Thickness Dependent Structural Evolution In Mg-Zn-Ca Thin Film Metallic Glasses

Mg-Zn-Ca bulk metallic glasses(BMGs)have recently received considerable attention,because they exhibit the Young's modulus of about 20-30 GPa,shear modulus of about 9-14 GPa,almost comparable with the moduli of human bones and a density of about 1.6-2.4 g/cm3,which have potential applications as bio-materials.Mg-Zn-Ca TFMGs can be coated on magnesium-based implants to improve the degradability,e.g.,during the healing period,subcutaneous gas pockets can be hindered.they also exhibit some superior properties,potentially useful in applications of micro-electro-mechanical system(MEMS)devices.As a member of the metallic glass family,no studies on the thickness dependent behavior of Mg-Zn-Ca TFMGs have been reported.Although bulk metallic glasses are generally brittle,when decreasing their size into nanoscale,the deformation mode can change from localized shear band mode to homogeneous flow.In this work,we prepared Mg-Zn-Ca TFMGs with various film thicknesses(40 mn-1600 nm)by magnetron sputtering deposition technique using an alloy target.The micrometer-scale structure,density,morphology,and roughness of Mg-Zn-Ca TFMGs with different film thicknesses were systematically investigated.By combining x-ray absorption fine structure(XAFS)technique with AIMD simulations,the film thickness dependent atomic packing in Mg-Zn-Ca TFMGs was also investigated.(1)In this paper,the surface morphology,cross-section morphology and roughness of a series of Mg-based amorphous alloy films were characterized by atomic force probe microscopy(AFM)and scanning electron microscopy(SEM).When the thickness of the film is deposited from 40 nm to 1600 nm,the roughness increases from about 1 nm to 2.5 nm in the small area and from 2 nm to 4 nm in the large region.The surface of the film was characterized by SEM,the mean particle size of the surface increased from 20 nm to 30 nm when the film thickness deposited from 40 nm to 400 nm.And then it remains almost constant above 400um films.In addition,the cross-section morphology of the as-deposited films were also examined by SEM,which exhibits a characteristic of columnar structure.the sizes of columnar structure for films are similar to those for the particle sizes.(2)To further understand the structure of Mg-Zn-Ca TFMGs,the x ray reflection(XRR),extended x-ray absorption fine structure(EXAFS)and ab initio molecular dynamics(Ab initio MD)method were invested.Studied by XRR found Mg based TFMG also has a turning point at 400 nm thickness,the coordination number around Zn atoms decreases with increasing film thickness,while the average distances of Zn-Zn and Zn-Mg bonds reduce slightly first and then are almost constant when the film thickness is over 400 nm.The differences in thinner films(<400 nm)could be ascribed to the different growth mode at the beginning of deposition compared to the thick films that is under an equilibrium deposition.The results confirm that when the film thickness is above 400 nm,the atomic structure in Mg-based TFMGs have no obvious difference,while below 400 nm the thinner films exhibit the denser atomic packing.Therefore,the thickness dependent mechanical properties of studied Mg-based TFMGs should be resulted from both the extrinsic size effect and the intrinsic atomic structure.