Wear Rate and Mechanism Maps for Stainless Steel at High Temperature
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Zhang, Chuchu
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Abstract
Contacting interfaces moving in a back and forth way relative to each other at high temperature is found in mechanical systems like internal combustion engine, aerospace propulsion system, energy generation system and metalworking equipment. Materials selection for low friction, low wear in high temperature applications is a complex problem because of the many influence factors and coupling effects especially when friction and wear control methods like lubricating cannot be used.
It was found that a compacted and sintered wear protective layer, namely “glaze layer” can be formed in many Co, Ni and Fe alloys under certain circumstances, and it is possible to take advantage of glaze layer formation to reduce friction and wear at high temperature. The dissertation is aimed for better understanding of glaze layer and its role in severe-to-mild wear transition, and utilize this knowledge to push forward development of practical methodologies to build a robust high temperature wear map that benefits high-temperature tribo-pair design. 310S austenitic stainless steel is chosen as a sample system to generate temperature-frequency wear mechanism map. This map covers considerably large temperature and frequency in its working condition than has previous been considered. It has been found that the environmental temperature plays the overall dominant role in determination of the wear mechanism, although in the transition zone, a coupling effect between temperature and frequency has been observed. The experimental data was also used to validate the applicability of a newly proposed critical cycle (N_GL) model for glaze layer formation.
In addition, we propose a novel way to understand the role of glaze layer using computer vision algorithms. Two workflows, one for quantitative glaze layer identification and the other for image alignment, have been developed. For glaze layer identification, we used computer vision concepts that considers the color and reflectiveness of glaze layer under optical microscope (OM). For image alignment, we developed a strategy to conduct pixel-to-pixel alignment of images acquired by multiple techniques (e.g., OM, scanning electron microscopy, 3D optical profilers) with sub-pixel error. As such, the correlation between the height map and locations of the glaze layer within the wear scar can be readily determined. The proposed methodology is applied to analysis the worn surface of 310S after high temperature fretting test, and the glaze layer is found to always occupy relatively high locations within wear scar. With temperature rise, the projected coverage of glaze layer follows the same increasing trend with three distinguishable stages, and the threshold temperature of the three stages matched with severe-to-mild wear transition. These results provide evidence that severe-to-mild wear transition resulted from spreading of glaze layer coverage, and glaze layer may reduce friction and wear by reducing real contact area.
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2020-12-06
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Dissertation