In this study, we investigate the nanoindentation characteristics and crack formation with flexure of nanoscale atomic layer deposition (ALD) alumina on polyamide-6 (PA-6) films. Initial ALD processing PA-6 is shown to form a subsurface hybrid layer by reaction between ALD precursors and the polymer backbone, followed by standard Al₂O₃ surface formation in subsequent cycling. Over an exposure temperature from 60 to 120 °C, the degree of hybrid layer formation varies significantly. Transmission electron microscopy shows that the thickness of the underlying hybrid layer is increased at lower temperatures, up to 120 nm for 100 ALD cycles of trimethylaluminum and water on PA-6. At 120 °C processing, no hybrid layer is observed. The elastic modulus and hardness evaluated by nanoindentation show a corresponding decrease in value with the thickness of the hybrid layer. Flexure testing shows that both crack density and critical tensile strain is decreased for films processed at lower temperatures that have an increased thickness of the underlying hybrid layer. This analysis shows that the nucleation behavior of the ALD thin films on polymers plays an important role in understanding the mechanical performance of the thin films. This work has important consequences in the how ALD materials need to be applied and evaluated on polymers for application as diffusion barrier layers.