Page 437 - Failure Analysis Case Studies II

420

Fig. 5. Optical micrograph of the cage and the silver coating

Optical microscopy of the cage shows a dual phase structure of the material (Fig. 5). The cage is
most probably cast and has a - 38 pm thick silver coating. Silver coating provides good resistance
to fretting and improves the bedding-in and running properties of harder bearing materials [2].
Microscopic study revealed that there were darkly etched spherical inclusions in the cage material;
the inclusions are up to - 8 pm in diameter and are Al-rich.
The bearing balls, outer and inner races were manufactured from a Cr-steel in the hardened and
tempered condition, having a fine distribution of Cr-carbides; the composition is summarized in
Table 1. From the point of wear similar material to the components which are in mutual contact
have a tendency to “smear” during the wear process [3].
Material of the bearing lock was found to be 38 Cr AI; Table 1. A section of the lock material
observed under the optical microscope revealed - 18 pm thick copper coating on the surface.

2.3. Material smeared on components
The material smeared on the surfaces of the components was analyzed following preparation of
smaller cross-sectional specimens of the component. These were analyzed in the scanning electron
microscope equipped with energy dispersive spectroscopy. The results are summarized in Tables 2
and 3.
The higher magnification view of the bearing ball specimen in Fig. 4d-iii shows the presence of
deposited material; this is especially true in the regions where the material close to the surface shows
heavy deformation bands. The smearing, confirmed from EDX analysis, was found rich in Fe, Cu,
Si, A1 and Cr while Ag and Mn were detected at isolated locations. Si is probably from the lubricant
or debris. At a location on the surface of the ball (location A in Fig. 4d-i) where it seemed that
material was removed due to impact, a high concentration of Fe, Cr, Si, A1 and Cu was detected.
It seems that the material of the cage was smeared probably at the inner race first and then extensive
compressive/shear stresses between the ball and the outer race resulted in chipping or shearing of
the ball material.
Smearing on the outer race is accompanied by deformation, as is evident from the deformation
bands near the surface in Fig. 6. Three different regions were analyzed and the results are summarized
in Table 2a; the first two regions were big enough to permit analysis at a couple of locations. In all
the locations cage material was smeared. Region I contained a significant amount of Ag showing
that the cage got smeared when the coating was intact. Analysis at Region 111, on the other hand,
did not show the presence of an Ag coating, indicating that the cage had already been distressed to
the level that its coating was completely stripped off before smearing at Region 111.
Micrographs in Fig. 4c show the condition of smearing on the inner race. The LHS was quite
deformed and damaged. Figure 4c-i shows the seat of the race where extensive smearing and
deformation is clearly visible. Four different locations, marked in Figs 4c-vi and c-vii, were analyzed