Comparison of HIP and VPS Tungsten Coating Behavior Using Laser Spallation

1. 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

2. Outline • The Laser Spallation Technique • HIP’d W-F82H measurements • Plasma Spray W-F82H measurements • Dynamic vs. Static Loading • Fracture Mechanics Calculations • Conclusion Dec. 13th, 2006 Jaafar A. El-Awady

3. Tension Nd:YAG Laser1064 nm Compression Coating Substrate SiO2 The Laser Spallation Technique Experimental Layout Dec. 13th, 2006 Jaafar A. El-Awady

4. D = 20 mm 1.1 mm 50 mm W coating F82H substrate HIP’d 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 Dec. 13th, 2006 Jaafar A. El-Awady

5. Laser Fluence: 1577 mJ 100mm W Crack nucleation F82H W 100mm F82H F82H Delemination W W F82H Laser Fluence: 1708 mJ F82H HIPPED W-F82H Bond Strength Measurements Laser Fluence effect on the failure of the bond W Dec. 13th, 2006 Jaafar A. El-Awady

6. Laser Fluence: 1329 mJ 100mm W F82H Interface Cracking W F82H MPa(80% dense W) MPa (Bulk W properties) HIPPED W-F82H Bond Strength Measurements 95% dense W propertiesW bulk properties80% dense W properties Dec. 13th, 2006 Jaafar A. El-Awady

7. Example of “Complete Delamination” of the VPS-W Coating Powder Feed Substrate Porosities Plasma Flame VPS-W coated F82H • Powder melts in Plasma Flame • Molten droplets are accelerated towards substrate • Droplets solidify on substrate • A new layer of molten droplets solidifies Example of “Popped”VPS-W Coating Dec. 13th, 2006 Jaafar A. El-Awady

8. Laser Fluence: 110.4 KJ/m2 Laser Fluence: 75.0 KJ/m2 VPS-W coated F82H Failure Strength Dec. 13th, 2006 Jaafar A. El-Awady

9. MPa (80% dense W (VPS)) MPa (Bulk W properties) 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 25~30 MPa Dec. 13th, 2006 Jaafar A. El-Awady

10. annealed ? 105~108 s-1 ~ ~ 3*103s-1 90% increase 10-3s-1 *Dümmer et. al. 1998 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. Dec. 13th, 2006 Jaafar A. El-Awady

11. 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 Dec. 13th, 2006 Jaafar A. El-Awady

12. Conclusions • We have successfully tested VPS and HIP’d Tungsten coated ferritic steel samples • HIP’d samples fail at the W-F82H interface while VPS samples fail in the W-coating itself • Failure strength in HIP’d samples is found to be about twice that in VPS samples • For VPS W-F82H the static strength is 25~30 MPa while the dynamic strength is about 450~550 MPa • Fracture mechanics gives similar results to our current experimental results • We are proposing the use of Fracture toughness instead of tensile properties Dec. 13th, 2006 Jaafar A. El-Awady