Anomalous viscoplasticity during nanoindentation of Al/SiC nanolaminated composites

Nanoindentation analysis is severely affected by time-dependent deformation (viscoplasticity) in materials and needs to be understood. Hence, in this work we carefully examine the viscoplasticity behavior of Al/SiC nanolaminates under nanoindentation loading and its effect on the measured mechanical properties. The viscoplastic behavior of Al/SiC nanolaminates was compared to deformation in Al thin films and a mechanism for deformation in nanolaminates was elucidated. Hold time during indentation has a pronounced effect of "creep" on unloading in pure Al. With zero hold time there is viscoplasticity during unloading due to a displacement lag. This is less predominant at larger hold times since the stresses are relaxed. This effect is minimal in the multilayered material because of the constraint from SiC. This constraint also has a significant effect during thermal drift measurement at low load. The material experiences "negative creep" due to push-back from the sample upon removal of the indenter. These experimental observations were validated by numerical analysis using finite element method.