Herbert A. Chin

Corrosion Assessment and Characterization of Aerospace-Bearing Steels in Seawater and Ester-Based Lubricants

The corrosion behavior of bearing steels was screened using potentiodynamic scans in seawater. The results of electrochemical testing provided a relative ranking of the bearing steels when tested in aqueous chloride-containing solution. The corrosion behavior of bearing steels in the lubricant environment has been observed to be quite different than in aqueous solution. Both the amount of water contamination in oil and chloride content of the water impact the observed corrosion rates in oil-water mixtures. All steel compositions tested demonstrated localized corrosion damage when exposed to oil with water added; however, inherently less corrosion-resistant alloys had more widespread shallow attack, whereas higher Cr-containing alloys displayed more localized severe attack. The mechanism of corrosion in the two-phase, oil plus aqueous phase system appears to be controlled by an aqueous corrosion process dependent on steel microstructure and emulsified or free water and likely limited by oxygen availability...

Finite Element Analysis of the Critical Flaw Size in Hybrid Silicon Nitride Bearing Ball

Hybrid bearings containing large silicon nitride balls are considered a critical technology for high speed turbine engine bearing applications. High costs of the balls as well as the lack of a reliable life prediction methodology have hindered extensive use of hybrid bearings in aerospace applications. The presence of surface cracks on silicon nitride balls necessitates the development of a fracture mechanics based approach for life prediction. The key element of the fracture mechanics based approach is the identification of a critical flaw size in silicon nitride balls. Finite element analysis was performed to parametrically vary the crack geometry and to determine the worst case crack geometry conditions. Stress intensity factors were computed for the worst case crack under Hertzian contact loading and in the presence of traction stresses. Failure maps were created that provide a prediction of the maximum permissible surface flaw in silicon nitride bearing balls. Single ball rig tests were performed with induced C-cracks to validate the predictions. Results from the single ball rig test were in good agreement with the results of the analysis for spontaneous spallation. The results of the analysis indicate that 100 μm deep cracks should not cause failure under nominal bearing operation conditions.Copyright

Rolling Textures in Compacts of Copper and Steel

This study was undertaken to determine whether differences exist in the rolling textures of materials rolled with and without cladding and whether such differences are affected by bonding the clad and core by hot isostatic pressing (HIP’ing) prior to rolling/ Both steel-clad copper and copper-clad steel were studied, using compacts having nominal volume fractions of core equal to.33 and.50. Unclad core material having the same initial thicnkess as the cores was rolled to similar reductions for comparison with material rolled with cladding. No significant difference in the deformation textures, compared to unclad material, were observed for steel-clad copper. However, for copper-clad steel, a pronounced deformation texture developed in the core at lower total reductions than those required to developed the same degree of texture in unclad material. In these cases, the cladding showed evidence of a shear texture. Prebonding of the compacts by HIP’ing intensified this effect.