서강대학교

초록/요약

Understanding the mechanical behavior of functional materials including ceramics, functional glasses, piezoelectric ceramics and metallic alloys etc. has been crucial for ensuring the reliability and performance of the engineering products. This necessity continues enhancements in mechanical testing techniques for simple and reliable prediction of mechanical properties. This study aims at enhancing (i) nano-indentation, (ii) nano-scratch and (iii) double-torsion (DT) techniques for evaluating mechanical (elastic modulus, hardness, fracture toughness) and tribological (friction and wear) properties of bulk metallic glasses (BMGs) and glass materials, which are very difficult to characterize with conventional techniques or for which no standard methods exist. Here, in-depth experimental and numerical investigations are performed to understand both the mechanical and tribological behavior of materials. Application of nanoindentation techniques to BMGs is assessed through a serious of nano-indentation experiments and simulations with zirconium-based BMG (Zr-BMG, Zr65Cu15Al10Ni10). Both Berkovich and sphero-conical diamond tips are employed in this study. The influence of loading rate and peak and cyclic loads on the evaluated nanomechanical properties by Oliver-Pharr (OP) method are investigated to reveal the indentation-size effect (ISE). According to OP method, hardness H becomes peak load-independent, whereas elastic modulus E increases with indentation load. The same phenomenon is observed with sinus and progressive multicyclic (PMC) modes of nanoindentation. However, observation of significant pile-up (the ratio of elastic and total energy We / Wt = 0.33 < 0.5) questions the application of OP method to Zr-BMG; therefore, enhanced nanomechanical properties are obtained by replacing the projected contact area calculated by OP method with those measured using atomic force micro-scope (AFM). Further, we investigate the tribological behaviors of the same Zr-based BMG using nano-scratch techniques. Since, frictional and wear mechanisms of certain bulk metallic glass (BMG) are mostly different from another BMG, it is improper to use the same technique for diverse BMGs. After assessing nanomechanical properties through the enhanced nanoindentation techniques, the nanoscratch tests are performed under multiple constant and ramp normal loads. Variations of lateral forces and coefficient of friction (COF) with scratch distance are investigated. It is found that COF values irregularly increase with normal load due to the roughness from fracture and chipping, which play crucial roles on the frictional behaviors of Zr-BMG. Finally, a novel attempt is made to perform numerical simulations of nanoscratch tests by employing damage model of shear failures; accordingly, the tribological potential of Zr-BMG is demonstrated. The direct application of DT test for characterizing the fracture behavior of BMGs is rather difficult due to the lack of standardized DT test procedure even for brittle materials. Therefore, we attempt at enhancing the analytical solutions of DT test based on three-dimensional XFE analyses with soda-lime glass specimens. By combining bending deformations of DT specimen with torsional deformations, previous analytical solutions are improved to accurately represent the load-displacement (P-h) relationship and stress intensity factor (SIF). Weighting factors are proposed as functions of thickness, moment arm and crack length. DT experiments were conducted with soda-lime glass specimens to further validate the proposed weighting factors. Finally, correction terms are provided for SIF and fracturs toughness evaluation in DT specimen with straight and curved crack fronts. Fracturs toughness values of soda-lime glass specimens with various thickness are consistent, and in good agreement with literature values. Suggested enhancements can be utilized while standardizing the DT fracture test for fracture toughness evaluation in various functional brittle materials. The assessment on the appropriateness of DT fracture test for BMGs is left as future work of this study. Observed material behavior and proposed enhancements can be further utilized in various engineering applications, which require the characteristics of the new glassy materials to show which characteristics are better or worse than those of existing materials.