Fabrication of surface microstructures by mask electrolyte jet machining

Highlights

• A novel electrochemical process for fabrication of micro surface structures.

• Assimilate the strong point of TMEMM and Jet-ECM, present a high-throughput process with adjustable flow field.

• The process shows potential especially considering large area complex geometries.

Abstract

This paper presents a novel electrochemical processing technique, mask electrolyte jet machining (MEJM), for the fabrication of surface microstructures. MEJM combines jet electrochemical micromachining (Jet-ECM) and through-mask electrochemical micromachining (TMEMM), combining the advantages of TMEMM, which is a high-throughput process, and of Jet-ECM, with its adjustable flow field. The effects of a mobile nozzle on electrolyte flow are investigated, and a new modeling approach for large translational movements is proposed. An analysis of the accuracy and reliability of the proposed method is presented. Microprotrusions and microdimples are produced to high precision and with excellent consistency of dimensional variation (maximum standard deviation 2.171 $\mathrm{\mu }$m). The results suggest the possibility of an affordable technique for batch fabrication of surface microstructures with high efficiency and precision.

Introduction

In the last few decades, there has been increasing interest in the application of surface structures, particularly on the micro- and nanoscale, owing, for example, to their tribological and wetting properties [1,2]. A variety of approaches to the fabrication of micro/nanoscale surface structures have been investigated. Electrochemical micromachining (ECM) is widely employed [[3], [4], [5], [6], [7]]. This is a noncontact and nonthermal process and thus does not lead to tool wear or to mechanical and thermal residual stresses that might initiate changes in material properties [8,9]. The material removal rate can be adjusted by controlling the applied current. A wide range of metallic materials, regardless of their hardness and thermal resistance, can be easily machined by ECM.

For the fabrication of surface structures, a number of techniques based on ECM are available, including jet electrochemical machining and through-mask electrochemical micromachining. Jet electrochemical machining (Jet-ECM) employs a jet of electrolyte: a stream of electrolyte flows through a nozzle and impinges directly on the workpiece, so that the electrochemical reaction is localized to a small region. Sen and Shan [10] investigated the effects of process parameters such as applied voltage, nozzle diameter, and electrolyte pressure on the quality of small holes produced by Jet-ECM. This technique has demonstrated its capacity to obtain various shapes down to the micrometer range [11], even its capability of fabricating microstructures on metal carbides [12]. However, Jet-ECM suffers from a relatively slow speed because of the need for sequential processing. Costa and Hutchings [13] employed textured tools consisting of arrays of holes to fabricate microdimples by Jet-ECM in a more efficient parallel machining process. However, the shape, size, and precision of microfeatures in Jet-ECM largely depends on the size of the nozzle: to generate smaller features, the inner diameter of nozzle must be reduced. Not only does this increase manufacturing costs, but it is extremely difficult to maintain a sustainable and stable electrolyte flow on a microscale. In addition, to generate different microsurface structures, such as triangles and squares, it is necessary to use nozzles of the corresponding shape, which poses further problems for nozzle manufacture. This is a natural limitation on the implementation of Jet-ECM for microfabrication.

Through-mask electrochemical micromachining (TMEMM), another common method, employs photolithography to produce micropatterns on photoresist coated on the workpiece surface, so that a large number of desired areas dissolve in parallel. This technique is precise and relatively fast, capable of generating well-defined surface textures with controlled size, location, and density. Using this method, Wang et al. [14] fabricated three-dimensional cylindrical microstructures with feature sizes as small as $40\mathrm{\mu }$m. Landolt et al. [15] carried out TMEMM of titanium using a laser-patterned oxide film. Qu et al. [16] developed a modified TMEMM technique for fabricating microdimple arrays with a re-useable polydimethylsiloxane mask. Ming et al. [17] used a patterned inert metal plate to fabricate metal through-hole arrays with double tapered openings. Qian et al. [18] were able to achieve a significant improvement in machining localization by employing the mask as an auxiliary anode.

All of these developments have widened the range of application of electrochemical micromachining processes and have improved their reliability to the extent that they are suitable for industrial implementation in surface fabrication for batch production. However, there remains the problem that the electrolyte flow direction in TMEMM is normal to the patterned photoresist. This leads to the formation of vortices in the electrolyte flow and consequently to a very low flow velocity in the mask holes [19]. This hampers the removal of electrolysis products and Joule heat by the electrolyte flow [20]. In addition, the nonuniform flow field as the electrolyte passes through the mask from one side to another can lead to poor consistency of material removal rate and to poor machining accuracy.

Wang et al. [21] investigated ways to improve electrolyte flow field during TMEMM and found that a mask with cone-shaped holes was beneficial to electrolyte flow. Chen et al. [22] investigated both the lateral and forward flow modes during TMEMM and proposed a modification to the latter mode in which the use of a multiple-slit structured cathode gave a better distribution of the electrolyte flow field.

This paper presents an approach, mask electrolyte jet machining (MEJM), that combines the advantages of Jet-ECM and TMEMM, namely, high throughput and an controllable flow field, respectively. This approach also does not suffer from problems of nonuniformity, difficulties in manufacturing and assembling jet nozzles at a micro-scale, or a lack of flexibility in micropattern design caused by the need to use nozzles of fixed shape.

To investigate the performance of the proposed approach, a mathematical multi-ion transport and reaction model (MITReM) is constructed [23]. In particular, the effects of a mobile nozzle on electrolyte flow are examined via numerical simulations for different nozzle travel speeds. The results show that the use of a mobile nozzle enhances the uniformity of the electrolyte flow field throughout the fabrication process. Experiments on fabrication of both concave and convex microstructures are carried out. The results suggest that the proposed approach should be suitable for batch fabrication of surface microstructures.