Measurement of Tungsten Armor - Ferritic Steel Interfacial Bond Strength Using a Nanosecond Laser Source

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Measurement of Tungsten Armor - Ferritic Steel
Interfacial Bond Strength Using a Nanosecond
Laser Source
Jaafar El-Awady, Sauvik Banerjee, Shahram
Sharafat, Nasr Ghoniem and Vijay Gupta
Mechanical and Aerospace Engineering Department
University of California Los Angeles, Los Angeles
Introduction
Determining Interfacial Stresses
The Laser Spallation Interferometer Experiment
Al layer melts and rapidly expands, causing the
sacrificial SiO2 layer to spall off and sending
strong compressive stress waves through substrate
into film layer

• Tungsten has been chosen as the primary
  candidate armor material protecting the low
  activation ferritic steel first wall (FW) chamber
• The tungsten armor is less than 1-mm thick and
  is applied by vacuum plasma spraying (VPS)
• Interface bond strength between the W-armor and
  the substrate needs to be quantified in order to
  provide guidance for further RD of the W-armor
  protected FW

6ns-duration
The compressive load from the laser source can be
related to the measured velocity of the front
surface
NdYAG Laser Continuum Corp. Model Precision
II 1064nm wavelength
Specimen Holder
Bonding Tungsten to Low Activation Ferritic
Steel (Romanoski et. al., Oak Ridge National
Laboratory)
Energymeter
Compressive waves are reflected as tensile waves
from free surface of film and cause tensile
failure at film/substrate interface
Beam Splitter for NdYAG
The stresses at any of the interfaces can be
related to the applied compressive load from the
laser source at the back face or in other words
to the velocity of the front face as follows
SiO2
Coating
He-Ne laser
CT
CR
Mirror
Advantages of The Laser Spallation Technique
T
Methods of Interface Bond Strength Measurements
Aluminum
Convex Lens
(Interferometer)
Substrate

• Because of the short rise time of the stress
  pulse, an interfacial region of approximately 70
  to 150 micrometers is stressed uniformly.
• High amount of Laser energy can be obtained by
  reducing the focus area
• The failure occurs at the weakest link in the
  region which is spanned by the coating, interface
  and the substrate material.
• Such a short pulse is able to invoke a rather
  local response from the interface such that
  minute structural and chemical changes are
  directly reflected in the measured strengths.

Pull-Off Adhesion Tester(www.defelsko.com)
Schematic of Tension Tester (Kobayashi, Vacuum,
73, 2004)
Magnified pictures of Laser-spalled area (Laser
energy 65mJ)
Sample
Failure in Cu(1400nm)/TiN(70nm)/Si System (a)
Failure inside Si (b) Failure at the interface of
Cu/Tin (Gupta et. al., UCLA, USA)
Three-point bend test (NPL,UK)
Adhesion Strength Measurement
Four-point bend testFracture Mechanics Approach
(Somerday et. al., SNL, USA)
Bond Strength Measurements for Different
Configurations
Summary
Properties of the materials Titanium h 200
?m, ? 4.5 g/cc, ? 3.3 mm/?s Bone Tissue h
6 ?m, ? 2 g/cc, ? 6.0 mm/?s

• Determine interface bond strength of
  W-armor/ferritic steels as a function of vacuum
  plasma spraying (VPS) parameters
•  
• Establish lifetime for W-armor/steel interface
  bond as a function of number of thermal cycles
  induced by (a) laser, and (b) x-rays simulated
  pulses, and (c) RHEPP ion pulses Develop
  low-cycle SN curve for W-armor delamination
•  
• Determine failure mechanism of W-armor
  delamination (a) interface fatigue crack
  nucleation/ propagation, and/or (b) surface crack
  nucleation and propagation to the interface
• Work will also includes microscopy and SEM of
  failed interfaces to determine failure mechanisms.

Configuration Bond Strength Measurement Method Reference
Porcelain/Metal 8-12 ksi 4 point flexural test (FEM stress analysis) DeHoff, 1982
gold alloy and pure titanium bars/ dimethacrylate polymer-glass fiber composite 10-35 MPa Push out test Villittu, 2003
Thin film polymer/ metal As a function of strain energy release rate 10-25 J/m2 4 point bend test (Fracture mechanics approach) Somerday, 2003
Zirconia composite coating/stainless steel 10-40 MPa Tension tester Kobyashi, 2004
Very strong and ultra thin film interfaces for device applications as high as 2.5 GPa ! Laser spallation technique Gupta, 2003
Laser Energy 65mJ
Calculated stress at interface Maximum tensile
stress at failure 200MPa
Compressive stress from laser source Maximum
1100 MPa