Abrasive waterjet micro-piercing of borosilicate glass

Abstract

Abrasive water jet (AWJ) machining of delicate materials is difficult due to the large pressures that commonly result in large-scale chipping and other target damage. The damage occurs mostly during the initial piercing operations when the jet flow creates a large stagnation pressure at the jet to surface interface at the bottom of blind holes. This paper examined the parameters that affect target damage during piercing operations in borosilicate glass. The effects of stand-off distance (SOD), dwell time and pressure for three nozzles sizes were compared. A novel aspect of the study is the use of a prototype 254 μm micro-abrasive waterjet machining (μAWJM) nozzle. The results of a parametric analysis identified machining configurations that resulted in the reduction of chipping and hole size, and an improved hole circularity. This paper also proposed and assessed two methods for the reduction of exit pierce chipping using the μAWJM nozzle. A finite element analysis showed that the use of stacked borosilicate glass plates reduced the exit stresses associated with through piercing, thus reducing the amount of exit hole chipping. An μAWJM/AJM (micro-abrasive waterjet machining/abrasive jet machining) hybrid operation was also shown to considerably reduce exit chipping and exit hole size.

Introduction

The material properties of high precision glass make them an attractive solution for many industrial applications. El-Meliegy and Noort (2012) and Wananuruksawong et al. (2011) have shown the diverse application of glasses in the medical and dental industries, respectively. The extensive use of glass in photonics has been discussed by Carvalho et al. (2008) and Richardson et al. (2010). Glasses are of particular importance to the micro machining industries, such as the production of lab-on-chip's as shown by Chen et al. (2012), micro-electro mechanical systems (MEMS) as utilized by Tiwari and Chandra (2014), and for precision optical and electonic applications. Wiederhorn (1971) states that unlike other brittle materials that can deform in a ductile manner when subjected to sufficient pressures, glass remains brittle even at very high pressures, is not capable of significant plastic deformation, and has been observed by Ali and Wang (2011) to respond to impacts by forming cracks and fracturing. These properties make it one of the most difficult materials to machine. Glass is usually cut by scoring and cleavage fracturing, but this is generally only effective for straight line cuts on thin plates. Cutting curves or milling of glass requires more advanced procedures. Piercing operations on glass are particularly challenging, given the propensity to create unacceptably large defects. Mitigation or elimination of piercing defects during micromachining processes is thus highly desirable.

Machining complex geometries in glass using traditional mechanical tools requires carbide or diamond tools; with a slow feed rate and adequate lubrication and cooling. Excessive heat generation and tool wear can develop as a result of friction between the tool and glass, resulting in material damage. For example, Zhimalov et al. (2006) have shown that mechanical cutting of glass reduces the strength of glass by 60% on average. In the mechanical drilling of soda-lime glass plates using a micro-drill bit, Park et al. (2002) found the amount of glass chipping on the exit side of the hole could be reduced by using a supporting glass plate. Meticulous mounting and stacking of the two glass plates was required in order to optimize the surface adhesion. Park et al. (2002) determined that the addition of the backing glass plate reduced the stress concentrations on the exit hole edges, resulting in a more gentle and defect free exit pierce. Although there was a significant reduction in chipping, the process was nevertheless relatively slow. Egashira et al. (2002) have shown that ultrasonic drilling is capable of producing high quality holes, and were able to produce 10 μm diameter ultrasonically drilled holes in glass using a micro tool; however, they were limited to small depth of cut (20 μm) and restricted by the size and stresses of the cutting. Chen et al. (2011), and Quan et al. (2010), produced small holes in glass using mechanical drilling–grinding tools, but were limited to meso features size (∼1 mm) due to the diameter of the tools, and slow drill speeds.

Laser beam machining (LBM) can also be used in glass substrate processing. For example, Zhimalov et al. (2006) demonstrated that cutting glass using LBM could produce better results than traditional mechanical methods. Machulka et al. (1972) cut glass with a CO₂ laser, and proposed a theoretical cutting rate model. Jiao and Wang (2009) utilized a novel dual laser beam approach to demonstrate its feasibility for machining glass substrates. Shah et al. (2001) used a femtosecond lasers to produce <100 μm holes in silicate glasses, with depths greater than 1 mm. Petkovšek et al. (2008) developed a novel method for monitoring the laser micro drilling process in glasses when using and excimer laser. Yu et al. (2009) produced nano-pores (90 nm) in a borosilicate glass membrane using an ArF excimer laser. While micro-holes machined by lasers are precise, they either lack a rapid drilling rate when using femtosecond lasers, or produce a heat affected zone during material ablation and vaporization.

Reactive ion etching (REI) is a well-established dry etching method that is commonly used for micro-fabrication of MEMS devices. Li et al. (2000) preformed inductive coupled plasma deep RIE on Pyrex and silica glass. Their experiments yielded smooth features (Ra ∼ 4 nm) with resolutions of 20 μm, and channel aspect ratios >10; however, these features were machined using very long etch times, with a maximum etch rate of 0.6 μm/min. Wet chemical etching utilizes acidic solutions for material removal and is usually coupled with masking using photolithography. Wet chemical etching is capable of producing very fine resolution features, but has extremely slow etch rates (maximum ∼ 30 nm/s), is restricted to low aspect ratio features as stated by Spierings (1993), and has a significant environmental cost.

Mochimaru et al. (2012) investigated the machining of micro holes in 160 μm glass using electro-chemical discharge machining (ECM) with an electrode diameter of 20 μm. While the resolution can be very fine, the holes produced experience considerable thermal damage.

When compared to the aforementioned technologies, abrasive waterjet machining (AWJM) of brittle materials is a more environmentally benign direct write (no mask required) process, with a much more rapid material removal process, producing no heat affected zone or changes to the material properties. Liu and Schubert (2008) state that the smallest features machined by current waterjet nozzles are approximately 200 μm. The extremely high etch rate of μAWJM (Table 1) makes it an attractive alternative for the micro-machining of glasses. Momber and Kovacevic (1998) have shown that abrasive waterjet (AWJ) systems have been developed to have a wide range of manufacturing abilities, which include milling of glass channels as shown by Dadkhahipour et al. (2012), turning of glass rod by Zhong and Han (2002) and cutting glass substrates by Miller (2004). For the bulk of these AWJM processes, the preliminary operation is piercing; however, the majority of the AWJ research on brittle/delicate materials has concentrated on channel milling, with very little focus on piercing. Various parametric analyses have been performed regarding the milling of channels into brittle materials with a focus on kerf characteristics and the effect of traverse speed. Zhu et al. (2008) AWJ machined channels in glass and evaluated the substrates machinability based on kerf width. Dadkhahipour et al. (2012) also AWJ milled channels in glass, but focused on the morphological aspects of the channel formation. Srinivasu et al. (2009) milled channels in silicon carbide, and developed a relation between the key kinematic parameters of the AWJ, and the dimensional characteristics of the kerf. Unlike AWJ channel milling/cutting, where the escaping slurry is deflected away from the incoming jet flow, the outbound slurry when piercing produces additional drag on the incoming jet. Wang et al. (2009) analyzed the formation of micro holes using abrasive slurry jet (ASJ) with a focus on blind hole morphology under various slurry pressures, particle sizes, and particle concentration conditions. Their results yielded several ways to increase material removal rate (MRR) and hole depth.

In abrasive water jet machining, pierced hole shapes are affected by four main process parameters: jet structure, target material, dwell time and stand-off distance (SOD) as characterized by Liu (2006). Liu (2006) also determined that material properties and wear resistance have a considerable effect. Liu and Schubert (2008) state that a drawback of AWJ piercing is that it frequently results in damage to the glass in the form of chipping at both the entry and exit, and is due to the high stagnation pressure that forms during the initial impact and at the bottom of blind holes. For example, Liu (2006) found that holes pierced in glass result in substantially more wear at the entrance of the pierce, causing a larger slope and greater wear radius around the top of the hole when compared to more ductile materials such as aluminum alloys. Liu and Miles (1998) have also found that the drilling rate decreases exponentially with an increase in hole depth, and that entrance wear increases with pressure. Based on those findings, it has been suggested by Liu (2006) that the AWJM of brittle materials such as glass should be performed at relatively low pressures (e.g. 35 MPa or lower) in order to reduce large-scale cracking and thus improve hole quality. However, lower pressures may cause inconsistent abrasive entrainment due to the relatively weak mixing chamber vacuum. Moreover, the lower kinetic energies increase the drilling times and may limit the maximum hole depth.

Although flash abrasive waterjet (FAWJ) and abrasive cryogenic jetting (ACJ) systems have been developed by Liu and Schubert (2008) to produce abrasive/vapor jets that reduce the stagnation pressure and thus hole chipping, the systems are highly complex, bulky, and their harsh operating environments result in cost ineffectiveness. The present study focuses on piercing operations in glass using a micro-abrasive waterjet (μAWJ), with an emphasis on determining the process parameters that most affect target damage, and developing methods to avoid it.