Challenges in Developing New Coatings

to Improve Performance

Ashutosh S. Gandhi

Department of Metallurgical & Materials Engineering

Indian Institute of Technology Madras

Chennai 600 036

a.s.gandhi@iitm.ac.in

4th Indo-American Frontiers of Engineering Symposium

1-3 March, 2012

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Outline

Aviation, Energy and Environment

Efficient Aero-engines

• High Temperature – High Efficiency

• Coatings Push Operating Temperatures Up

State-of-the-Art: Thermal Barrier Coatings

Challenges in Coatings Technology

Future Coatings Technology

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Aviation, Energy and Environment

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Alternative Energy Vehicles

http://evworld.com

BSA Electric Scooter Hydrogen Powered Car (BMW 7)

Latest Passenger Aircraft

All_Nippon_Airways_Boeing_787-8_Dreamliner

Flies on

Aviation Turbine Fuel

Need to maximize fuel efficiency

Aero-Engines

High Temperature – High Efficiency

Lightweighting – Advanced light-weight materials

Increasing the Engine Operating Temperature

• Ideal Brayton Cycle Efficiency: = 1 – (Tatmospheric/Tcompressor exit)

• Higher specific power output with higher turbine inlet temperature

Alloy temperature capability has reached its maximum

• Cooling technology has helped increase temperature

• Coatings allow further increase in temperature

4

Enhancing Aircraft Fuel Efficiency

Thermal Barrier Coating System Critical Enabling Technology

5

Heat Transfer

TGO: Thermally grown oxide (AlO1.5 i.e. Al2O3) Turbine blade with TBC (ZrO2-YO1.5)

Temperature Capability Over the Years

Turbine airfoil gas inlet temperature capability has steadily improved from the 1940’s.

The processing of superalloys has changed from wrought to cast (equiaxed) to directionally solidified to the present-day single crystal alloys.

The cooling of the turbine blades has steadily changed from convection cooling to film cooling to convection + impingement + film cooling

Thermal barrier coatings have facilitated further increase in temperature

The turbine inlet temperature has increased from ~1000 to ~1500C.

6 Clarke and Levi, 2003; Kelly, 2006.

Zirconia (ZrO2) TBC’s

A TBC material should have low thermal conductivity (k).

TBC coefficient of thermal expansion (CTE) should be as close to that of the superalloy as possible

Engine superalloy CTE is in ~15x10-6 K-1

Low k materials include glasses, but their CTE is 3x10-6 to 8x10-6 K-1. Glasses also have low melting points

Zirconia has conductivity of ~2 Wm-1K-1 and CTE ~10x10-6 K-1

Zirconia melting point is ~2700C

Hence is a suitable material as a TBC.

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How to Lower Thermal Conductivity?

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Guidelines for Developing Low-k TBC’s

Two modes of heat transfer:

• Conduction (phonons) and radiation (photons)

• Equal contribution at service temperature

Low conductivity may be achieved if material has:

• Large molecular weight

• Complex crystal structure

• Non-directional bonding, and

• Large number of different atoms per ‘molecule’

• Large number of point defects, grain boundaries, pores

Low radiative heat transfer may be achieved by

• Mixture of phases with different refractive indices

Ideally, pores of ~0.5 μm diameter

Microstructural modulations between to /4

Clarke (2003)

Conductivity of Co-Doped Zirconia

Addition of lanthanides such as Gd, Yb, Nd, La Er, etc. decreases conductivity

Co-doping with Y also decreases conductivity

Co-doping with multiple lanthanides without Y is also successful in decreasing thermal conductivity

Pyrochlore zirconates RE2Zr2O7 (RE = lanthanide elements) have low conductivity

Rare-earth co-doping of zirconia is a good strategy for reducing thermal conductivity.

Rare-earth zirconates other candidates.

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Microstructure of EBPVD TBC’s

Electron beam physical vapor deposition (EBPVD) gives rise to a columnar structure of the TBC

The line-of-sight deposition combined with rotation of the substrate ensures that a textured coating with inter-columnar gaps is deposited

Typical TBC thickness is 150 to 250 m, column diameter is 5-10 m, with ~1-2 m gaps

Each column has a feathery surface as well as internal porosity due to preferred growth directions during EBPVD

These features obstruct heat transfer, hence coating conductivity is only about 50% of the monolithic material conductivity

Inter-columnar gaps reduce in-plane elastic modulus. This is good for minimizing thermal stresses.

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Microstructure of Plasma Sprayed

TBC’s

Powder of stabilised zirconia is pushed through a plasma

Powder particles melt and impinge on the susbstrate

Droplets spread and are quenched to bring about rapid solidification

Coatings have porosity and cracks parallel to the substrate

Thermal conductivity is lower than EBPVD but compliance is poorer

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Durability and Lifetime Prediction

Prime reliance on coatings essential for enhanced operating temperatures

Non-empirical lifetime prediction models depend on understanding the durability issues

Durability is governed by:

• Processes within each layer, and

• Interaction between layers

• Interaction with engine environment

Need to understand materials science and mechanics

Need for condition monitoring and online diagnostic technologies for remaining life assessment

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Durability Issues in the Top Coat

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Phase Changes in t’-Zirconia

t’

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• The t’ phase is supersaturated with YO1.5.

• High temperature makes atomic movement fast enough to reject excess YO1.5 as cubic zirconia.

• Tetragonal phase leaner in YO1.5.

• Finally, monoclinic phase may form during cooling.

• Kinetics of t’t+c studied using Johnson-Mehl-Avrami-Kolmogorov analysis.

Archana & Gandhi

1150C

1250C

fc = f0 [1- exp(-ktn)]

Gd2Zr2O7/7YSZ Bi-Layer Growth

BEI SEI

Gandhi et al.

Thermochemical Compatibility with TGO

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• If a large amount of stabilizer is added to zirconia, it may react to form aluminates, e.g. GdAlO3 (Leckie et al, 2005)

• TBC should not react with TGO (AlO1.5)

• New TBC’s containing large amount of multiple stabilizers may react with the TGO.

• Interlayer of conventional material to prevent the reaction with TGO

• Deposition of bi-layer coatings to ensure continuous column growth.

YSZ

Gd2Zr2O7

Continuous column growth

YSZ

Gd2Zr2O7

Failure of TBC’s

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Dominant Failure Mechanisms Intrinsic Failure: Cracks propagate through the TBC near

the interface

Extrinsic Failure: Fracture near the top

Fracture toughness of TBC material needs to be as high as possible

18 18 T.A. Schaedler, Ph.D. Thesis, UCSB, 2006

Fracture Toughness of ZrO2-Based TBC’s

• Cyclic life is compromised with

increasing stabilizer content for

singly- as well as co-doped ZrO2

• Intrinsic fracture toughness of 7YSZ

higher than 20YSZ

• Ferroelastic toughening mechamism

(Evans et al. 2008)

Thermal Cyclic Life of TBC’s: Plasma Sprayed & EBPVD

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Fracture Toughness Variation During Thermal Exposure

ZrO2-8 mol% YO1.5

Archana & Gandhi

1250C

1150C

• Toughness changes during lifetime of

the TBC

• Even transformation toughening can

operate in aged TBC

Extrinsic Failure of TBC’s

Dust ingested into the gas turbine consists of calcium aluminium magnesium silicates (CMAS)

The melting point is ~1240C

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• Volcanic ash and sand behave differently: Different compositions, melting points and viscosities

Molten Dust (CMAS) Attack…

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CMAS partially infiltrates the TBC

During thermal cycling, CMAS solidifies and causes thermal stresses

CMAS problem more severe for future TBC’s owing to higher operating temperatures

CMAS attack mitigation by using over-layers of oxides that increase CMAS viscosity

CMAS/volcanic ash problem more severe for future TBC’s operating at higher temperatures.

Erosion and Foreign Object Damage

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Small solid particles (~10m) cause erosion of TBC

Large particles (~100m) cause foreign object damage

Need for improved fracture toughness at operating temperature

TBC’s: Multifunctional Layered System

Low thermal conductivity: ZrO2-7wt% YO1.5 (7YSZ)

Electron beam physical vapor deposition (EBPVD)

Columnar microstructure: strain compliance

Porosity: lower conductivity

Phase compatibility with underlying layer (TGO)

Phase stability & fracture toughness

Erosion and foreign object damage

CMAS (dust) and sulphate/vanadate attack

Morphological evolution & sintering

Supplies Al for formation of -AlO1.5 TGO

Slow oxidation kinetics

Ni(Pt)Al or MCrAlY (M = Ni, Co, Fe)

≤1050°C

~1150°C

Co

oli

ng

Thermally grown oxide (TGO, -AlO1.5) provides

oxidation protection

Ni-Based superalloy (CMSX-4, René-N5)

~1350°C

“TBC’s contain all materials science” – Arthur H. Heuer

Inherently Metastable

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Low-k TBC

Future TBC System

Diffusion barrier to minimize bond coat – superalloy interaction

YSZ interlayer to prevent reaction with TGO

Luminescent layers for monitoring remaining life

Top layer with erosion and CMAS resistance

• Combination of materials

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Erosion/CMAS resistant Layer

Luminescent Layer

Luminescent Layer

YSZ Interlayer

TGO (AlO1.5)

Bond Coat

Diffusion Barrier

Superalloy

Beyond Superalloys

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Si-Based Ceramics & Composites:

Environmental Barrier Coatings Si-Based Non-oxide Ceramics &

Composites (Si3N4, SiC, Si-B-C-N) • High temperature strength exceeding Ni-

superalloys

• Future Gas Turbine Engines, Re-entry Vehicles, Scramjet (Hypersonic Planes)

Si-Based Ceramic

Composite

Si “Bond Coat”?

EBC

TBC

Material?

>1200°C?

>>1350°C

Susceptible to Oxidation & Water Vapour Attack (High Velocities) e.g. SiC + 3H2O = SiO2 + 3H2 (g) + CO (g)

SiO2 + 2H2O = Si(OH)4 (g)

Requirements for an EBC • Resistance to Water Vapour

• Thermochemical Compatibility with the Substrate

• Thermal Expansion Match with the Substrate

Credits Boeing 787 Image -

http://commons.wikimedia.org/wiki/File:All_Nippon_Airways_Boeing_787-8_Dreamliner_JA801A_OKJ.jpg

BMW 7 - http://en.wikipedia.org/wiki/BMW_Hydrogen_7 Gas tuebine cross-section -

http://en.wikipedia.org/wiki/File:Jet_engine.svg Blade coated with TBC -

http://en.wikipedia.org/wiki/File:ThermalBarrierCoating.JPG Padture et al., 2002: Science; 296:280. Sampath et al., 1999: Mat. Sci. Eng. A272 (1999) 181. Clarke & Levi, 2003: Annu Rev Mater Res; 33:383. Clarke, 2003: Surface and Coatings Technology 163 –164, 67–74 Kulkarni et al., 2003: Materials and Engineering A359, 100-111. Levi, 2004: Curr Opin Solid State Mater Sci; 8:77. Leckie et al., 2005: R.M. Leckie, S. Krämer, M. Rühle, C.G. Levi, Acta

Materialia 53, 3281–3292 Kelly, 2006: J. Mater. Sci., 41 905-912. Renteria & Saruhan, 2006: Journal of the European Ceramic Society

26, 2249–2255 Evans et al. 2008: Evans AG, Clarke DR, Levi CG. J Eur Ceram Soc

2008; 28:1405.

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