The ferrography technique was developed in the 1970s to overcome the large particle detection deficiencies of spectrometric oil analysis. It is a technique used for the separation of particles from fluids for microscopic examination and subsequent analysis. The name ferrography derives from the initial development of the methods to precipitate ferrous wear particles from engine lubricating oil. The original analytical ferrograph used a strong magnetic field gradient to separate wear particles from the used oil sample for subsequent microscope examination.
Figure 1: Example of a Ferrogram. On the lower right is an image
of the Spectro Scientific Thistle Tube Ferrography Lab including
ferrography slide maker, microscope and wear particle atlas (WPA)
software with reference pictures.
Since that time, a number of different instruments have been introduced such as the rotary particle depositor which also separates wear particles for microscopic examination along with other instruments which quantify the amount of ferrous wear debris in an oil sample. The primary instrument in ferrography is the analytical ferrograph which is used to prepare a ferrogram. A non-wetting barrier is painted on one surface of a microscope slide. This coating traps the fluid delivered by a peristaltic pump. In use, the slide is mounted at a slight horizontal angle, permitting the fluid to flow by gravity along the glass but within the barrier, where it finally is picked up by a drain tube.
The slide is mounted above two permanent magnets which are separated by an aluminum sheet. The poles of the magnets are counter posed. That is, where one magnet pole is considered north, the pole of the other magnet across the aluminum strip is south. Positioning the magnets in this way causes a strong magnetic field gradient to be created in the vertical direction above the aluminum strip. Magnetic particles in the fluid experience a strong downward force. These particles migrate through the fluid down to the glass surface, where they are deposited in strings perpendicular to the direction of fluid flow (Figure 1).
After all the fluid in a given sample has run across the slide, a fixer solution is passed over the slide to remove residual fluid. When the fixer has evaporated, the slide is ready for observation using the microscope. Ferrous particles are deposited on the slide according to size. The force acting on a particle is proportional to volume, but the viscous resistance of the suspending fluid is proportional to surface area. Therefore, for spheres, force increases with the cube of the diameter (volume) but resistance increases only with the square of the diameter (contact surface) .
The largest ferrous particles, therefore, are deposited at the entry region of the slide where the lubricating oil first touches down on the glass surface. At a position farther along the slide, all ferrous particles larger than a characteristic size already will have been precipitated. For non-ferrous particles, such as aluminum, brass, white metal, etc., precipitation will often occur because these materials are weakly magnetic. However, the deposition of these materials will be less size-selective. Consequently, large particles of nonferrous metal may be found anywhere along the length of the slide as also shown in Figure 1.
Figure 2: Example of cutting wear, severe sliding wear and
Examination of the slide in a bichromatic microscope reveals details of size, shape, and number of particles. From this information the condition of oil-lubricated parts may be assessed . Machines operating normally usually generate small flat particles at a slow steady rate. If the number of particles increases, and particularly if the ratio of large to small particles increases, we have an indication that a more severe mode of wear has begun. The generation of large severe wear particles signals the imminent failure of the wearing surface.
Different wear mechanisms each generate characteristic particles. Six regimes of wear have been identified with sliding wear. Miscellaneous shaped particles are associated with break-in wear. Small platelets are associated with normal rubbing wear. As operating parameters become more arduous, the metallic rubbing wear particles increase in size and become oxidized. Finally, just prior to failure, large, metallic, severe wear particles are produced.
Abrasive wear, analogous to a crude machining process, generates particles in the form of loops, spirals, and bent wires. Increase in the number and size of these particles shows that an abrasive wear mechanism is progressing rapidly.
In addition, two types of particles are associated with rolling mechanisms. These are fatigue chunks and laminar particles. Fatigue chunks represent material removed as rolling elements spall. Laminar particles, which are large, thin plates, result from material being passed through the rolling contact. Combined rolling and sliding, as in gears, produces scuffing particles and fatigue chunks. Determination of the composition of particles can establish their origin. The site of deposition, reflectivity, and color of particles aid in their identification. Figure 2 shows examples of abrasive wear, sliding wear and fatigue wear.
Ferrography can be useful to detect situations that are not obvious, such as water in oil as described below: