Understanding the Machining Process of Hierarchical Micro/Nanograting Structures Used for Optical Variable Device

Abstract

Hierarchical micro/nanograting structures have attracted increasing attention owing to their significant applications in the fields of structural coloring, anti-counterfeiting, and decoration. Thus, the fabrication of hierarchical micro/nanograting structures is important for these applications. In this study, a strategy for machining hierarchical micro/nanograting structures is developed by controlling the tool movement trajectory. A coupling Euler-Lagrange finite element model is established to simulate the machining process. The effect of the machining methods on the nanograting formation is demonstrated, and a suitable machining method for reducing the cutting force is obtained. The height of the nanograting decreases with an increase in the tool edge radius. Furthermore, optical variable devices (OVDs) are machined using an array overlap machining approach. Coding schemes for the parallel column unit crossover and column unit in the groove crossover are designed to achieve high-quality machining of OVDs. The coloring of the logo of the Harbin Institute of Technology and the logo of the centennial anniversary of the Harbin Institute of Technology on the surface of metal samples, such as aluminum alloys, is realized. The findings of this study provide a method for the fabrication of hierarchical micro/nanograting structures that can be used to prepare OVDs.

1. Introduction

Hierarchical micro/nanograting structures have significant applications in numerous fields, particularly structural coloring [1–5]. In recent years, optical variable devices (OVDs) have been widely used in the anti-counterfeiting [6–9] and decoration fields because their appearance changes with the angle of view or lighting conditions. Although the feasibility of generating complex patterns and grayscale images from surfaces with micro and nanostructures has been demonstrated [10, 11], few studies have attempted to process OVDs using machining methods based on structural coloring.

However, it has been proven that the mechanical machining method is feasible for preparing micro and nanostructures [12–17]; the size of the processed structures can be less than 100 nm [18–28] and this method has been successfully applied in the field of structural coloring. Yang et al. [29] used elliptic vibration-assisted machining technology to process the color palette from 400 nm to 760 nm in increments of 40 nm on a brass surface; when viewed at an angle of 70°, the entire visible spectrum could be identified using bright and interesting colors. Wang et al. [30] further improved the processing technology and pattern-generation strategy to achieve complex structural colors. Zhou et al. [31, 32] proposed an axial feed fly cutting method to realize the machining of micro and nanostructures. In this machining method, the tool was installed in a rotating spindle and the workpiece was fixed on the horizontal displacement stage. The micro/nano structural unit was machined in the form of a "shuttle groove" array on the surface of nickel phosphide and the control of the structural color was realized.

In our previous study [33], we proposed that controlled periodic nanostructures with structural color characteristics could be fabricated by combining the morphology of the groove and the pile-up of the groove machined using the AFM tip-based nano scratching approach. However, the current mechanical machining methods of micro and nanostructures have shortcomings in terms of tip wear, limitation of the depth of the machined structure, the machining accuracy limited by the tool radius and motion, and difficulty in fabricating hierarchical micro and nanostructures. Recently, researchers have used the tool rotation trajectory method to prepare micro/nano composite structures [5, 34–36]. Tool-path-based machining technology utilizes a diamond nanoindenter as a machining tool.