Surface properties of copper-based bulk metallic glasses
Student thesis: Doctoral Thesis
The effects of abrasion and oxidation on the surface properties of copper-based bulk metallic glasses (Cu-based BMGs) have been studied. Four types of Cu-based bulk metallic glasses, namely Cu-Zr-Ti, Cu-Zr-Hf-Ti, Cu-Zr-Ti-Y and Cu-Zr-Al alloys, were used in this investigation. Crystallized Cu-Zr-Ti alloys were used for comparison as well. The wear and oxidation behaviors of all the four metallic glasses and crystallized Cu-Zr-Ti alloys were studied while the Cu-Zr-Ti bulk metallic glass was used to demonstrate the improved oxidation resistance by surface modification technique. The abrasive wear behaviors of the four copper-based bulk metallic glasses, the crystallized Cu-Zr-Ti alloy and 304 stainless steel have been investigated using pin-on-disc measurement. The volume loss increases with both sliding distance and applied load. However, the wear resistances are not proportional to the hardness and do not follow the standard wear law. Generally, the wear rate increases in the order, 304 stainless steel, 30 % crystallized alloy, metallic glasses, 50 % crystallized alloy. The difference in the wear behaviors between metallic glasses and crystalline alloys are attributed to the fact that the wear mechanism of the former involves crack initiation and propagation. Analysis of the worn surfaces subject to higher applied load showed that the metallic glasses suffer from severe wear compared with that of crystalline alloys. The dependences of the wear mechanisms of different materials on the applied loads are clearly presented. The oxidation behaviors of Cu60Zr30Ti10 bulk metallic glass and its crystalline counterpart have been compared over the temperature range of 573 to 773 K. The oxidation kinetics, measured with thermogravimetric analysis (TGA), of the metallic glass follows a linear rate law between 573 and 653 K and a parabolic rate law between 673 and 733 K. Similar trend was also observed for the crystallized sample while its oxidation rate is lower than that of metallic glass. Characterization and identification of the oxide films was done by surface analysis and X-ray diffraction, respectively. The formation of various oxides is explained based on the physical properties such as atomic size, electronegativity and heat of formation of the constituents in metallic glasses. Particular attention is drawn to the segregation of copper in the metallic glass induced by oxidation. Results showed that segregation is strongly dependent on annealing environment. Various oxides with metallic copper are formed after annealing in oxygen atmosphere but only crystalline phases are found under vacuum annealing. The effect of the segregation phenomenon of metallic glass during oxidation is discussed in view of composition, atomic mobility and chemical affinity of elements in the alloy system. The oxidation resistance of Cu60Zr30Ti10 bulk metallic glass can be improved by alloying with a small amount of elements. The effects of the addition of hafnium or yttrium into the Cu60Zr30Ti10 bulk metallic glass on oxidation kinetics, oxide compositions and structures have been studied. The oxidation resistance is improved by adding hafnium but deteriorated by alloying with yttrium. A breakaway oxidation was observed in the (Cu0.6Zr0.3Ti0.1)98Y2 metallic glass oxidized at 673 K and its poor oxidation resistance was due to the porous oxide structure revealed from the microscopic examination. Apart from alloying, substitution of titanium with aluminum into Cu60Zr30Ti10 bulk metallic glass was also found interesting in this study as it resulted in a substantial improvement on the oxidation resistance. The relationship between the alloying elements and the oxidation behaviors of metallic glasses under different temperatures is discussed in terms of the oxide structures as well as the thermal stabilities of metallic glasses. A very effective way to enhance the oxidation resistance while not affecting the glass forming ability (GFA) of the metallic glasses is by means of surface modification. Surface modification of Cu60Zr30Ti10 bulk metallic glass by plasma immersion ion implantation (PIII) was used in this study to understand the influence of the implanted surface on the protection against oxidation. Surface characterization of the implanted metallic glass revealed the formation of zirconium nitride in the implanted layer and a copper depleted surface layer. The results showed that not only the surface composition and structure of the nitrogen ions implanted metallic glasses were different from those of the as-cast one, but also the oxidation resistance was increased by a thin layer of zirconium nitride. The implanted metallic glass exhibits improvement on surface properties combined with the retention of the original properties of the bulk.
- Surfaces, Metallic glasses