Date of this Version
Hassebrook, A, 2017, "Applications of Femtosecond Laser Processed Metallic Surfaces: Leidenfrost Point and Thermal Stability of Rare Earth Oxide Coatings," M.S. thesis, Mechanical and Materials Engineering, University of Nebraska-Lincoln.
In this thesis, micro/nano structured surfaces were created through the use of Femtosecond Laser Surface Processing (FLSP). In the first part of the thesis, an experimental investigation of the effects of droplet diameters and fluid properties on the Leidenfrost temperature of polished and nano/microstructured surfaces has been carried out. Leidenfrost experiments were conducted on a stainless steel 304 polished surface and a stainless steel surface which was processed by a femtosecond laser to form Above Surface Growth (ASG) nano/microstructures. Surface preparation resulted in a root mean square roughness (Rrms) of 4.8 µm and 0.04 µm on the laser processed and polished surfaces, respectively. To determine the Leidenfrost temperatures, the droplet lifetime method was employed using Deionized (DI) water and HFE 7300DL. A precision dropper was used to vary the size of DI water droplet diameters from 1.5 to 4 millimeters. The Leidenfrost temperature was shown to display increases as high as 100 °C on the processed surface over the range of droplet sizes, as opposed to a 40 °C increase on the polished surface over the same range of droplet sizes. Average increases of the Leidenfrost temperature between polished and processed samples were as high as 200 °C. The experiment was repeated with HFE 7300DL; however, no noticeable changes of the Leidenfrost temperatures with droplet size were observed, either on the polished or the processed surface. The difference in the Leidenfrost behavior between DI water and HFE 7300DL and among the various droplet sizes can be attributed to the nature of the force balance and flow hydrodynamics at temperatures slightly below the Leidenfrost point.
In the second part of this thesis, a method of generating nearly superhydrophobic surfaces from FLSP metallic substrates, and a study of their thermal stability at elevated temperatures are presented. Using FLSP, hierarchical micro/nano structures were fabricated on stainless steel 316 after which a 200 nm Cerium Oxide (CeO2) film was sputtered onto the surface. Before CeO2 deposition, the contact angle of the sample was measured. Post CeO2 deposition, the contact angle was measured again. As a result of the CeO2 deposition, the contact angle of the originally hydrophilic FLSP surface changed to nearly superhydrophobic, with a contact angle of approximately 140o. Subsequently, the coated surface was annealed in air. The surface maintained its high contact angle from room temperature to about 160oC, after which it lost its hydrophobicity due to hydrocarbon burn off. For each annealing temperature, the chemical composition for the cerium oxide-coated FLSP surface was monitored using energy dispersive x-ray spectroscopy (EDS) and X-ray diffraction (XRD). Under a nitrogen rich annealing environment, the nearly superhydrophobic FLSP metallic surface maintained its high contact angle up to temperatures as high as 265oC. To further understand the physics behind the observed phenomenon, we investigated two additional samples of polished stainless steel 310 again coated with 200 nm of CeO2. Once again, the sample heated in nitrogen showed improved thermal stability over the sample heated in oxygen. Additionally, hydrophobicity loss again occurred at approximately 200˚C confirming that hydrocarbon adsorption is the underlying mechanism for hydrophobicity in rare earth oxide ceramics.
Advisors: Sidy Ndao and George Gogos