A) Ring-Cone
2. Time is needed to develop a critical nucleus
2.7 Compositional Dependence of Crack Initiation Behavior
2.7.2 Crack Initiation Resistance and Glass Composition
Wada et al.41 studied the crack initiation resistance of several series of silicate glasses by conventional Vickers indentation testing. The load at which there was 50% median-radial cracking (an average of two corners exhibiting median-radial cracks) around an indentation after testing, was used as a measure of the crack resistance of each glass. Their data are shown in Table IV, along with the normalized glass compositions. All the glasses studied by Wada et al.41 behaved ‘normal,’ since according to the researchers no glasses exhibited ring-cone fracture. As shown in Figure 3, the glasses with higher normalized-network-former ratio in general show higher load at
Table IV. Cracking Data of Wada et al.*
Researcher and Glass
Glass
Type RO2 R2O3 RO R2O R2O5
Density (g/cm3)
Load At 50% Crack
Formation (g) Wada et al.
BSIL N 7.46 2.57 0.00 1.00 2.30 1000
ALBSIL 1 N 7.73 1.37 0.20 0.80 2.38 800
ALBSIL 2 N 5.13 0.72 0.36 0.64 2.46 240
ALBSIL 3 N 2.01 0.56 1.00 0.00 2.59 110
ALSIL 1 N 2.50 0.43 1.00 0.00 2.61 150
ALSIL 2 N 2.79 0.05 0.46 0.54 2.48 60
LDSIL 1 N 3.26 0.03 0.45 0.55 3.05 63
LDSIL 2 N 2.76 0.09 0.49 0.51 3.21 60
LDSIL 3 N 2.58 0.09 0.57 0.43 3.20 32
LDSIL 4 N 1.97 0.00 0.83 0.17 4.31 28
ALSIL + P N 6.21 1.40 0.14 0.86 0.08 2.45 800
*BSIL= borosilicate, ALBSIL=aluminoborosilicate, ALSIL = aluminosilicate, LDSIL=lead silicate, ALSIL+P = phosphorous aluminosilicate.
50% crack formation, i.e., increased resistance to crack initiation. Figure 4 shows that as glass density increases, the tendency is for the load at 50%
fracture to decrease. A large gradient in the initiation load centered at a density of ~ 2.5 g/cm3is apparent. From Table IV it is seen that glasses with the lowest densities tend to have the highest NNF amounts. This can be understood by considering that, as network-modifying ion concentration decreases relative to network-former concentration, there should be more open space in the glass structure, leading to lower density.
The crack initiation test results of Wagner9 are shown in Figure 5. The normalized initiation load for the first median-radial cracks to initiate on unloading for the three glass series tested is shown as a function of the NNF ratio. In general, the glasses containing the highest normalized amount of network-forming oxide in each series, i.e., the ‘anomalous’ glasses, had greater resistance to median-radial crack initiation on unloading (lower normalized loads correspond to increased resistance to initiation). Wagner found that v-SiO2 initiated median-radial cracks at ~ 14% of Fmax on unloading, which is near the lowest of the soda-lime-silicate glasses, although some median-radials were also seen on loading.
Closer inspection of Wagner’s data revealed several more interesting trends regarding the crack initiation load and glass composition. For the soda-lime-silicate glasses, increases in the percent NBO content correlated with decreasing resistance to crack initiation, as shown in Figure 6. The
Figure 3. Indentation load for 50% crack formation as a function of the normalized-network-former ratio from the data of Wada et al.41
Figure 4. Indentation load for 50% crack formation as a function of the glass density from the data of Wada et al.41
100 1000
2 4 6 8 10
IndentationLoadat 50%CrackFormation(g)
(RO2 + R
2O
3 + R
2O
5) 20
400
50 200
100 1000
2 2.5 3 3.5 4 4.5
IndentationLoadat 50%CrackFormation(g)
Density (g/cm3) 20
50 200 400
Figure 5. The normalized initiation load for the first median-radial cracks to initiate on unloading as a function of the NNF ratio for the three series of glasses tested by Wagner9. ‘A’ indicates the ‘Anomalous’ compositions.
Maximum indentation load was 1400 g. Curves are guides for the eye. Data from Wagner9.
Figure 6. The normalized initiation load for the first median-radial cracks to initiate on unloading as a function of the percent NBO content of the soda- lime-silicate glass series tested by Wagner9. Data from Wagner9.
0 20 40 60 80 100
0 4 8 12 16 20 24 28 32
v-SiO
2
(F init/F max)*100
Percent NBO (%)
A A
A
0 20 40 60 80 100
2 4 6 8 10 12 14 16 18
SLS ALS BS L
(F init/F max)*100
(RO2+R
2O
3)
A A
A A A A
normalized initiation load increased rapidly accompanying the transition from ‘anomalous’ to ‘normal’ as the percent NBO increased and then leveled off after ~ 20% NBO. Interestingly, the initiation resistance of v-SiO2 (0%
NBO) was similar to the glass containing ~ 15% NBO. This possibly suggests that crack initiation load in soda-lime-silicate glasses is independent of NBO content below a certain threshold.
For the sodium-borosilicate glasses the normalized initiation load for median-radial cracks increased rapidly, then leveled off as the (Na/B) molar ratio increased, as shown in Figure 7. This is presumably also related to the concentration of NBO’s in the structure. As the (Na/B) molar ratio increases above a threshold, NBO’s are created, resulting in ‘normal’ behavior and decreased resistance to median-radial crack initiation on unloading.
Another interesting trend arises when the normalized initiation load is plotted as a function of the (Na/Al) molar ratio for the sodium-aluminosilicate glasses of Wagner, as shown in Figure 8. As the (Na/Al) molar ratio increases above 1, NBO’s are introduced into the structure, and the normalized initiation load increases, i.e., decreased resistance to crack initiation. When the (Na/Al) molar ratio equals unity, no NBO’s should be in the structure, and the normalized initiation load is a minimum. For (Na/Al) < 1 the Al3+ion most likely acts as a network modifier with the creation of NBO’s.
The above results suggest that oxide glasses with low NBO content are better able to resist median-radial crack initiation than similar glasses with
Figure 7. The normalized initiation load for the first median-radial cracks to initiate on unloading as a function of the (Na/B) molar ratio for the sodium-borosilicate glasses of Wagner9. Data from Wagner9.
Figure 8. The normalized initiation load for the first median-radial cracks to initiate on unloading as a function of the (Na/Al) molar ratio for the sodium-aluminosilicate glasses of Wagner9. Data from Wagner9.
20 40 60 80 100
0 0.5 1 1.5 2 2.5 3 3.5 (F init/F max)*100
(Na/B) Molar Ratio
A A
10 20 30 40 50 60
0.5 1 1.5 2 2.5 3 3.5 4 4.5 (F init/F max)*100
(Na/Al) Molar Ratio
A A
high NBO content. However, such glasses become more susceptible to ring- cone crack initiation. Vitreous B2O3 may be one exception. Although this glass should in theory contain no NBO’s, it behaves ‘normal.’7 This could be due to the relatively weak van der Waals bonding between sheets of BO3
triangles, which facilitates shear flow, and likely reduces surface elastic tensile stresses such that ring-cone crack formation is suppressed.
Cook and Pharr11 found that median-radial cracks initiated on loading in v-SiO2 and borosilicate glass at 938 g and 1630 g, respectively. Wagner9 found that lateral cracks for all three series of glasses he tested initiated at less than 5% of Fmax. Of the three major crack systems, ring-cone, lateral, and median-radial, the last system is the most detrimental to glass strength, and hence assumes more importance in indentation testing.