Radley Scientific started out as the Research and Development (R&D) department of an ultrasonics consultancy over which was formed 40 years ago on a farm on the edge of Dartmoor.
Since then they have successfully developed products that are marketed across the globe. Such developments include the LOTUS ultrasonic scalpel (shown to the right), which utilises a torsional ultrasonic mode of resonance making it one of the most efficient in the world, as well as the next generation of orthopaedic revision surgical tools in TORS.
What did they want?
Radley wanted some explorative analysis carried out on their piezo electric ceramic (PZT) ring component to analyse the grain structure and possibly the internal stresses, before and after a treatment that mimics the effect of ageing through relief of internal stresses. PZT material is made electromechanically active by aligning the crystallographic structure of the material to a very high voltage field, applied across opposite faces of the rings, after these faces are made conductive by coating with silver.
Agreed analysis plan
It was agreed that electron backscatter diffraction (EBSD) would be carried out on both samples and the results would be compared in both the parallel and perpendicular planes to the plated silver surface. Although EBSD can’t directly measure stress the resulting maps should be able to show the comparative level of stress in the samples.
Un-aged PZT results
EBSD analysis was carried out both parallel and perpendicular to the plating, for comparison; one plane the structure naturally aligned to the applied voltage field, and the other the structure forced to align to the field. The parallel EBSD maps (1st image) were all highly stressed, which was apparent by the fact the grains were not a single colour. Analysing the patterns, showed that there was misorientation within the grains which indicated that the colour was due to stress. The perpendicular maps (2nd image) seemed to be less stressed compared with the parallel maps, which was apparent by the presence of less colour variation within grains. Radley expected the plane parallel to the plating should be more stressed since this contains the realigned structure, induced by the applied field during manufacturing. The plane perpendicular to the plating has a structure that was closer aligned to the applied field, therefore retains less stress after the applied field.
Further analysis looked at an aged sample to observe what had happened to the stress after aging. The first EBSD image shows the results obtained from parallel to the surface and the second EBSD shows the results obtained from perpendicular to the surface. It was observed that the stress in both the analysed planes were similar, which was indicated by the degree of "speckling" being similar in both EBSD images. It was also observed that the amount of "speckling" was reduced compared with the un-aged parallel sample. This could indicate that the aging process has reduced the overall stress in the sample and caused whatever stress is left to become more uniform in both directions; this interpretation fits with Radley Scientifics understanding of the change in PZT performance over time.