dependence of fracture toughness of ceramic thermal barrier coatings on microstructure: electron...

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