Glass strengthening with an ultrafast laser, страница 3

Both values are dropping significantly when the pitch increases from 6 µm to 30 µm. The biggest drop in both values occurs when the pitch changes from 6 µm to 9 µm. Assuming that the stress produced by a single track is the same, the cumulative stress decreases because there are fewer laser-written lines in the same area. The further these lines are separated the smaller the produced effect is. At 30 µm spacing, the effect vanishes and the fracture toughness is reduced to the one of the untreated benchmark samples.

0317 soda aluminosilicate glass

The laser-tempering effect in the 0317 glass has the maximum like in the soda-lime glass followed by a drop and then the increase again (Fig. 6).

Fig. 6. Residual stress and AFT as a function of laser pulse energy in 0317 glass.

The magnitude of strengthening effect is ~ 46% and the strength maximum is observed at pulse energies around 1 µJ. The growth in strength and stress at 3 µJ is not statistically proven because of the variability in the material and the errors in measurements.

9964 Spinel glass

In Spinel glass, we were able to induce much higher residual stress of 15 MPa compared with 0215 and 0317 glasses (Fig. 7)

Fig. 7. Residual stress and AFT as a function of laser pulse energy in 9964 glass.

The maximum AFT increase achieved was 1,532 MPa*m^1/2, which is about 42%. Unlike 0215 and 0317 glasses, we did not observe the saturation of stress and strength with laser pulse energy.

9964 Spinel GC

In Spinel GC, there is no tendency for saturation as in the 9964 glass. The experiments were carried out with pulse energies up to 6 µJ when we started observing the significant structural damage in the material (Fig. 8). The increase in strength is more than 65%.

Fig. 8. Residual stress and AFT as a function of laser pulse energy in 9964 GC.

Discussion

The materials we used in this work fall in two groups according to their initial AFT (see Table 2).

Table 2. AFTs of the studied materials before and after laser treatment.

0215 and 0317 glasses have approximately the same initial AFT of ~ 0.77 MPa*m^1/2 while 9664 materials are “stronger” with initial AFT in the 1.08 – 1.13 MPa*m^1/2 range. 

Fig. 9. Normalized AFT vs. stress

The response of these materials to laser irradiation differs between the two groups. In 0215 and 0317 glasses, the residual stress peaked at 0.8 – 1 µJ while in 9664 glass and GC we did not observe stress saturation. It may be explained by significant difference in laser damage threshold in these materials. At pulse energies above ~ 1.5 µJ we saw noticeable damage in 0215 and 0317 glasses, which probably was relieving the stress.

The maximal residual stresses depended on the above group breakdown. 0215 and, especially, 0317 glasses have noticeably smaller stress than 9664 glass and GC, which may be caused by their high CTE.