Date of Award

Winter 2009

Project Type

Dissertation

Program or Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

First Advisor

James E Kraznowski

Abstract

The tribological behavior of composite coating incorporating solid lubricant reservoirs has been investigated in this study. A new method, termed " microbeading" was devised to create microscopic reservoirs on the surface of sputter-deposited hard coatings. Pin-on-disk tests were used to evaluate the lubricating performance of these newly devised composite coatings. Three sizes of the reservoirs were investigated: 1.5, 5 and 10 microm. The 10 microm-sized-reservoirs performed better in terms of reducing friction and extending wear life. These coatings were also evaluated with finite element models. It was found that during ball-coating surface interaction, the lowest stresses and calculated work done were obtained at a critical location at the bottom of hole on the coating with 10 microm holes.

Further investigations were conducted using photolithography to fabricate the coatings, which allows a more controlled reservoir distribution on the surface. For these tests, silicon wafer substrates were used with masks prepared by photolithography. Reservoir diameter (4 and 9 microm) and area coverage (2% and 10%) were the primary variables, and it was found that the larger diameter reservoirs (9 microm) and higher area coverage values (10%) were more beneficial in providing lubricant storage and replenishment in the wear track. These results were correlated with observations of wear tracks after the tribological tests, particularly the aspects of microreservoir filling and spreading on the track.

The microbeading coating method was also applied on machining tool inserts and their performance was investigated by lathe turning tests. Indium was chosen as the solid lubricant for this test, and hardened 4340 steel was used as the workpiece. All coatings with indium (with or without reservoirs) showed substantially better wear lives than TiN alone under wet cutting test conditions, and slightly higher wear life under dry cutting conditions. Further research was conducted with machining simulation tests and it was found that the frictional lubricity of the indium coating was lost at high temperatures (above 600°C). X-ray photoelectron spectroscopy (XPS) was conducted on the surfaces of the tool inserts after dry cutting, and it was found that some indium was still present. Based on these results, indium has been demonstrated to be capable of providing increased lubricity when cutting under lubricated conditions.

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