1 15 th High Average Power Laser Workshop General Atomics San Diego, CA Aug. 9 – 10, 2006 W-Steel Interfacial Bond Strength, & MC-Simulation of IEC He-Implantation Shahram Sharafat *, with significant contributions from A. Hyoungil, A. Takahashi 1, J. El-Awady, Q. Hu, J. Qua, G. Romanowski 2, and N. Ghoniem, and collaborative interactions with G. Kulcinski 3, R. Radel 3, S. Gulobov 2, N. Parikh 4, and L. Snead 2 Mechanical and Aerospace Engineering Department University of California Los Angeles 1 Tokyo University of Science 2 Oak Ridge National Laboratory 3 University of Wisconsin – Madison 4 University of North Carolina at Chapel Hill This work was supported by the US Navy/Naval Research Laboratories through a grant with UCLA.

2 OUTLINE 1.W – Steel Interface Bond Strength  Review HIP’d W-F82H  Report on VPS-W 2.Monte Carlo Simulation of IEC Results  Issues for Low-Energy He Implantation  Survey of Low-E He Implantation  KMC Simulation Results

3 The Laser Spallation: Determine Interface Bond Strength 1.Al layer melts and rapidly expands 2.Launching a compressive stress waves through substrate into film layer 3.Compressive waves are reflected as tensile waves from free surface 4.If tensile stress is sufficient interface failure will occur. SiO 2 Substrate Al Layer Nd:YAG Laser 1064 nm Coating Tension Compression Photodiode voltage is used to determine the Displacement profile of the coating surface Coating surface velocity is calculated by differentiating the displacement profile The stress can then be calculated using:  = ½(  c v) Finally, tensile failure stress failure is then evaluated using FEM Density and Elastic Properties of both coating and substrate are required to determine accurate failure stresses

4 HIP’d W-F82H Sample (ITER, JAEA) o Hot Isostatic Pressure (HIP) bonded Tungsten to F82H 1.1 mm W coating ~50 um thick F82H substrate D = 20 mm Since Elastic properties of the coating depend on processing in a statistical manner  Predicted interfacial strengths will have statistical variations Reported 14th HAPL Bond Strength: Depends on Coating Elastic Properties & Density }  bond 450 MPa 1050 MPa F82H W (HIP) Interfacial Crack Time: Sample from ITER Development JAEA (Japan)

5 VPS-W Test Matrix (PPSI/ORNL) o Vacuum Plasma Sprayed (VPS) samples supplied by PPI (S. O’Dell).  W-Coatings were polished to ~50  m thickness at ORNL (G. Romanoski) TEST MATRIX: VPS W-Coated Steel Samples

6 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 Powder Feed Plasma Flame Substrate Plasma Spray Coating Process Modified from Ghansen Comp. In Phys. COMPUTERS IN PHYSICS, VOL. 12, NO. 1, JAN/FEB 1998

7 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 Powder Feed Plasma Flame Substrate Plasma Spray Coating Process Modified from COMPUTERS IN PHYSICS, VOL. 12, NO. 1, JAN/FEB 1998

8 Interface between Substrate & Coating is Porous Simulated Plasma Sprayed Coating Al 2 O 3 ZrO 2 Substrate Pore 500 X Interface Pore *S. Sharafat, Vacuum 65 (2002) 415 Plasma Sprayed Micro-composite Thermal Barrier Coatings (TBC)* Substrate – Interface Porosity Substrate Substrate – Interface Porosity

9 Factors Influencing Coating Bond Strengths o Elastic properties of the coating depends on processing: — HIP’ing results in high density (  ) high Young’s modulus (E) — Plasma Spray results in low  and low E o Substrate/Coating interface topography: — HIP’ing results in high fraction of interface coverage — Plasma Spray shows reduction of interface coverage o Present analysis based on: — Published VPS-W E*: E VPS = 54 GPa (E Solid = 410 GPa) — Coating Density Range: 80% – 60% — Interface Coverage Range: 100% – 40 % *Matejicek, 2005; **Estimate PropertiesF82HW (Bulk) 80% Dense W (VPS) 60% Dense W (VPS) Young’s Modulus (GPa) *27** Poisson’s ratio Density (kg/m 3 ) Wave speed (km/s) Nano-porous VPS-W Deposits 1 1 S. O’Dell, PPI 2004 We need coating properties (E,  ) and interface topography

10 60% Dense W-Coating VPS-Sample #02:  = 11,548 kg/m 3 E = 27 GPa * E Laser = mJ Preliminary Interfacial Strengths of VPS-W VPS-Sample #02:  = 15,397 kg/m 3 E = 54 GPa * E Laser = mJ 80% Dense W-Coating VPS-W Coating Delamination Stress*: 140 MPa – 616 MPa *Based on 3 VPS-W samples and uncertain coating material properties and interface topgraphy HIP-W Interface Cracking Stress** : 450 MPa – 1050 MPa **Based on 1 HIP-W sample uncertain E.

11 Example of “Popped” VPS-W Coating 2 mm VPS-W Coating Failure Bonding of Plasma Sprayed Coating is weaker than that of the HIP’d Coatings: May require development of Interface Layer VPS-W Coating Surface Example of Complete Coating Delamination 60% Dense Coating IP: Interface Porosity 80% Dense Coating No Interface Porosity HIP (avg.)

12 Future Work o Elastic properties of coatings need to be measured including: — Densities, Young’s modulus, and Interface Bond Coverage o Determine minimum stress for ONSET of interface cracks in VPS-W (reported results are for complete failure of coatings) o Cross section and micrograph tested VPS samples (# 01, 02, 05)  ORNL o Test remaining samples (# 03 – 04 & 06 – 09) o Present/Publish at the TOFE 17 (ANS, Nov 13-14, Albuquerque NM)

13 OUTLINE o W – Steel Interface Bond Strength — HIP’d W-F82H Sample (ITER, JAEA) — VPS W-Steel Sample (PPS/ORNL) o Monte Carlo Simulation of IEC Results — Issues for Low-Energy He-Implantation — Survey of Low-E He Implantation — Simulation Results

14 Issues for Low-Energy He Implantation o Results can not be explained by conventional rate theory models because pores sizes are too large ( X 10) and densities are tool low. o Speculations regarding effects of sputtering on surface: — Sputtering increases with decreasing incidence angle (avalanche) — Surface Erosion Via Ion-Sputtering*: From initial ripples morphology to a rough morphology o Surface Temperatures are too low for rapid and large bubble formation (usually occurs above ~0.6 TM; for W: 730 o C ~ 0.26 TM) o Need to measure/calculate the residual trapped Helium. * R. Cuerno, H. A. Makse, S. Tomassone, S. Harrington, and H. E. Stanley, Stochastic Model for Surface Erosion via Ion- Sputtering: Dynamical Evolution from Ripple Morphology to Rough Morphology, Phys. Rev. Lett. 75, (1995);Stochastic Model for Surface Erosion via Ion- Sputtering: Dynamical Evolution from Ripple Morphology to Rough Morphology

15 OUTLINE o W – Steel Interface Bond Strength — HIP’d W-F82H Sample (ITER, JAEA) — VPS W-Steel Sample (PPS/ORNL) o Monte Carlo Simulation of IEC Results — Issues for Low-Energy He-Implantation — Survey of Low-E He Implantation — Simulation Results

16 Low Energy He Implantation of Cu* 30 keV He x10 16 /cm 2 annealed 973 K for 1800 s. He range ~130 nm; peak 1.68 at.%. Pinhole Diam avg ~ 150 nm (predicted ~ 14 nm) *Evans, Nuclear Instruments and Methods in Physics Research B 217 (2004) 276–280 Copper: … surprising result that after annealing at 973 K, 80% of the helium was released and surface pinholes seen, even though the average bubble size predicted from migration and coalescence theory was 14 nm …*

17 Survey of Low Energy He Implanted Tungsten AuthorTungsten Surface Temperature (K) He-Energy (eV) Flux (He/m 2 -s) Fluence (He/m 2 ) Nishijima et al.(2004) PM, SC (purity 99.95%) 1300 – eV4×10 22 – 3.7× ×10 25 – 2.6×10 27 Tokitani et al.(2005) PM, SC (purity 99.95%) >1300 (deduced) eV1×10 22 Tokunaga et al.(2003) PM (99.99%), VP-Spray (92.5% dense) 705 – eV3.83×10 21 – 1.2× ×10 25 – 1.11×10 26 Iwakiri et al.(2003) PM (99.95%)293 – eV 8 keV –2.5×10 21 (8.0×10 21 ) Tokunaga et al.(2004) PM (99.99%)1073 – – 19 keV2× – Survey of Low Energy He Implanted Tungsten

18 Tungsten 10 – 30 eV Helium (Nishijima, 2004) o Low Energy Helium (~ 10 to 30 eV) on PM Tungsten; high dose 2.6 x He/m 2 D. Nishijima et al. / Journal of Nuclear Materials 329–333 (2004) 1029–1033 X 5000 o Cross section of sample W1 which shows holes or passage to neighboring bubbles.

19 Tungsten 19 keV Helium (Tokunaga, 2004) o SEM images of surface (a) and cross section (b) taken from the sample irradiated to 3.3 x He/m 2 at the peak temperature of 2600 ℃. o The energy of He is 19 keV. He beam flux and heat flux at the beam center is 2.0 x He/m 2 s and 6.0 MW/m 2, respectively. Beam duration is s and interval of beam shot start is 30 s. 2m2m (a) (b) K. Tokunaga et al. / Journal of Nuclear Materials 329–333 (2004) 757–760

20 IEC Results ( Cipiti & Kulcinski, 2004 ) : 1  m 1160 °C 2.6x10 16 He/cm 2 -s 2.5 min. 990 °C 8.8x10 15 He/cm 2 -s 7.5 min. 1  m 730 °C 2.2x10 15 He/cm 2 -s 30 min. Steady State: 40 KeV He 5 He/cm 2 Temperature Pore Size Pore Density d ave ~15 nmd ave ~50 nmd ave ~150 nm

21 IEC Results ( Cipiti & Kulcinski, 2004 ) : Pore Diameter vs. Temperature Pore Diameter vs. Time Temperature Time to Pore 730 C1160 C150 s1800 s

22 OUTLINE o W – Steel Interface Bond Strength — HIP’d W-F82H Sample (ITER, JAEA) — VPS W-Steel Sample (PPS/ORNL) o Monte Carlo Simulation of IEC Results — Issues for Low-Energy He-Implantaion — Survey of Low-E He Implantation — Simulation Results

23 MC Simulation of IEC He-Implantation  Migration and Coalescence (M&C) of He-bubbles is based on Brownian bubble motion  Initial bubble density and avg. bubble radius from HEROS code  Differentiate between near surface and bulk processes by calculating He-pressure based on: – Ideal gas law (near surface) – Hard-sphere model (bulk material)

24 Initialize Model Calculate diffusion probability of He-bubble Sum probabilities: Diffusion, Coalescence & Implantation for each He Examine one event (Diffusion of a bubble or Implantation) Jump with constant distance Check Coalescence t n+1 = t n +  t Grow He-bubbles by implantation MC – Calculation Procedure Diffusion Migration: Coalescence: Growth by Implantation: Es : Activation energy, 2.5eV* D 0 : Pre-expon 1.25x10 -2 cm 2 /s Surface diffusion rate Bubble diffusion rate Instantaneous Equilibrium Size: R : Uniform random number (0:1) *Evans, 2004

25 KMC Model for the 730 o C IEC Case: He TemperatureBubble Density He-Implantation Init. Radius Simulation Volume 730 C /cm x /cm 2 -s 0.5 nm 0.2 x 1.0 x 1 mm C /cm x /cm 2 -s 1.0 nm 0.2 x 2.5 x 1 mm C /cm x /cm 2 -s 1.5 nm 0.2 x 5.0 x 1 mm 3 Gaussian Density Distribution Front ViewSide View 1  m 0.2  m Helium Simulation Volume 40 keV He – W: Range: 1.6 nm Straddle: 0.63 nm

26 Evolution of Bubble Size for the 730 o C IEC Case (~2000 s): Bubble Color: Red = Matrix Bubble Blue = Surface Pore Side View ( 0.2  m) Front View ( 1  m) Helium

27 Time Sequence of Pore Evolution (730 o C IEC) 3e-4 s1.5 s68 s 2000 s562 s 383 s

28 KMC Simulation of ICE Experiments 1  m 2.5  m 5  m T = 730 o C t = 1800 s 990 o C 450 s 1160 o C 150 s Pore Diameter vs. Time

29 Surface Pore Evolution o KMC simulates the trend of surface pore size and density IEC

30 Bubble Size Near Surface vs Bulk * *CHERNIKOV, JNM 1989 Near Surface Bulk 1000 appm He Implanted in Ni at RT. Uniform He implantation using degrader Al-foil (28 MeV He) Annealing time: 0.5 – 1.5 hr Abundance of Near Surface Vacancies promotes rapid and large bubble growth

31 Sub-Surface Break Away Swelling Contribution  BREAK-AWAY Swelling (very rapid growth of bubbles) occurs at the subsurface  However, because the bubbles bisect the surface the swelling is stopped by venting He.  Time to BREAK-AWAY swelling DECREASES with higher Temps. Sub-Surface Break-Away Swelling Surface Pore Formation He Avg. Bubble Radius

32 Probable Explanation of IEC Results o Abundance of near surface vacancies allow bubbles to grow rapidly to equilibrium size:  Large bubbles & low He-pressure o Near the surface, Migration & Coalescence (M&C) plus rapid growth results in super-size bubbles. o Super-large bubbles bisect the surface, thus providing a probable explanation for surface deformation and large subsurface bubbles. o A network of deep interconnecting surface pores is rapidly set up which results in drastic topographical changes of the surface

33 Abstracts Submitted to TOFE “Surface Roughening Mechanisms for Tungsten Exposed to Laser, Ion, and X-ray Pulses” Michael Andersen and Nasr M. Ghoniem 2. “Modeling Space-Time Dependent Helium Bubble Evolution in Tungsten Armor under IFE Conditions” Q. Hu, S. Sharafat, and N. Ghoniem 3. “Measurement of Interface Bond Strength between Tungsten Coatings and Steel Substrates for HAPL FW Armor” Jaafar El-Awady, Jennifer Quang, Shahram Sharafat, and Nasr Ghoniem 4. “MC Simulation of Tungsten Surface Pores Formed by Low- Energy Helium Implantation” Akiyuki Takahashi, Shahram Sharafat, Nasr Ghoniem, J. Kulcinski, and R. Radel

34 Backup Slides

35 VPS Interface Bond Strengths E W = 410 GPa No Interface Pores  = 15,397 kg/m 3 *Matejicek, 2005; **Estimated PropertiesF82HW (Bulk) 80% Dense W (VPS) 60% Dense W (VPS) Young’s Modulus (GPa) *27** Poisson’s ratio Density (kg/m 3 ) Wave speed (km/s) E W = 54 GPa No Interface Pores  = 15,397 kg/m 3

36 VPS-W Coating Failure HIP Coating: 1050 mJ to Onset of Failure VPS Coating: 167 mJ to Complete Delamination Laser Energy Compr. Stress to Failure (GPa) Bonding of Plasma Sprayed Coating is weaker than that of the HIP’d Coating Example of “Popped” VPS-W Coating VPS-W Coating Surface 2 mm Example of Complete Coating Delamination

37 Plasma Sprayed Tungsten Coating Material Properties 1. H. You, T. Hoschen, S. Lindig, “Determination of elastic modulus and residual stress of plasma-sprayed tungsten coating on steel substrate,” J. Nucl. Mater. 348 (2006) 94– Jirı Matejıcek,Yoshie Koza, Vladimır Weinzettl, “Plasma sprayed tungsten-based coatings and their performance under fusion relevant conditions,” Fus. Eng. Des. 75–79 (2005) 395–399 “The aim of this work is to measure Youngs modulus of a plasma-sprayed thick porous tungsten coating deposited on a steel (F82H) substrate.” [1]

38 Low Energy He on W Experiments: Nishijima(2004) Tokitani (2005) Tokunaga (2003) Iwakiri (2003)

39 GOAL of STUDY o IEC implants Low-Energy (<110 keV) Helium in Tungsten o All forms of W examined at 730 °C °C showed extensive surface deformation (E He : keV). o Both, steady state or pulsed operation show deformation: — Steady 6 mA ≈ /cm 2 s; — Pulsed 10Hz, 1 ms, 60 mA ≈ /cm 2 per pulse OBJECTIVES:  Provide a potential explanation for the development of MASSIVE Surface Pores (~10 X predicted He-bubble size).  If possible, provide mitigating measures against these MASSIVE surface deformations.

40 SEM image (High temp./Low fluence) ~2600 ℃、 1.7x He/m 2 3.5s/30s( 8S) 18.7 keV, 6.7x He/m 2 s WF-6(20x20x0.1mm) 20mm  The color of surface becomes to be white from metallic sliver color by the irradiation up to ～ He/m 2.  Fine uneven morphology and small holes are observed on the surface. 2μm Slide from: K. Tokunaga a ICFRM-11, Dec. 7-12, 2003, Kyoto, Japan For HAPL: R=6.5m Chamber: ~8x10 22 He/m 2 /day

41 SEM image (High temp./High fluence) ～ 2600 ℃、 3.3x He/m 2 3.5s/30s( 145S) 18.7 keV, 6.7x He/m 2 s WF-2(20x20x0.1mm) 20mm  When fluence is beyond ～ He/m 2, the color of surface becomes to be black  The surface is modified resulting in a fine uneven morphology and holes with a diameter of about 50 nm are observed on the surface. 2μm 1μm Slide from: K. Tokunaga a ICFRM-11, Dec. 7-12, 2003, Kyoto, Japan For HAPL: 3.3x10 23 He/m 2 in ~4.5 day

42 SEM image of cross section ~2600 ℃、 3.3x He/m 2 3.5s/30s( 145S), 18.7 keV, 6.7x He/m 2 s, WF-2(20x20x0.1mm)  Grain growth by re-crystallization occurs.  Many horn-like protuberances with a width of about 300 nm and a length of about 1 μm are observed at the surface. In addition, He bubbles with a diameter of about nm are observed near surface.  The surface modification is considered to be formed by the He bubbles and their coalescence, the migration of He bubbles near surface. Surfac e 20μm1μm K. Tokunaga a ICFRM-11 Dec. 7-12, 2003 Kyoto Japan

43 Low E-He on Copper UKAEA FUS 499 EURATOM/UKAEA Fusion Kinetics of bubble growth and point defect migration in metals J.H. Evans October 2003

44 Low E-He on Copper (Evans, 2004) Evans, Nuclear Instruments and Methods in Physics Research B 217 (2004) 276–280

45 New HEROs code gives information about pore sizes: R ~ 16 nm R ~ 50 nm R ~ 100 nm

46 Bubble Density …(corrected!) 6E13 5E14 4E15