Instable Microdeformation and Strain Recovery in Amorphous LiPON Thin Layer

Lithium phosphorus oxynitride (LiPON) is a crucial electrolyte for all-solid-state thin-film batteries due to its sufficient ionic conductivity. Understanding the mechanical behavior of LiPON films is crucial for further technological development. Previous studies noted unexpected ductility and strain recovery in amorphous LiPON during sharp-ended tip indentations revealing pile-up formation and densification as the main deformation mechanisms. Our work presents nanoindentation experiments including spherical tips, revealing a novel mechanical behavior of a sudden deformation event followed by slower but complete strain recovery during unloading. This unique deformation phenomenon is likely linked to the material's special structure, featuring isolated phosphate tetrahedra P(O,N)₄ embedded in an amorphous Li matrix with occasional N bridge bonds between tetrahedra. In this study, the authors report on a range of nanoindentation experiments, examining how instability depends on strain rate and the indenter's tip geometry. It is found that instability occurs only within a specific range of deformation velocities and strongly depends on the indenter's tip sharpness. Assuming the mobility and the capability of the cooperative movement of the tetrahedra, the measured novel deformation method, and other, deformation-attached properties of the LiPON can be explained.