Reverse plasticity in single crystal silicon nanospheres

Nanoparticles in the range of 20 to 100 nm in size can be deposited, isolated, and individually probed for their mechanical properties. With a hypersonic plasma particle deposition technique, this has been successfully accomplished for silicon and titanium. We have already shown that silicon nanoparticles are superhard in the 30 to 50 GPa range after work hardening (Gerberich, W.W., Mook, W.M., Perrey, C.R., Carter, C.B., Baskes, M.I., Mukherjee, R., Gidwani, A., Heberlein, J., McMurry, P.H., Girshick, S.L., 2003a. Superhard silicon nanospheres. J. Mech. Phys. Solids 51, 979). At the same time when small nanospheres are compressed, a fraction of the plastic strain is reversed after unloading. Initially, the amount of reverse dislocation motion was small but appeared to accelerate once a threshold strain was reached. The cumulative reverse plastic strain from repeated loading of the same nanosphere appeared to increase from less than 0.04 to approximately 0.4 as cumulative strain increased from 0.2 to 0.6. For large strains then, it appears that a greater amount of plastic strain is recovered after unloading. This can at least partially be understood in terms of the enormous back stress developed at the small scale when dislocations are only a few nm apart. As the ramifications to nanoscopic features on MEMS, micromachines and magnetic recording devices is considerable, it is desirable to understand if a length scale can be developed for such phenomena. In terms of classic dislocation theory an attempt is made. Problems and prospects are discussed with regards to predictive models for hardness and reverse plasticity.