Date of Award

Summer 8-8-2025

Document Type

Thesis (Ph.D.)

Department or Program

Engineering Sciences

First Advisor

Ian Baker, D.Phil.

Second Advisor

Francis E. Kennedy, Ph.D.

Third Advisor

Rebecca A. Gallivan, Ph.D.

Abstract

Cryogenic tribological applications have become increasingly important with the development and innovation of advanced equipment in aerospace, polar regions, superconductivity, and more. Traditional lubrication methods are not viable under these harsh environments, so metallic materials must be able to maintain excellent mechanical properties and withstand the constant impact and wear alone. High entropy alloys (HEAs) have emerged as potential candidates for these applications because of their wide variety of excellent room temperature properties and apparent improvement of mechanical properties of some HEAs at extremely low temperatures. Despite their promising nature, very limited work has been done on understanding the wear behavior of these HEAs under cryogenic sliding conditions and even less work has focused on offering comparisons to other HEAs or traditionally-used materials.

This thesis investigates the tribological properties and specific wear mechanisms experienced by two single-phase face-centered cubic (f.c.c) HEAs, an equiatomic CoCrMnNiFe (Cantor HEA) and 1.1 at% C-doped Fe40.4Ni11.3Mn34.8Al7.5Cr6 (CHEA), as well as a two-phase HEA, Fe28.2Ni18.8Mn32.9Al14.1Cr6 (two-phase HEA), under cryogenic temperatures and room temperature. Overall, it was found that the two-phase HEA outperformed the 316 SS by up to 5x and the single-phase HEAs by up to 10x under the same sliding conditions at 77 K. It was observed that the better wear performance of the two-phase HEA was directly related to its sub-surface microstructural gradient and more even strain distribution. Amongst the single-phase HEAs, the CHEA showed the best wear resistance at cryogenic temperature due to the formation of stable and adherent oxides and the exclusive appearance of twins and secondary phases.

An additional chapter also explored the potential of utilizing a gas nitriding treatment to improve the wear resistance of the two-phase HEA. It was found that the nitrided pins only performed better than their as-cast counterparts under low sliding speed. This was due to the thickness, continuity, and chemistry of the mechanically mixed layer formed on each wear pin surface as well as changes in hardness.

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