For indentation of the thread region of the leadscrew, a specialist dual axis goniometer jig was sourced and adapted to allow the tapered screw thread to be mounted parallel to the nanoindenter tip. Five sets of five horizontal indents, 30 µm apart, were made at 250µm intervals from the screw thread root, along the thread flank, to the thread crest. The goniometer was adjusted between indents to ensure contact between the screw and indenter tip was perpendicular.
To investigate the crest feature, a region rich in these features was removed from the leadscrew and then sectioned longitudinally, mounted in resin and polished. Several of the largest crest features were imaged using scanning electron microscopy (SEM), and several locations around two of the features were also examined with energy-dispersive X-ray spectroscopy (EDX) to determine the composition of the material inside this region. SEM analysis on the cross-sections showed that there were large voids, from which fine cracks extended through the bulk material.
The EDX compositional analysis taken from the pores and notches were characteristic of stainless steel, however, the investigation also evidenced the presence of calcium, which would not otherwise be expected. This suggested that the feature was an artefact of the thread forming process, a known consequence of roll forming threads. Roll forming results in high levels of plastic deformation at both the thread crest and the thread root. Whilst no features were visible at the thread root, the nature of deformation at this point suggested it was the most likely to be caused by the presence of the contaminating material observed.
This knowledge has given Waters a better understanding of their materials which has allowed them to make informed choices about the components used in future work. This should increase not only the lifetime of the component but also the system as a whole, and increase production as the system should stop seizing during service.
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