Dependence of Fracture Toughness of Ceramic Thermal Barrier Coatings on Microstructure: Electron Beam Physical Vapor Deposition vs. Air Plasma Spray Project submitted for MASTER OF MECHANICAL ENGINEERING RPI East Hartford, CT Presented by Danh Tran 6/29/2015

Outlines Objectives Thermal Barrier Coatings Processes - Air Plasma Spray (APS) - Electron Beam Physical Vapor Deposition (EB-PVD) Fracture Toughness Measurement using Vickers Nano-indentation method Microstructure of samples - APS - EB-PVD Results - Compare Fracture Toughness (K IC ) of APS vs. EB-PVD

Objectives To Compare Fracture Toughness of ceramic layer deposited by different Thermal Barrier Coatings processes: –Air Plasma Spray (APS) vs. Electron Beam – Physical Vapor Deposition To Observe crack characteristics of ceramic layer by both processes

Introduction TBCs reduces metal temperature of gas turbine blades Ceramic Bond Coat Substrate x Active

TBCs processes - Air Plasma Spray (APS) APS provides multi-layer ceramic with splat structure

TBCs processes – Electron Beam Physical Vapor Deposition (EB-PVD) A simple EB-PVD process: Under vacuum (10 -4 to torr) Bending of the electron beam is obtained by a magnetic field perpendicular to the drawing. EB-PVD provides ceramic with columnar structure ± 30°

where: K IC : Fracture Toughness(MPa-m 1/2 ) K : empirical constant(no unit) E : Young’s Modulus(GPa) H V : Vickers Hardness(GPa) P : Load(N) c : crack length(m) K = (Ref.) Fracture Toughness (K IC ) The fracture toughness K IC, is a measure of the material’s resistance to the propagation of a crack. c Vickers Test Diagram

TBCs Microstructures EB-PVD Columnar microstructure APS microstructure APS process provides multi-layer ceramic with splat structure EB-PVD provides columnar structure

Crack length & Fracture Toughness Experiment Prepare two Thermal Barrier Coatings samples –One from APS process –One from EB-PVD process Apply loads on samples at multiple locations (using Vickers Hardness Tester): –25gf, 50gf, 100gf and 200gf (*) (*) NOTE: Applied load based on tester’s minimum load as starting point Record Hardness data from applied loads Measure & compare crack lengths under microscope between two processes

Measured crack’s length on APS samples: Load = 25gf Mag = 500X Crack length & Fracture Toughness Experiment (cont’) Ceramic from APS, cracks widely spread to surrounding area Vickers Test Diagram c

Ceramic from EB-PVD, cracks propagate within grain boundary Crack length & Fracture Toughness Experiment (cont’) Measured crack’s length on APS samples: Load = 25gf Mag = 500X Vickers Test Diagram c

Results Average crack from APS is longer than EB-PVD at each applied load Reason: APS has lower fracture toughness

Results (cont’) Average Vickers Hardness from APS is lower than EB-PVD’s

Slope  111 Slope  314 Results (cont’) Smaller slope requires bigger load to increase crack length Ceramic from EB-PVD process is harder than APS’s

Conclusions For each applied load: - Vickers hardness of ceramic from APS is lower than EB-PVD - Average crack from APS is longer than EB-PVD’s Ceramic from APS has lower K IC than EB-PVD’s Empirical constant, K, which was determined from literature, is applicable for this experiment Ceramic from APS process is more brittle compare to EB-PVD’s under constant loading condition