In-Situ Tribology of Engineered Surfaces with 3D-Printed Micro/Nano-Textures

Author

Mahyar Afshar Mohajer, University of Arkansas, FayettevilleFollow

Date of Graduation

12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Zou, Min

Committee Member

Millett, Paul C.

Second Committee Member

Huitink, David

Third Committee Member

Meng, Xiangbo

Fourth Committee Member

Prinz, Gary S.

Keywords

additive manufacturing; friction; superhydrophobic; surface engineering; tribology; two-photon lithography

Abstract

Surface texturing is an effective way to reduce the adhesion and friction between two contacting surfaces by reducing the real area of contact between them, providing a potential solution for adhesion and friction-induced failures, for example, failures in micro-/nano-electromechanical systems (MEMS/NEMS). Surface texturing is also essential to the realization of functional surfaces such as superhydrophobic/philic and icephobic surfaces. However, textures are prone to deformation due to being subjected to higher contact pressures. The advancement of 3D nanoprinting by two-photon lithography (TPL), combined with the ability to make an in-situ observation of the interplay between the forces and the sliding surfaces at a single micro/nano structure level, provides opportunities for gaining a better understanding of textured surfaces, enabling better engineering of novel 3D micro-/nano-textures. Using TPL, textures were fabricated with precise shape, dimension, and position control for a systematic investigation of the effects of texture three-dimensionality by comparing 3D textures (truncated cones) to 2.5D textures (cylinders and rods). Moreover, macro- and micro-scale tribological testing and in-situ monitoring of the experiments using a digital microscope at the macro-scale and a scanning electron microscope (SEM) at the micro-scale provided unique insights into the multi-scale tribological properties of the textured surfaces by real-time monitoring of the interplay between the forces and the sliding surfaces down to a single micro-scale structure level. Macro-scale tests showed that cones not only had a lower coefficient of friction due to their reduced area of contact but also slide more smoothly and are more durable. Micro-scale tests shed new light on the relationship between friction and microstructure deformation by in-situ SEM monitoring of texture-counterface interactions. Micro/nano-hierarchical textures play essential roles in realizing the functionalities of surfaces. Their friction and deformation behavior at the nanoscale are relatively unknown. Targeted friction testing of individual micro/nano-hierarchical structures (micropillars covered with nanohairs) inside an SEM, helped show the coupling between micropillar deformation and nanohair height. It was also found that the bending of long nanohairs can provide assistive sliding forces and that buckling of the long nanohairs resulted in the development of lateral forces under only normal loading and before sliding started. Varying the tapering angle of the micropillars and the length of the nanohairs enabled control over the effective stiffness of the micro/nano-hierarchical structures. It was revealed that changes in the structure stiffness by varying the tapering angle affected the onset of sliding motion, friction force, and coupling between the deformation of the nanohair and the micropillar. Additionally, it was shown the nanohair buckling was delayed as a result of increasing the stiffness of the micropillar underneath the nanohair. Finally, atomic layer deposition (ALD) of ZnO on micro/nano-hierarchical structures was done and in-situ SEM mechanical and tribological tests were carried out to shed light on the effect of different ZnO thicknesses and polydopamine (PDA) underlayer. PDA showed improvements in the performance of structures at the lowest ZnO thickness (10 nm), while as the thickness increased to 30 nm, whether or not PDA underlayers existed, coatings exhibited increased failure and cracking.

Citation

Afshar Mohajer, M. (2022). In-Situ Tribology of Engineered Surfaces with 3D-Printed Micro/Nano-Textures. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/4711