Date of Award

12-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Jing Zhang

Second Advisor

Yung C. Shin

Committee Chair

Jing Zhang

Committee Co-Chair

Yung C. Shin

Committee Member 1

Peter J. Schubert

Committee Member 2

Yung C. Shin

Committee Member 3

Jing Zhang

Committee Member 4

Fu Zhao

Abstract

Thermal barrier coatings (TBCs) are refractory materials deposited on gas turbine components, which provide thermal protection for metallic components at operating conditions. The current state-of-art TBC material is yttria-stabilized zirconia (YSZ), whose service temperature is limited to 1200 celsius, due to sintering and phase transition at higher temperatures. In comparison, lanthanum zirconate (La2Zr2O7, LZ) has become a promising candidate material for TBCs due to its lower thermal conductivity and higher phase stability compared to YSZ.

The primary objective of this thesis is to design a novel robust LZ-based TBC system suitable for applications beyond 1200 celsius. Due to LZ’s low coefficient of thermal expansion and fracture toughness, which cause poor thermal cycling performance, two TBC architectures are proposed: (1) multiple layered coating, and (2) LZ/8YSZ composite coating.

In this work, LZ powders are fabricated using the solid-state reaction method, and all of the coatings are deposited using air plasma spray (APS) technique. The physical, thermal and mechanical properties of the sprayed LZ coatings have been systematically investigated, including temperature-dependent thermal conductivity, coefficient of thermal expansion, density, hardness, Young’s modulus, bond strength, and erosion resistance. The durability of the coatings in various thermal and mechanical conditions is also investigated, including furnace cycling test, thermal gradient mechanical fatigue test, and jet engine thermal shock test. The results show that for the layered TBCs, porous YSZ + LZ has reasonably good thermal cycling performance. For the composite TBCs, LZ/8YSZ (vol. % is 50:50) with a thin buffer layer LZ/8YSZ (vol. % is 25:75) has the greatest thermal cycling performance, comparable to pure 8YSZ coatings. The improved performance is explained by the graded coefficient of thermal expansion and enhanced fracture toughness.

In parallel to experimental investigations, a multi-scale modeling approach is employed to study the fundamental thermal and mechanical properties of LZ crystal and coatings. Physics-based models are developed, including using density functional theory (DFT), molecular dynamics (MD), and finite element (FE) methods. The nanoscale tensile and shear deformations of LZ single crystal are simulated using DFT calculations with the generalized gradient approximation (GGA) functional. The anisotropic Young’s moduli are studied using two approaches: (1) stress-strain curve of large deformation, and (2) analytical method in small deformation. Additionally, the nanoscale tensile and shear large deformations of LZ single crystal are simulated using the MD method with Buckingham and Coulomb potentials at room temperature (300 K). Both DFT and MD results show that LZ has strong anisotropic Young’s modulus with the ranking [111] > [110] > [100]. The shear modulus in {111}direction is slightly larger than that in {111}. Both Bader charge transfer and electron charge density analyses indicate that the electron interactions between O and Zr ions in LZ are stronger in [111] for tensile and in {111}for shear deformation.

For thermal properties, the temperature-dependent thermal conductivities of LZ coating are calculated using a multiscale approach. First, the thermal conductivity of LZ single crystal is calculated using a reverse non-equilibrium molecular dynamics (reverse NEMD) approach. The single crystal data is then passed to an FE model which takes into account realistic TBC microstructures. The predicted thermal conductivities from the FE model are in good agreement with experimental validations using both flash laser technique and pulsed thermal imaging-multilayer analysis.

Furthermore, the mechanical properties at the ceramic-metal (C-M) interface in TBCs are investigated. The nanoscale tensile and shear deformations of the ZrO2/Ni interface, an approximation of the interface between the top and bond coats, are performed using both DFT and MD calculations. The DFT results indicate that the elastic modulus, ultimate strength, and toughness of the C-M interface increase with the decrease of the Ni layer thickness. The charge transfer analysis and the charge density distribution show that a thin interface layer exhibits a strong interaction between Ni and O ions. The MD simulations using COMB3 potential show that the Young’s modulus of ZrO2/Ni interface in [111] direction is larger than that in [100] direction, and the shear modulus in {111}direction is larger than that in {111}direction.

In summary, this thesis work provides important thermomechanical properties of LZ-based thermal barrier coatings and can serve as a design tool for future advanced coating systems.

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