Fabrication of micro-pin array with high aspect ratio on stainless steel using nanosecond laser beam machining

Abstract

In this paper, a micro-pin array with a high aspect ratio was fabricated on AISI 304 using laser beam ablation for attachment to a vertical wall. In recent times, there has been research in various fields, including robotics and bio-MEMS, regarding attachment to vertical walls, and micro-pin arrays may offer the best solution. For vertical wall attachment, the micro-pin should have a high aspect ratio, long length, and sharp tip. The recast layer could be piled due to the chromium oxide with high surface tension and viscosity of chromium oxide, and it composed the micro-pins with high aspect ratio. X-ray photoelectron spectroscopy (XPS) was used to identify the characteristics of the piled recast layer. The machining characteristics for a high aspect ratio micro-pin array were investigated according to laser beam machining parameters. In addition, experiments for attaching force relative to the surface roughness of the subject plane were carried out.

Introduction

Over the last decade, climbing and attaching to vertical walls has been one of the most challenging topics in the robotics research field. Various mechanisms have been used for vertical wall attachment depending on the subject wall properties. In the case of a rough wall, like a concrete or brick wall, the climbing robot uses the spines on its feet, which are inspired by beetles [1], [2], [3]. The principle of attaching by spines is that the sharp tips of the spines engage with the micro irregularities on the subject rough surface. Through this mechanism, the spine should be sharp enough to attach to the fine surface because the irregularities on the fine surface are small. In addition, less force is generated for attachment on the small irregularities. It follows that existing robots, which use spines, are only able to attach to rough surface walls because the spine tip is generally too large. Therefore, the micro-pin needs to be sharp and long for attaching to a fine vertical surface, like machined metal or plastics. The micro-pin array can attach on the fine surface by virtue of its sharp tip. In addition, it can generate more force due to the greater number of pins.

There has been much interest in the fabrication of the micro-pin array. It can contribute to microelectromechanical systems (MEMS) and the field of bio-engineering for use on super-hydrophobic surfaces, for DNA separation, and as a micro channel mixer. To manufacture a micro-pin array, various manufacturing processes have been proposed using photo electrochemical etching [4], photolithography [5], polydimethylsiloxane (PDMS) replication [6], and deep reactive ion etching (DRIE) [7]. However, these techniques have the disadvantage of high associated costs and a limitation on material applicability—they can only be used with polymer or silicon materials. Because the micro-pins have physical contact with the subject surface, the micro-pin array should have the appropriate mechanical property, like high elasticity, for attaching to a vertical wall. However, polymers have low stiffness, and silicon can easily be broken due to its high brittleness. For these reasons, polymer and silicon are not suitable materials for an attachment application. On the other hand, a metallic material could be a good candidate because of its mechanical properties.

Several machining technologies can be proposed to machine metallic materials. In particular, the non-traditional machining process can be applied to machine micro products [8]. Micro electrical discharge machining (EDM) and micro electrochemical machining (ECM) have been researched for the micro-machining of metallic materials [9], [10]. These technologies are very precise, but they have a low machining speed. Therefore, they are not suitable for application in large area machining, such as in the fabrication of micro-pin arrays. On the other hand, laser beam machining (LBM) has large area machining capabilities due to its high machining speed. However, for example, a micro-pillar array was fabricated on a metallic alloy by laser-assisted surface modification, but the pins had an irregular shape and were tangled with neighboring pins [11]. For precise machining, an ultra-short pulsed laser beam, which has a pulse duration of picoseconds and femtoseconds, is generally used. But its material removal rate is so low that it is not suitable for large area machining. Thus, a machining process for micro-pin array fabrication, which has both machining speed and accuracy, is needed.

Nanosecond pulsed laser beam machining has high productivity due to high machining speed [12], [13]. On the other hand, it has a precision machining weakness due to a long pulse in comparison to the ultra-short pulsed laser. For this reason, nanosecond pulsed laser beam machining is recognized as an inadequate process for fabricating micro-pin arrays with a high aspect ratio. In this study, a new process using a nanosecond pulsed laser was proposed to solve the aforementioned problems.

In the nanosecond pulsed laser process, a heavy recast layer is formed due to the high pulse energy, and this is the main reason precision machining is not obtainable. But the recast layer could be utilized for specific purposes, such as the pin-shaped structure fabrication in this research. In this study, a laser machining process using recast layer piling was researched for the manufacture of sharp long micro-pin arrays with a high aspect ratio. The laser beam machining parameters were investigated to fabricate micro-pin arrays with a high aspect ratio. Lastly, the fabricated micro-pin array was attached to a vertical wall.