Fabrication of ordered hierarchical structures on stainless steel by picosecond laser for modified wettability applications

Micro/Nano Periodic Surface Structures and Performance of Stainless Steel Machined Using Femtosecond Lasers

The machining of micro/nano periodic surface structures using a femtosecond laser has been an academic frontier and hotspot in recent years. With an ultrahigh laser fluence and an ultrashort pulse duration, femtosecond laser machining shows unique advantages in material processing. It can process almost any material and can greatly improve the processing accuracy with a minimum machining size and heat-affected zone. Meanwhile, it can fabricate a variety of micro/nano periodic surface structures and then change a material's surface performance dramatically, such as the material's wetting performance, corrosive properties, friction properties, and optical properties, demonstrating great application potential in defense, medical, high-end manufacturing, and many other fields. In recent years, the research is gradually deepening from the basic theory to optimization design, intelligent control, and application technology. Nowadays, while focusing on metal structure materials, especially on stainless steel, research institutions in the field of micro and nano manufacturing have conducted systematic and in-depth experimental research using different experimental environments and laser-processing parameters. They have prepared various surface structures with different morphologies and periods with sound performance, and are one step closer to many civilian engineering applications. This paper reviews the study of micro/nano periodic surface structures and the performance of stainless steel machined using a femtosecond laser, obtains the general evolution law of surface structure and performance with the femtosecond laser parameters, points out several key technical challenges for future study, and provides a useful reference for the engineering research and application of femtosecond laser micro/nano processing technology.

Fabrication of an ordered micro-/nanotextured titanium surface to improve osseointegration

Ordered microscale titanium (Ti) surface topography with homogeneous cell-sized microholes (20 µm in diameter) was fabricated using simple electrochemical etching. The as-prepared surface imposed with uniform titania nanotubes (TNTs, 70 nm in diameter) through electrochemical anodization showed no considerable change in the initial microscale morphology. Bone marrow mesenchymal stem cells (BMSCs) were used in evaluating the bioactivity. Compared with polished Ti and unordered microtextured Ti, the ordered microtextured Ti formed by electrochemical etching remarkably promoted cell attachment, alkaline phosphatase activity, collagen secretion, extracellular matrix mineralization, and osteogenesis-related gene expression but considerably inhibited cell proliferation. After TNTs were introduced to the ordered microtextured Ti, cell attachment and osteogenic differentiation indexes were further enhanced, and cell proliferation recovered over time. The ordered micro-/nanotextured Ti surface was more conducive to the cell attachment, proliferation, and osteogenesis of BMSCs than polished Ti with and without TNTs, unordered microtextured Ti with and without TNTs, and unitary ordered microtextured Ti. Thus, the novel ordered bio-inspired micro-/nanotextured structure composed of cell-sized microholes and TNTs on the Ti surface possessed a favorable interfacial environment that improved osseointegration, potentially optimizing Ti implant surface topography.

Wettability Transition of the Picosecond Laser-Ablated 304 Stainless-Steel Surface via Low-Vacuum Heat Treatment

Inspired by natural creatures, superhydrophobic surfaces with various adhesion behaviors have attracted significant scientific interest. In this study, by controlling the laser fluence, the scanning times, and the subsequent cleaning method, microcolumn arrays with different morphologies were fabricated on 304 stainless-steel surfaces using picosecond laser direct writing. To achieve wettability transition, the laser-processed samples were then subjected to heat treatments (120 °C) in air and in a low vacuum environment (6 kPa). The results show that after heat treatment in different environments and with various time lengths, the laser-processed surfaces become hydrophobic surfaces with different adhesion properties. It is worth noting that while surfaces heat-treated in air exhibit weak wettability transition potential and high adhesion, the surfaces heat-treated in a low vacuum environment present superhydrophobic and low adhesion properties with a minimum sliding angle of about 3.14°. Moreover, the low-vacuum heat-treated surfaces retain good superhydrophobic properties after 1 month of observation as well as an abrasion test. These transitions in hydrophobic behavior and adhesion properties may be mainly attributed to the heat treatment-induced (in the air or in a low vacuum environment) redistribution of surface compounds and the microstructure-induced alternation of the solid-liquid contact state. By controlling the laser processing parameters and the heat treatment time and environment, stable wettability transition and flexible adhesion control of stainless steel can be easily achieved.

A Simple Model to Predict Machined Depth and Surface Profile for Picosecond Laser Surface Texturing

A simple mathematical model was developed to predict the machined depth and surface profile in laser surface texturing of micro-channels using a picosecond laser. Fabrication of micro-craters with pulse trains of different numbers was initially performed. Two baseline values from the created micro-craters were used to calculate the estimated simulation parameters. Thereafter, the depths and profiles with various scanning speeds or adjacent intervals were simulated using the developed model and calculated parameters. Corresponding experiments were conducted to validate the developed mathematical model. An excellent agreement was obtained for the predicted and experimental depths and surface profiles. The machined depth decreased with the increase of scanning speed or adjacent interval.