Sand in the Gearbox – Abrasive Minerals Invisible in Contaminant Particles?

It is often surprising for our customers when we find hard mineral particles on their components during technical cleanliness analysis. Customers ask: "How can that be? We didn't find anything in our own cleanliness tests."

The contamination of hard minerals is so important, because these particles have an abrasive effect and thus massively damage mechanical systems such as gears and motors.

Our customers mostly use light microscopy for their own cleanliness analysis. This is exactly the problem: mineral abrasives are mostly transparent or light gray. Microscopic detection, however, requires a sufficient contrast between the light-colored filter membrane and the particles. In the case of abrasive minerals, the optical contrast in the light microscope is too weak to enable automatic detection. For this reason, our laboratory only uses scanning electron microscopy (SEM) for the detection of abrasives particles.

To illustrate the effect, we analyzed a particle-loaded membrane with scanning electron microscopy (SEM) and light microscopy (LM) and compared the results.

In the image displayed, we classified particles using element spectroscopy (EDX) divided into four material classes: aluminium, corundum, glass blasting material and steel. In the SEM image, the cellulose membrane is dark and the four particle classes are shown with good contrast. The filter background is light in the light microscopic image. Only the metallic particles show a sufficient optical contrast to the membrane. The hard mineral particles are difficult to detect and are not detected by the automatic particle analysis. This problem was already addressed by the "Analytics" working group when revising the VDA Volume 19.1. The "Limits of light-optical analysis" are described in the appendix to chapter 8.2. The following figure shows abrasive particles that cannot be detected by the binarization threshold.
VDA-19.1 Grenzen lichtoptischer Systeme
We recommend all customers who need reliable evidence of abrasive mineral particles as part of component cleanliness to resort to an analysis using SEM-EDX. The experts from our test laboratory are happy to further advise you. We look forward to hearing from you.