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Abstract: Two sets of deep groove ball bearings 6214 (one set of bearing rolling elements is steel ball, and the other set of bearing rolling elements is ceramic ball) with WS2 solid lubricating film sputtered on the inner and outer ring channel surfaces in aqueous medium were tested separately. After the experiment, it was found that although WS2 transfer film was formed on the rolling element surfaces of both sets of bearings, the WS2 film on the steel ball surface was better than that on the ceramic ball surface.
Keywords: deep groove ball bearings; WS2 ; Ceramic ball; Steel ball; Transfer film
The deep groove ball bearing 6214 used by a certain host adopts a self-lubricating cage due to special working conditions, but in the initial stage of operation, there is no lubrication between the rolling elements and the ring groove. To solve the early lubrication problem of the bearing, WS2 solid lubricating film was sputtered on the inner and outer ring channel surfaces.
1. Experimental conditions and results
The test bearings consist of two sets of deep groove ball bearings 6214, with one set made of silicon nitride as the rolling element material and the other set made of 9Cr18 stainless bearing steel. During the bench test, the bearing speed was 20000 r/min, the radial load was 10 kN, the working medium was water, and multiple startups were performed.
After the experiment, the structures of the two sets of bearings were intact and operated flexibly. After disassembling the sleeves, a visual inspection was conducted on the inner and outer ring grooves, ceramic balls, and steel balls of the two sets of bearings. The results showed that there were a large number of pits on the surface of the outer ring grooves of the ceramic ball bearings, and the WS2 solid lubricating film layer on the groove surface was severely worn; The outer ring groove and steel ball of the steel ball bearing are in good condition, and a good WS2 solid transfer film is formed on the surface of the steel ball, with less wear on the inner film layer of the groove.
2. Appearance inspection of rolling elements
Take one rolling element from each of the two sets of test bearings, use a 400x optical microscope to detect its surface morphology, and compare the morphology of ceramic balls and steel balls before and after ultrasonic cleaning.
Figures 1 and 2 show the morphology of different parts of the same ceramic ball. From the figure, it can be seen that there is no transfer film formed on some surfaces of the ceramic ball, while WS2 transfer film is present on some surfaces, and the transfer film is distributed on both sides with deeper wear marks.
Figures 3 and 4 show the surface morphology of the same steel ball before and after the experiment. Comparing Figure 3 and Figure 4, it can be seen that a relatively dense WS2 transfer film was formed on the surface of the steel ball after the experiment, even in areas with severe friction where WS2 transfer film still exists.
Figure 1: Morphology of ceramic balls without WS2 transfer film formation
Figure 2 Partial morphology of WS2 transfer film formed by ceramic balls
Figure 3: Morphology of Steel Balls Before Experiment
Figure 4: Morphology of the steel ball after the experiment
After 10 minutes of ultrasonic cleaning in alcohol, the surface morphology of ceramic balls and steel balls is shown in Figures 5 and 6. It can be clearly seen that there is basically no WS2 solid transfer film on the surface of the ceramic ball, while the WS2 transfer film on the surface of the steel ball still exists.
Figure 5 Surface morphology of ceramic balls
Figure 6 Surface morphology of steel ball
3. Appearance and composition detection
3.1 Optical Microscope Inspection
Using a 30x optical microscope to inspect the morphology of the inner and outer ring grooves of the bearings, it was found that there were no obvious wear marks on the surface of the inner ring grooves of the two sets of bearings, and the surface condition was good. The morphology of the outer ring groove is shown in Figures 7 and 8. There are a large number of wear marks (pits) in the outer ring groove of the ceramic ball bearing, while the outer ring groove of the steel ball bearing is in good condition.
Figure 7 Surface morphology of outer ring groove of ceramic ball bearing
Figure 8 Surface morphology of outer ring groove of steel ball bearing
3.2 Scanning Electron Microscopy and Energy Spectrometer Detection
The morphology and composition of the outer ring groove of steel ball and ceramic ball bearings were detected using an S-4800 scanning electron microscope. Due to the non-conductive nature of ceramic balls, charge accumulation occurs during scanning electron microscopy detection, making it impossible to detect the surface morphology and composition of ceramic balls.
3.2.1 Steel Ball
The microstructure of the steel ball is shown in Figure 9. From the figure, it can be seen that the surface condition of the steel ball is good, and no adhesive wear phenomenon was found, and a substance similar to a film layer was found on the surface of the steel ball.
Figure 9: Morphology of Steel Balls
Using an energy spectrometer, the composition of the micro area surface of the steel ball shown in Figure 9 was detected at three points, and the main element contents are shown in Table 1. From the values in the table, it can be seen that a layer of WS2 solid lubrication transfer film has formed on the surface of the steel ball.
Table 1 Composition of Steel Balls (Mass Fraction)
3.2.2 Ceramic ball bearing outer ring channel
The microstructure of the outer ring channel of the ceramic ball bearing is shown in Figure 10. As shown in the figure, there are many pits at the bottom of the channel, where the WS2 solid film layer has been worn through and at the edges, which is consistent with the results observed by the optical microscope.
Figure 10 Morphology of the Outer Ring Channel of Ceramic Ball Bearings
4. Nano hardness testing of steel ball surface
Three points on the surface of the steel ball were subjected to nanohardness testing using a load control mode, with a controlled load of 4 N. The resulting load displacement depth curve is shown in Figure 11, and the elastic modulus and nanohardness values of the three points are shown in Table 2.
Figure 11 Load displacement curve
Table 2 Elastic modulus and nanohardness of 3 points
The hardness of the three points controlled by load is all greater than 10 GPa. The difference in hardness between point 1 and point 2 is very small, while the hardness of point 3 is much higher than that of point 1 and point 2, which may be due to hitting the hard phase, resulting in a large deviation. The load displacement depth curve also shows a good overlap between the curves at point 1 and point 2, while the curve at point 3 deviates significantly, which is consistent with the hardness display results. After the experiment, there was no significant change in the elastic modulus and hardness of the steel ball, and the surface condition of the steel ball was good.
5. Conclusion
When bearings with WS2 solid lubricating film sputtered on the inner and outer raceway surfaces work in aqueous media, steel balls are more conducive to the formation of surface lubricating film than ceramic balls.
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2024-09-14