Lubricants
What is it?
The Bruker Contour GT-K profilometer uses the principles of interferometry to produce a 3‑dimensional representation of a surface. The surface of any test piece can be mapped to nanometre precision using this instrument.
Why do we use it?
An important aspect of tribology is the amount of material removed from a test piece after sliding against another surface. The removal of material in this manner is commonly termed “wear”. The amount of wear can be estimated from the 2-dimensional size of a wear “scar” demonstrated in the image to the right, however this does not tell the whole story. Using 3D microscopy it is possible to accurately measure the volume of material that has been removed or displaced. This enables us to generate high quality quantitative data on the anti-wear properties of a given lubricant.
The image below shows the wear scar formed by EU 5W30 engine oil (a and c) compared to that with 0.5% polymeric friction modifier added (b and d).
Bruker Contour GT-K Profilometer:
In the image above, the appearance of the scar (c and d) gives a visual indication of wear but only the 3D images (a and b) reveal the true scale of the difference.
How does it work?
The Bruker Contour GT-K uses two basic interferometric techniques; vertical scanning interferometry (VSI), and phase scanning interferometry (PSI). The choice of technique is dictated by the surface in question. PSI gives the highest vertical resolution, however it is unsuitable for very rough surfaces where there is a large height difference between the top and bottom surface. A recently developed third method, VXI, uses the same measurement as VSI but with a much more sophisticated analysis algorithm that can reach a height resolution approaching that of PSI.
All of the techniques are based on the principle of interference. When two light beams combine they can interfere constructively or destructively, depending on whether they are in or out of phase. A classic example of this is the double slit experiment shown on the right. A single light source passes through two parallel slits creating waves which propagate outwards. These waves interact to generate a pattern of light and dark fringes on the detector screen.
The double slit experiment:
Interference Microscopy:
In interference microscopy, a beam splitter, splits incident light into two beams; a reference beam and a measurement beam. The reference beam is reflected off a mirror and recombined with the measurement beam before reaching the detector, shown in part a of the diagram to the left. In this way the path length of the reference beam is always known.
The measurement beam is reflected from the sample surface before being recombined with the reference. The interference pattern is measured and from this the path length difference between the two beams can be calculated.
It is this difference which enables the height or depth of the surface to be measured, as shown in part b of the diagram to the left.