Thermo-elastic Stressesin

High Temperature Coatings

Steven LySpring 2009

Thermo-elastic Stresses in High Temperature Coatings

• An analytic and finite element method study of residual stresses in single layer and multi-layer thermal barrier coatings using Fortran and ANSYS.

• Used in aerospace, automotive and industry to protect parts from wear and temperatures.– Increase system efficiencies– Prolong, extend and restore part life– Increase operational envelopes

Thermal Barrier Coatings

Overlay Coating Types

1.Air Plasma Spraya.Plasma created in inert gas by electrical arc between electrodes.b.Metallic Powder inject into plasmac.Particles impacted onto substrate.d.Heat transferred to substrate.e.Coating particles deform, splatter, solidify, shrink creating an interspersed coating with cracks.

2.High Velocity Oxygen Fuela.Mixture of oxygen and fuel (hydrogen, kerosene, propylene, Propane) injected thru a sonic convergent-divergent nozzle to achieve supersonic velocity.b.Due to high velocity and low residence time in air, oxide content is reduced. c.Dense coatingd.Lower coste.Cannot melt ceramics

3.Low Pressure Plasma Spraya.Performed in low pressure chamberb.Powder fed thru plasma spray gunc.Component is cleaned prior to coating.d.Component preheated prior to coatinge.Coating follows cleaning

Diffusion Coatings

1.Electron Beam Physical Vapor Depositiona.Lasts longer

2.Physical Vapor Deposition a.Pack

i.More cost affective than other coating typesb.Chemical Vapor Depositionc.Slow, inefficient

3.Hot dipping

Coating Materials

Nickel-AluminumCobalt-AluminumYttriumMCrAlY (NiCrAlCoY + Si, Hf)CoCrAlYPt-Al – type 1 corrosionAluminideCr-Al -coating – type 1, 2 corrosion, used in diffusionY2O3, ZrO2

1. Analytic vs. FEM comparison of residual stresses

1.1 Fortran model of Kroupa method for analytic comparison

1.2 Axis-symmetric FEM model for comparison to Kroupa method

2. Two layer coating axis-symmetric FEM model for comparison to

Zhang analysis

3. Two layer coating with varying thicknesses

3.1. 2% coating thickness

3.2. 4% coating thickness

3.3. 6% coating thickness

3.4. 8% coating thickness

3.5. 10% coating thickness

4. Two layer coating with varying temperatures.

4.1. 6% thickness, 1000º C to 25 º C.

4.2. 6% thickness, 700º C to 25º C.

4.3. 6% thickness, 427º C to 25º C.

Cases Studied

Material Properties

ANSYS

Fortran

substrate

bond coattop coat

Finite Element Modeling

Mesh w/B.C.’s:

Axis-symmetric model:

Representative Cross-section:

Radius 0.0125 m

Substrate thickness 0.010 m

coating thickness 0.006 m

Radial width element size 100

Substrate thickness element size 80

Bond coat thickness element size 20

Top coat thickness element size 20

Mesh characteristics

Fortran Analytic Model Stress Results

-2000

-1500

-1000

-500

0

500

1000-1

-0.9

-0.9

-0.8

-0.8

-0.7

-0.7

-0.6

-0.6

-0.5

-0.5

-0.4

-0.4

-0.3

-0.3

-0.2

-0.2

-0.1

-0.1 -0 0

0.06

Substrate / Coating Thickness

Stre

ss [M

Pa]

10% homogeneous coating thickness stress10% functionally gradient coating thickness stress

 

10% thick homogeneou

s [MPa]

10% thick functionally grandient

[MPa]

Difference [MPa]

% Differen

ce

Max 638.5 637.8 0.75 0.12

Min -1561.7 -1549.1 12.61 0.81

STRESS

10% thick homogeneou

s Fortran model

ANSYS model

Difference

% Diff

Substrate interface in tension 638.5 585.7 52.78 8.3

Coating interface in compression -1561.7 -1489.3 72.39 4.6

Fortran & ANSYS results comparison

Homogenous and functionally gradient Fortran stress comparison

Fortran Results

 

Ly model [MPa]

Zhang model [MPa]

Difference [MPa] % Diff

Radial stress, max 55.9 52 3.9 7.5

Radial stress, min -104.3 -110 5.7 5.2

Axial stress, max 16 17 2 5.9

Axial stress, min -195.5 -198 2.5 1.3

Shear stress, max 109.3 112 2.7 2.4

Shear stress, min 0 0 0 0.0

Ly, Zhang comparison

Correlation to Zhang results

Residual Stress in varying coating thicknesses with constant substrate thickness

-6.00E+08

-5.00E+08

-4.00E+08

-3.00E+08

-2.00E+08

-1.00E+08

0.00E+00

1.00E+08

2.00E+08

Location into substrate/coating (m)

Stre

ss (P

a) 2%4%6%8%10%

Strain in varying coating thicknesses with constant substrate thickness

-7.00E-03

-6.00E-03

-5.00E-03

-4.00E-03

-3.00E-03

-2.00E-03

-1.00E-03

0.00E+00

1.00E-03

1.10E

-03

1.65E

-03

2.20E

-03

2.75E

-03

3.30E

-03

3.85E

-03

4.40E

-03

4.95E

-03

5.50E

-03

6.05E

-03

6.60E

-03

7.15E

-03

7.70E

-03

8.25E

-03

8.80E

-03

Thickness through substrate/coating (m)

Stra

in

2% tc/ts4% tc/ts6% tc/ts8% tc/ts10% tc/ts

 

Max Radial Stress [MPa]

Min Radial Stress [MPa]

2% 22.9 -490.1

4% 44.0 -484.2

6% 81.2 -478.5

8% 115.1 -466.4

10% 147.8 -457.5

Comparative stresses in variable coating

thicknesses.

Varying initial temperature

-6.00E+08

-5.00E+08

-4.00E+08

-3.00E+08

-2.00E+08

-1.00E+08

0.00E+00

1.00E+08

2.00E+08

1.59E

-03

2.12E

-03

2.65E

-03

3.18E

-03

3.71E

-03

4.24E

-03

4.77E

-03

5.30E

-03

5.83E

-03

6.36E

-03

6.89E

-03

7.42E

-03

7.95E

-03

8.48E

-03

9.01E

-03

9.54E

-03

1.01E

-02

1.06E

-02

thickness through material

Stre

ss (M

Pa)

427 deg C 700 deg C 1000 deg C

Conclusions

)( if TTE

• Residual stresses are due to varying coefficients of expansion between substrate and coating(s), driven by temperatures during/at adherence and room temperatures and operating temperatures, Hooke’s Law:

• In this case, the coating went into high compression and the substrate went into tension.

Increasing thickness of coating increased tensile stress in the substrate, which could lead to spallation.

Stress in coating is mostly unaffected with varying thickness and driven by change in temperature.

Changing thermal difference proportionate affects residual stresses in coating.