**2. Experimental procedure**

The RCF test rig in Figure 1 was assembled using off the shelf components purchased from leading vacuum and mechanical equipment vendors (Danyluk and Dhingra, 2012a). For example, a KollmorgenTM servo motor (model number AKM21E) is used to rotate the rod. The drive motor is mounted underneath the chamber and motor torque is delivered to the test rod inside the vacuum chamber using a ferro-fluidic rotary feed-through device similar to Kurt J. Lesker VacuumTM part number FE121099. High vacuum is applied using a VarianTM V-81M turbo pumping system as shown in the right panel of Figure 1.

**Figure 1.** Support hardware and UHV-RCF chamber: turbo pumping system, residual gas analyzer, servo motor drive, and ferro-fluidic rotary feed through.

The RCF hardware inside the chamber is a modification of the three-ball-rod test rig of reference (Hoo 1982). The test-head hardware components were fabricated from 304L stainless steel and are positioned inside the UHV chamber as shown in Figure 2c. A component view of the test chamber is shown in Figure 2a, with emphasis on hardware type and assembly order. The RCF test elements: the balls, rods, and races, are shown in Figure 2b. The races are press fit into the test fixture and held stationary while the balls rotate between the fixed races and the rotating rod. A typical test consumes 5 or 6 balls depending on which ball size is used: 12.7 mm or 7.94 mm. For example, when testing with 12.7 mm diameter balls as shown in Figure 2c, five balls, two races and one rod will constitute a test. If 7.94 mm balls are used then six balls are required. A description of test element combinations that may be carried out using this platform is presented in Table 1.

Rolling Contact Fatigue in Ultra High Vacuum 99

Rex 20 Race

9.53 mm Rex20 Rod

M50 Race

geometry influences surface hardness and it is to be expected that ball, rod, and race geometries will influence the hardness measurement as well. Table 2 presents hardness and material property data for the test elements presented in Figure 2b. Table 3 contains measured surface roughness data. When collecting hardness and surface roughness data, it is recommended to carryout repeated measurements over multiple samples. For example, the data presented in Tables 2 and 3 were repeated three times per sample on five samples

and the average was taken based on 15 measurements of each test element.

**Table 2.** Hardness and material property data for RCF test elements in Figure 2b.

7.94 mm T5 Ball

**Table 3.** Average surface roughness Ra data in microns for RCF test elements.

9.53 mm Diameter Si3N4 Rod

HRC (measured) 63.2 74.7 61.3 61.9 65.8 44.1

(GPa) 235 310 214 203 235 203

Rex20 Race

Poisson Ratio 0.29 0.25 0.29 0.29 0.29 0.29

0.31 0.32 0.04 0.15 0.05 0.11

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.

7.94 mm Steel ANSI T5 ball

12.7 mm Steel M50 ball

9.53 mm Si3N4 Rod

9.53 mm Diameter Rex 20 Rod

M50 Race

Material Property

Elastic Modulus

12.7 mm M50 Ball

**2.2. Test preparation** 

**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.

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 purchased from KoyoTM.


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