Date of Graduation

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Nair, Arun K.

Committee Member

Zou, Min

Second Committee Member

Huitink, David

Third Committee Member

Millett, Paul C.

Fourth Committee Member

Chen, Jingyi

Keywords

Coefficient of friction; Core-shell nanostructures; Dislocation starvation; Multi-asperity surfaces; Nanoindentation; Texture surface

Abstract

Tribology is the study of surfaces where two objects are sliding against another. Significant energy is lost due to friction between the sliding surfaces. Therefore, developing or designing surfaces to minimize friction is critical for the durability and reliability of the mechanical components. Several researchers have identified that surface texturing at the nanoscale (nanotexture) would reduce the friction between the contacting surfaces. The nanotextured surfaces have several applications in microelectromechanical systems and nanoelectromechanical systems. This dissertation employs molecular dynamics simulations to investigate the frictional and mechanical response of nanotextured aluminum (Al) and Al/amorphous silicon (a-Si) composite surfaces.

This study determines the effective geometry (spherical or cylindrical) for texturing an Al surface that lowers the coefficient of friction of the nanotextured surface compared to a smooth surface. The results suggest that as the counter surface radius increases, the coefficient of friction decreases. For the lower counter surface radius, the coefficient of friction of the textured surface is higher than the smooth surface. But, after a specific increase in the radius of the counter surface, the coefficient of friction of the textured surface is lower than the smooth surface.

The nanotextured surface consisting of Al has lower mechanical strength, which results in permanent failure even at low contact forces. Thus, a nanotextured hemispherical Al core surface is coated by an a-Si to protect the nanotextured surface from plastic deformation, and they are named as core-shell nanostructures (CSNs). The CSNs has previously shown remarkable deformation recovery to compression loading beyond the elastic limit. This study finds an optimum coating thickness that would protect the core from plastic deformation. i.e., the ratio of core radius to shell thickness should be between 0.5 and 2.0 to have deformation resistant CSNs.

Additionally, this research investigates the core (single crystal and grain boundary) and substrate (crystalline and amorphous) material that affect the mechanical behavior of the CSNs subject to indentation. The results from this study conclude that CSNs with a single crystal core and crystalline substrate are more reliable for deformation-resistant behavior than those that contain grain boundary core and amorphous substrate.

From our previous studies, it is clear that not all textured surfaces will have a lower coefficient of friction. The coefficient of friction also depends on the indenter or counter surface radius. Therefore, we investigate the relationship between surface texture (r, L) and counter surface (R) variables. The results from this study suggest that the counter surface radius should be greater than the difference between twice the pitch length and radius of the asperity (R > (2L -r)) in order to have lower COF for the textured surface compared to a smooth surface. The relationship found between the textured surface and indenter surface variables is also confirmed for CSNs. Further, the relationship established in this study is also verified using experiments.

This work provides the groundwork in designing the textured surfaces as well as deformation-resistant core-shell nanostructures that has both lower COF and deformation-resistant behavior. Additionally, this research finds the mechanisms behind the deformation-resistant behavior of the CSNs.

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