Strain-rate sensitivity of scratch hardness and deformation mechanism in nanocrystalline Ni under micro-scratch testing

To investigate the strain-rate sensitivity in nanocrystalline (NC) materials using a single experimental technique over a wide range of strain rate, the micro-scratch testing technique was selected to obtain consistent and systematic data of the strain-rate sensitivity in a fully dense, high purity, and well-characterized electrodeposited NC Ni sample. The scratch characterizations and mechanical properties of the sample under the different scratch velocities were discussed in details. First, some critical parameters under the micro-scratch testing, such as strain rates, scratch ditch widths, were confirmed. Then, the scratch hardness under the different scratch velocity was investigated. From the results, the sample exhibits strain-rate sensitive mechanical properties. Further, the average values of strain-rate sensitivity of the sample were calculated. The values increase with increasing scratch velocity, and exhibit higher values that are of the order of 0.03-0.1. The greater strain-rate sensitivity exponents indicate that the NC Ni samples have different fundamental physical deformation mechanisms. Finally, the fundamental physical deformation mechanism under scratch testing was inspected using SEM and TEM technique. From the SEM and TEM morphologies, the grain boundary dislocation pile-ups should be a carrier of plastic flow under scratch testing in the NC Ni sample.