Dissertation
Quantitative analysis of surface characteristics produced by laser powder bed fusion and their role in surface functionalization
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2025
DOI: 10.25820/etd.008143
Abstract
Additive manufacturing, particularly laser powder bed fusion (L-PBF), has emerged as a transformative technology in various industries due to its ability to produce complex geometries with high precision and efficiency. Notwithstanding all the benefits of L-PBF, one of its primary drawbacks is the increased surface roughness and morphology inherent to the process. A novel approach to laser-assisted functionalization has been developed by the author’s group which can create the desired surface characteristics and/or precondition and attain the desirable surface chemistry over large areas of metal surfaces. However, preliminary investigations have revealed that when the novel approach to laser-assisted surface functionalization is applied to L-PBF surfaces, it does not always yield the same result as when the laser-assisted functionalization is applied to metals produced by conventional manufacturing processes (i.e. rolling, casting, forging, milling, etc.). This present work shows that the surface roughness and morphology inherent to L-PBF influence the application and results of the novel approach to laser-assisted surface functionalization.
Chapter 3 of this thesis characterized the as-printed surfaces of AlSi10Mg, Ti6Al4V, 4340 steel, and 316L stainless steel parts fabricated using L-PBF. The results of the analysis showed surface roughness and morphology varied considerably between the materials. Considering the variation with a given material, the surfaces fabricated with no part inclination (0°) can vary substantially from the surfaces of parts produced non-zero inclination (45°, 60°, or 90°), depending on the material. However, there is less variation in the surface roughness and topography among parts with non-zero inclinations. The greatest variation in surface roughness and topography among parts with non-zero inclinations is generally due to surface location (upskin versus downskin). Downskin surfaces exhibit significantly greater roughness and surface areas than upskin surfaces due to the increased presence of partially fused particles and spattered droplets.
Analysis of water contact angles showed that wettability shortly after fabrication is hydrophilic or superhydrophilic. However, when parts are stored in the air for several weeks, the wettability transitions towards slightly hydrophilic or slightly hydrophobic. Notwithstanding, when an extended cleaning method is applied to the parts exposed to air for several weeks, the wettability returns to values close to those observed on thoroughly clean surfaces shortly after fabrication. This shows that the unstable wettability is due to contamination from airborne organic compounds. Furthermore, the increased surface roughness and topography inherent to L-PBF present locations and opportunities for the contamination to remain on the surfaces.
Chapters 4 through 6 focus on applying laser-assisted functionalization to surfaces produced by L-PBF, using nanosecond laser texturing, either exposed to air (aNLT) or confined by water (wNLT), followed by chemical treatments. Chapters 4 and 5 used the chlorosilane reagents [CN(CH2)3SiCl3], also known as CPTS, and [CF3(CF2)5(CH2)2SiCl3], also known as FOTS, were used to create superhydrophilic and superhydrophobic surfaces, respectively. Results showed that the increased surface roughness and morphology of as-built L-PBF surfaces provided bonding sites, although limited, for the functional groups in FOTS to attach to the surface. However, the greater surface roughness and surface area also created a Wenzel state of wetting that resulted in high-adhesion superhydrophobicity (rose petal effect). In contrast, disrupting the L-PBF surfaces with aNLT or wNLT provided abundant bonding sites for the functional groups in FOTS, resulting in low-adhesion superhydrophobicity (lotus leaf effect).
Instead of using chlorosilane reagents, Chapter 6 focused on combining laser-assisted functionalization with gel-like carbon dots (G-CD). Although chlorosilane reagents are effective at modifying surface chemistry, they belong to the class of per- and polyfluoroalkyl substances (PFAS)—commonly known as "forever chemicals"—which raise environmental and health concerns due to their persistence. G-CD are an organic and environmentally friendly alternative to PFAS. The results showed that G-CD are effective at functionalizing the L-PBF surfaces with superhydrophilic properties and that the superhydrophilicity is very stable, even left exposed to atmosphere for extended periods of time. Results also showed that increased the concentration of G-CD from 1% to 2% improved surface wetting and thermal stability of the G-CD coating. Finally, it was also shown that increased surface roughness and surface area improved the effectiveness of G-CD in reducing the water contact angle.
Details
- Title: Subtitle
- Quantitative analysis of surface characteristics produced by laser powder bed fusion and their role in surface functionalization
- Creators
- Benjamin D. Nelson
- Contributors
- Hongtao Ding (Advisor)Albert Ratner (Committee Member)Jia Lu (Committee Member)Shaoping Xiao (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Summer 2025
- DOI
- 10.25820/etd.008143
- Publisher
- University of Iowa
- Number of pages
- xviii, 147 pages
- Copyright
- Copyright 2025 Benjamin D. Nelson
- Language
- English
- Date submitted
- 07/22/2025
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (pages 122-147).
- Public Abstract (ETD)
3D printing with metal powders has revolutionized additive manufacturing by fabricating metal parts with the level of customization and flexibility only attainable by 3D printing. Despite the advantages of 3D printing with metal powders, one of its major drawbacks is the surface roughness of the finished parts inherent of metal powders.
Engineering metal surfaces to have extreme wetting characteristics is an area of research that has received considerable attention. Two main methods of engineering metal surfaces to have these extreme wetting characteristics are to change surface roughness by laser surface texturing and to change the surface chemistry by chemical treatments. The findings of this research show that the inherent roughness of 3D-printed metal surfaces has a substantial effect on the ability to engineer extreme wetting properties.
When the as-printed surfaces of 3D printed metal parts are chemically treated to be superhydrophobic, the resulting surfaces show a high adhesion behavior where water droplets will ball up on the surface, but are unable to roll off. When the roughness of the as-printed surfaces is altered by laser surface texturing, subsequent chemical treatment yields a low adhesion superhydrophobic surface where the water balls up and easily rolls off the surface.
The chemicals traditionally used to alter the surface chemistry of metals are coming under increasing regulatory pressure due to long lasting persistence in the environment. Carbon nanoparticles (gel-like carbon dots) were explored in this research as a more ecologically and environmentally friendly chemical surface treatment. The results showed the gel-like carbon dots were effective in producing superhydrophilic surfaces when treating both as-printed and laser surface textured 3D printed metal parts.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9984948641302771
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