Machinery Lubrication

Machinery Lubrication July August 2021 Digital Edition

Machinery Lubrication magazine published by Noria Corporation

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Page 37 of 56 | July - August 2021 | 35 (ppm) or just ferrous debris (wear particle concentration), amongst others. All of these debris monitoring methods are great for initial detection and trending data. However, they each have their limitations and may not provide much investigative evidence on where the particles came from and how they were produced. W hen abnormal trends arise during routine debris monitoring, it's often necessary to prompt additional analysis (called exception testing) to answer some of these questions: What is the root cause? How threatening is the condition? What is the failure mode? How far has the wear mode progressed? Even if a failure has already occurred, the remaining debris can shed light on what caused the failure and if it could have been avoided. ese questions often require techniques with microscopic debris analysis to provide the answers. Crucial evidence to answer million- dollar questions about machine failure modes lies within the hidden depths of the particles. What is Scanning Electron Microscopy? When observing an object, what we see with our eyes is the light reflected off the object. Our unaided eyes have a resolution; at about 100 microns (0.1 millimeters), we begin losing the ability to distinguish individual objects. Optical microscopes, which use lenses to focus light, allow us to see smaller features and objects. Light travels in waves with a specific wave- length, which is the distance that light takes to complete one oscillation. e wavelength of the visible light spectrum is between 400 and 700 nanometers (or 0.4 and 0.7 microns). As we attempt to magnify objects with an optical microscope, we encounter a resolution problem: the viewing area size of the object we are looking at becomes close to the wavelength of light. In other words, the image does not look clear, and we cannot distinguish indi- vidual features. To visualize objects (or features on an object) smaller than the limits of the visible light spectrum, we must apply a different tech- nology that uses a smaller wavelength, such as the Scanning Electron Microscope (SEM). Instead of light, an SEM uses electrons that travel with a much smaller wavelength (about 0.01 nanometers). An electron column focuses and shoots a beam of electrons at the surface of an object, often in a vacuum-controlled chamber. Detectors measure the electrons that are reflected off the surface or knocked out of the atomic orbitals near the surface of the material; this information is then used to generate an image of the material. Because the SEM uses electrons to produce an image, the object being observed must be conductive. e electron column and specimen may need to be kept under a high vacuum in order to minimize interference of air molecules and other airborne contaminants during high magnification (greater than about 5000x). However, for analysis purposes that require the magnification of typical wear debris (2500x to 5000x), a low vacuum option (along with special- ized laboratory techniques) can be used to more easily emphasize the particle's characteristics. In order to obtain SEM images of the particulate in an oil sample, the solid debris gathered from the oil sample (or elsewhere, such as a used filter) is collected onto a patch and, using a conductive adhesive, is adhered to an aluminum specimen mount. If sputter coating is used, a nanometer-scale layer of ions of a conductive material is deposited onto the surface of the sample, creating a conductive pathway for the electrons. Common materials for sputter coating include carbon, aluminum and gold. Once the sample on the mount is prepared, it can be placed onto the SEM sample holder. e sample holder is on a movable stage © Copyright. 2018 The University of Waikato - Te Whare Wananga o Waikato. All rights reserved. www. ML ML

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