Thermomechanical properties of novel lanthanum zirconate based thermal barrier coatings - an integrated experimental and modeling study

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] \u3e [110] \u3e [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

First principles study of thermodynamic properties of lanthanum zirconate

Lanthanum zirconia (La2Zr2O7) has become an advanced thermal barrier coating material due to its low thermal conductivity and high temperature stability. In this work, the first principles calculations were used to study the thermodynamic properties of the material. Lattice parameters, bulk and shear modulus, and specific heat of La2Zr2O7 were calculated by means of density functional theory (DFT). Hydrostatic pressure-dependent elasticity constants and bulk modulus were also studied. The thermal conductivity was calculated based on the Fourier's law. The calculated properties are in excellent agreement with the experimental and calculation results in literature

Abrasive Resistant Coatings—A Review

Abrasive resistant coatings have been widely used to reduce or eliminate wear, extending the lifetime of products. Abrasive resistant coatings can also be used in certain environments unsuitable for lubrications. Moreover, abrasive resistant coatings have been employed to strengthen mechanical properties, such as hardness and toughness. Given recently rapid development in abrasive resistant coatings, this paper provides a review of major types of abrasive coatings, their wearing mechanisms, preparation methods, and properties

A BP-MF-EP Based Iterative Receiver for Joint Phase Noise Estimation, Equalization and Decoding

In this work, with combined belief propagation (BP), mean field (MF) and expectation propagation (EP), an iterative receiver is designed for joint phase noise (PN) estimation, equalization and decoding in a coded communication system. The presence of the PN results in a nonlinear observation model. Conventionally, the nonlinear model is directly linearized by using the first-order Taylor approximation, e.g., in the state-of-the-art soft-input extended Kalman smoothing approach (soft-in EKS). In this work, MF is used to handle the factor due to the nonlinear model, and a second-order Taylor approximation is used to achieve Gaussian approximation to the MF messages, which is crucial to the low-complexity implementation of the receiver with BP and EP. It turns out that our approximation is more effective than the direct linearization in the soft-in EKS with similar complexity, leading to significant performance improvement as demonstrated by simulation results.Comment: 5 pages, 3 figures, Resubmitted to IEEE Signal Processing Letter

3D Printed ABS and Carbon Fiber Reinforced Polymer Specimens for Engineering Education

Three 3D printed plastic materials, ABS, ABS plus, and CFRP, have been studied for their potential applications in engineering education. Using tensile test, the stress strain curves of the materials have been measured. The Young’s modulus, ultimate strength, and fracture toughness of the materials are calculated from the stress strain curve. The results show that CFRP has the highest stiffness or Young’s modulus. ABS plus has strongest mechanical properties, with highest ultimate strength and fracture toughness. With the measured properties, the 3D printed samples are a viable solution for engineering students to learn mechanical properties of materials

Thermal properties of La2Zr2O7 double-layer thermal barrier coatings

La2Zr2O7 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

Microstructural non-uniformity and mechanical property of air plasma-sprayed dense lanthanum zirconate thermal barrier coating

Lanthanum 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

Lanthanum Zirconate Based Thermal Barrier Coatings: A Review

This 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

Molecular Dynamics Simulation of Electrical Resistivity in Sintering Process of Nanoparticle Silver Inks

A molecular dynamics (MD) model is developed to simulate low temperature sintering of silver nanoparticles and resultant resistivity. Due to the high surface to volume ratio, nanoparticle silver inks can sinter at low thermal curing temperatures, which are used in intense pulsed light (IPL) sintering process. In this study, the configurational change of nanoparticle silver during sintering is studied using the MD model. Then the resultant electric resistivity is calculated using the Reimann-Weber formula. The simulation results show that the resistivity decreases rapidly in the initial sintering stage, due to the fast neck formation and growth. Additionally, the predicted temperature-dependent resistivity evolutions are in good agreement with both experimental measurements and analytical sintering model, indicating that the resistivity decreases with increasing sintering temperature. The model provides a design tool for optimizing IPL process

STUDY OF THE THERMAL AND MECHANICAL PROPERTIES OF LA2ZR2O7 USING FIRST PRINCIPLE METHOD

poster abstractAs 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