**2.2. Test preparation**

98 Performance Evaluation of Bearings

purchased from KoyoTM.

**2.1. Test material measurements**

If 7.94 mm balls are used then six balls are required. A description of test element

**Figure 2.** RCF test chamber and hardware: a) chamber sections assembly order, b) Rex 20 and M50 balls, races and rods, c) M50 balls against a Si3N4 rod with M50 outer races inside the vacuum chamber.

Config. Ball Material Rod Material Race Material Lubrication Ball Size

Referring to Table 1, the Si3N4 rods were purchased from CeradyneTM Inc, and were processed using the EKasinTM method from Ceradyne. These rods were chosen for their high temperature and high thermal shock resistance. The Rex20 rods and races, along with the 12.7 mm diameter M50 steel balls were purchased from TimkenTM and are used extensively for RCF testing with oil based lubrication. The 7.94 mm diameter ANSI T5 balls were

1 M50 Steel Rex 20 M50 Steel Silver 12.7 5

2 ANSI T5 Rex 20 Rex 20 Ni/Cu/Ag 7.94 6

3 M50 Steel Si3N4 M50 Steel Silver 12.7 5

**Table 1.** Test configurations for RCF testing of test elements shown in Figure 2b in ultra-high vacuum.

Coating survival and RCF test life is dependent on coating and substrate material properties. For example, a thin film deposited onto an M50 Steel substrate is likely to spall if the applied contact stress during the RCF test exceeds the ultimate strength of the substrate. When considering the ultimate strength and hardness of rolling bearing elements it is best to directly measure surface hardness and then approximate the ultimate strength. Substrate

(mm)

Number of balls

combinations that may be carried out using this platform is presented in Table 1.

Preparation and process history of the test elements have significant influence on RCF life. Thin film coatings on the order of 500 nm thick are very susceptible to surface contamination from exposure to air, or contact with volatile substances such as organic compounds along with low melting temperature metals such as tin or indium for example. Component cleanliness and honest adherence to good vacuum procedures are required to ensure validity of test results. For example, prior to silver deposition all of the balls were cleaned in an ultrasonic bath in methylene chloride for 20 minutes to remove oil and particulates. Prior to coating, the balls were also outgassed inside a UHV chamber at 10-7 Pa for 24 hours and then scrubbed with argon plasma before approximately 200 nm of silver was deposited on to the ball surface. Steps were taken to insure even coating thickness during deposition. The rods and races were cleaned similarly and all test components were stored in warm dry nitrogen. Pre-coated T5, 7.94 mm diameter balls that were purchased from Koyo were vacuum-packed and then stored in warm dry nitrogen until loaded into the test rig. Each time the test elements are exposed to air an out-gassing procedure should be applied prior to the start of the test. For example, after the balls and rod were loaded into the UHV-RCF test chamber, the system was outgassed for 12 hours at 10-7 Pa prior to starting the test.
