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Effect of B 2 O 3 content on crack resistance was investigated by indentation tests of glass samples with various compositions of B 2 O 3 . The ternary SiO 2 B 2 O 3 Na 2 O glass system (SBN series) and non-alkaline aluminoborosilicate glass system (SAB series). When B 2 O 3 is substituted with SiO 2 in the SBN system ("SBN1" series), crack resistance has a relationship with density. In a series of the SBN system where density did not change with B 2 O 3 content ("SBN2" series), crack resistance decreased with increasing B 2 O 3 content. On the other hand, crack resistance increased with increasing B 2 O 3 content in the SAB series, where density did not change. According to the results of NMR measurement, boron in 4-cordination state ([4] B) increased in the SBN2 series while boron in 3-cordination state ([3] B) increased in the SAB series with increasing B 2 O 3 content. Therefore, crack resistance increases with increasing [3] B and decreases with increasing[4] B. The difference in structure between [3] B and [4] B containing glasses leads to different effect on residual stress around the indentation, resulting in difference in crack resistance.

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