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 AngelesConfiguration Bond Strength Measurement Method Reference Reference Porcelain/Metal8-12 ksi 4 point flexural test (FEM stress analysis) DeHoff, 1982 gold alloy and pure titanium bars/ dimethacrylate polymer-glass fiber composite MPaPush out testVillittu, 2003 Thin film polymer/ metal As a function of strain energy release rate J/m 2 4 point bend test (Fracture mechanics approach) Somerday, 2003 Zirconia composite coating/stainless steel MPa Tension testerKobyashi, 2004 Very strong and ultra thin film interfaces for device applications as high as 2.5 GPa ! Laser spallation techniqueGupta, ns-duration Beam Splitter for Nd:YAG Energymeter Mirror Convex Lens Specimen Holder SiO 2 Substrate Coating He-Ne laser (Interferometer) CTCT CRCR T Aluminum Nd:YAG Laser Continuum Corp. Model: Precision II 1064nm wavelength Al layer melts and rapidly expands, causing the sacrificial SiO 2 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 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=65m J Magnified pictures of Laser-spalled area (Laser energy = 65mJ): Sample: Summary 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 W F82H Steel Fe 7 W 6 W F82H Steel Bonding Tungsten to Low Activation Ferritic Steel (Romanoski et. al., Oak Ridge National Laboratory ) Introduction Pull-Off Adhesion Tester ( 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 Interface Bond Strength Measurements Bond Strength Measurements for Different Configurations The Laser Spallation Interferometer Experiment x S1S1 S n-1 SnSn SNSN h n-1 hnhn f(t) nnnn Back Face Front Face The compressive load from the laser source can be related to the measured velocity of the front surface: 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: 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 source Maximum ≈ 1100 MPa Calculated stress at interface Maximum tensile stress at failure ≈ 200MPa Laser Energy = 65mJ Determining Interfacial Stresses Advantages of The Laser Spallation Technique Adhesion Strength Measurement