Three-dimensional information on the distribution of elements or phases within materials is critical when dealing with compounds that are anisotropic or heterogeneous in nature. This information is required when modeling properties in materials or for predicting synthetic routes.

A group of scientists in the United States has developed a tomography technique harnessing the characteristics of femtosecond laser ablation to build 3D datasets. This automated technique provides a new way of imaging complex materials in a fraction of the time when compared to existing technologies, such as mechanical or focused ion beam techniques.

Schematic of the instrumentation. Image courtesy of McLean Echlin.

It is well known how tomographic imaging can elucidate problems in medicine, geology, oceanography, and materials science. 2D slices of samples that can be successfully reconstructed into 3D rich datasets may be acquired with a wide variety of techniques that use electrons, neutrons, x-rays, ions, visible light, or acoustic waves. However, this technique is accompanied by many restrictions, in terms of the resolution, quality of data, sample preparation and of course acquisition time. It is also a very skilled and labor intensive procedure.

In this study the scientists, from UC Santa Barbara and the University of Michigan, have succeeded in overcoming many of these obstacles. The newly developed femtosecond laser based technique involves laser ablation followed by optical imaging of the ablated surface, with no subsequent surface preparation required. These steps are repeated for the number of slices required to section a predetermined volume of material.

The ablation event and incoming laser are orthogonal to the plane of the sample surface. Images are captured optically during the sectioning experiment using a high resolution CCD detector. Statistical analysis of the datasets was then carried out.

With this set up the scientists were successfully able to distinguish TiN particles in steel, with a diameter larger than 1 micron. Imaging enhancements, including in situ SEM and the integration of laser induced breakdown spectroscopy, are currently on-going in order to improve the resolution by an order of magnitude.

This new tomography method is ideal for imaging multiphase systems containing phases with similar densities that are inherently difficult to image using other techniques, such as x-ray based measurements.

Many experiments and observations still need to be carried out; for instance, making use of the non-contact mode of laser machining, which will allow materials to be sampled in vacuum, and to take advantage of other analysis techniques such as EBSD and EDS.

Source: Materials Today