Page 334 - Handbook of Materials Failure Analysis

10 Copper Alloy Wear Particles 331

100µm 500µm
(a) (b)
FIGURE 13.28
The wear particles covering with milky layers (200 ): (a) a fatigue wear particle from a
gearbox lube with 25% free water; (b) a fatigue wear particle from another gearbox lube with
extremely high moisture content (>10,000 ppm).

9 THE SPHERICAL PARTICLES
Spherical particles are often found in fatigue failures of rolling element bearings and
gears.Itisgenerallybelievedthatthe sphericalparticlesareformedfrommoltenmetals.
Like the water bead in nature, the formation of the spherical particles is to minimize
surface energy. Figure 13.31 shows a partial molten wear particle found in Case Study
2. Figure 13.31a is the particle under stereomicroscope. Figure 13.31b reveals the par-
ticle details under ESEM. As the wear particle was partially molten, the top portion
appearsthewaterbeadfeaturewhereasthebottomportionretainstheunmoltenfeatures.
This evidence confirms that the in situ temperature can be so high in some particles.
The diameters of spherical particles can range from 5 μm to several hundred
microns. In oil samples, the spherical particles from 10 to 50 μm are commonly found.
Figure 13.32a is a spherical particle (D100 μm) under stereomicroscope (45 ).
The particle was from the Case Study 2. Figure 13.32b shows another spherical particle
(D50 μm) under metallographic microscope. The particle was from an oil sample taken
from a rolling element bearing lube.
The formation of spherical particles is not limited to steels. Brass and bronze
components can also generate spherical particles. Figure 13.33 is a bronze spherical
particle found on an oil filter of the journal bearings of a geothermal steam turbine.
In addition to metals, oil degradation products can form spherical particles.
Figure 13.34 shows the two varnish spherical particles with different colors. The par-
ticles were from two different hydroturbine oil samples.

10 COPPER ALLOY WEAR PARTICLES
Copper alloy, particularly bronze, is widely applied in the machinery components,
such as bearing cage, worm gear, and some slide bearing pads. Hence, copper alloy
wear particles are not uncommon on filtergrams. Generally, copper alloy wear par-
ticles are easily recognized by their reddish-yellow color.