Nanoindentation study on helium-irradiated graphene–aluminum composite: Indentation size effect, creep behavior, and their implications for coating–substrate systems

We carried out systematic nanoindentation measurements on nanolaminated graphene (reduced graphene oxide, RGO)–aluminum (Al) composite irradiated with high-energy helium ions at room and elevated temperatures. We revealed a clear transition in the deformation mechanism with the increase in indentation depth, within the micron-sized irradiation damage layer. At shallow indentations, the deformation of the irradiated layer was dominated by the indentation size effect, while the substrate effect started to interfere when the indentation depth exceeded a critical depth. Subsequent room temperature creep tests were conducted at maximum indentation depths that were meticulously chosen to avoid the interference of the substrate effect. The fitted creep stress exponents and the extracted apparent activation volumes reflected different operating deformation mechanisms among various sample sets. Our study provides a practical approach to deconvolute the indentation size effect and the substrate effect, which may lead to an improved accuracy in the interpretation of indentation data. Graphical abstract