Actions for Microstructural Evolution Under Loss of Lubriction Conditions in Aerospace Gear Steels

Email RIS file
Bookmark
Report an Issue

Microstructural Evolution Under Loss of Lubriction Conditions in Aerospace Gear Steels

Author
Isaacson, Aaron
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2024.
Physical Description
1 electronic document
Additional Creators
Palmer, Todd
Access Online

Availability

I Want It
I Want It

Finding items...

Graduate Program
Restrictions on Access
Restricted (PSU Only).
Summary
An important design criterion for aerospace grade gear steels involves their performance under the high temperatures and stresses during loss of lubrication incidents. In the absence of oil, scuffing on the highly loaded and improperly lubricated gear tooth surfaces occurs quickly as the friction produces high temperatures, leading to softening of the high carbon martensite structure. Once scuffing of the surface occurs, catastrophic failure of the gears rapidly follows. The ability of the gear steel to resist softening at the high temperatures experienced during these loss of lubrication events is referred to as hot hardness and it has traditionally been used as the metric to gauge performance of a material during loss of lubrication. Improvements in hot hardness have been a focus with the introduction of each new generation of aerospace gear steel. The strengthening mechanisms used to attain higher hot hardness levels have become increasingly complex, utilizing refinement of the martensitic structure and the introduction of complex carbides stable at high temperatures. Even with the evolution of these materials, catastrophic failures still occur under these loss of lubrication conditions at times long before the specified level of 30 minutes. There is a perception that better hot hardness leads to longer gearbox operation after loss of lubrication. The progression from scuffing initiation to catastrophic failure is not understood and there is a disconnect between this perception and reality. The objective of this research was to advance the body of knowledge surrounding the material failure progression during loss of lubrication in aerospace gear steels. Previous works have focused on prediction of scuffing initiation, but the mechanisms occurring during the seemingly stable progression from scuffing initiation to catastrophic failure have not been studied. Loss of lubrication performance has historically been measured using full-scale gearbox testing for each aircraft platform that often screen several variables simultaneously, making accurate assessment of individual factors difficult. In this work, three economical test methods were developed to study specific aspects that contribute to better understanding the failure progression during gear loss of lubrication. The tests can generally be described as follows, in-situ hot hardness testing, loss of lubrication gear testing, and friction dependency on temperature testing. Three generations of gear steel were studied using these techniques, specifically SAE/AISI 9310 (generation 1), Pyrowear 53 (generation 2), and Ferrium C-64 and Pyrowear 675 (generation 3). Specifics of each evaluation method are presented in detail. Hot hardness testing clearly demonstrated greater hardness retention at higher temperatures with each successive alloy generation. However, the hot hardness results did not correlate with those from the loss of lubrication gear testing, confirming contributions from other mechanisms. Detailed characterization of gear failure surfaces confirmed presence of a distinct oxide layer with high hardness. The final set of experiments was developed to measure how the coefficient of friction changed as the oxide layer developed on the alloys. A very distinct reduction in sliding friction was demonstrated in all of the alloys investigated as a result of the surface oxide formation. The benefits of the oxide layer are further demonstrated through performance of loss of lubrication gear tests in an inert atmosphere. The microstructural evolution of the high carbon surface region was studied for each alloy generation. The differences in carbide precipitates formed for each material are investigated using a multi-scale characterization and computational thermodynamics approach. The primary focus was study of carbide precipitation and surface oxidation of Pyrowear 53 due to short term high temperature exposure representative of loss of lubrication events.
Other Subject(s)
Genre(s)
Dissertation Note
Ph.D. Pennsylvania State University 2024.
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
View MARC record | catkey: 44541256