Ultra high strain rate compressions on Ag nanocubes using a high-speed nanomechanical low-load sensor

This project aims to develop a testing methodology for the nano-scale samples inside an SEM using a high-speed nanomechanical low-load sensor (nano-Newton load resolution) and high-speed dark-field differential phase contrast imaging-based scanning transmission electron microscopy (STEM) sensor.

Testing nanoscale objects such as nanowires or nanocubes at higher strain rates requires a unique combination of stiffness as well as high sensitivity of the load sensor. In this work, a piezoresistive strain gauge-based silicon microcantilever was implemented as a load sensor. The signal-to-noise ratio of this Si-based nano-load sensor was found to be ~150nN whereas the resonance frequency of the cantilever is in the range of ~1 MHz which makes the load cell suitable for force sensing at very high speeds up to 20 mm/s. This roughly translated to a strain rate of 100,000 s^-1 and 25000 s^-1 for a nanoparticle of height 200 nm and 750 nm respectively. Prior to the testing, the nano-load sensor was shaped to require size using focused ion beam milling to obtain the flat-punch configuration for compression of micro/nanoparticles. Subsequently, the low load sensor is calibrated using a pre-calibrated load sensor on a fused silica sample. During the actual testing, the piezo-tube sensor was used as the displacement actuator as well as the sensor. Both the load and displacement signals were captured using oscilloscopes with a sampling frequency up to ~1GHz. Recently, as a case study, the rate-dependent properties of single crystal Ag nanocubes (~200nm facet size) were ascertained, for the first time, across 8 orders of strain rate magnitude from 0.001 s^-1 till 100,000 s^-1 and the stress-strain signatures are currently being investigated. Currently, efforts are being made to combine high-speed STEM sensor in conjunction with nanoparticle compressions at high strain rates to capture the in situ microstructural evolution during mechanical testing.