Solid Skin Rectification

The elasticity and hardiness of a skin depend on several factors [18] (1) the surface curvature, (2) the nature of the bond involved, and (3) the ratio between the operating temperature and the skin melting point. Normally, the surface of a solid 

As shown in Fig. 24.2c, the tangent modulus of a CNT turf (a complex structure of intertwined nanotubes cross-linked by adhesive contact) was measured to be one order higher near the surface and the modulus drops with the penetration depth [31]. The modulus near the terrace edge is even higher than the flat surface. 

Ideally, an enhancement of the elasticity and hardness are readily observed from the skins of compounds, alloys, or specimens with high melting points, whereas the enhancement is not seen so often using indentation method for specimens with lower melting points such as Sn, Zn, Al, and organic specimens unless they are chemically passivated [34]. 

The measured skin hardening or softening also varies with methods of detection. For instance, the modulus of nanocrystalline of 50-800-nm-thick Ni films, measured using acoustic AFM [35], is lower than that of the bulk, as no artifacts such as accumulation of dislocations are involved in the non-contact acoustic AFM method. SiTiN films are measured approximately 10-20 % harder using nanoindentation than the values obtained using SAWs methods [36]. The additional pressure due to compression will cause deviation of the intrinsic values. Therefore, 

Normally, deformation resistance from the effect of the pile up of dislocations, strain gradient work hardening, and the artifacts due to indentation tip shapes, strain rates, loading scales and directions, etc., involved in the contact mode of indentation would play a role of significance [39]. The surface smoothness has less influence on the measurement [40]. 

---