Transition from surface damage to subsurface shielding in multilayer graphene oxide coatings under contact loading

Polylactic acid (PLA) reinforced with conformal graphene oxide (GO) coatings has recently demonstrated significant improvements in stiffness and surface mechanical properties. However, beyond the initial enhancement achieved by a single GO layer, the mechanisms governing damage evolution and protection in multilayer coating systems remain unclear. In particular, it is not yet established whether additional GO layers provide merely incremental reinforcement or fundamentally alter load transfer and failure behavior under contact loading. In this work, molecular dynamics (MD) simulations are employed to systematically investigate the mechanical response of PLA with single-layer, bilayer, and trilayer GO coatings, focusing on the evolution of failure modes and protection capability with increasing layer numbers. The results show that while additional GO layers provide marginal increases in effective contact modulus, they fundamentally alter the deformation and failure behavior of the coating system. Under progressively loaded nanoscratching, the critical force for crack initiation increases from ∼135 nN in single-layer systems to ∼160 nN and ∼195 nN for bilayer and trilayer coatings, respectively, accompanied by a transition from immediate surface failure to progressive, layer-by-layer damage. Notably, in trilayer configurations, the intermediate layer remains largely intact up to ∼280 nN load-controlled nanoscratching, indicating effective shielding of subsurface regions. These findings indicate that increasing the number of GO layers leads to more effective protective behavior under contact loading.