Network Glasses Under Pressure: Permanent Densification in Modifier-Free ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ Systems

${\mathrm{SiO}}_2$, ${\mathrm{P}}_2{\mathrm{O}}_5$, ${\mathrm{B}}_2{\mathrm{O}}_3$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$ are all well-known network formers in glasses, but the structure and properties of mixed ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses without the presence of network modifiers are poorly understood. The relatively low atomic packing density of these glasses should favor network densification when subjected to high local stress (e.g., indentation) at room temperature, and it is therefore interesting to examine their structural response to high-pressure treatment. In the present study, we investigate the pressure-induced changes in volume, structure, and mechanical properties (hardness and crack resistance) of five ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses with varying $\mathrm{Si}:\mathrm{P}$ ratio. The glasses are isostatically compressed at 1 GPa at the glass transition temperature, enabling permanent densification of large (approximately ${\mathrm{cm}}^2$) sample specimens. In the as-prepared glasses, boron atoms become partially converted from the threefold- to the fourfold-coordinated state when $[{\mathrm{P}}_2{\mathrm{O}}_5]>[{\mathrm{Al}}_2{\mathrm{O}}_3]$, with all ${\mathrm{Al}}_2{\mathrm{O}}_3$ maintained in tetrahedral groups. For $[{\mathrm{P}}_2{\mathrm{O}}_5]>([{\mathrm{Al}}_2{\mathrm{O}}_3]+[{\mathrm{B}}_2{\mathrm{O}}_3]$), boron is exclusively found in fourfold coordination, while the aluminum coordination number increases, and all aluminum atoms are preferentially associated with phosphorus as next-nearest-neighbor cations compared to silicon. Upon isostatic compression, the glasses permanently densify up to approximately 6%, leading to an increase in hardness and a change in the indentation cracking pattern. We discuss these pressure-induced changes in glass properties in relation to the structural changes quantified through Raman and ${}^{11}\mathrm{B}$, ${}^{27}\mathrm{Al}$, and ${}^{31}\mathrm{P}$ NMR spectroscopy.

Figure 1

Ternary diagram showing the relative ratios of ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{B}}_2{\mathrm{O}}_3$, and ${\mathrm{P}}_2{\mathrm{O}}_5$ (mole fractions) in the investigated ${\mathrm{Al}}_2{\mathrm{O}}_3{\text{−}\mathrm{B}}_2{\mathrm{O}}_3{\text{−}\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glass compositions (Table 1).

Figure 2

(a) ${}^{27}\mathrm{Al}$ MAS NMR spectra of as-prepared (solid lines) and compressed (dashed lines) ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses. (b) Total isotropic projections of ${}^{27}\mathrm{Al}$ 3QMAS NMR spectra of as-prepared (solid lines) and compressed (dashed lines) ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses.

Figure 3

${}^{11}\mathrm{B}$ MAS NMR spectra of as-prepared (solid lines) and compressed (dashed lines) ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses.

Figure 4

Fraction of tetrahedral boron ($N_4$) as a function of ${\mathrm{P}}_2{\mathrm{O}}_5$ content for as-prepared and compressed ${\mathrm{Al}}_2{\mathrm{O}}_3{\text{−}\mathrm{B}}_2{\mathrm{O}}_3{\text{−}\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses.

Figure 5

${}^{31}\mathrm{P}$ MAS NMR spectra of as-prepared (solid lines) and compressed (dashed lines) ${\mathrm{Al}}_2{\mathrm{O}}_3{\text{−}\mathrm{B}}_2{\mathrm{O}}_3{\text{−}\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses. Asterisks denote spinning sidebands, which are present only for the spectra at slower (10 kHz) sample spinning.

Figure 6

Raman spectra of the as-prepared (solid lines) and compressed (dashed lines) ${\mathrm{Al}}_2{\mathrm{O}}_3\text{−}{\mathrm{B}}_2{\mathrm{O}}_3\text{−}{\mathrm{P}}_2{\mathrm{O}}_5\text{−}{\mathrm{SiO}}_2$ glasses.

Figure 7

(a) Density, (b) molar volume and APF, and (c) plastic compressibility as a function of the molar concentration of ${\mathrm{P}}_2{\mathrm{O}}_5$ for both as-prepared and isostatically compressed glasses. The dashed lines are guides for the eye. The errors associated with the density values are smaller than the size of the symbols. Plastic compressibility is calculated from the slopes of the linear fits to density vs pressure for each composition.

Figure 8

Vickers hardness $H_V$ (at $P=0.2\mathrm{kgf}$) as a function of the molar concentration of ${\mathrm{P}}_2{\mathrm{O}}_5$ for both as-prepared and isostatically compressed glasses. Inset: Dependence of the slopes ($d{H}_V/dp$) on the plastic compressibility of the glasses. The dashed lines are guides for the eye.

Figure 9

(a) Load dependence of Vickers hardness $H_V$ for the $P0$ glass exhibiting the well-known ISE. (b) Bernhardt coefficient ($a_1$) as a function of the ${\mathrm{P}}_2{\mathrm{O}}_5$ concentration of glasses. $a_1$ is a measure of the extent of the indentation size effect and is calculated using Eq. (1). The dashed lines are guides for the eye.

Figure 10

Vickers indentations performed at various loads ($P$) of the as-prepared glasses.

Figure 11

Vickers indentations performed at various loads ($P$) of the compressed glasses.

Figure 12

Crack resistance (against medial or radial cracks) as a function of the molar concentration of ${\mathrm{P}}_2{\mathrm{O}}_5$ for both as-prepared and isostatically compressed glasses. There are no data points for the $P2$ and $P4$ as-prepared glasses, as these glasses do not exhibit any radial or median cracks (see Fig. 10). Inset: Crack resistance of the as-prepared glasses as a function of their plastic compressibility.