Scuffing characteristics of high-load rolling/sliding contacts operating in liquid oxygen–-effects of materials and surface roughness

This research reports on an experimental study of the effects of materials and surface roughness on the scuffing characteristics of rolling/sliding contacts cooled and lubricated with liquid oxygen. Experiments were carried out under heavy loading with a Hertzian pressure in the range of 2.0 to 3.0 GPa and with a high rolling velocity of up to 48 m/s. For contacts between AISI 440C stainless-steel elements, the results showed that the scuffing behavior of the system was fairly consistent under a wide range of rolling velocity. Scuffing commenced at a small slide-to-roll ratio of around 0.02, and the scuffing behavior of the contact was not sensitive to surface roughness for the test-sample RMS roughness ranging from 0.02 to 0.10 μm. For contacts between 440C and Si₃N₄ elements, on the other hand, the scuffing behavior of the system was not very consistent and was somewhat unpredictable. The results were sensitive to surface roughness, particularly that of the Si₃N₄ test sample. With well-polished test samples, consistent results were obtained; the level of traction was lower than that with a 440C toroid and scuffing did not take place up to a slide-to-roll ratio of near 0.03. The results strongly suggest that significant hydrodynamic effect can be generated by liquid oxygen under heavy loading and high velocity conditions. The results also suggest that the hydrodynamic action is likely generated by the conventional viscous mechanism as it can be largely destroyed by a narrow circumferential surface scratch running through the central region of the contact.