Measurements of Surface Diffusion on Metallic Glasses

The surface mobility of glasses plays a crucial role in various applications, including friction, sintering, heterogeneous catalysis, and thin-film deposition. This area has garnered significant attention over the past decades, predominantly focusing on organic glasses. Metallic glasses (MGs), characterized by their relatively simple atomic arrangement and a range of unique properties, are considered ideal model systems for studying the fundamental properties and behaviors of glassy materials. However, the surface diffusion on MGs has remained largely unexplored due to their multicomponent, highly reactive chemical nature. Building on the pioneering work of long-term surface dynamics measurement on a Pd₄₀Ni₁₀Cu₃₀P₂₀ MG in 2015, we investigated the surface diffusion on Au₆₀Cu_15.5Ag_7.5Si₁₇ and Pd_77.5Cu₆Si_16.5 MGs at temperatures well below their glass transition temperatures (<i>T</i>_gs) using surface grating decay and surface scratch decay methods, respectively. The decay of these surface nanostructures with increasing annealing time was analyzed using the Mullins model, enabling the derivation of temperature-dependent surface diffusion coefficients. Specifically, surface diffusivities were evaluated to range from (9.0 ± 2.1) × 10⁻²⁰ m² s⁻¹ and (2.6 ± 0.5) × 10⁻¹⁸ m² s⁻¹ in the temperature range from 0.83<i>T</i>_g to 0.94<i>T</i>_g for the Au-based MG, and from (8.66 ± 0.80) × 10⁻¹⁹ m² s⁻¹ and (5.90 ± 0.60) × 10⁻¹⁸ m² s⁻¹ within 0.83<i>T</i>_g to 0.91<i>T</i>_g for the Pd-based MG. By fitting the temperature dependence of surface diffusivities with the Arrhenius equation, the average activation energies for surface diffusion were determined to be approximately 0.67 eV for the Au-based MG and 0.92 eV for the Pd-based MG. Notably, the activation energy for the Pd-based MG is about half the value for bulk diffusion, aligning with theoretical predictions. Furthermore, our experimental results support the correlation between enhanced surface diffusion and liquid fragility. These findings not only pave the way for measuring surface diffusion on other MGs but also contribute to a broader understanding of surface mobility in glassy materials.