorption energies of CO2, O2, SO2 and CH4 on (001), (011) and (111) planes in different sites were studied as well. The results show the most favorable gas adsorption sites for CO2, O2, SO2 and CH4 occur at 3-fold and 4-fold sites. The most favorable gas adsorption plane for CO2, O2, SO2 and CH4 is (111) plane.Item Ideal tensile strength and shear strength of ZrO2(111)/Ni(111) ceramic-metal Interface: A first principle study(Elsevier, 2016-12) Guo, Xingye; Zhang, Yi; Jung, Yeon-Gil; Li, Li; Knapp, James; Zhang, Jing; Department of Mechanical Engineering, School of Engineering and TechnologyThe ideal mechanical strengths of ZrO2(111)/Ni(111) ceramic-metal (C-M) interface are calculated through simulated tensile and shear deformations using the first principles calculations. The structures of ZrO2(111)/Ni(111) interfaces with 1- and 3-layer Ni thicknesses are optimized and the mechanical properties are investigated. For tensile deformation in [111] direction, the Young's moduli of the 1-layer Ni and 3-layer Ni M-C models are 139.9 GPa and 60.2 GPa, respectively; and ultimate tensile strengths are 11.6 GPa and 7.9 GPa, respectively. For shear deformation in {111} 〈110〉 system, the shear moduli of the 1-layer Ni and 3-layer Ni M-C models are 43.9 GPa and 30.4 GPa, respectively; and ultimate shear strengths are 7.0 GPa and 3.0 GPa, respectively. For shear deformation in {111} 〈11View the MathML source〉 system, the shear moduli of the 1-layer Ni and 3-layer Ni M-C models are 30.9 GPa and 17.3 GPa, respectively; and ultimate shear strengths are 6.0 GPa and 1.8 GPa, respectively. Overall, 1-layer Ni C-M interface models have better mechanical properties than those of 3-layer models. The observed strengths are explained by using charge distribution, electron localization function, and Bader charge transfer analyses. The results are important for designing robust thermal barrier coating through optimizing bond coat thickness.Item Lanthanum Zirconate Based Thermal Barrier Coatings: A Review(Elsevier, 2017-08) Zhang, Jing; Guo, Xingye; Jung, Yeon-Gil; Li, Li; Knapp, James; Department of Mechanical Engineering, School of Engineering and TechnologyThis review article summarizes the latest information about the manufacturing techniques of lanthanum zirconate (La2Zr2O7, LZ) powder and La2Zr2O7 based thermal barrier coatings (TBCs). Lanthanum zirconate is a promising candidate material for TBC applications, due to its lower thermal conductivity and higher thermal stability compared to other traditional TBC systems. In this work, the physical, thermal, and mechanical properties of the powder and coatings are evaluated. The durability experiments of the TBCs in various thermal, mechanical, and corrosive conditions are also reviewed. In addition, theoretical studies on the powder and coatings properties are presented. Finally, future research directions of lanthanum zirconate as TBC applications are proposed.Item Mechanical Properties of Layered La2Zr2O7 Thermal Barrier Coatings(Springer, 2018-04) Guo, Xingye; Li, Li; Park, Hyeon-Myeong; Knapp, James; Jung, Yeon-Gil; Zhang, Jing; Engineering Technology, School of Engineering and TechnologyLanthanum zirconate (La2Zr2O7) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La2Zr2O7 and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La2Zr2O7 + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La2Zr2O7 TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La2Zr2O7. The erosion rate of La2Zr2O7 TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La2Zr2O7. DCL porous 8YSZ + La2Zr2O7 had a lower erosion rate than other SCL and DCL La2Zr2O7 coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.Item Microstructural non-uniformity and mechanical property of air plasma-sprayed dense lanthanum zirconate thermal barrier coating(2014) Zhang, Jing; Guo, Xingye; Jung, Yeon-Gil; Li, Li; Knapp, JamesLanthanum zirconate is a promising thermal barrier coating material. In this work, imaging technique was used to characterize the microstructural non-uniformity of the coating. The imaging analyses revealed that, along the thickness of the coating, the cracks were primarily horizontal in the top and middle regions, while vertical cracks became dominant in the bottom region. The calculated porosities showed a non-uniformity (4.8%, 5.3%, and 5.5% in the top, middle, and bottom regions, respectively). They were lower than the experimentally measured one, 7.53%, using the Archimedes method. This is because imaging analysis does not take internal porosity into account. Additionally, the measured Vickers hardness was 5.51±0.25 GPa, nanoindentation hardness was 8.8±2.1 GPa, and Young's modulus was 156.00±10.03 GPa.Item Novel Lanthanum Zirconate Thermal Barrier Coatings for Gas Turbines(Office of the Vice Chancellor for Research, 2015-04-17) Zhang, Jing; Guo, Xingye; Jung, Yeon-Gil; Li, Li; Knapp, JamesPyrochlore lanthanum zirconate (La2Zr2O7) is a very promising candidate material for thermal barrier coating (TBC) applications. Layered microstructure was designed and prepared using air plasma spray to improve the thermal durability. The physical, mechanical and thermal properties of the coatings were measured. La2Zr2O7 shows outstanding phase stability in high temperatures and lower thermal conductivity than commercial 8YSZ TBCs material. Coating lifetime performance was also estimated through the JETS (Jet Engine Thermal Simulation) and TGMF (Thermal Gradient Mechanical Fatigue) tests.Item STUDY OF THE THERMAL AND MECHANICAL PROPERTIES OF LA2ZR2O7 USING FIRST PRINCIPLE METHOD(Office of the Vice Chancellor for Research, 2014-04-11) Guo, Xingye; Knapp, James; Li, Li; Jung, Yeon-Gil; Zhang, JingAs an advanced thermal barrier coating, Lanthanum zirconia (La2Zr2O7) has been studied in this paper using first principle calculations. La2Zr2O7 crystal bulk was used in this calculation. The lattice parameter, mechanical and thermal properies of La2Zr2O7 were investigated by means of density functional theory (DFT). Hydrostatic pressure-dependent elasticity constant, bulk modulus were calculated. The thermal conductivity was calculated based on fick’s law using a 20 layers supercell. La2Zr2O7 coating samples were spraied by APS equipment, the coating samples were identified by XRD and observed by optical microscope. The thermal effect of Ce doping of the La2Zr2O7 were studied by ab initial calculations. The calculated properties have considerable good agreement with others experimental and calculation results.Item Study of Thermomechanical Properties of Lanthanum Zirconate Based Thermal Barrier Coatings(Office of the Vice Chancellor for Research, 2016-04-08) Guo, Xingye; Jung, Yeon-Gil; Li, Li; Knapp, James; Zhang, JingAbstract La2Zr2O7 is a promising thermal barrier coating material due to its low thermal conductivity and superior high temperature stability. In this work, La2Zr2O7 and conventional 8 wt% yttria stabilized zirconia (8YSZ) coatings were deposited using plasma spray technique. Both singlelayer La2Zr2O7 and double-layer 8YSZ/La2Zr2O7 coating architectures were designed. Thermal conductivity and coefficients of thermal expansion (CTE) of the coatings were measured. The measured average thermal conductivity of La2Zr2O7 was about 25% lower than that of porous 8YSZ. The CTE values of La2Zr2O7 were about 10% lower than that of 8YSZ. Thermal cycling and thermal shock tests were used to evaluate coatings’ thermomechanical properties. All La2Zr2O7 coats were delaminated in the furnace cycle test in less than 20 cycles. This is because residual thermal stresses accumulated during thermal cycling. In the jet engine thermal shock (JETS) tests, the single-layer La2Zr2O7 and double-layer dense 8YSZ/La2Zr2O7 coatings were fully delaminated. In contrast, the double-layer porous 8YSZ/La2Zr2O7 coating sample only partially delaminated on the edge. This is because the porous 8YSZ layer provides strain compliance. The findings will help to propose efficient thermal barrier coating systems.Item Thermal properties of La2Zr2O7 double-layer thermal barrier coatings(Taylor & Francis, 2018) Guo, Xingye; Lu, Zhe; Park, Hye-Yeong; Li, Li; Knapp, James; Jung, Yeon-Gil; Zhang, Jing; Mechanical and Energy Engineering, School of Engineering and TechnologyLa2Zr2O7 is a promising thermal barrier coating (TBC) material. In this work, La2Zr2O7 and 8YSZ-layered TBC systems were fabricated. Thermal properties such as thermal conductivity and coefficient of thermal expansion were investigated. Furnace heat treatment and jet engine thermal shock (JETS) tests were also conducted. The thermal conductivities of porous La2Zr2O7 single-layer coatings are 0.50–0.66 W m−1 °C−1 at the temperature range from 100 to 900°C, which are 30–40% lower than the 8YSZ coatings. The coefficients of thermal expansion of La2Zr2O7 coatings are about 9–10 × 10−6 °C−1 at the temperature range from 200 to 1200°C, which are close to those of 8YSZ at low temperature range and about 10% lower than 8YSZ at high temperature range. Double-layer porous 8YSZ plus La2Zr2O7 coatings show a better performance in thermal cycling experiments. It is likely because porous 8YSZ serves as a buffer layer to release stress.Item Thermal Stability of Novel Multilayer Lanthanum Zirconate Based Thermal Barrier Coatings(Wiley, 2016) Guo, Xingye; Lu, Zhe; Jung, Yeon-Gil; Li, Li; Knapp, James; Zhang, Jing; Department of Mechanical Engineering, School of Engineering and TechnologyThis chapter discusses the thermal stability of SCL and DCL porous La2Zr2O7 coats by depositing three types of SCL and DCL La2Zr2O7 and 8YSZ structures in an experiment. Heat treatment, cyclic furnace cycle test and jet engine thermal shock test were conducted for the thermal barrier coatings (TBC) samples. The heating and cooling cycles were repeated until cracks or spallation were spotted on the TBC surface. Both the front and back surface temperatures were measured using pyrometer. The delamination is caused by the thermal residual stress induced by CTE difference between La2Zr2O7 and 8YSZ. All the La2Zr2O7 coats were delaminated in the furnace cycle test in less than 20 cycles. This is because residual thermal stresses accumulated during thermal cycling. Microstructure images of heat treated samples were taken using a scanning electron microscope.