Research ArticleMATERIALS SCIENCE

Organic monolayers disrupt plastic flow in metals

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Science Advances  16 Dec 2020:
Vol. 6, no. 51, eabc8900
DOI: 10.1126/sciadv.abc8900
  • Fig. 1 Using SAMs to probe MC effect.

    (A) Schematic of a molecule adsorbed on metal surface showing head and terminal groups, and hydrocarbon chain. (B) Procedure to deposit SAMs: For the silane head molecules, workpiece is first plasma-treated to increase hydroxyl group density at surface and then immersed in ethanol solution containing the molecule. For the stearic acid SAM, workpiece is directly immersed in ethanol solution containing stearic acid. (C) SAM-deposited workpiece surfaces characterized by contact angle measurement. (D) Surface energy obtained from the contact angle using the Owens-Wendt theory (34). Because surface energy is determined purely by terminal group, all the SAMs show a uniform reduction of ∼37% in this energy. (E) Model 2D experimental (simple shear) configuration used to explore MC effect with SAM films (red). A high-speed camera, coupled to an optical microscope, is used to image material flow and characterize plastic flow field.

  • Fig. 2 Characterizing MC effect using cutting force.

    (A) Comparison of force with and without SAM films. A large force decrease (∼85%), typical of the MC effect, is seen with the long-chain SAMs [CH3(10) and SA(17)]. (B) Plot of the ratio (Γ)—maximum cutting force with a specific SAM film to the maximum cutting force without any film (bare)—as a function of chain length. A sharp decrease in Γ from 1 to ∼0.15 occurs as chain length increases from 6 to 10, indicating the importance of chain length in controlling the MC effect.

  • Fig. 3 High-speed imaging of (contrasting) plastic flow modes reveals origin of MC effect.

    Select images, with strain-field (background color) and streaklines superimposed, show development of flow with [bottom row, SA(17)] and without (top row, bare Al) long-chain SAMs. (Top) With bare Al workpiece, the shearing results in sinuous flow with thick-chip, wavy streaklines, and heterogeneous straining. Flow development is tracked via green, yellow, and red points (initially collinear, frame A1). A bump forms on workpiece surface ahead of chip in frame A2. The bump grows in amplitude, rotates, and shears to form a fold in frame A3. The final chip is a stack of folds, with fold interfaces resembling notches. (Bottom) With a long-chain SAM [SA(17)], the shearing results in segmented flow—typical of MC effect. Movement of green, yellow, and red points in frames B1 and B2 is similar to frames A1 and A2 (top row). However, between frames B2 and B3, a crack starts from workpiece surface and propagates toward wedge tip, causing a large separation between yellow and green points. This cracking, which arrests the sinuous flow in its incipient stage, is recurrent and results in segmented flow.

  • Fig. 4 Continuum and atomistic models to study adsorbate-induced embrittlement.

    (A) Schematic of adsorbate-covered notch in Al subject to remote shear loading. Notch tip is assumed locally flat with a narrow region (2d) free of adsorbate. The adsorbate induces a surface stress f on the surface, with the inhomogeneity resulting in a force dipole M = 2fd. A virtual dislocation is shown at point P (distance ρ from O) situated on a slip plane making angle φ with horizontal. (B) Equilibrium position of dislocation ξ* versus φ for various values of nondimensional surface stress η. ξ* > 2 (<2) implies brittle (ductile) behavior. As η increases, model predicts brittle behavior. (C) Atomistic simulations of notch behavior in ductile Al subject to remote tensile loading with and without adsorbate. Top: In the absence of adsorbate, notch emits dislocations (core marked yellow) that travel along slip plane; this emission blunts the notch (ductile). Bottom: With an adsorbate that induces surface stress f = + 3 N/m, the atoms give way for a crack to propagate (brittle).

Supplementary Materials

  • Supplementary Materials

    Organic monolayers disrupt plastic flow in metals

    Tatsuya Sugihara, Anirudh Udupa, Koushik Viswanathan, Jason M. Davis, Srinivasan Chandrasekar

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