Modeling Nanowire Indentation Test With Adhesion Effect

Because of the large aspect ratio of its length to radius and the large surface area to volume ratio, the nanowire is highly flexural and susceptible to the adhesion influence. The bending deflection of nanowire and its adhesion effect make the previous indentation models inappropriate for the nanowire indentation test. In this paper, a new model incorporating the nanowire bending deflection, loading symmetry/asymmetry, and adhesion effect is presented and compared with the previous models. Because of the bending deflection of the flexural nanowire, the nanowire may lift-off/separate from its contacting elastic medium and the localized contact effects may thus be induced. The localized contact effects as predicted by this new model can cause the relatively large deflection difference of the nanowire in test as compared with those obtained by the previous models, which impacts directly and significantly on the interpretation of the indentation experimental data. The nanowire is modeled as a cylinder/beam and the indentation force is modeled as a concentrated force. The elastic medium is modeled as an elastic foundation. The elastic foundation behaves as a linear spring in nonadhesive Hertz contact and as a nonlinear softening spring in adhesive contact. In the Hertz contact, due to lift-off, the contact length is independent of the load. However, in adhesive contact, larger load results in smaller contact length. Unlike the Hertz contact in which lift-off always occurs when adhesion force is too large for bending cylinder to overcome, there is no lift-off for cylinder and the full contact scenario is thus formed.