Glass strength/strengthening and flaws, indentation hardness and cracking (site sponsor, C.R.'Chuck' Kurkjian)

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Stresses in indented and scratched glasses
link to the Power Point presentation at GOMD 2008: /Documents/gomd bf short final modified3.pdf
 
On this page we will describe our work on the measurement of stresses induced in glasses due to indentation and scratching or by the application of an externally applied load.  
 
As-drawn, flat silica and soda-lime-silica glass fibers, i.e. 'perfect surfaces' (~300 microns) were indented with a diamond cone (Vickers-like, ~138 degrees) at 50 grams load.  The retardation/azimuth images were obtained with an   Abrio system (http://cri-inc.com/) in transmission by R. Oldenbourg at the Woods Hole Marine biology Laboratory in Woods Hole, MA.
Figure 1.  Operation of the Abrio system
 
Figure 2.  50 gram un-cracked cone indent in glass
 
 
Figure 3.  Retardation and azimuth in an uncracked soda-lime-silica glass perpendicular to the load direction (50 grams).
Figure 4.  Retardation and azimuth in an un-cracked silica glass perpendicular to the load direction (50 grams).
 
Figure 5.  Retardation in silica glass parallel to the load direction.
Figure 6.  Azimuth in silica glass parallel to the load direction.
 

 

From these images, it can be seen that there are substantial differences in the magnitude and distribution of the stress birefringence due to indentation in these two glasses ( as expected, see Arora et al. JNCS 31 415-428, 1979). This is also to be expected  based on the portions of  flow due to shear and compaction in the two glasses (see Yoshida et al, J. Mater. Res, 20 (12) 3404-3412, 2005), as well as earlier work of Peters, Hagan and others.  Detailed  calculations of the 3D stresses are in progress and will be presented soon.

 

It is clear that an understanding of such indentation processes is critical to the development of more robust glasses as well to the development  of appropriate scoring cracks  for the 'thermal cutting' of substrate glasses used for liquid crystal displays.   Such quantitative work has not been done to date.  The availability of the CRI Abrio system as well as the analyses of Prof. H. Aben and A. Errapart at the Institute of Cybernetics at Tallinn University of Technology in Estonia (see 'links') have made this work possible.

 

SUMMARY AND NEW IMAGES(08 02 09), COURTESY OF

 R. OLDENBOURG, WOODS HOLE MA.

 

Below are images (first perpendicular and then parallel to the indentation direction) of two glasses recently imaged.  As with the above soda lime and silica  glasses, 50 gram loads were applied to a cone indenter.  It can  be seen that substantial cracking occurred (compare with the 'uncracked' images above).  Both of these glasses had been found to be 'less brittle' when indented with a Vickers diamond (see Ito et al., ito.pdf).  This illustrates the complexity of the indentation problem, as well as the need to study it in great detail.  This is necessary in order to understand it and to improve the 'robustness' of such oxide glasses (see also Yoshida, PACRIM8-24-yoshida.pdf).. 

glass #1 perpendicular

 

glass#1 parallel

 

glass#2 perpendicular

 

 

glass#2 parallel