Experimental and Numerical Investigation on the Effect of Scratch Direction on Material Removal and Friction Characteristic in BK7 Scratching

Abstract

In order to study the influence of scratch direction on the deformation characteristics and material removal mechanism of optical glass BK7, nanoscratching experiments were conducted on a Nano indenter using Vickers indenter. Results indicate that the face-forward scratch is more likely to induce the initiation and propagation of lateral cracks, which is found to be more beneficial to material removal processes; in contrast, small chips and debris are released from the machined grooves without introducing lateral cracks in the edge-forward condition, leading to poor material removal efficiency. In addition, the choice of scratch direction can make differences to the elastic recovery rate of optical glass BK7. The results revealed that both the elastic recovery rate and the residual stresses of the material under the face-forward scratching are greater than those of the edge-forward scratching. A theoretical model for coefficient of friction (COF) under different scratch directions was established. It is found that the COF between indenter and workpiece in the edge-forward scratching is larger than the face-forward scratching under otherwise identical conditions, this finding is consistent with experimental results. A stress field analysis using finite element method (FEM) was conducted to understand the different crack initiation and propagation behaviors from different scratch directions. The current study discusses the significance of scratch direction on material removal behavior of optical glass BK7, and the results would encourage further research on investigating the connections between tool geometry and material removal mechanism.

Surface damage resistance and yielding of chemically strengthened silicate glasses: From normal indentation to scratch loading

“…This enables the evaluation of subtle changes in the lateral force response during the rate‐controlled scratching experiment. Furthermore, it avoids issues associated with the orientation of the indenter tip relative to the scratching direction 107,108 . Examples of ramp‐load scratch tests on the commercial soda‐lime silicate glasses in the as‐received state and after chemical strengthening, respectively, are presented in Figure 4, together with images of the resulting scratch morphologies.…”

“…e ., behind the sliding indenter tip 27,38,110,112 . However, theoretical considerations suggest that micro‐cracks may also be generated during loading 108,113,114 . This discrepancy underlines the need for combined in situ and post‐scratch monitoring of scratch‐induced surface fracture 111 .…”

“…Furthermore, it avoids issues associated with the orientation of the indenter tip relative to the scratching direction. 107,108 Examples of ramp-load scratch tests on the commercial sodalime silicate glasses in the as-received state and after chemical strengthening, respectively, are presented in Figure 4, together with images of the resulting scratch morphologies. Postmortem optical inspection of the residual scratch groove on the as-received soda-lime silicate glass (SLS, Figure 4A) reveals notable variations in the scratching behavior with increasing load level.…”

“…27,38,110,112 However, theoretical considerations suggest that micro-cracks may also be generated during loading. 108,113,114 This discrepancy underlines the need for combined in situ and post-scratch monitoring of scratch-induced surface fracture. 111 In the present case, we identify microcracking only from ex situ analysis of the scratch morphology (using optical imaging and post-scratch scanning, e.g., Figure 4 and Figure 5).…”

“…[15][16][17][18][19]21 However, in the more sensitive experiment of blunt scratching, the reduced crack resistance is very well resolved. Finite element modeling of the stress field during lateral loading 108,112,113 may provide further insight on this effect.…”