Abstract
Additive manufacturing technologies are developing at a fast pace and are rapidly becoming prevalent in the aerospace, medical, nuclear, defence, automotive, marine, space and manufacturing sectors. As technologies are created, commercialised and adopted, many areas of research and development arise. One of these is the surface topography of as-built AM components. Currently, these surfaces are generally considered negative aspects of an AM component, to be either corrected or removed, as they do not conform to current standards for surface roughness.
This thesis explores an alternative solution, that inverts our relationship with AM surfaces into a positive one. Designed surface topographies may be integrated into the surfaces of components, that produce performance enhancements for the intended application. This research project focuses on the use of designed surface topographies to control the wettability of as-built stainless steel structures. Inspiration for these surfaces is taken from the field of Biomimetics that studies, engineers and replicates aspects of the surface topography of naturally occurring functional structures.
This study investigates the capability of current generation L-PBF technologies to manufacture designed surface topographies on the scale of the process melt-pool, and the repeatability of the process, using EOS M270 and Aconity Mini additive manufacturing machines. The capacity for control of manufacturing parameters such as laser power and speed to affect the dimensional accuracy of built features and wettability of the surfaces is also studied through a DoE study using Minitab 2017 software. The surface topographies are measured using white light focus variation microscopy on an Alicona Infinifocus G5, and static water contact angle measurements are acquired using a Kruss FM40 Easydrop goniometer. Finally, the surface chemistry and chemical states present on the surfaces of test specimens is assessed using a Thermo Scientific K-Alpha X-Ray Photoelectron Spectrometer to determine how this has affected the resulting wettability of these surfaces.
It was found that designed surface topographies can be produced, and that these structures show high levels of geometric repeatability. Water contact angle results showed significant variability that includes step changes in wettability from hydrophobic to superhydrophilic, and these were thought to have a strong relationship with surface topography. A central composite design of experiment found no statistically significant relationship between wettability and laser power and speed, which are input parameters directly affecting feature geometry. Analysis of the surface chemistry and chemical states do not indicate a strong relationship between the average thickness of the hydrocarbon layer on the surface and the water contact angle results for those surface topographies. Composite O1s and C1s XPS peaks suggest that chemical states within these binding energy ranges may be responsible for the step changes in wettability observed, however further work is required to confirm this hypothesis.
This research project completes an initial investigation into current L-PBF capability, producing these surface structures and a foundation for further research, based on its conclusions and the many potential threads of research that it offers.