Date of Graduation

5-2021

Document Type

Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Min Zou

Committee Member

Arun Nair

Second Committee Member

Xiangbo Meng

Keywords

Coefficient of friction, Core-shell nanostructure, Normal load, Surface uniformity, Tip radius

Abstract

Friction tests are a beneficial means to analyze the tribological characteristics and advantages of materials and textured surfaces. However, the selected test parameters can significantly influence the results. This work explores the significance of the friction testing parameters on the frictional performances of core-shell nanostructure-textured surfaces (CSNTSs). Several applied normal loads (10 μN, 100 μN, and 500 μN) and diamond counterface indenter tip radii (1 μm, 5 μm, and 20 μm) were selected for the testing of Al/diamond-like-carbon (DLC) and Al/amorphous silicon (a-Si) CSNTSs. The measured friction values of the CSNTSs were then compared to a matching Al/DLC film and Al/a-Si film, respectively, and to each other.

At the lowest normal load, due to no permanent deformation of the core-shell nanostructures (CSNs), the presence of adhesion force had its greatest effect, causing the highest coefficient of friction (COFs) on all surfaces with all tip sizes. At the higher normal loads, permanent deformation occurred on all surfaces with all tips, and the COF was considerably lower due to the lessened effect from the adhesion force. Greater deformation was experienced on both CSNTSs than their respective matching films due to the smaller contact areas. Also, for each tip size, greater deformation was observed on the Al/DLC CSNTS than the Al/a-Si CSNTS.

The tip size also had a significant effect on the frictional and deformation behavior of the CSNTSs. The 1 μm tip experienced large vertical movements while travelling along the CSNs because of its small tip size, causing an interlocking interaction between the tip and CSNs. The deformation is also more severe due to the smaller contact area. Both the interlocking and deformation contributed to higher COFs than that of the 5 μm tip. The 5 μm tip displayed the lowest COFs on both CSNTSs at the higher loads due to its more ideal combination of contact area and pressure. The deformation of the 20 μm tip was similar to that of the 5 μm tip at the higher normal loads, however, the COF was greater due to its larger contact area.

Additionally, the material and uniformity of the CSNTSs significantly affected the measured friction. Al/DLC CSNTS had lower COFs than Al/a-Si CSNTS since DLC has a much lower COF than a-Si, except for at high contact pressures, resulting in more severe deformation and thus higher COFs. On the uniform Al/DLC CSNTS, the COF exhibited a steady and predictable trend. With the 1 μm tip, the COF was lower on the Al/DLC film, while with the larger tips, the COF was lower on the Al/DLC CSNTS. However, on the Al/a-Si CSNTS, the larger tips also experienced significant interlocking while sliding on the non-uniform Al/a-Si CSNs. Consequently, there was no apparent trend with increasing tip size between the COFs of the different Al/a-Si surfaces.

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