Development of a high-speed cryogenic tribometer: Design concept and experimental results

In order to evaluate the tribological properties of ball bearings used in high-speed cryo-turbopumps of liquid rocket engines, a tribometer to operate at high sliding speed in cryogenic environment is required. In order to meet the above requirements, a new high-speed cryo-tribometer has been designed. With this new tribometer, the evaluation of friction and wear mechanisms of various self-mated materials and solid lubricant coatings under high sliding speeds (>0.5–45 m/s) with high Hertzian contact stresses (up to 2.5 GPa) in cryogenic fluid is now possible. Two additional features available with this new cryo-tribometer are: (a) a mono-ball rolling-on-disk tester under the above conditions for the analysis of rolling friction and (b) the testing of unlubricated and lubricated ball bearing (10 mm diameter bore) submerged in liquid nitrogen or liquid helium under very high-speed (36,000 rpm). An additional provision has been made for the continuous measurement of bearing frictional torque at a maximum axial load of 50N. The design details of the new tribometer are the major focus of discussion in this paper. This paper also reports the results of a very first set of experiments conducted on self-mated 440C steel and AISI 304 steel under liquid nitrogen (LN2) immersed condition with selected testing parameters. Testing of ball bearings with Pb-IP coating at 36,000 rpm under liquid nitrogen also has been carried out. Additionally, the wear mechanisms for self-mated steel in cryogenic liquid are also discussed.