Surface Engineering by Expanded Austenite
Roughness Surface Testing (RST)
The surface roughness of the post carburised samples were of particular interest as it was anticipated that the expanded austenite would have a significant impact upon it. Previous to the samples being sent to the carburising company, each sample was polished on one face.
The RST Plus system was used to measure the surface roughness of the samples. The RST Plus is a non-contact optical profiler, so all measurements are performed optically and therefore the surface of the sample is never touched. There are two ways of measuring surface roughness using the RST; the first way is using the phase-shifting interferometery (PSI) mode, which is used to measure very smooth surfaces. However the samples used in this project were not expected to be smooth enough for this mode, therefore the vertical-scanning interferometry (VSI) mode was used. This mode is used for rough surfaces and steps.
The VSI works by splitting half of a white-light beam onto a reference surface and the other half onto the sample surface. Both white-light beams are reflected and recombine at the beam splitter to produce an interference pattern. The best contrast between interference fringes are found when the optical lenses are at optimum focus. The RST then measures the degree of fringe contrast (modulation).
Once the optical microscope is fully focused and the feedback monitor only appears as an interference pattern, the surface roughness measurements are ready to be taken. The reference arm where the reflected white-light beam is interfering, moves vertically, scanning the surface at different heights. Due to the nature of white light having a short coherence length, the interference pattern only interacts over a very shallow depth of a focal position. The reference arm starts above the focal position and moves through the focus point, while the contrast between the interference pattern reaches a peak and the reference arm moves past the focal point. During this time the camera is capturing the interference data at each even step. This information is then sent to the computer software which performs a complicated algorithm with the data in order to calculate the surface height. The programme also calculates information regarding the surface which includes the average surface roughness.
A link to the RST analysis can be found by pressing the button below:
