Elsevier

CIRP Annals

Volume 63, Issue 1, 2014, Pages 329-332
CIRP Annals

Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC

https://doi.org/10.1016/j.cirp.2014.03.031Get rights and content

Abstract

The production of extremely thick silicon carbide (SiC) has recently become possible with the advent of a specific chemical vapor deposition process. Ultra-precision machining of high-purity SiC has been performed by using a polycrystalline diamond (PCD) micromilling tool to investigate the machining characteristics. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are thin enough to achieve ductile mode machining. Micron-sized wells and groove structures with nanometer-scale surface roughness were successfully machined by using the PCD tool. In addition, a new electrochemically assisted surface reconditioning process has been proposed to remove the contaminant material adhered onto the PCD tool surfaces after prolonged machining.

Introduction

Recently, silicon carbide (SiC) has been widely used in a variety of applications, including advanced micro-optics and molds, micro-medical devices and micro-ceramic reactors for chemical analysis, primarily owing to their high wear resistance and thermal stability. Thus far, several methods have been adopted to deposit SiC by chemical vapor deposition (CVD). The deposition rate is typically of the order of 1 μm h−1. However, higher deposition rates are often demanded for depositing thicker SiC. Lately, researchers have demonstrated the feasibility of depositing extremely thick (up to 20 mm) SiC by using a specific CVD process [1]. The resultant material was found to exhibit superior properties when compared to those of SiC produced by conventional sintering, especially in terms of high purity (99.9995%) and homogeneous structure.

Meanwhile, SiC is characterized by extreme hardness and brittleness, making it a difficult-to-machine material. This has increased the demand for the development of a suitable micromachining method that is capable of forming SiC with required surface quality and geometric accuracy [2], [3], [4], [5]. In general, hard and brittle materials such as SiC are machined by grinding and polishing processes to achieve a high-quality surface [2], [3]. However, these methods have certain inherent limitations, especially when it comes to the fabrication of fine complex structures. Turning is yet another machining approach that has been applied to SiC [5]. Although this method is considered to be suitable for the production of symmetric structures such as lens molds, it is highly challenging to produce asymmetric structures, such as microchannels. On the other hand, micromilling using polycrystalline diamond (PCD) is considered to be an attractive method for the fabrication of three-dimensional structures in hard and brittle materials. This method offers the advantages of high surface integrity via ductile mode machining [6], [7], [8].

In this study, ultra-precision machining of high-purity SiC has been performed using a PCD micromilling tool to investigate the machining characteristics. In general, when a tool is used for prolonged machining operations and for producing concave and intricate shapes in multiple samples, it becomes highly important to maintain the surface conditions of the tool. Therefore, an electrochemically assisted method for periodic reconditioning of the tool surface is also proposed.

Section snippets

Coordination of experiments

The sample analyzed in this study is high-purity bulk SiC produced by a specific CVD process. In addition, bulk SiC prepared by conventional sintering process was considered for comparison. The dimension of the sample was 10 mm × 10 mm × 5 mm, and the properties of the samples are summarized in Table 1.

Micromilling was performed by using a specially designed square-shaped PCD end mill, as shown in Fig. 1. The PCD diamond grains of average size 1.0 μm were sintered with metallic cobalt under high

Basic experimental results and machined surface analysis

The surface quality of the sample micro-machined using a PCD was investigated by performing machining experiments with various feed rates in the range of 5–50 mm/min. The machining conditions are listed in Table 2. Fig. 3 shows the average surface roughness (Ra) and peak-to-valley surface roughness (Rz) of the high-purity SiC and conventionally sintered SiC, as measured by SWLIM. The feed per tooth (f) determined using Eq. (1) is also shown in the figure, where F is the feed rate, S is the tool

Machining of micron-sized well structures

While machining microconcave or intricate shapes, there is always a concern that the surface quality of the machined bottom portion is degraded because of the relatively low chip evacuation. Therefore, the micromachining performance of the PCD tool was verified by machining micron-sized well structures on both the high-purity SiC and sintered SiC.

As can be observed from the schematic of the micromachining contour shown in Fig. 6, the tool path for machining a well structure follows a spiral

Investigation of material adhered onto the tool surface

The material adhered onto the surface of the PCD tool was investigated by SEM and EDS analyses. Typical SEM images of the PCD tool after repeated machining are shown in Fig. 14(a) and (b). As can be seen from the figure, the entire surface of the tool is covered with a thick film. In particular, the cutting edge itself is completely covered with the adhered material and contains small cracks, as shown in Fig. 14(c). The high-magnification SEM image of the tool surface shown in Fig. 14(d)

Electrochemically assisted reconditioning process

EDS analysis of the PCD tool surface suggests that the contamination is primarily due to the adhesion of silicon-based material, which was assumed to be amorphous SiO2. In general, such contaminations can be removed by brushing and ultrasonic cleaning in an acetone solution. However, in the case of a micron-sized tool, it is difficult to fully remove it using these physical approaches. Therefore, an electrochemical approach was attempted for cleaning and reconditioning of the PCD tool.

Fig. 16

Conclusions

Ultra-precision machining of high-purity SiC was performed by using a PCD micro-milling tool and their milling characteristics have been investigated. Results indicate that a high-quality surface (Ra = 1.7 nm) can be obtained when the removed chips are sufficiently thin to achieve ductile mode machining. Although the micron-sized well and groove structures with nanometer-scale surface roughness were successfully machined, the surface roughness gradually deteriorated and the depth of the well

Acknowledgements

The authors would like to thank NS Tool Co. Ltd. for their continued support.

References (11)

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