Thermal Fatigue of Several Babbitts on Flat Plates: Part I — Unloaded Expansion

Fatigue strength is often a limiting factor for the application of babbitt to hydrodynamic bearings. Many studies address mechanical fatigue, but fewer studies address thermal fatigue. In the United States, the selection of babbitt for hydrodynamic bearings has become increasingly uniform based on the ASTM B-23 specification, with the majority of bearings using ASTM B-23, alloy #2. Alternative tin-based alloys some containing cadmium, but all with higher antimony and copper content, leading to higher compressive load capacity have been popular with European-designed bearings. In light of environmental concerns with cadmium, one significant babbitt (whitemetal) vendor, developed an alternative design alloy replacing the cadmium with silver and zinc. All of these alloys are predominantly tin, which has a non-cubic thermal structure and anisotropic thermal expansion. In equilibrium at high antimony contents, three phases of antimony compounds are present, including increasing amounts of tin-antimony cuboids. As a result thermal expansion results in stress concentration along phase and grain boundaries. Increasing temperatures can allow for alloy migration within the tin matrix. These factors can affect the application of babbitts subject to repeated thermal transients, as can occur during the typical startup of a heavily loaded thrust bearing. Accelerated thermal fatigue tests without load have been performed on six different babbitt compositions statically cast on steel plates. Measurements of thermal deformation and bond strength were made at periodic intervals, as well as visual examination of metallurgical changes. The results indicate that tin-alloys with lower alloy contents are subject to increased surface deformation, on the order of the hydrodynamic film thickness. Tin-alloys with higher alloy content show increased alloy migration and more tin-antimony intermetallic formation. Intermetallic compounds concentrated at the steel-babbitt interface grew during the testing. These phase changes reduce the toughness of the babbitt-steel interface due to embrittlement, raising the potential for bond failure. The need for testing in a loaded condition to help quantify the relevance of these indications to loaded hydrodynamic conditions is identified.