Sub-Micron Feature Patterning of Thermoplastics using Multi-Scale BMG Tooling
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Sub-Micron Feature Patterning of Thermoplastics using Multi-Scale BMG Tooling
Dermot J. Stratton, Cormac Byrne, James Mulcahy and David J. Browne Engineering & Materials Science Centre, University College Dublin, Belfield, Dublin 4, Ireland ABSTRACT One potential application for Bulk Metallic Glasses (BMGs) is in dies with micro- and nano-sized features. Three basic characteristic sets inherent to BMGs make them ideal materials for micro/nano-tooling applications: (1) excellent compressive strength, wear and corrosion resistance; (2) amorphous structure which presents no microstructural length scale limitation to cutting and forming operations; (3) the presence of a glass transition temperature above which they can be easily formed. There are many potential applications for multi-scale BMG tooling, including in production of microfluidic and other precision biomedical devices. In the current work, discs were cut from 5 mm diameter cylindrical specimens of Zr44Cu40Al8Ag8 BMG produced via arc melting and casting into water-cooled copper molds. The cylindrical specimens were then thermoplastically formed into thin coin-like disc samples. The thin disc-shaped plates were then ground and polished to create a smooth flat surface. Sub-micron-sized features were patterned into the plates via a focused ion beam. We demonstrated that such feature sizes are not achievable in conventional crystalline metallic tool materials. The patterned BMG tools were then set in a compression press where the platen temperature was precisely controlled and a series of load-controlled embossing trials were carried out in which the features of the BMG tooling were replicated in poly(methyl methacrylate) (PMMA) sheet. An exercise in mapping out the size limitation of such a multi-scale embossing operation is reported. INTRODUCTION In this paper, the focus is placed on three specific characteristics of the BMG material that make it an ideal material choice for micro- and nano-featured dies for use in production of polymeric devices: (1) excellent mechanical properties, (2) amorphous structure free of microstructural details, and (3) presence of a glass transition temperature. The excellent mechanical properties of these alloys have been studied extensively [1]. The high compressive yield strength, large elastic strain limit, high wear resistance and good corrosion resistance of these BMGs are all properties ideal for a mold material. Another important characteristic of BMGs is their amorphous structure. In crystalline materials, the size of feature that can be milled or formed is limited by the size of the microstructural features (grain boundaries, lattice defects, etc.). Since such microstructural features are absent in BMGs, they can be milled or formed down to the nanoscale. Another characteristic resulting from the amorphous nature of BMGs is the presence of a glass transition. BMGs can be formed at relatively low pressures at temperatures above their glass transition temperature. As a result, BMGs can first be cast into simple cylindrical
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