Comparison of HIP and VPS Tungsten Coating Behavior Using Laser Spallation

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Comparison of HIP and VPS Tungsten Coating
Behavior Using Laser Spallation

• Jaafar El-Awady
• with significant contributions fromH. Kim, J.
  Quan, S. Sharafat, V. Gupta, G. Romanowski1
• and N. Ghoniem
• Mechanical and Aerospace Engineering Department
• University of California Los Angeles
• 1Oak Ridge National Laboratory

16th High Average Power Laser Workshop Princeton
Plasma Physics Laboratory Princeton, NJ Aug. 12
13, 2006
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Outline

• The Laser Spallation Technique
• HIPd W-F82H measurements
• Plasma Spray W-F82H measurements
• Dynamic vs. Static Loading
• Fracture Mechanics Calculations
• Conclusion

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The Laser Spallation Technique
Experimental Layout
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HIPd W-F82H Joint
HIP (Hot Isostatic Pressure)W-F82H joint is
fabricated with HIP conditions of 1243K, 143MPa
and 2 hour holding time.
Akiba and Hirose (JAEA)
Time
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HIPPED W-F82H Bond Strength Measurements
Laser Fluence effect on the failure of the bond
Laser Fluence Failure
613 mJ No Failure
1065 mJ No Failure
1329 mJ Some Crack generating at the interface
1577 mJ Severe Damage
1708 mJ Severe Damage
1737 mJ Severe Damage
W
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HIPPED W-F82H Bond Strength Measurements
95 dense W propertiesW bulk properties80
dense W properties
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VPS-W coated F82H

1. Powder melts in Plasma Flame
2. Molten droplets are accelerated towards
   substrate
3. Droplets solidify on substrate
4. A new layer of molten droplets solidifies

Example of PoppedVPS-W Coating
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VPS-W coated F82H Failure Strength
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VPS-W coated F82H Failure Strength
Failure strength of the coating is
at 1/3 of the thickness
Static test results (Greuner et. al. 2005)
Failure occurs in the coating and not at the
interface The failure strength is estimated to
be 2530 MPa
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WHY? Dynamic vs. Static Loading

• The material undergoes a ductile-to-brittle
  transition as the strain rate is increased.
• The yield stress increases significantly and the
  work-hardening rate decreased as the strain rate
  increases.
• In dynamic loading the fracture toughness is
  independent of any plastic deformation and
  geometry effects on the contrary with static
  loading.

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Back of the Envelop Calculations of the Required
Stress for Fracture (Fracture Mechanics)

• The stress required to propagation a crack in a
  brittle material can be calculated using an
  elastic strain energy model
• For a 1 mm initial size crack in an 80 dense
  coating

E modulus of elasticity g specific surface
energy a one half the length of an internal
crack
80 dense W (Analytic result)
MPa
(Experimental result)
MPa
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Conclusions

• We have successfully tested VPS and HIPd
  Tungsten coated ferritic steel samples
• HIPd samples fail at the W-F82H interface while
  VPS samples fail in the W-coating itself
• Failure strength in HIPd samples is found to be
  about twice that in VPS samples
• For VPS W-F82H the static strength is 2530 MPa
  while the dynamic strength is about 450550 MPa
• Fracture mechanics gives similar results to our
  current experimental results
• We are proposing the use of Fracture toughness
  instead of tensile properties