Computational Nano & Micro Mechanics Laboratory UCLA

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

ConfigurationConfiguration Bond StrengthBond Strength Measurement Method Measurement Method Reference Reference

Porcelain/Metal 8-12 ksi4 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/ metalAs 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

6ns-duration

Beam Splitter for Nd:YAG

Energymeter

Mirror

Convex Lens

Specimen Holder

SiO2

Substrate

CoatingHe-Ne laser

(Interferometer)

CTCR

T

Aluminum

Nd:YAG Laser

Continuum Corp.

Model: Precision II

1064nm wavelength

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 Compressive waves are reflected as tensile waves from free surface of film and cause tensile failure at film/substrate interface

f

•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.

1mm

Laser=65mJ

Magnified pictures of Laser-spalled area (Laser energy = 65mJ):

Sample:

SummarySummary

• 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 R&D of the W-armor protected FW

WF82H

SteelFe7W6

W

F82H Steel

Bonding Tungsten to Low Activation Ferritic Steel(Romanoski et. al., Oak Ridge National Laboratory)

IntroductionIntroduction

Pull-Off Adhesion Tester(www.defelsko.com)

Schematic of Tension Tester (Kobayashi, Vacuum, 73, 2004)

Three-point bend test (NPL,UK)

Four-point bend test:Fracture Mechanics Approach (Somerday et. al., SNL, USA)

Methods of InterfaceMethods of InterfaceBond Strength MeasurementsBond Strength Measurements

Bond Strength Measurements for Different Bond Strength Measurements for Different ConfigurationsConfigurations

The Laser Spallation Interferometer ExperimentThe Laser Spallation Interferometer Experiment

x

S1Sn-1

Sn SN

hn-1 hn

f(t)

n

n

BackFace

FrontFace

The compressive load from the laser source can be related to the measured velocity of the front surface:

i

xvPxuPf NN

NN

,ˆ,ˆˆ 21

21

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:

,ˆˆ,ˆ 21

21

21NNnn xv

i

Af

P

Ax

• 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.

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)

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

Compressive stress from laser sourceMaximum ≈ 1100 MPa

Calculated stress at interfaceMaximum tensile stress at failure ≈ 200MPa

Laser Energy = 65mJ

Determining Interfacial StressesDetermining Interfacial Stresses

Advantages of TheAdvantages of TheLaser Spallation TechniqueLaser Spallation Technique

Adhesion Strength Measurement Adhesion Strength Measurement