Crack-resistant Al2O3–SiO2 glasses

Obtaining “hard” and “crack-resistant” glasses have always been of great important in glass science and glass technology. However, in most commercial glasses both properties are not compatible. In this work, colorless and transparent xAl2O3–(100–x)SiO2 glasses (30 ≤ x ≤ 60) were fabricated by the aerodynamic levitation technique. The elastic moduli and Vickers hardness monotonically increased with an increase in the atomic packing density as the Al2O3 content increased. Although a higher atomic packing density generally enhances crack formation in conventional oxide glasses, the indentation cracking resistance increased by approximately seven times with an increase in atomic packing density in binary Al2O3–SiO2 glasses. In particular, the composition of 60Al2O3•40SiO2 glass, which is identical to that of mullite, has extraordinary high cracking resistance with high elastic moduli and Vickers hardness. The results indicate that there exist aluminosilicate compositions that can produce hard and damage-tolerant glasses.


Results
The Al 2 O 3 -SiO 2 binary glass system is important in geosciences, glass-ceramics, and is a base composition for fabricating multiple commercial glasses. It has been widely studied owing to its interesting structure and phase-separation phenomena. In particular, binary aluminosilicate glasses with more than 30 mol% of Al 2 O 3 are difficult to fabricate in the bulk form by conventional melting processes because of their low glass-forming ability and high melting temperatures. Al 2 O 3 -rich glasses have been prepared by splat quenching, flame spraying, roller quenching, sol-gel, and containerless processing [26][27][28][29][30][31][32][33] . Most techniques produce thin flakes or small particles, whereas containerless processing has the advantage of producing bulk glasses by solidification without heterogeneous nucleation at the surface of the melt. Recently, it has been reported that TiO 2 -, Nb 2 O 5 -, WO 3 -, and Al 2 O 3 -based glasses without any network formers have been successfully fabricated with this method [34][35][36][37][38][39] . Weber et al. obtained Al 2 O 3 -SiO 2 glasses with Al 2 O 3 content up to 67 mol% by aerodynamic levitation (ADL); however, the diameter of the glass samples was at most 1 mm 33 . In this work, we used the ADL technique for fabricating bulk Al 2 O 3 -SiO 2 glasses with high alumina content. Transparent and colorless xAl 2 O 3 -(100-x)SiO 2 glasses were obtained in the range of 30 ≤ x ≤ 60. At x ≥ 60, mullite and α -Al 2 O 3 directly crystallized from a melt. The properties of the glasses are summarized in Table 1. Figure 1A shows the composition dependence of density ρ and atomic packing density C g . Both ρ and C g increase linearly with Al 2 O 3 content and are in good agreement with the previous results obtained from roller-quenched and splat-quenched amorphous flakes 28,29 . The glasses become more packed and rigid as alumina content increases. Figure 1B shows that all the elastic moduli increase linearly with x. Young's modulus E increases from 102.9 GPa to 134.2 GPa, bulk modulus K increases from 65.6 to 99.0 GPa, shear modulus G increases from 41.5 GPa to 52.7 GPa, and Poisson's ratio ν increases from 0.239 to 0.274 with increasing alumina content. The values of ρ, C g and elastic moduli seem to extrapolate linearly to the pure SiO 2 composition. In Fig. 1C, the Vickers hardness H V increases monotonically with increasing Al 2 O 3 content from 7.23 to 8.07 GPa in agreement with the elastic moduli trends. The increase in elastic moduli and hardness of xAl 2 O 3 -(100-x)SiO 2 glasses with increasing x follows the expected relation between elastic moduli and hardness as well as the atomic packing density and dissociation energy per unit volume of the components. The increase in Al 2 O 3 content enhances the atomic packing density and total dissociation energy of the glass because of the large dissociation energy of Al 2 O 3 per unit volume (G Al2O3 = 131 kJ/cm 3 ) compared with SiO 2 (G SiO2 = 68 kJ/cm 3 ) 40 . As a result, the glass with the highest Al 2 O 3 content (x = 60) with mullite composition shows the highest elastic moduli and Vickers hardness. These values are comparable to hard glasses, such as R 2 O 3 -SiO 2 -Al 2 O 3 or CaO-SiO 2 -Al 2 O 3 glasses, and are much larger than those of crack-resistant silicate or borosilicate glasses [7][8][9][10][11][12][13][18][19][20][21][22][23][24][25] .
In order to rule out any surface effect in the glasses obtained by the levitation system we compared the hardness and indentation imprints of pure SiO 2 glass as prepared by the aerodynamic levitation system with a reference SiO 2 glass (Edmund optics). The values of indentation hardness H IT were 8.69 ± 0.08 GPa and 8.69 ± 0.07 GPa for the levitation SiO 2 glass and the reference SiO 2 glass respectively. Also, the indentation imprints displayed the same cracking patterns with cone cracks typical of anomalous glasses and similar cracking frequency. These results suggest that there is no apparent surface effect on the indentations properties of the glasses obtained through the levitation system. Figure 2 shows the Vickers imprints on xAl 2 O 3 -(100-x)SiO 2 glasses for various loads. It is noted that with increasing Al 2 O 3 content, the glasses become more resistant to radial cracking (observed at the imprint corners) although the atomic packing density increased. Normal (radial cracks) behavior is observed in all samples for x = 30, 40, 45, 50, and 55, whereas no cracks were observed in more than 50% of the indentation imprints for glasses with x = 60 even using an indentation load of 49.03 N. To quantify the resistance to fracturing, cracking probability curves are shown in Fig. 3. The data were fitted by using a sigmoid function. From the fitting curve, the loading force required to generate a 50% cracking probability or 2 radial cracks in average (cracking resistance CR) was estimated. As shown in the inset, CR increases drastically at x ≥ 50. The CR value of 55.4 N for the x = 60 glass is considerably larger than those of the crack-resistant glasses. For example, Asahi's LB glass had an indentation cracking resistance of approximately 30 N measured in nitrogen  Table 1. Density ρ, atomic packing density C g, sound velocities, elastic moduli, and indentation properties of xAl 2 O 3 -(100-x)SiO 2 glasses including pure SiO 2 glass. The density measurement error was less than 0.01 g/cm 3 . The standard deviation of the sound velocities, elastic moduli, Poisson's ratio ν and Vickers hardness H V were within ± 0.03 km/s, ± 1 GPa, ± 0.001 and ± 0.06 GPa, respectively. nd: not determined.
(N 2 ), a 80SiO 2 •10Al 2 O 3 •10CaO glass had CR of approximately 10 N under N 2 , an aluminoborosilicate commercial glass had CR of 11 N at 30% relative humidity, and a 80SiO 2 •15Na 2 O•5CaO glass had 10 N CR at 30% relative humidity 18,19,22,41 . It is well known that the indentation cracking resistance in silicate glasses strongly depends on the atmosphere during measurements and that humidity decreases CR 42,43 . Therefore, it is likely that the CR of the x = 60 glass will increase when measured in a nitrogen atmosphere. The large CR of the x = 60 glass is comparable to chemically strengthened soda lime glass 17 .

Discussion
Oxide glasses with large CR usually have enough free space to dissipate the mechanical stress owing to densification (compaction) and to lesser extent shear deformation. Therefore, large CR in oxide glasses should correlate to open structure (less-packed) 18  However, it is observed that CR increases with alumina content. The increase in the CR of binary aluminosilicate glasses is the opposite of what is expected. Apparently, other mechanism to prevent crack initiation should be taken in consideration. Shear deformation is an alternative mechanism in conventional oxide glasses. In oxide glasses, such deformation likely occurs in the vicinity of atoms that are weakly bonded owing to the existence of non-bridging oxygen, as observed in sodium silicate glasses 45 . In case of binary Al 2 O 3 -SiO 2 glasses, non-bridging oxygens were not confirmed by XPS measurements 46 . Thus, shear deformation owing to the movement of non-bridging oxygens is rather unlikely. Shear deformation will also occur in borate glasses where easy-slip units such as boroxol rings exist or where BO 3 -BO 4 species exchange occurs under applied stress 47 . Recently, similar shear deformation processes have been proposed to occur in densified silica glass 48,49 . As in the case of BO 3   based on oxygen diffusion in melts, it was suggested that AlO 5 units in Al 2 O 3 -SiO 2 glasses are trapped in the glass structure in a meta-stable state between the AlO 4 in liquids and AlO 6 in crystalline materials like mullite 33,51 . In this sense, it is probable that the multiple coordination environments of Al atoms as well as the mid-range structure around these units play a role on the enhancement of the cracking resistance through shear deformation processes. Although the overall mechanism is still not clear it is important to note that shear deformation processes are favored as the packing density and Poisson's ratio increases which is observed in the studied system as the alumina content increases. Accordingly, there is a possibility that plastic deformation may be aided not only by densification but also by shear deformation processes in Al 2 O 3 -SiO 2 glasses.

Conclusion
The elastic properties and indentation of xAl 2 O 3 -(100-x)SiO 2 glasses prepared by aerodynamic levitation were investigated. All glasses in the range 30 ≤ x ≤ 60 were colorless and transparent. The elastic moduli and hardness increased monotonically with alumina content from x = 30 to x = 60. The steady increase of the elastic moduli and hardness can be explained by the increasing atomic packing density and the high dissociation energy per unit volume of Al 2 O 3 compared to SiO 2 . Furthermore, it was found from the indentation imprints that the glass cracking resistance increases with increasing alumina. As a result, alumina-rich Al 2 O 3 -SiO 2 glasses are strong materials because of their high hardness and high indentation cracking resistance. In particular, the 60Al 2 O 3 •40SiO 2 glass displayed the highest indentation cracking resistance, elastic moduli, and hardness in the binary system. The increase in cracking resistance cannot be explained only by densification as it is widely accepted for conventional oxide glasses. Thus, it is proposed that the local structure of aluminum atoms as well as the structure around these units may play a role in the increased cracking resistance of alumina-rich Al 2 O 3 -SiO 2 glasses through shear deformation processes.

Methods
Glass synthesis. The glasses were fabricated using an aerodynamic levitation furnace described elsewhere 52 . Alumina

Indentation behavior.
In order to rule out any surface effect in the glasses obtained by the levitation system we compared the hardness and indentation imprints of pure SiO 2 glass as prepared by the aerodynamic levitation system with a reference SiO 2 glass (Edmund optics). Spherical SiO 2 samples obtained by aerodynamic levitation were mirror-polished into a disk shape with a thickness of 500-μm. The indentation experiments were made using a dynamic indenter (Shimadzu DUH-211) loaded with a diamond Berkovich 115° indenter in an atmosphere with 60% relative humidity. The loading and unloading rate was set to 70.067 mN/s using a dwell time of 15 s. The indentation hardness was calculated from the equation H IT = F max /A p , where F max is the applied load and A p = h max − 0.75(h max − h r ) is the projected contact area, where h max is the maximum penetration at F max , and h r is the point of intersection between the tangent of the linear section of the unloading curve and the indentation depth axis. Ten indentations were performed for each SiO 2 glass sample. The resulting imprints were observed by optical microscopy. For the Al 2 O 3 -SiO 2 glasses indentation experiments were performed using a Vickers hardness tester at 23 °C and 60% relative humidity. Mirror-polished glasses with approximately 500-μm thickness were used. A Shimadzu DUH HMV-1 Vickers tester and an Akashi AVK-C2 Vickers tester were used for indentation loads below 19.6 N and over 19.6 N, respectively. The dwell time was 15 s. Vickers hardness H V was calculated from the diagonal length of the imprints at a load of 4.903 N. To evaluate the cracking resistance CR to radial cracks, the number of corners (four) divided the number of cracks for each indentation and the results were averaged by the number of indentation tests. The averaged value is the cracking probability at specific load. Cracking probability curves were obtained by plotting the cracking probability as a function of the loading force.
Scientific RepoRts | 6:23620 | DOI: 10.1038/srep23620 The indentation cracking resistance (CR) is defined as the load required for generating two radial/median cracks on average or associated with 50% cracking probability 55 . Three specimens were used for each composition. At least 20 indentation imprints were used to calculate the H V and CR at each load.