Atomic force microscopy study of nanoindentation creep on the (100) face of MgO single crystals

Abstract

The experimental results obtained on nanoindentation creep and evolution of indentation impressions made on the (100) face of MgO crystals from their in-situ observations by atomic force microscopy are described and discussed. The indentations were made by square pyramidal Si tips at a constant load of 2 μN and indentation times, t ind, between 0.001 and 10 s. It was found that: 1. the initial depth, d i, of indentations is practically constant for t ind<0.1 s but increases with increasing t ind for t ind>0.1 s; 2. the initial indentation diameter, a i, practically does not depend on t ind; 3. an indented surface always tends to heal with time, but the time, t, of healing increases with penetration depth of d (i.e. on indentation time); 4. the dependences of both d and a on recovery time, t, exhibit three recovery stages — an initial transient stage, a steady-state stage and a slow stage — and follow a power-law dependence in these stages; and 5. indentation creep occurs for t ind>0.1 s. Analysis of the kinetics suggests that the recovery of indentations and the indentation creep occur by local reorganization, and volume and surface diffusion processes, but the indentation creep is caused above a threshold value of the indentation pressure gradient.