Pathways to improve the sliding wear behavior of metallic glasses: A case study of Cu- and Fe- based bulk metallic glasses

Mechanical properties of metallic glasses are quite distinct from their crystalline counterparts, which makes them attractive for several applications. Since metallic glasses exhibit high strength, hardness, and large elastic strain limits, they are expected to show superior wear resistance. The low viscosity in the super-cooled liquid region is exploited for the fabrication of metallic glasses in various shapes, for example, as high precision gears. Enhancements in the wear properties of metallic glasses are considered in this dissertation by partial devitrification and suitable micro alloying. Partial devitrification following controlled heat treatment of bulk metallic glass (BMG) provides an opportunity for microstructure control and thereby for optimizing the wear properties. There are studies showing that a small suitable alloying addition of few at. % (micro alloying) to the BMG results in an increase of the glass forming ability and also the mechanical properties, but studies on the effect of minor-alloying on wear characteristics are scarce. The focus of this dissertation is directed towards designing an optimal microstructure for enhanced wear resistance of Cu50Hf41.5Al8.5 BMG and Fe48Cr15Mo14C15B6Er2 BMG. The effect of yttrium micro alloying in the Cu-based BMG (Cu50Hf41.5−xAl8.5Y x, x =0, 2, 5, 8, 10, 15 at. %) and the erbium micro alloying in the Fe-based BMG (Fe50–xCr15Mo 14C15B6Erx, x = 1, 2 at. %) on wear properties has been studied. The role of toughness and hardness on the sliding wear properties of these metallic glasses has been examined. A critical load for crack nucleation and propagation has been computed and compared with the applied load during wear. An emphasis is also placed on the occurrence of dynamic effects such as sliding-induced crystallization and their role on the wear behavior of these metallic glasses. ^ In summary, for Cu50Hf41.5Al8.5 BMG, annealing at 510 °C for 300 min rendered a structure consisting of an amorphous matrix with uniform distribution of nano-crystalline particles with a size of 20-30 nm, revealed the best wear resistance. Sliding-induced crystallization was observed in the as-cast and structurally relaxed Cu50Hf 41.5Al8.5 BMG, which resulted in an improvement of wear behavior. Up to 5 at. %° of Y additions in Cu50Hf 41.5–xAl8.5Yx resulted in improvement of wear resistance and further additions of Y, deteriorated it. In the Fe-based BMG, addition of Er up to 2 at. % resulted in the enhancement of wear resistance and the Fe48Cr15Mo14C15B 6Er2 BMG sample, annealed for 10 min at 580 °C exhibited the best wear resistance. There was no evidence of sliding wear induced crystallization but a shear band formation was observed for the Fe-based BMG, indicating a good thermal stability of the alloy. The results demonstrate that suitable micro-alloying is an useful approach for improving the wear resistance of bulk metallic glasses and that the combination of micro-alloying and annealing can substantially improve the wear resistance.