**3.2. RCF post test autopsy**

Figures 8 through 10 present post-test photos of the ball, rod, and races for two types of failures and for two RCF loads. Configuration 1 of Table 1 was used for all three tests. Figure 8 contains results from a suspended test after 1.7 hours. A ball-rod-race model is presented in the center panel of Figure 8 to highlight the silver and the wear tracks on the ball and rotating rod. Inspection of the ball surface reveals that silver is still present on the ball surface, as illustrated by the scratch-test results shown in the right panel of Figure 8. Inspection of the rod surface shows traces of silver transfer to the rod wear track. Figure 9 contains results from a life test that eventually failed for silver depletion after 20.6 hours of testing, corresponding to 9.6 x 106 stress cycles accumulated on the rod wear track. Information related to silver transfer and the movement of silver on the wear track is highlighted as in Figure 9. The significance of silver transfer from the ball surface to both the rod and race wear-tracks suggests that a third-body-transfer model may be used to model the UHV-RCF test method. Concerning the rod and race information in Figure 9, the solid silver lubricant was depleted by incipient sliding between the rod-ball and race-ball contacts such that all of the silver was transferred out of the third-body storage areas. For the RCF platform, the third-body storage areas may be represented on the rod and race wear tracks.

Rolling Contact Fatigue in Ultra High Vacuum 105

rotating. The debris from the spall remains within the wear track resulting in significant plastic deformation of all contact surfaces before the test is stopped. Coating spall failure resulted in higher friction and stress leading ultimately to surface yielding. The silver depletion failure mode resulted in increased friction and vibration as well but without surface yielding before the test was stopped. However, if allowed to continue without lubrication the increased

**Figure 9.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 3.61 GPa loading. Silver depletion failure after 20.6 hours of rotation at 130 Hz, accounting for approximately 9.6 x 106 rod

**Figure 10.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 4.0 GPa loading. Coating spall failure after 9.1 hours of rotation at 130 Hz, accounting for approximately 4.2 x 106 rod

stress cycles.

stress cycles

friction would accelerate the onset of subsurface spall of the ball and rod.

**Figure 8.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 3.61 GPa loading. Suspension after 1.7 hours of rotation at 130 Hz, accounting for approximately 7.8 x 105 rod stress cycles.

Figure 10 contains an early life spall failure of the silver coating and subsequent damage to the ball and rod surfaces. Enlargement of the rod wear track is to be expected for this failure mode since plastic yielding has occurred due to increased friction associated with spall failures. Once a coating spall occurred on at least one of the balls, the surface is damaged due to yielding and the resulting contact area with the rod increases rapidly. The test is halted after the stopping criteria, vibration levels in excess of 0.35 g, is exceeded for 1 minute. With the rod rotating at 130 Hz and a deceleration time of about 3 seconds, approximately 390 rod-rotations will be added to the failed surfaces before the rod stops rotating. The debris from the spall remains within the wear track resulting in significant plastic deformation of all contact surfaces before the test is stopped. Coating spall failure resulted in higher friction and stress leading ultimately to surface yielding. The silver depletion failure mode resulted in increased friction and vibration as well but without surface yielding before the test was stopped. However, if allowed to continue without lubrication the increased friction would accelerate the onset of subsurface spall of the ball and rod.

104 Performance Evaluation of Bearings

**3.2. RCF post test autopsy** 

Figures 8 through 10 present post-test photos of the ball, rod, and races for two types of failures and for two RCF loads. Configuration 1 of Table 1 was used for all three tests. Figure 8 contains results from a suspended test after 1.7 hours. A ball-rod-race model is presented in the center panel of Figure 8 to highlight the silver and the wear tracks on the ball and rotating rod. Inspection of the ball surface reveals that silver is still present on the ball surface, as illustrated by the scratch-test results shown in the right panel of Figure 8. Inspection of the rod surface shows traces of silver transfer to the rod wear track. Figure 9 contains results from a life test that eventually failed for silver depletion after 20.6 hours of testing, corresponding to 9.6 x 106 stress cycles accumulated on the rod wear track. Information related to silver transfer and the movement of silver on the wear track is highlighted as in Figure 9. The significance of silver transfer from the ball surface to both the rod and race wear-tracks suggests that a third-body-transfer model may be used to model the UHV-RCF test method. Concerning the rod and race information in Figure 9, the solid silver lubricant was depleted by incipient sliding between the rod-ball and race-ball contacts such that all of the silver was transferred out of the third-body storage areas. For the RCF platform, the third-body storage areas may be represented on the rod and race wear tracks.

**Figure 8.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 3.61 GPa loading. Suspension after 1.7 hours of rotation at 130 Hz, accounting for approximately 7.8 x 105 rod stress cycles.

Figure 10 contains an early life spall failure of the silver coating and subsequent damage to the ball and rod surfaces. Enlargement of the rod wear track is to be expected for this failure mode since plastic yielding has occurred due to increased friction associated with spall failures. Once a coating spall occurred on at least one of the balls, the surface is damaged due to yielding and the resulting contact area with the rod increases rapidly. The test is halted after the stopping criteria, vibration levels in excess of 0.35 g, is exceeded for 1 minute. With the rod rotating at 130 Hz and a deceleration time of about 3 seconds, approximately 390 rod-rotations will be added to the failed surfaces before the rod stops

**Figure 9.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 3.61 GPa loading. Silver depletion failure after 20.6 hours of rotation at 130 Hz, accounting for approximately 9.6 x 106 rod stress cycles.

**Figure 10.** Autopsy data for RCF elements in configuration 1 of Table 1 operated at 4.0 GPa loading. Coating spall failure after 9.1 hours of rotation at 130 Hz, accounting for approximately 4.2 x 106 rod stress cycles

Post-test autopsy and Scanning Electron Microscopy (SEM) results may be used to approximate the amount of silver remaining on the ball surface at the end of each test. Table 4 presents element composition of one non-coated and two silver coated balls after testing. These balls account for three types of SEM test results: before coating, after silver depletion failure, and after early spall failure. The surface composition of each ball was derived from energy spectroscopy analyses attached to the SEM instrument. Concerning Table 4, post-test SEM data from a silver depletion failure is shown in column 3 with no silver present. All of the silver was transferred from the ball surface on to the rod and race surfaces, representing termination of the third body transfer mechanism suggested in (Higgs and Wornyoh, 2008). The results of column 4 however show some silver was still present on the ball surface following early spall failure after 9.1 hours. A theoretical analysis for a third-body transfer model using UHV-RCF data collected from suspended tests is presented in the next section.

Rolling Contact Fatigue in Ultra High Vacuum 107

(1)

shaft. The rate of silver transfer is related to: i) the force of the contact, ii) the surface roughness and speed of the shaft, and iii) the rate at which excess silver is pushed out of the contact area. Concerning the RCF tests in Figures 8 through 10, the source of the solid lubricant is the amount of thin-film coating on the balls at the beginning of the test, approximately 200 nm. The film lubricant is transferred to the rod and race contact areas, which are considered the third-body volumes. Depending on surface roughness, more or less lubricant may accumulate in the valleys between surface asperities on the rod and race surfaces. The solid lubricant on the ball surface represents the source or input to the third-

The control volume fraction coverage model (CVFC) has been presented and explained in (Higgs and Wornyoh ,2008), however, some parts of that formulation are presented here for clarity. The assumptions of the CVFC model for solid lubrication transfer to the third-body volumes are as follows: i) the ball/rod and ball/race contact surfaces are flat within their contact areas, ii) incipient sliding occurs between surfaces due to elastic deformation, iii) the fractional response and friction of the interfaces is primarily a function of the amount of silver present in the third-body volumes of the race and rod, and on the surface of the ball.

A conservation of mass formulation for the transfer of film lubricant from the ball surface to

. *Third Body Third Body Third Body Storage Rate Input Rate Output Rate* 

The output rate in equation (1) is driven by the load between the ball-rod and ball-race that forces some of the solid silver out of the wear track. Examination of the wear tracks on the races and on the rod and race in Figures 8 through 10 illustrate that silver is pushed outside of the CVFC volume over time, and hence removed from the third-body storage volumes. The input rate to the third-body storage volumes of the rod and race contact zones is influenced by the fiction coefficient between the solid lubricant and contact area. Concerning RCF contact and Figures 8 through 10, incipient sliding between the ball-rod

Equation (1) may be described as the rate of change of the fractional coverage, *X(t)* of the third-bodies on both the rod and race wear tracks. For the present study, *X(t)* will be normalized to the average surface roughness of the race and rod as presented in Table 3, or approximately 250 nm and represents the maximum asperity height defined as, *h*max. The asperity depth is about the same as the initial silver coating thickness on the balls as well, approximately 200 nm. Following the form of (Higgs and Wornyoh, 2008), and (Danyluk

> max , *<sup>h</sup> <sup>X</sup>*

where *h* is the local height of silver coating in the third-body volumes. Archard's volume

wear rate law is used to account for surface wear interactions and is defined as,

*<sup>h</sup>* (2)

body concept.

the wear tracks of the race and rod is as,

and ball-race is assumed throughout this formulation.

and Dhingra, 2012a) the fractional coverage variable is defined as,


**Table 4.** Element composition of one non-coated ball and two coated balls after testing. Composition derived from energy dispersive spectroscopy using a SEM.
