dependence of fracture toughness of ceramic thermal barrier coatings on microstructure: electron...
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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 byDanh Tran
04/18/23
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 (KIC) 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 10-5 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:
KIC : Fracture Toughness (MPa-m1/2)
K : empirical constant (no unit)
E : Young’s Modulus (GPa)
HV : Vickers Hardness (GPa)
P : Load (N)
c : crack length (m)
K = 0.036 (Ref.)
Fracture Toughness (KIC)
The fracture toughness KIC, is a measure of the material’s resistance to the propagation of a crack.
23
21
c
P
H
EKK IC
c
Vickers Test Diagram
2854.1
d
F
A
FHV
TBCs Microstructures
EB-PVD Columnar microstructureAPS 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
ResultsAverage 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 KIC 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