Page 332 - Handbook of Materials Failure Analysis

8 The Oxide Wear Particles 329

(a) 200µm (b) 200µm

FIGURE 13.25
The black oxide particles from different machines (200 ): (a) a black oxide particle from an
air compressor; (b) a black oxide particle from a roller bearing.

Figure 13.25 shows the typical black oxide wear particles from the oil samples
with normal moisture contents. The particle in Figure 13.25a was from an oil sample
taken from an air compressor. Figure 13.25b was a particle from an oil sample taken
from the return line of a roller bearing in a Yankee machine.
The influence of the moisture on the particle oxidation is complex. Generally, the
moisture forms red oxide scale on the wear particle surface, changing the particle’s
colors from orange to dark brown. On the other hand, the improper lubrication would
form black oxide scale. Hence, the multi-oxide scales can form on the single-particle
surface. Figure 13.26 shows the wear particles with mixture of red oxide scale and
black oxide scale. These two particles were from the same oil sample with extremely
high moisture content (Figure 13.24).
In high moisture condition, the red oxide crystals (like the rust) can further grow
on wear particle surfaces. Figure 13.27 shows the red oxide crystals on the wear par-
ticle surface. Under polarized reflected light, the red oxide crystals remain with their
colors and brightness (Figure 13.27b).
In lubricating oil with extremely high moisture contents, a milky oxide layer can
be formed on the wear particle surfaces. Figure 13.28 displays wear particles cover-
ing with milky layers. These particles were from two different gearboxes in a paper
mill. Both oil samples contained extremely high moisture contents due to the seal
failures. In oil sample of Figure 13.28a, 25% free water was found in the oil sample
bottle. In oil sample of Figure 13.28b, the moisture content was over 10,000 ppm.
The nature of the milky layer has not been studied. It could be iron hydroxide or
the oil degradation products generated by the extremely high moisture.

CASE 4: THE WEAR PARTICLE WITH MULTI-OXIDE SCALES FROM AN
OIL SAMPLE WITH HIGH MOISTURE CONTENT

In a paper mill, a main lubricant tank (3000 l capacity) supplied the oil to lubricate
the two major roller bearings in a Yankee machine, and also other rolling element
bearings in the production line. In any event of seal failure, water would infiltrate