In situ measurement of bulk modulus and yield response of glassy thin films via confined layer compression

Abstract

Abstract

Extrinsic and Intrinsic Mechanical Responses of Confined Indented Thin Films

“…where G is the elastic shear modulus [17]. At yield, shear stress is capped on the Von Mises yield surface 𝑄 = 𝑌 0 (dotted black line), allowing only the hydrostatic stress to increase with further loading.…”

“…At a stress of 𝜎 𝑧𝑧 = 𝑌 𝑓𝑙𝑜𝑤 = 0.65 GPa, the confining material yields leading to extrusional flow and gross deviation from the CC state. This confinement failure value has been shown to depend on the geometric and material parameters of the test [17]. Until Yflow, the deformation is highly one dimensional as demonstrated in Fig1d &1e, which show stressstrain curves and AFM residual topography maps for three separate indents of a h0 = 190 nm aPS film.…”

“…[16] In contrast, we recently have shown that a nearly uniform state of confined compression uniaxial strain (CC) can be achieved in glassy thin polymer films through an indentation technique we call the layer compression test. [17] Geometric confinement allows isolation and controlled study of distinct elastic and elastic-plastic states at the nanoscale with sensitive, low stiffness instruments. Prior to this, CC had only been demonstrated in granular or porous materials due to difficulties in preventing Poisson driven expansion and/or volume preserving plastic flow.…”

“…For an elastic perfectly plastic material, the stress-strain relation is 𝜎 𝑧𝑧 = 𝐾𝜀 𝑧𝑧 , where K is the elastic bulk modulus. [21] The layer compression test [17] realizes CC by indentation of an aligned, cylindrical flat punch whose diameter is many times the initial film thickness (Fig. 1b).…”

Effect of cross-linking and hydration on microscale flat punch indentation contact to collagen-hyaluronic acid films in the viscoelastic limit

“…The use of a cylindrical flat punch with a flat film-like sample has a advantages over other forms of indentation testing at small scales where direct means to characterize the contact geometry are difficult or impossible to realize due to adhesive forces and pile-up or sink-in effects 37 . While in principle suffering a stress singularity at the contact edge, in practice the film geometry and mild punch edge rounding 38 mitigate this, leading to a nearly uniform stress distribution across the contact 39 .…”