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Mechanical Robustness of ALD/MLD-Based Barrier Coatings
Keywords: atomic layer deposition (ALD), molecular layer deposition (MLD), fracture strength
The mechanical robustness of atomic layer deposited (ALD) alumina and recently developed molecular layer deposited (MLD) films as well as laminated composites composed of both materials was characterized using a novel fluorescent taggant. All coatings were deposited on polyethylene naphthalate (PEN) substrates and demonstrated a similar evolution of damage morphology according to applied strain, including: channel crack formation, crack densification up to saturation, followed by transverse crack formation. From the crack density characterization, the mode I critical fracture toughness of separate ALD and MLD films (KIC=1.890.10 and 0.170.02 MPam0.5, respectively) was evaluated at the point of steady-state propagation, whereas the critical interfacial shear (c=39.58.3 and 66.66.1 MPa, respectively) was estimated according to the saturated crack density. The toughness of nanometer-scale ALD alumina was comparable to that of alumina thin films grown using other techniques, whereas MLD aluminum alkoxide (alucone) was quite brittle. The use of MLD alucone as a spacer layer situated between ALD alumina films, was found to be detrimental to the overall strength of the composite coatings. The hindered performance is attributed to the limited toughness of alucone, confirmed using a fracture mechanics based formalism that also verified the experimentally observed through-thickness damage morphology. To aid future study, the increase in the toughness and elastic modulus of the spacer layer required to render improved critical strain at fracture were identified. These results may be applied to a broad variety of multilayer material systems composed of ceramic and spacer layers, rendering robustness coatings for use in barrier applications.
David C. Miller, Scientist II
University of Colorado
Golden, CO

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