Nanomechanical Fracture Testing of Diatoms

Abstract

A way to test the fracture properties of the foramen layer has been developed and tested. Cantilevers made out of the diatom silica were produced with the Focused Ion Beam (FIB). These had notches milled close to the base of the cantilever and were then loaded to fracture using a picoindenter. Challenges and issues during testing have also been discussed in order to assist in any experiments that may continue the described diatom testing.

Unfortunately, only one cantilever yielded usable results. Finite element models were created in order to analyse the result, as well as conventional fracture toughness formulas. However, the cantilever had been sloped upwards during the production because of internal stresses, which had to be taken into account. Two models were made: one of which was based on no internal stresses while the other was based on the internal stresses being nullified when the cantilever was horizontal. These two models resulted in a conservative estimate and an over-idealised estimate of the fracture toughness and thus yielded inconclusive results. The real value should be somewhere between the two extremes, whose values ended up at 0,31 MPa√m and 2,03 MPa√m.

A dimensionless geometry factor for the optimal cantilever shape was also obtained. This was shown to be considerably lower than the values from theory, much due to the smaller vertical sides of the pentagonal cross section.