Glass strengthening with an ultrafast laser, страница 4

Fig. 9 allows for making an important observation that despite scattering in data points there is the obvious of AFT being proportional to the laser-induced residual stress for all the studied materials. The slope of these functions and the magnitude of AFT increase differ from glass to glass. 0317 glass has the steepest slope then goes 0215 glass and 9664 glass and GC have roughly the same slope.

The maximal relative AFT increase that we measured did not tie to the two above groups. We saw the smallest increase of ~ 42% in 9664 glass and the biggest change of 65% in 9664 GS. 0215 and 0317 were in between.

Despite the noted differences between the studied glasses we believe that the key factor in explaining laser tempering is the macroscopic effect of induced stress. This is also proven by the decaying stress/AFT dependence on the separation between the laser-written lines. If the crack-arrest effect was microscopic in origin and individual defects could stop the crack as proposed in [1], then we should see the AFT increase as a function of pulse energy because damage spots become larger and the probability (cross-section) of inhibiting the crack propagation increases. On the other hand, there should be no or little dependence on the spacing between the laser-inscribed lines.

This argument is further supported by the observations made in Ref. [4]. They studied the toughening of glassbased composite materials with different sizes of particles dispersed in them. One of the mechanisms of crack arrest is thermal contraction mismatch resulting from the disparity of thermo-mechanical properties between the particles and the host. The language of [4] and of this study becomes similar if we make laserinduced defects equivalent to these particles. Moreover, the authors of [4] were using the same DT toughness test method as we did.

Conclusion

We tempered several glasses and glass-ceramics with the femtosecond laser and observed common response in compressive stress within the laser-treated area. In these areas, higher loading was required in the DoubleTorsion test to propagate the crack. From the analysis of apparent fracture toughness dependencies on the irradiation pattern and femtosecond-laser pulse energy, we conclude that laser-induced compressive stress is the cause of the increases apparent fracture toughness.

References

[1]  Hirao K,, Shimotsuma Y., Qiu J., and Miura K. (2005) Femtosecond laser induced phenomena in glasses and photonic device applications. Mater. Res. Soc. Symp. Proc. 850, 13-23. 

[2]  Streltsov A. and Borrelli N. (2002) Study of femtosecond-laser-written waveguides in glasses, JOSA B 19, 2496-2504. 

[3]  Oldenbourg. R.  (1996) A new view on polarization microscopy, Nature 381, 811-812. 

[4]  Nadeau J.S. and Dickson J.I. (1980) Effects on internal stresses due to a dispersed phase on the fracture toughness of glass, J. Amer. Cer. Soc. 63, 517523. 

[5]  Salem J.A., Radovic M., Lara-Curzio E., and Nelson G. (2006) Fracture toughness of thin plates by the Double-Torsion test method, Ceramic Engineering and Science Proceedings 27, 63-73. 

Meet the Author

Alexander Streltsov graduated from Moscow State University with a PhD degree in physics and mathematics. He conducted research in institutes and universities in Russia, the Netherlands, and in the United States. In recent years, he worked in the industrial R&D at Intel and at Corning. His research interests are primarily in laser processing of materials.