1 / 19

Thin Film Cracking Modulated by Underlayer Creep

Thin Film Cracking Modulated by Underlayer Creep. Rui Huang The University of Texas at Austin Collaborators: J. Liang, J.H. Prevost, Z. Suo. SiN film on Al (ratcheting) Huang, Suo, Ma, J. Mech. Phys. Solids 50, 1079 (2002). Creep and ratcheting induced cracks in thin films.

thisbe
Download Presentation

Thin Film Cracking Modulated by Underlayer Creep

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Thin Film CrackingModulated by Underlayer Creep Rui Huang The University of Texas at Austin Collaborators: J. Liang, J.H. Prevost, Z. Suo

  2. SiN film on Al (ratcheting) Huang, Suo, Ma, J. Mech. Phys. Solids50, 1079 (2002) Creep and ratcheting induced cracks in thin films SiGe film on glass Huang, et al., Acta Mechanica Sinica (2002).

  3. Film on elastic substrate a Film on viscous layer Stress relaxes in crack wake, but intensifies at crack tip; Gradual loss of constraint (G2  G1) Viscous layer   h Gradual loss of constraint due to creep Free-standing film  a  h l ~ a

  4. Viscous layer  Cracking of a brittle film on a viscous layer • Will a pre-exist crack grow? • When will a pre-exist crack grow? • How fast will a crack grow?

  5. Elastic film: plane stress dx  h  H Viscous layer: pure shear Diffusion-like equations, 2D Shear Lag Model Elsasser, 1969. Rice, 1980. Freund and Nix, 1999. Xia and Hutchinson, 2000. Huang et al., 2001.

  6. Length scale = Dimensional consideration: Analytical solution: (Laplace transform) K • Gradual loss of constraint: • When t = 0, K = 0 • When t  ∞, K  ∞ • Long crack will grow after a delay (when K = Kc) Stationary long crack

  7. When t = ∞, 1 When t  0, 0 Very short cracks will never grow, Stationary short crack Longer cracks are subject to delayed fracture.

  8. Cracks never grow Crack grows t Kc Delayed fracture tm 0 a 0 ac a Delayed fracture

  9. y L time x L 2a L L t = 10.0 K = 0.441 t = 0.05 K = 0.264 t = 1.0 K = 0.604 t = 3.0 K = 0.722 Effect of edge relaxation Huang, et al., Acta Mater. 50, 4137 (2002).

  10. Crack growth criterion: Length scale: Time scale: Representative values  = 500 MPa E = 100 GPa, h= 10 GPa-s,h = 0.1 m, and H = 1 m  = 4 m, t0 = 16 s Growing Cracks

  11. Stationary crack Steady state growth Numerical simulation of crack growth Transient state

  12. Viscous layer  Steady state velocity of crack growth h H Steady velocity is approached after the crack grows by a distance ~ L. Crack velocity can be readily measured experimentally, and can be use to determine the viscosity of the underlayer. Liang, Huang, Prévost, Suo, Experimental Mechanics. In press.

  13. Kc Crack Velocity, V Subcritical V-K curve Kth Stress Intensity Factor, K Subcritical cracking • Steady state set by two kinetic processes: • underlayer creep • Subcritical bond break • Know the subcritical cracking V-K curve of the brittle film • Measure crack velocity to determine the underlayer viscosity.

  14. 0 Brittle film L L Crack Velocity, V Viscous layer Substrate Stress Intensity Factor, K Crack Velocity, V Critical legnth: Lc Bridge length, L Crack in a micro-bridge

  15. Elastic underlayer (Xia and Hutchinson, 2000) No initiation viscoelstaic rubbery Kc Delayed fracture glassy Instant initiation days weeks years 0 a Viscoelastic underlayer Suo, prevost, Liang, submitted.

  16. Power law creep: Stationary long crack: Steady state velocity: Nonlinear creep Measure crack velocity to determine the creep law (B, n) for the underlayer. Liang, Zhang, Prevost, Suo, submitted to Acta Mater.

  17. Uni-directional shear stress cyclic stress metal film Y E cyclic temperature substrate strain Ratching-creep analogy: Strain per cycle Thin Film Ratcheting Huang, Suo, Ma, Acta Materialia49, 3039-3049 (2001).

  18. Tensile Film Ratcheting Layer Cyclic temperature Stress intensity factor of a stationary long crack: Steady state growth rate: Ratcheting-induced crack Liang, Huang, Prevost, Suo, Experimental Mechanics, in press.

  19. Summary • Underlayer creep induces loss of constraint on cracks in thin films. • A long crack starts to grow after a delay. • Subcritical cracking, modulated by underlayer creep, attains a steady state crack velocity. • Extensions to viscoelastic and nonlinear creep underlayers. • Ratcheting-induced crack by analogy.

More Related