Surface hardening by gaseous oxidizing of (Zr₅₅Cu₃₀Al₁₀Ni₅)₉₈Er₂ bulk-metallic glass

The present investigation addresses surface hardening and the oxidation behavior of (Zr₅₅Cu₃₀Al₁₀Ni₅)₉₈Er₂ bulk-metallic glass (BMG) during gaseous oxidizing below the glass transition temperature (<Tg). The BMG was thermochemically surface engineered in controlled gaseous atmospheres imposing either an extremely low or an extremely high oxygen partial pressure. The hardened oxygen-containing case developing during oxidizing the BMG was characterized with X-ray diffraction, (electron) microscopy, energy dispersive spectroscopy and micro-hardness indentation. It was observed that oxidizing at a high resulted in the formation of an internal and an external oxidized zone; the latter can be minimized by applying a low. The inner oxide zone (IOZ) consisted mainly of a nano-crystalline dispersion of tetragonal ZrO₂, while a porous CuOx network developed in the outer oxide zone (OOZ). The formation of copper oxide islands at the surface is interpreted as the result of outward diffusion of metallic Cu driven by a compressive stress gradient in the IOZ caused by volume expansion associated with ZrO₂ (and Al₂O₃) formation. The results demonstrate that a hardened case (the IOZ) with a maximum hardness of ∼12 GPa can be achieved. The depth distribution of oxygen is explained in terms of a concurrent crystallization of the BMG below T_g.