Viscous flow and viscosity measurement of low-temperature imprintable AuCuSi thin film metallic glasses investigated by nanoindentation creep

Thin film metallic glasses are a class of new materials in metallic glasses, which have received much attention owing to their huge potential for applications in catalysts, micro-electromechanical systems and nano-structured devices. Reference to their smooth surface and the viscous flow characteristics in the supercooled liquid region, thin film metallic glasses are appropriate for the application as thermal imprinting materials. Imprinting materials should have polymer-like viscous flow behavior. Polymethyl methacrylate is a well-known conventional polymer having superior transcribe ability. Unfortunately, its mechanical strength is inadequate at room temperature. As opposed to polymers, therefore, metallic glass applied for imprinting resumes its metallic behavior such as high photoemission, excellent electrical conductivity, and high strength after imprinting.

Gold, lanthanum, and Ytterbium based metallic glasses possessing polymer-like features in the supercooled liquid region have also been developed for potential application as high performance microstructures. Gold and cerium-based bulk metallic glasses are suitable for thermal imprinting owing to their low process temperatures. Unfortunately, their high cost limit their extensive implementation. Therefore, using thin film metallic glasses instead of bulk metallic glasses can be appropriate to minimize material cost and maintain excellent mechanical attributes.

Researchers led by Professor Chun-Hway Hsueh at National Taiwan University prepared four compositions of amorphous Au-Cu-Si thin film metallic glasses through magnetron sputtering. Their main aim was to analyze the rheological characteristics of Au-Cu-Si thin film metallic glasses in their supercooled liquid region. They also analyzed the phase, composition, and microstructure. Their research work is published in Materials and design.

The authors deposited four varying compositions of fully amorphous Au-Cu-Si thin film metallic glasses using magnetron sputtering. The researchers obtained glass transition temperature from nanoindentation tests at varying temperatures using nose-like over-shooting phenomena upon unloading and crystallization temperature from the surface morphology change with temperature. The glass transition temperatures for the four samples labelled R30, R40, R50, and R55 were 90-100, 60-70, 60-70, and 50-60 °C, respectively, while crystallization temperatures were 180, 130, 90, and 70 °C, respectively.

The authors observed that the crystallization and glass transition temperatures and their thermal stability decreased when the content of the base metal in the thin film metallic glasses was increased. Nanoindentation creep measurements performed with spherical and Berkovich tips in the 50-170 °C range could be used to derive the viscosity of the thin film metallic glasses within their supercooled liquid regions and viscosities in 10¹¹-10¹³ Pa.s range were realized.

The researchers also demonstrated that the Newtonian viscous flow in Au-Cu-Si thin film metallic glasses as well as activation energy of viscous flow could be computed from the nanoindentation measurements conducted. They also realized that the results obtained from the spherical and Berkovish tips were in agreement with each other. Au-Cu-Si thin film metallic glasses indicated superior thermal imprinting capability in the supercooled liquid region. Therefore, the films are good candidates for application in the relevant industries.

NOTE: The work was jointly supported by the Ministry of Science and Technology, Taiwan under Contract no. MOST 102-2221-E-002-063-MY3 and Excellent Research Projects of National Taiwan University under Project no. 105R8918