Densification Deformation Research Articles

Sink-in/pile-up formation and crack nucleation mechanisms of high purity fused silica and soda-lime silica glass during nanoindentation experiments

To improve the fabrication qualities and service performance of optical glass, the mechanical response mechanisms of two typical optical glass materials, high purity fused silica (HPFS) and soda-lime silica glass (SLSG), are investigated by nanoindentation experiments under loads below 500 mN. The mechanical properties of glass are reflections of its molecular structure. The molecular structure of HPFS is an amorphous SiO2 3D crosslink network structure with free volume and nonbridging oxygens (NBOs). For SLSG, due to the metallic ions packed into the free volume of the SiO2 network and the directionless property of the ionic bonds, its elastic modulus, Poisson's ratio and densification deformation resistance are higher than those of HPFS. The hardness and shear flow resistance of SLSG are lower than those of HPFS. An analytical model of the strain field after the nanoindentation unloading process was developed considering the permanent densification deformation. The mechanisms of the sink-in and pile-up formations and crack nucleation were explored using the analytical model of the strain field. The borderline cracks nucleate at the sinking region of the HPFS during the unloading process due to the free volume stacked at the surface. The mechanisms of radial crack nucleation in SLSG are the tensile stress concentration at the elastic-plastic interface near the surface induced by residual stress.

Effect of scratch direction on densification and crack initiation of optical glass BK7

It is known from nano-indentation studies that optical glass material undergoes a certain densification deformation during ductile scratching. In this study, nano-scratch experiments were conducted under different scratch directions (face-forward and edge-forward of the same indenter) and normal loads to explore the differences in densification of material in ductile removal mode. Furthermore, inherent differences in densification deformation were utilized to explore the effect of scratch direction on the crack resistance (CR) or ductile-to-brittle transition (DBT) of material under brittle removal mode. Based on quantitative evaluation of densification deformation induced by indentation and scratching, a high-temperature annealing procedure was employed to determine the changes of the scratch groove geometry and volume before and after the release of the scratch-induced densification deformation under different scratch directions (edge-forward and face-forward). The geometric parameters of scratch grooves under edge-forward and face-forward directions showed similar changes after annealing. It is found that in ductile removal mode, the densification ratio to the total scratching volume in edge-forward scratching is greater than that in face-forward scratching. It is likely that the greater densification deformation of edge-forward scratching led to stronger crack resistance and smaller tensile residual stress. Therefore, lateral cracks are more likely to initiate and propagate under face-forward scratching under brittle removal mode, which is consistent with the experimental results.

Compressive properties of porous metal fiber sintered sheet produced by solid-state sintering process

A novel porous metal fiber sintered sheet (PMFSS) with three-dimensional reticulated structure was fabricated by using solid-state sintering method of copper fibers. Uniaxial compressive test was carried out to investigate the effects of porosity and manufacturing parameters on the compressive properties of PMFSS. During the compressive process, it was found that the PMFSS initially exhibited short-term elastic deformation, and then quickly entered into the compact densification deformation stage. The stress–strain plots showed no obvious yield stage in the whole uniaxial compressive process. Under given stress, the PMFSS with higher porosity exhibited higher strain, hence implying lower effective stiffness. Additionally, our results showed that higher sintering temperature or longer sintering time would soften the PMFSS.