Effect of nanoscopic defects on barrier performance of thin films deposited by plasma-enhanced atomic layer deposition on flexible polymers

Abstract Penetration paths caused by nanoscopic defects in plasma-enhanced atomic layer deposited (PEALD) Al2O3 thin films could increase their transmission rate, mechanical limitations could impair their encapsulation utility. In contrast, this phenomenon could be beneficial in the case of polymer electrolyte fuel cells containing a proton exchange membrane, where water retention in the membrane is crucial for efficient transport of hydrated ions. Nanocracks in the deposited layer can act as nanovalves at low humidity levels and maintain the suitable level of hydration for the membrane. This paper presents the results of a unique, in situ oxide film cracking-gas permeation measurements on 25–100 nm Al2O3 coated low-density polyethylene. The developed measurement chamber enables the bulging of the substrate/coating system, which initiates the cracking of the coating. Under the same mechanical load, the permeability decreases exponentially with film thickness: quadrupling the film thickness reduced it by a tenth. Simulation-based empirical models on gas diffusion through defected barrier layers are summarized, and an analytical model is constructed instead. We have also carried out simulations to help understand the processes. The derived analytical equations perfectly describe the literature data, our simulations, and experimental results. The introduced technical setup, experimental and theoretical work may open further perspectives in understanding the behavior of barrier layers under mechanical loading.