Low friction and severe wear of alumina in cryogenic environment: A first report

Abstract

Structural ceramics are considered as potential candidate materials for use in hybrid bearings in rocket turbopumps, operated under high stress in cryogenic environment. The friction and wear-related surface failure is considered as one of the critical factors in selecting the materials for cryo-turbopumps of Space Shuttle Main Engine (SSME). To obtain fundamental understanding of the tribological properties of ceramics in cryogenic environment, a very first set of sliding wear tests were carried out on self-mated Al2O3, a model brittle ceramic material, in liquid nitrogen (LN2) under varying load (2–10 N) and high rotational speed of 2550 rpm, using a newly designed cryogenic tribometer. The present research attempts to answer some important questions: (i) What would be the influence of LN2 on frictional and fracture behavior at sliding contacts? (ii) How does the material removal process occur in LN2 environment? Our experimental results reveal that self-mated alumina exhibits low steady-state coefficient of friction ∼0.13–0.18 and suffers from high wear rate (10−5 mm3/Nm) under the selected testing conditions. The novelty of the present work also lies in presenting some interesting results, for the first time, concerning the deformation and fracture of alumina at cryogenic temperature under high speed sliding conditions. Detailed scanning electronic microscope observation of the worn surfaces indicates that severe damage of both ball and flat occurs in cryogenic environment by transgranular and intergranular fracture. The observed wear behavior is explained in terms of thermal heat dissipation and brittle fracture of alumina in LN2.