The compressive strength of sub-micrometre diameter magnesium oxide crystals

Abstract

The compressive strength of individual sub-micrometre diameter MgO crystals has been determined by direct measurement inside an electron microscope. The micro-loading device used was capable of applying forces down to 2 × 10⁻⁴ g f (20 μN) and the crystal size ranged from 0·2 μm to 0·4 μm. The crystals were cube-shaped and were prepared by burning magnesium to form a smoke. Compression was carried out in a 〈100〉 direction. The results show that the crystals behave in a fully plastic manner on this scale without cracking, up to ∼70% strain. The measured strengths showed considerable variation between crystals but were typically 180±40 kg/mm² (18 MN/m²). No work-hardening was evident. The strength of the micro-crystals is approximately 15 times greater than that of bulk MgO crystals but only approximately 1/10 that of the theoretical strength of MgO. The crystals were not sufficiently small to obtain good electron transmission and it was not possible to observe clearly the changes taking place within the crystals during compression. Comparison of the results with those of previous experiments on sub-micrometre gold crystals shows that the strengths are very similar when expressed in terms of the elastic modulus. It is suggested that, as with gold, the increased strength at small sizes is associated with the operation of harder dislocation sources necessary to produce slip on a finer scale.