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
7/12/2016
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
x
Ceramic
Bond Coat
Substrate
Active
TBCs processes - Air Plasma Spray (APS)
APS provides multi-layer ceramic with splat structure
TBCs processes –
Electron Beam Physical Vapor Deposition (EB-PVD)
EB-PVD provides ceramic with columnar structure
± 30°
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.
Fracture Toughness (KIC)
The fracture toughness KIC, is a measure of the material’s resistance to the
propagation of a crack.
K IC
E
 K 
H
1
2
 P 
 3 
 2
c 
where:
KIC : Fracture Toughness (MPa-m1/2)
K : empirical constant
(no unit)
E : Young’s Modulus
(GPa)
HV : Vickers Hardness
(GPa)
P : Load
(N)
Vickers Test Diagram
c : crack length
(m)
F
F
H V   1.854 2
A
d
K = 0.036 (Ref.)
c
TBCs Microstructures
• APS process provides multi-layer ceramic with splat structure
• EB-PVD provides columnar structure
APS microstructure
EB-PVD Columnar microstructure
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
Crack length & Fracture Toughness Experiment (cont’)
Ceramic from APS, cracks widely spread to surrounding area
Vickers Test
Diagram
c
Measured crack’s length on APS samples:
Load = 25gf
Mag = 500X
Crack length & Fracture Toughness Experiment (cont’)
Ceramic from EB-PVD, cracks propagate within grain boundary
Vickers Test
Diagram
c
Measured crack’s length on APS samples:
Load = 25gf
Mag = 500X
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
Results (cont’)
•
Smaller slope requires bigger load to increase crack length
•
Ceramic from EB-PVD process is harder than APS’s
Slope  314
Slope  111
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