Page 317 - Handbook of Materials Failure Analysis

314 CHAPTER 13 A concise filtergram wear particle atlas

network. The artificial neural network was trained by the particle library obtained
from ferrographic techniques [12].
As the morphology of the oil degradation particles is diverse, some of these var-
nish particles share the similar profiles with the metallic wear particles [13], i.e., the
pseudo-metal wear particles. Under optical microscope, the pseudo-metal wear par-
ticles can mislead even the experts. Hence, computer image analysis would misiden-
tify some vanish particles as fatigue wear particles.
Benefiting from its high efficiency, LNF is used to process large numbers of oil
samples for identifying the abnormal wear. However, as recommended by the LNF
manufacturers [12], a follow-up technique is needed if LNF detects the abnormal
wear. Manual microscopic examination should be carried out to validate the particle
classifications. Therefore, the goal of replacing experts with computer image anal-
ysis may be realized in the near future.
In fact, filtergram is still a valuable technique in today’s wear particle analysis.
Different from ferrography, filtergram can display detailed features of individual
particles. From this point, a concise wear particle atlas from industrial case studies
is presented in this chapter.

3 RUBBING WEAR PARTICLES
Rubbing wear particles are the most common wear particles, but not important in
wear diagnosis. Even in an ideal lubrication condition, such as hydrodynamic lubri-
cation, rubbing wear particles are still generated. It is believed that rubbing wear par-
ticles are usually generated from the shear mixed layer (Bielby layer). The shear
mixed layer is the metal outer surface on which the atoms are amorphous [14]. Under
contact stress, the normal rubbing wear particles detach from the shear mixed layer
due to the crystalline structural difference from the bulk metal.
The sizes of the rubbing wear particles range from 0.5 to 15 μm[2]. Figure 13.1a
shows individual rubbing wear particles (bright) on filtergram. Some of the small-size
particlesinthebackgroundwerethevarnishparticles(brown).Theseparticleswerefrom
an oil sample of a rolling element bearing. The same oil sample was also prepared on a
ferrogram. On the ferrogram (Figure 13.1b), therubbing wear particle formed the strings
under magnetic fluxes, whereas most of the varnish particles had been washed away.
In some lubricants with EP (Extreme Pressure) additives, or other additives to
form metallic organic compounds, the metallic salt layer is formed on the steel sur-
face under extreme pressure. The tiny wear particles generated from the metallic salt
layer resemble rubbing wear particles under optical microscope. Strictly speaking,
the particles from metallic salt layer do not belong to the rubbing wear particles.
Figure 13.2a shows the tiny metallic salt layer wear particles on a filtergram. The
particles were from a rolling element bearing oil sample with the EP additives. On the
filtergram, it is difficult to differentiate the metallic salt layer wear particles from the
tiny varnish particles. However, on ferrogram the metallic salt wear particles forms
strings (Figure 13.2b) whereas the varnish particles had almost washed away entirely.