Challenges in Developing New Coatings to Improve Performance Ashutosh S. Gandhi Department of Metallurgical & Materials Engineering Indian Institute of Technology Madras Chennai 600 036 [email protected]

4th Indo-American Frontiers of Engineering Symposium 1-3 March, 2012 1

Outline  

Aviation, Energy and Environment Efficient Aero-engines • High Temperature – High Efficiency • Coatings Push Operating Temperatures Up

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State-of-the-Art: Thermal Barrier Coatings Challenges in Coatings Technology Future Coatings Technology

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Aviation, Energy and Environment Alternative Energy Vehicles BSA Electric Scooter

Hydrogen Powered Car (BMW 7)

http://evworld.com

Latest Passenger Aircraft Flies on Aviation Turbine Fuel Need to maximize fuel efficiency

All_Nippon_Airways_Boeing_787-8_Dreamliner

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Aero-Engines High Temperature – High Efficiency Enhancing Aircraft Fuel Efficiency 

Lightweighting – Advanced light-weight materials

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Increasing the Engine Operating Temperature • Ideal Brayton Cycle Efficiency:  = 1 – (Tatmospheric/Tcompressor exit) • Higher specific power output with higher turbine inlet temperature

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Alloy temperature capability has reached its maximum • Cooling technology has helped increase temperature • Coatings allow further increase in temperature 4

Thermal Barrier Coating System Critical Enabling Technology

Heat Transfer

TGO: Thermally grown oxide (AlO1.5 i.e. Al2O3)

Turbine blade with TBC (ZrO2-YO1.5)

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Temperature Capability Over the Years 

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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 ~1500C. Clarke and Levi, 2003; Kelly, 2006.

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Zirconia (ZrO2) TBC’s 

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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 ~2700C Hence is a suitable material as a TBC. 7

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

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Low conductivity may be achieved if material has: • • • • •

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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  

Clarke (2003)

Ideally, pores of ~0.5 μm diameter Microstructural modulations between  to /4 9

Conductivity of Co-Doped Zirconia 

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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 

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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. 11

Microstructure of Plasma Sprayed TBC’s 

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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 

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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

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Need to understand materials science and mechanics Need for condition monitoring and online diagnostic technologies for remaining life assessment 13

Durability Issues in the Top Coat

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Phase Changes in t’-Zirconia

1250C

t’

1150C

fc = f0 [1- exp(-ktn)] Archana & Gandhi

• 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. 15

Thermochemical Compatibility with TGO • 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.

Gd2Zr2O7/7YSZ Bi-Layer Growth BEI

SEI

Gd2Zr2O7

Gd2Zr2O7

YSZ

YSZ Gandhi et al.

Continuous column growth

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Failure of TBC’s

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Dominant Failure Mechanisms 

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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

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

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Fracture Toughness of ZrO2-Based TBC’s Thermal Cyclic Life of TBC’s: Plasma Sprayed & EBPVD

Fracture Toughness Variation During Thermal Exposure ZrO2-8 mol% YO1.5

• Cyclic life is compromised with increasing stabilizer content for singly- as well as co-doped ZrO2 • Intrinsic fracture toughness of 7YSZ higher than 20YSZ

1250C

1150C

• Ferroelastic toughening mechamism (Evans et al. 2008) Archana & Gandhi

• Toughness changes during lifetime of the TBC • Even transformation toughening can operate in aged TBC 19

Extrinsic Failure of TBC’s 

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Dust ingested into the gas turbine consists of calcium aluminium magnesium silicates (CMAS) The melting point is ~1240C

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

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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.

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Erosion and Foreign Object Damage

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Small solid particles (~10m) cause erosion of TBC Large particles (~100m) cause foreign object damage Need for improved fracture toughness at operating temperature

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

TBC’s: Multifunctional Layered System Inherently Metastable Cooling

~1350°C ~1150°C

≤1050°C

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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

 Thermally grown oxide (TGO, -AlO1.5) provides oxidation protection  Supplies Al for formation of -AlO1.5 TGO  Slow oxidation kinetics  Ni(Pt)Al or MCrAlY (M = Ni, Co, Fe)  Ni-Based superalloy (CMSX-4, René-N5)

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Future TBC System Erosion/CMAS resistant Layer

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Luminescent Layer Low-k TBC

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Luminescent Layer

YSZ Interlayer

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TGO (AlO1.5)

Bond Coat Diffusion Barrier Superalloy

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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 24

Beyond Superalloys

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Si-Based Ceramics & Composites: Environmental Barrier Coatings 

• High temperature strength exceeding Nisuperalloys • Future Gas Turbine Engines, Re-entry Vehicles, Scramjet (Hypersonic Planes)

>>1350°C

TBC Material?

 EBC

>1200°C?

Susceptible to Oxidation & Water Vapour Attack (High Velocities) e.g. SiC + 3H2O = SiO2 + 3H2 (g) + CO (g) SiO2 + 2H2O = Si(OH)4 (g)

Si “Bond Coat”?

Si-Based Ceramic Composite

Si-Based Non-oxide Ceramics & Composites (Si3N4, SiC, Si-B-C-N)

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Requirements for an EBC • Resistance to Water Vapour • Thermochemical Compatibility with the Substrate • Thermal Expansion Match with the Substrate

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Credits 

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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. 27