Rolling Contact Fatigue of Superelastic Intermetallic Materials (SIM) for Use as Resilient Corrosion Resistant Bearings

Superelastic intermetallic materials (SIM), such as 60NiTi, are emerging as candidates for corrosion and shock-resistant rolling element bearings. Compared with metals, the intermetallic materials are more brittle and may be prone to rolling contact fatigue degradation. In this paper, a series of three ball-on-rod rolling contact fatigue tests were conducted using polished steel balls and NiTi rods prepared by vacuum casting and powder metallurgy techniques. The test protocol matched that used in ASTM STP 771 except that the steel balls were not intentionally roughened. In general, the NiTi rods exhibit fatigue damage at much lower stress levels than commercial bearing steels. At the lowest stress level tested (1.7 GPa), 60NiTi rods that were largely free from processing defects gave acceptably long lives, and testing was terminated without failure after 800 h. At elevated stress (2.5 GPa), failure occurred for some specimens, while others reached the preset test length goal of 800 h. Improperly prepared 60NiTi rods that had unconsolidated particles or significant ceramic inclusions occasionally experienced surface fatigue prior to completion of the test period even at the lowest stress level. Alloyed NiTi rods containing small amounts of Hf as a microstructural processing aid generally endured higher stress levels than the baseline 60NiTi composition. Two predominant fatigue failure mechanisms were observed: intergranular (grain boundary) fracture and intragranular (through the grains) crack propagation. The results suggest that further fatigue capability improvements could be obtained through process improvements, microstructural refinements and alloying. SIM currently available are recommended for mechanically benign applications involving modest stress levels and rates of stress cycle accumulation. Applications that include high continuous loads (stress) and high speeds for long durations should be avoided.