Discovery of Ultra-Crack-Resistant Oxide Glasses ... - ACS Publications

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Discovery of Ultra-Crack-Resistant Oxide Glasses with Adaptive Networks Kacper Januchta,† Randall E. Youngman,‡ Ashutosh Goel,§ Mathieu Bauchy,∥ Stephan L. Logunov,‡ Sylwester J. Rzoska,⊥ Michal Bockowski,⊥ Lars R. Jensen,# and Morten M. Smedskjaer*,† †

Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark Science and Technology Division, Corning Incorporated, Corning, New York 14831, United States § Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States ∥ Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095, United States ⊥ Institute of High-Pressure Physics, Polish Academy of Sciences, 01-142 Warsaw, Poland # Department of Mechanical and Manufacturing Engineering, Aalborg University, 9220 Aalborg, Denmark ‡

S Supporting Information *

ABSTRACT: Despite their transformative role in our society, oxide glasses remain brittle. Although extrinsic postprocessing techniques can partially mitigate this drawback, they come with undesirable side effects. Alternatively, topological engineering offers an attractive option to enhance the intrinsic strength and damage resistance of glass. On the basis of this approach, we report here the discovery of a novel melt-quenched lithium aluminoborate glass featuring the highest crack resistance ever reported for a bulk oxide glass. Relying on combined mechanical and structural characterizations, we demonstrate that this unusual damage resistance originates from a significant self-adaptivity of the local atomic topology under stress, which, based on a selection of various oxide glasses, is shown to control crack resistance. This renders the lithium aluminoborate glass a promising candidate for engineering applications, such as ultrathin, yet ultrastrong, protective screens.

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INTRODUCTION The resistance of materials to fracture is one of the most crucial characteristics defining the ranges of materials applicability in modern industries and infrastructure. There is thus a need to develop new materials that exhibit a combination of high strength, low density, and high fracture toughness (i.e., the ability of a material with a pre-existing crack to withstand a given load without fracture). In oxide glasses, surface flaws can lead to catastrophic failures, as tensile stress concentrates at the tips of the flaws.1,2 As these amorphous solids do not have a stable shearing mechanism, this seriously limits the scope of their applications. Increasing the inherent damage or crack resistance of oxide glasses is thus of the utmost importance in order to minimize the number of cracks and their propensity to propagate from flaws created during handling. To this end, various extrinsic post-treatment methods have been developed, such as chemical strengthening3 that leads to formation of a compressive stress layer at the surface, confining any propagating cracks. However, postprocessing is expensive and typically comes with undesirable side effects.4,5 There is, therefore, an increasing interest in enhancing the intrinsic mechanical properties of glasses by compositional design.6 To evaluate the damage resistance of glasses, instrumented indentation is the method of choice, since sharp contact is the © 2017 American Chemical Society

primary failure mode of glasses for many applications. Indentation thus mimics real-life damage incidents under controlled conditions.7 Upon indentation, stress induces some local structural rearrangements, which, in turn, can affect glasses’ properties. For example, Lee et al.8 showed through inelastic X-ray scattering that, when subjected to pressure at room temperature, vitreous boric oxide features an increase in the average coordination number of boron atoms, with irreversible changes starting at 4−7 GPa. This magnitude of stresses is typically reached in indentation experiments; for instance, the measure of hardnesswhich characterizes the resistance to elastoplastic deformationstypically involves the application of stress values of 1−10 GPa for oxide glasses.9 Therefore, understanding the response of glassy solids to pressure is of paramount importance for developing new damage-resistant glass compositions. However, the complexity of the stress field generated by the indenter tip has largely limited our understanding of glasses’ behavior under load,10 since structural characterizations methods typically require homogeneous samples, that is, a homogeneous pressure in the Received: March 6, 2017 Revised: June 21, 2017 Published: June 21, 2017 5865

DOI: 10.1021/acs.chemmater.7b00921 Chem. Mater. 2017, 29, 5865−5876

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Chemistry of Materials

water is known to break network bonds through stress corrosion.34 An even higher CR value has recently been reported for a modifier-free Al2O3−SiO2 glass35 (CR = 55 N and ∼11% crack probability at 19.6 N), but due to its extremely low glass-forming ability and high liquidus temperature, it cannot be melt-quenched and has to be prepared using special techniques such as aerodynamic levitation, thus limiting sample size and eliminating the vast majority of industrial applications. The advantage of the present aluminoborate composition lies in its good glass-forming ability and glass stability (Figures S1 and S2 in the Supporting Information), enabling easy manufacturing of bulk glasses that can be shaped and processed into any size. Other previously reported oxide glass compositions with high intrinsic crack resistance are listed in Table S1 in the Supporting Information. The correlation between the chemical composition and fracture-related properties remains poorly understood; e.g., there is a poor correlation between CR and various material properties as illustrated in Figure S3 in the Supporting Information. In this paper, based on indentation methods, Brillouin spectroscopy, micro-Raman spectroscopy, and solid state nuclear magnetic resonance (NMR) of the asprepared and hot compressed glasses, we reveal the structural origin of the high CR value of the lithium aluminoborate presented herein. We demonstrate that the crack resistance is controlled by the ability of the glass to adapt its local atomic structure and densify under pressure.

present case. However, reaching homogeneous pressures above 5−10 GPa (e.g., in a diamond anvil cell) invokes a significant size limitation on the samples. Alternatively, the effect of pressure on structure and mechanical properties can be studied at higher temperature. Indeed, it has been shown that structural changes observed under high pressure (e.g., change of coordination numbers) can occur at significantly lower pressures if compression is performed at around the glass transition temperature (Tg). For instance, boroaluminosilicate11 and soda lime borate glasses12 have been shown to present an increase of the coordination of boron atoms under pressures as low as 0.1 GPa. Hence, hot-compression offers an attractive method to understand the effect of stress on the atomic structure of samples with volumes (∼cm3) that are suitable for, e.g., indentation experiments, while limiting the applied pressure to around 1−2 GPa. In order to understand the origin of high inherent damage resistance in oxide glasses, we select a lithium aluminoborate glass (24 mol % Li2O−21 mol % Al2O3−55 mol % B2O3) for this study. The design of this composition is based on the principles of topological engineering.13 According to the viewpoint of topological constraint theory, atoms in network glasses are constrained by their chemical bonds and bond angles, and the strength of these constraints depends on the local topology and the chemical nature of the elements.14−17 By counting the number of constraints around both networkforming and network-modifying atoms as a function of both composition, temperature, and pressure, it is possible to make quantitative connections among composition, structure, and mechanical properties.18−20 On the basis of previous studies on the structure and topology of alkali aluminoborate glasses,21−24 we expect the structure of this glass to (1) be free of nonbridging oxygens, which tend to facilitate isochoric shear flow, resulting in subsurface shear faulting damage,25,26 and (2) predominantly contain trigonal boron units (BIII), which tend to promote densification.27,28 These characteristics would, in turn, imply low level of residual stress induced by indentation, which is widely considered as the driving force for cracking in oxide glasses.10 In order to enable the melt-quenching of the glass and thus the preparation of bulk samples, a network modifier such as an alkali oxide is necessary, as it lowers the working temperature of the melt, but at the same time weakens the network structure due to the ionic nature of the alkali− oxygen bonds.29 The selection of Li2O over other alkali oxides assures the strongest possible weak bonds, thus securing a relatively high resistance to deformation.30 As expected, the lithium aluminoborate glass studied here displays (1) a remarkably high crack resistance (CR)in excess of 30 N and (2) an unusually low crack probability of only ∼5% under 19.6 N load when measured under ambient conditions. Here, crack probability is defined as the amount of observed cracks emanating from the corners of a Vickers indent divided by the highest possible amount of cracks, i.e., four per indent. CR corresponds to the load at which 50% crack probability is recorded.31,32 To the authors’ knowledge, the studied glass exhibits the lowest crack initiation probability ever recorded for any meltquenched (i.e., bulk) oxide glass at this high load. Although the “less brittle glass” by Seghal and Ito33 has been reported to display a slightly more favorable cracking behavior (CR = 34 N and ∼0% crack probability at 19.6 N), these values were recorded under a dry N2 atmosphere. The lack of atmospheric water significantly enhances crack-related properties,32 since

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EXPERIMENTAL SECTION

Sample Preparation. The glass was synthesized using the traditional melt-quenching technique. Li2CO3, Al2O3, and H3BO3 powders were mixed according to the target composition, melted in a Pt-Rh crucible at 1400 °C, and quenched by pouring the hot melt onto a steel plate. The glass was then transferred into a preheated annealing furnace at an estimated Tg based on data in the SciGlass database. The chemical composition was measured using flame emission and inductively coupled plasma spectroscopy and found to be 24 mol % Li2O−21 mol % Al2O3−55 mol % B2O3. To check for traces of bonded water in the glass due to the decomposition of H3BO3, we also acquired an attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectrum using a Spectrum One spectrometer (PerkinElmer) on a Ge crystal under ambient conditions with subsequent background subtraction. The spectrum shows no signs of OH groups or molecular water (Figure S4 in the Supporting Information). Characteristic temperatures of glass transition (Tg), crystallization (Tc), and melting (Tm) were determined from differential scanning calorimetry measurement (DSC 449C, Netzsch) for a glass specimen with a known thermal history (i.e., with a preceding cooling rate of 10 K/min). The intercept between the tangent to the inflection point and the extrapolated isobaric heat capacity was interpreted as the onset of each transition. The DSC output was calibrated against a sapphire specimen of similar dimensions and mass as the glass sample following the same temperature program. Following determination of Tg, the bulk glass was then reannealed for 1 h at the Tg value and cooled down to room temperature at a cooling rate of approximately 3 K/min. A powdered glass sample was used for X-ray diffraction measurement (Empyrean XRD, PANalytical), showing no signs of crystallization (Figure S1 in the Supporting Information). The sample homogeneity was confirmed by recording 54 Raman spectra (inVIA Raman, Renishaw) from the as-prepared glass separated by a distance of 50 μm over a 400 × 250 μm2 area (laser excitation area is ∼5 μm2). No significant changes in the spectra as a function of surface location were found (Figure S5 in the Supporting Information). Four glass specimens were subjected to an isostatic N2-mediated pressure treatment at 0.5, 1.0, 1.5, and 2.0 GPa, respectively. The 5866


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pulse widths (1 μs), delays of 10 s, and 32 acquisitions. MAS NMR spectra from these four nuclei were processed without any additional apodization, plotted using the normal shielding convention and with shift referencing to aqueous boric acid (19.6 ppm), aqueous aluminum nitrate (0 ppm), and aqueous LiCl (0 ppm). 11B and 27Al MAS NMR data were fit with DMFit utilizing second-order quadrupolar lineshapes for 11B and the CzSimple model for 27Al.41 In the case of 11B MAS NMR data, the overlapping satellite transition for the BIV resonance was also fit and subtracted from the integration, yielding accurate site intensities for all BIII and BIV peaks. 11 B and 27Al triple quantum MAS (3QMAS) NMR experiments were conducted using the hypercomplex shifted-echo pulse sequence.42 For 27Al 3QMAS NMR, RF pulses were calibrated to provide optimized signal-to-noise ratios, resulting in hard pulse widths of 2.8 and 1.1 μs, and a soft z-filter reading pulse of 15 μs. 120 acquisitions were collected for each of 256 t1 points, with an isotropic sweep width of 100 kHz. These pulse sequence parameters were similarly optimized to 3.2, 1.2, and 20 μs, respectively, for 11B. For 11B 3QMAS NMR experiments, 24 scans at each of 128 t1 points (80 kHz sweep width) were utilized. All 3QMAS NMR data were processed using commercial software and with minimal line broadening in both dimensions. Raman spectra for the as-prepared glass and compressed glasses were acquired using a micro-Raman spectrometer (inVIA Raman, Renishaw). The sample surface was excited by a 532 nm green HeNe laser for an acquisition time of 10 s. The range of the spectrum was from 242 to 1928 cm−1 and the resolution was better than 2 cm−1. Spectra from five different locations on the glass were accumulated for each specimen to ensure homogeneity. Furthermore, for the asprepared glass, the micro-Raman spectrometer was utilized in the mapping mode to acquire spectra at different positions around a Vickers indent produced at 19.6 N. The distance between the centers of the laser spot for two subsequent spectra was 5 μm, while the halfdiagonal of the indentation was approximately 45 μm. The spectra were collected at increasing distances from the center of the indent, until no further spectral difference compared to the spectrum of the bulk as-prepared glass could be detected. Five spectra were accumulated in each position to minimize the signal-to-noise ratio. All spectra were uniformly treated in a MATLAB script for background correction and area normalization.

compression was carried out by maintaining the given value of pressure at Tg for 30 min, and subsequent quenching with an initial cooling rate of 60 K/min. The pressure chamber was then decompressed at a 30 MPa/min rate. This method is described in more detail in ref 36. Property Characterization. Density values of the glass specimens were determined using Archimedes’ principle of buoyancy. The weight of each specimen (at least 0.30 g) was measured in air and ethanol 10 times. Brillouin spectra were measured with a 6 pass tandem interferometer from Table Stable Ltd. The excitation source used was single mode with a line width ∼ 10 MHz 100 mW Spectra Physics Excelsior laser. Both backscattering and 90° scattering geometry were used to obtain longitudinal and transverse frequency. Measurement of longitudinal acoustic frequency in two geometries helped to improve the accuracy of the test. The range of the interferometer was set to be ±45 GHz. The spectra collected for 3 samples with different pressures are shown in Figure S6b in the Supporting Information for backscattering and 90° geometry. Refractive indices of the samples were measured at different wavelengths in the visible range using the Pulfrich refractometer method.37 Elastic moduli and Poisson’s ratio were calculated using the refractive indices, density, as well as the measured longitudinal and transverse velocities (see Figure S6 in the Supporting Information). For both the as-prepared specimen and all compressed specimens, microindentation (Duramin 5, Struers) measurements were performed. Prior to indentation, specimens were grinded and polished in water using SiC adhesive discs with increasing grit size. The final steps of polishing were carried out in a water-free diamond suspension on a polishing cloth in order to prevent surface hydration. Thirty symmetrical indents were produced at 8 different loads (245 mN to 19.6 N) with a loading time of 10 s. The indents were evaluated after each indentation in terms of optical microscopy. The indent diagonal length as well as the number and the length of the radial/median cracks emanating from the indent corners were recorded. Following the method described in ref 38, the extents of indentation-induced densification and shear flow were quantified for the as-prepared sample. The topography images of 10 indents produced at 245 mN were acquired using atomic force microscopy (Ntegra, NT-MDT) before and after a thermal annealing at 403 °C (i.e., 0.9Tg in K) of 2 h duration. Silicon tip cantilevers (NSG10, NTMDT) were used in semicontact mode with a scanning frequency of 0.5 Hz to create 20 × 20 μm2 images with a resolution of 256 × 256 pixels. The acquired images were then treated using WSxM software39 and a custom-written MATLAB script to quantify the volume recovery ratio. Nanoindentation measurements (Nano Indenter XP, MTS) were performed for the as-prepared specimen and for the specimen compressed at 1.0 GPa. A Berkovich geometry tip was applied with increasing load monitored continuously along with the displacement into the surface of the glass. The target displacement was 2000 nm in each case. Hardness and reduced Young’s modulus were determined from the top part of the unloading curve according to Oliver−Pharr methodology40 (see Figure S7 in the Supporting Information). Structural Characterization. For three of the glass specimens (as-prepared, 1.0 GPa, and 2.0 GPa), a commercial spectrometer (VNMRs, Agilent) and magic angle spinning (MAS) NMR probes (Agilent) were used to acquire MAS and multiple-quantum MAS NMR spectra at an external magnetic field of 16.4 T. Glass samples were powdered with an agate mortar and pestle, and then packed into 3.2 mm zirconia rotors, with sample spinning of 22 kHz for 11B, 7Li, and 27Al, and 15 kHz for 6Li NMR measurements. 11B and 27Al MAS NMR data were collected at resonance frequencies of 224.5 and 182.3 MHz, respectively, using short rf pulses of 0.6 μs (π/12 tip angles), with 4 s recycle delay and signal averaging of 1000 acquisitions for 11B MAS NMR, and a pulse delay of 2 s and collection of typically 600 scans for 27Al MAS NMR. 6Li NMR spectra, at a resonance frequency of 103.0 MHz, were acquired using pulse lengths of 1.5 μs (π/6 tip angle), a recycle delay of 600 s, and 100 acquisitions. 7Li MAS NMR data, resonance frequency of 272.0 MHz, were also collected using π/6

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RESULTS Density. Archimedes’ principle allows determination of density of the studied glass, which is found to be 2.241 g/cm3. Furthermore, the knowledge of the glass’ chemical composition, and thus its molar mass, enables the calculation of the molar volume (Vm), which, in the case of the as-prepared glass, is found to be 29.52 cm3/mol. Finally, on the basis of the ionic Shannon radii of atoms43 and the assumption of spherical atoms, the lowest theoretical volume occupied by the atoms can be calculated. The ratio between this volume and Vm is defined as the atomic packing density (Cg). Cg for the as-prepared glass is estimated to be 0.55 by assuming 6-fold coordination for Li, 2-fold coordination for O, while the coordination numbers for B and Al are taken from the NMR results (see details below). Low-Cg glasses with significant voids in their structure such as glassy silica are prone to densification during indentation, as they can be compacted by mechanical loading.10,38 On the other hand, denser high-Cg glasses such as soda-lime silicates tend to primarily deform through an isochoric shear-flow mechanism, as the interstices in the glassy network are occupied by modifier cations hindering densification.38 The relation between the calculated Cg value and the indentation deformation mechanism for the studied glass will be discussed below. We also observe that, when the studied lithium aluminoborate glass is exposed to N2 gas at pressure in the GPa 5867


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Figure 1. (a) Pressure dependence of density (ρ) and Vickers hardness (HV). (b) Pressure dependence of elastic moduli (Young’s (E), shear (G), and bulk (B)) and Poisson’s ratio (ν).

Figure 2. (a) Optical images of indents produced at 19.6 N on the surface of the as-prepared (top), the 1 GPa (bottom left), and the 2 GPa (bottom right) glasses, respectively. (b) Crack probability as a function of applied indentation load for the as-prepared and compressed lithium aluminoborate glasses, as well as the former record bulk oxide glass27 for comparison. Inset: pressure dependence of crack resistance (CR).

range at its Tg, the glass structure responds by compacting its network, which manifests itself as a permanent density increase (Figure 1a), in agreement with previous studies on other oxide glasses.44,45 As a consequence, Vm decreases and Cg increases with increasing pressure (Table S2 in the Supporting Information). Indentation Hardness. Vickers hardness (HV) exhibits the same pressure dependence as density (Figure 1a), which is also in agreement with earlier findings.44,45 Both the relative increase in HV and in density are the highest values ever reported for hot-compressed oxide glasses45 (70% and 12% upon compression under 2 GPa, respectively). Upon densification, the packing density increases and hence the number of atomic bonds per unit volume also increases. In addition, the coordination numbers of the network-forming nuclei increase upon compression (see results and details below), which results in more bonds per atom. The increases in bond density and network connectivity are thus responsible for the pressure-induced increase in hardness.45 To support the trend recorded for HV, nanoindentation measurements yielding load−displacement information have been performed for the as-prepared glass and the glass compressed at 1 GPa. The compressed glass requires a higher load in order for the indentation probe to penetrate the glass surface to the same depth, indicating a higher resistance to deformation compared with the as-prepared glass (see Figure

S7 in the Supporting Information). Hence, a higher nanohardness value is recorded for the compressed glass than for the asprepared glass (7.6 and 5.3 GPa, respectively). Brillouin Scattering. Next, we use Brillouin light scattering to determine the elastic moduli and Poisson’s ratio (ν) for the as-prepared and hot compressed glasses. We generally observe a pressure-induced increase in all elastic moduli consistent with previous findings,44,45 but a non-monotonic change in ν (Figure 1b). The measured refractive indices and the Brillouin spectra used to calculate the elastic constants can be found in Figure S6a,b, respectively, in the Supporting Information. The observed trend in Young’s modulus (E) using Brillouin spectroscopy (Figure 1b) agrees with that in the reduced modulus (Er) found from nanoindentation measurements (see Figure S7 in the Supporting Information). We note that the elastic moduli also exhibit record high changes as a result of compression (Figure 1b), which is in agreement with the high extent of permanent volume densification exhibited by the studied glass composition in comparison to other oxide glasses.44,45 Although the elastic moduli are thus easily tuned by compression (e.g., 46% increase in E upon compression at 2 GPa), hardness is even more sensitive to the pressure treatment as noted above. This results in a decrease in the elastoplastic ratio E/H with increasing pressure (not shown), corresponding to an increasing elastic recovery upon unloading of the indenter apparent from the load−displacement curves (Figure S7 in the 5868


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Figure 3. (a) Schematic representation of the approach suggested by Yoshida et al.38 to determine the indentation deformation mechanism. (b) Topography of an indent produced at 0.25 N in the as-prepared glass shown as cross sections before and after annealing at 0.9Tg for 2 h. The respective 3D images are also shown.

Figure 4. (a) Micro-Raman spectrum of the as-prepared lithium aluminoborate glass, with indications of the main bands and assignment to selected molecular structures (from left to right: AlO4, triborate, AlO4-BO3, and BO3). (b) Micro-Raman spectra recorded for the as-prepared lithium aluminoborate glass and for the specimens hot compressed at 0.5, 1.0, 1.5, and 2.0 GPa. Inset: ratio between the relative intensities recorded at 1000 cm−1 (Band IIIb) and 900 cm−1 (Band IIIa). (c) Micro-Raman spectra for the as-prepared glass recorded at increasing distances from the center of an indent produced at 19.6 N. Inset: top view of an indent produced at 19.6 N with marked positions of laser focus. (d) Pressure (dashed line) and distance to center of the indent (solid line) dependences of the relative area fractions of the four main Raman bands.

Indentation Cracking. The knowledge of E/H and ν allows for the prediction of the characteristic cracking behavior of the studied glass. That is, Sellappan et al.47 successfully assigned the cracking patterns of different glass compositions to the regions of the three-dimensional stress field around a Vickers indent expected to drive ring, median, radial, or lateral cracks, based on the linear elasticity theory calculations of

Supporting Information).10,46 We also note the more pronounced pressure-induced increases in Young’s and shear (G) moduli occurring from 0 to 1 GPa compared to that from 1 to 2 GPa, while the opposite is found for bulk modulus (B) (Figure 1b). This manifests itself as the abrupt change in ν upon 2 GPa compression. 5869


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Chemistry of Materials Vickers stress fields by Boussinesq48 and Yoffe.49 The relatively high ν of 0.28 for the as-prepared glass (Figure 1b) suggests that radial cracks should be predominant. The few cracks recorded in this glass at 19.6 N are indeed emanating from the corners of the indent (Figure 2a). When the glass is subjected to hot compression at 1 GPa, the residual stress driving indentation cracking is higher than in the as-prepared glass, resulting in a larger extent of cracking and a higher level of birefringence (Figure 2a). Hence, the material cannot dissipate mechanical energy by densification to the same extent,50 leading to lower CR. However, the cracking pattern is not changed as ν is only slightly lowered. Upon hot compression at 2 GPa, ν increases to 0.32, resulting in lateral cracks that can lead to chipping off of the material close to the surface of glass, resulting in a characteristic bright pattern between two radial cracks as illustrated in Figure 2a. In addition to studying the cracking patterns, we also probe the extent of cracking by indentation through determination of CR. The CR of the present glasses is determined by counting the number of radial/median cracks at systematically increasing indentation load, which is challenging due to the remarkably resilient character of this composition. At the highest applicable load of the utilized indenter (19.6 N), only ∼5% crack probability is recorded, which prevents an accurate determination of CR. A sigmoidal function is then fitted to the data points in order to extrapolate the data, as shown in refs 50 and 51, resulting in an estimated CR value of ∼31 N (Figure 2b). The uncertainty in the CR value makes the comparison with other glass compositions challenging, but as shown in Figure 2b, the measured crack probability of the present glass at 19.6 N (∼5%) is significantly smaller than that of the previous record bulk oxide glass27 (∼46%). We find that the high CR of the present glass strongly decreases with increasing pressure applied during quenching (inset of Figure 2b). Crack probability rapidly increases with increasing extent of predensification, which has been found to be due to the reduced capability of the hot-compressed glass to densify.50 This is also evident from the birefringence, originating from residual stress, visible in the indent images produced at 19.6 N in the two glasses compressed at 1 and 2 GPa (Figure 2a). The large extent of permanent densification (12% upon compression at 2 GPa) suggests that the as-prepared glass exhibits a significant capacity for accommodating applied mechanical energy during indentation, which would result in a low level of residual stress driving crack initiation. This is, in turn, correlated to its ability to undergo structural transformations under an applied pressure or load, as discussed in detail below. Atomic Force Microscopy. To understand the role of densification during indentation on the high CR of the present glass, we determine the indentation deformation mechanism in the following. The extent of indentation-induced densification is quantified from the ratio between the densification and the total indentation volumes, and is defined as the volume recovery ratio (VR). This value can be determined following the method schematically presented in Figure 3a.38 By annealing the indented glass at 0.9Tg, the temperature is sufficiently high to activate local structural rearrangements and thus recover the indentation-induced densification of the glassy network,52 while the viscosity is too high for any significant viscous flow during the time scale of the annealing (2 h).53,54 Hence, this annealing approach combined with atomic force microscopy (AFM) measurements of the indent topography enables decoupling and quantification of the two indentation deformation

mechanisms. VR for the as-prepared glass is found to be 0.69; i.e., 69% of the permanently displaced volume during indentation has been densified within the surrounding plastic zone, while the remaining 31% of the volume has been subjected to volume-conservative shear flow. Raman Scattering. The acquired Raman spectrum for the as-prepared lithium aluminoborate glass is shown in Figure 4a. The spectrum exhibits many features characteristic of alkali borate glasses,55−57 but additional bands from other structural units are also expected. We divide the spectrum into four main bands enumerated I through IV, with the expected assignments outlined in the following. Band I (∼280 to 625 cm−1) is expected to contain contributions originating from B−O−B, Al−O−Al, and B−O−Al stretching in BIV and AlIV units.58,59 In addition, vibrations due to superstructural units such as pentaborates may occur in this region.56 Band II (∼625 to 815 cm−1) is characteristic for B2O3-rich glasses, since peaks in this frequency range are typically assigned to borate superstructures such as chain and ring metaborates, di-triborates, and penta-, tetra-, or triborates, as well as boroxol rings.55−57,60 However, we do not expect metaborate structures as these contain NBOs. The presence of triborates (∼770 cm−1) and ditriborates (∼755 cm−1) is more plausible, considering the high intensity in this range of wavenumbers and the fact that they consist of both BIII and BIV units. Furthermore, the high Al2O3 content of the present glass should result in similar ring structures containing both BIII and AlIV units, since high abundance of structures containing only boron units would imply separation of B2O3-rich and Al2O3-rich regions in the glass. Such occurrence of aluminoborate structures could explain the broadening of band II toward lower frequencies, although we note that this assignment has not previously been reported. The presence of boroxol rings (∼807 cm−1) in the glass is likely, but presumably only in a small quantity considering the low intensity at this frequency. Deconvolution of band II into modes originating from different structural units would provide additional insight into the intermediate range order of the studied glass. However, the large number of different units mentioned above (with additional interference due to mixing of Al and B) would make the deconvolution too uncertain. Band III is expected to result from vibrations of AlIV units (∼900 cm−1), borate superstructures (∼930 cm−1), and the AlIV-BIII network (∼1000 cm−1) as observed in lanthanum aluminoborates.56,58,59 Finally, band IV is expected to be dominated by signal contributions from vibrations of BIII units.57 The expected band assignment is also summarized in Table S3 in the Supporting Information. Micro-Raman spectra have been acquired for the hotcompressed samples (Figure 4b) to investigate the influence of pressure on the glass network structure. Using micro-Raman mapping of the indented as-prepared glass, we also acquire spectra with increasing distance from the center of a Vickers indent impression produced at 19.6 N following the method applied in refs 61 and 62 (Figure 4c). We observe similar changes in the structure as a function of increasing pressure and increasing proximity to the center of the indent, i.e., where the stress is assumed to be the largest. That is, the structural changes occurring at room and elevated temperature appear to be qualitatively similar (Figure 4d and Figure S8 in the Supporting Information). In detail, the area fractions of bands I and II are suppressed with increasing pressure/stress, indicating that the abundance of superstructural borate (and possibly aluminoborate) units is reduced. The suppression of these 5870


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Figure 5. (a) 11B MAS NMR spectra for the as-prepared glass along with spectral deconvolution (top) and for the glasses compressed at 1 and 2 GPa (bottom). (b) 27Al MAS NMR spectra for the as-prepared glass along with spectral deconvolution (top) and for the compressed glasses (bottom).

Figure 6. 11B 3QMAS NMR spectrum of the as-prepared glass. δCS, CQ, η, and ssb are the isotropic chemical shift, quadrupolar coupling constant, asymmetry parameter, and spinning sidebands, respectively.

bands is accompanied by an increase in the area of the band IV as a result of both compression and indentation and in that of band III in case of compression (Figure 4d). Surprisingly, indentation has no significant effect on the area of band III, indicating dissimilar compression- and indentation-induced changes. This suggests that the two routes of applying pressure do differ when considering certain structural units. However, we also observe that the shape of band III is changing similarly in both cases (Figure 4b,c). That is, upon increased pressure or stress, a sub-band situated around 1000 cm−1 appears (denoted IIIb), which may correspond to the AlIV-BIII network according to ref 59, indicating a more random mixing of the constituents upon hot compression. The relative intensity ratio between band IIIb and the highest intensity of band III around 900 cm−1 (band IIIa) is increasing as a function of pressure and of proximity to the center of the indent (inset of Figure 4b). That is, even though the Raman analysis of band III indicates some differences between the pressure- and indentation-induced structural changes, the structural units giving rise to the shifts (i.e., bands IIIa and IIIb) exhibit similar changes. In conclusion, besides some minor differences in the Raman spectra due to indentation and compression, we observe mostly similar structural changes to the network of the lithium

aluminoborate glass induced by indentation and by compression at Tg. Nuclear Magnetic Resonance Spectroscopy. We perform MAS NMR spectroscopy measurements on the three cationic nuclei. 11B and 27Al MAS NMR spectra are given in Figure 5a,b, respectively, along with the deconvoluted signals originating from the different chemical environments of the cations. The speciation of boron and aluminum is given in Table S4 in the Supporting Information. The network of the asprepared glass consists primarily of BIII and AlIV, although higher coordinated species are present in minor quantity. In sodium aluminoborate glasses, there is a preference for the modifier cations to charge-balance aluminum tetrahedra rather than boron tetrahedral.21−24 Hence, for the lithium aluminoborate glass, there should be a limited number of residual lithium cations available for converting BIII to BIV, as these cations are already associated with AlIV units. The obtained NMR results are thus consistent with the current structural models for alkali aluminoborate glasses. The local chemical environment of the network-former atoms is also highly sensitive to permanent densification. Both the 11B and 27Al MAS NMR data show that the average coordination numbers of both network-forming cations increase significantly with increasing pressure (Figure 5a,b). 5871


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Figure 7. (a) Correlation between crack resistance (CR) and volume recovery ratio (VR) for different oxide glass systems,24,27,47,50,51,73−75 including the lithium aluminoborate glass studied herein. Note that, in ref 47, the minimum load for generation of four cracks rather than two cracks is reported. (b) Correlation between CR and the atomic self-adaptivity under pressure ((Δ⟨n⟩Δρ)/(ρ0P)) upon hot compression at 1 GPa for different oxide glass systems,24,28,64,80 and the lithium aluminoborate glass studied herein. Note that the Na-aluminosilicate glasses appear to deviate from this correlation, presumably since the changes in coordination numbers are very subtle; i.e., densification is more efficiently achieved through other structural rearrangements. The dashed line is a guide for the eye. Inset: correlation between CR and the relative change in density (Δρ/(ρ0P)) upon hot compression at 1 GPa for the same glasses. 27

Both boron and aluminum have been reported to increase their coordination number as a result of permanent densification,28,63,64 but the extent of the changes in network-former speciation are the largest ever reported for any oxide glass compressed under similar conditions.45 This dramatic change in the local atomic structure upon densification agrees with the large permanent density increase following compression. In other words, the pressure dependences of the macroscopic volume and the local atomic structure are parallel. The increase in coordination numbers of boron and aluminum is accompanied by a slight increase in 6Li MAS NMR shift (Figure S9 in the Supporting Information), which we interpret as a shortening of the Li−O bond.65−67 The partial negative charge on the oxygen atoms surrounding boron increases along with the pressure-induced B III to B IV conversion, which is expected to increase their chargecompensating demand. Hence, the interactions between oxygen and lithium ions in the network become strengthened with higher applied pressure, leading to a shrinkage of the modifying site and in agreement with the evolution of the chemical environment of 6Li nuclei. The pressure-induced increase in 6Li MAS NMR shift may also originate from a reassociation of Li cations from AlIV to BIV sites, as Al enters a higher coordination state when the glass is compressed. This is discussed in more detail in Figure S9 in the Supporting Information. A more detailed analysis of the 11B MAS NMR spectra shows that the distribution of the trigonal boron sites is subject to a pressure-induced modification. In the as-prepared glass, the majority of the trigonal boron signal is assigned to ring sites (BIIIring), typically found in borate networks.68,69 The remaining signal is assigned to nonring sites (BIIInonring),70,71 as shown in Figure 5a. Both BIII sites are associated with bridging oxygens only, which is in agreement with the expectation for this composition.21−24 This assignment, based on the quadrupolar parameters used in the MAS NMR deconvolution, is supported by analysis of the 11B 3QMAS NMR data (Figure 6), where isotropic projections were fit to two overlapping Gaussians to determine isotropic shift of the two BIII peaks, and then MAS NMR slices were taken at these positions and analyzed with DMFit. The remaining 3QMAS NMR spectra of both 11B and

Al nuclei are provided in Figure S10 in the Supporting Information. Upon densification, the relative fraction of BIV increases at the expense of BIII. The spectral deconvolution shows that the extent of the decrease of [BIII] is most pronounced for the BIIInonring sites, even though they are the least abundant BIII units in the as-prepared glass (Figure 5a). The pressure dependence of the abundance of all three boron sites as well as the ratio between the two BIII sites is provided in Table S4 in the Supporting Information. We observe that the [BIIIring]/[BIIInonring] ratio increases from 3.4:1 for the asprepared glass to 5.7:1 for the glass hot-compressed at 2 GPa. We thus infer that trigonal boron sites, which are not associated with ring structures of the borate network, are more prone to undergo a change in coordination number compared to the BIIIring sites, as illustrated in Figure S11 in the Supporting Information.

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DISCUSSION The high crack resistance of the lithium aluminoborate glass is discussed by considering the studied glass as a mechanical energy-absorbing system. Upon application of an external force, such as sharp contact loading, the glass attempts to accommodate the energy supplied by the indenter primarily through localized densification beneath the contact surface. This implies that glasses with lower Cg values should have higher capacity for densification (i.e., displaying higher VR values), and thus be more crack-resistant. Indeed, an approximate negative correlation between VR and Cg has been reported, but the data are scattered and the majority of available data points are found within a very narrow range of Cg values (0.45−0.60).10,51 In turn, there is also an approximate positive correlation between CR and VR for oxide glasses, as illustrated in Figure 7a. However, the studied lithium aluminoborate glass does not follow this trend. Although a substantial fraction of the indentation volume is found to be recovered during the sub-Tg annealing treatment with VR = 0.69 (Figure 3b), the value is not high enough to explain the glass’ record high CR value. Compared to the similar sodium aluminoborate glasses reported recently24 (VR ∼ 0.80 for a glass with the same content of B2O3 and Al2O3), the present glass is 5872


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Chemistry of Materials deforming through shear flow to a greater extent. This is an interesting observation since a lower VR value is normally expected to result in a lower CR considering that there is a higher driving force (i.e., more residual stress) for indentation cracking.47,72 Nevertheless, the present lithium-containing glass has superior crack resistance as compared to the corresponding sodium-modified glass (CR of 31 and 19 N, respectively). Hence, our study shows that VR cannot be used as a universal parameter for predicting the composition dependence of CR. In order to understand the high CR of the lithium aluminoborate glass from a structural point of view, we have probed the short and intermediate range order changes induced by both indentation and hot compression through microRaman spectroscopy. The structural changes occurring during compression at room temperature and around Tg are not necessarily equivalent. For example, Mackenzie76 claimed that the process of compression at ambient temperature fundamentally differs from the process of compression at Tg. Furthermore, room temperature densification at moderate pressure (typically < 10 GPa) is reversible,52,77 whereas the densification obtained through pressure treatment at Tg is permanent.11,78 On the other hand, a recent comparative study has shown that similar structural changes occur in cold and hot compressed borate glasses.79 In this work, we have found that the structural rearrangements induced by the two densification routes are qualitatively similar. Furthermore, the small differences in band IIIb of the Raman spectra (Figure 4d) are interpreted as a change in the degree of mixing of the aluminoborate network,59 which is not related to changes in the coordination numbers of Al and B. This allows us to use the NMR results to understand the short-range structural changes induced by indentation, although it is impossible with this technique to directly probe the chemical environment of the indentation-induced densified volume. NMR has the advantage of providing quantitative information regarding the densification dependence of the short-range order of the glassy network, and is therefore used as a qualitative surrogate for correlating the localized indentation-induced structure and property changes in the following. Energy dissipating processes that can occur through different structural rearrangements include an increase in the coordination number of the network-forming species in the glass, as shown in this study. The present glass exhibits high CR and a high degree of pressure-induced change in coordination number of the network-forming cations (B and Al). We suggest that higher CR values are correlated with the ability of the atomic network to densify by self-adapting under stress, through some rearrangements in the short-range order structure. Densification mechanisms under load typically comprise: (1) decrease in bond angles, (2) increase in coordination numbers, and (3) decrease in bond length.45 Here, we define the potential for such topological self-adaptivity of a given atomic network as (⟨Δn⟩Δρ)/(ρ0P), where ⟨Δn⟩ is the change in coordination number as determined by NMR, Δρ is the change in density, ρ0 is the initial density, and P is the applied pressure. As such, the ability to self-adapt results from the balance between the energy cost associated with each densification mechanism (inversely correlated to the extent of pressure-induced change in coordination number ⟨Δn⟩/P), and its efficiency in increasing the packing density (proportional to the accompanying change in density Δρ/ρ0). Considering the previously studied oxide glasses,24,28,64,80 we find a positive correlation between CR and the atomic self-adaptivity (Figure

7b). For instance, increasing the coordination number of Si atoms requires a significant activation energy.45 Hence, silicate glasses typically densify under load through a decrease of the inter-tetrahedral Si−O−Si angle81 and/or change in ring size distribution.82 This results in a low self-adaptivity in silicate glasses, leading to lower CR values. In contrast, the energy associated with the transformation of BIII to BIV is significantly lower;28 i.e., glasses with high BIII content tend to exhibit high CR.27 Similarly, Al atoms can feature various coordination environments (from 4- to 6-fold), which also suggests that an increase in their coordination number comes with a low energy cost. The relation between the energy cost for structural rearrangement, densification, and crack resistance is illustrated in Figure S12 in the Supporting Information. Altogether, the ease of B and Al atoms to increase their local packing density with a low energy cost explains the high propensity for densification of the present aluminoborate glass, which, in turn, enables its unique resistance to cracking. The ability of the amorphous network to adapt its local bonding environment of the network-forming species may also originate from the reassociation of Li cations from AlIV to BIV sites when the network is subjected to pressure, as discussed in Figure S9 in the Supporting Information. We have thus shown that the high crack resistance of the present 24Li2O−21Al2O3−55B2O3 glass originates from the finding that the increase in network-forming cation coordination number constitutes the predominant structural rearrangement mechanism used by the network to dissipate elastic energy. This highlights that the driving force for indentation cracking depends not only on the extent of densification (commonly quantified through VR27,47) but also on the structural mechanisms facilitating the densification. In other words, future search for damage resilient glass compositions should focus on those, which are prone to self-adapt by changing their chemical bonding environment when subjected to an external force.

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CONCLUSIONS

In this work, we have reported the discovery of a meltquenchable lithium aluminoborate glass with a record high resistance to indentation cracking, as the measured crack probability at a load of 19.6 N is ∼5%. By using micro-Raman scattering experiments, we have shown that similar pressureinduced structural changes occur during sharp-contact mechanical loading and gas-mediated isostatic compression at elevated temperature. Moreover, solid state 11B and 27Al NMR studies have revealed that the cation-oxygen coordination numbers of both boron and aluminum exhibit pronounced increases upon compression. These structural changes manifest themselves in pressure-induced changes in material properties, such as Vickers hardness, Young’s modulus, and density, which all exhibit a very large increase upon hot compression (70, 46, and 12% upon compression at 2 GPa, respectively) relative to previously studied oxide glasses. On the basis of the pressure dependence of density and the short-range order of boron and aluminum, we have introduced a new parameter (atomic selfadaptivity) for predicting high crack resistance in oxide glasses. In addition to possible applications of the present lithium aluminoborate glass, our study has thus identified the structural signatures of high crack resistance, which will facilitate the future search for highly damage-resistant bulk oxide glasses. 5873


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00921. Physical properties of as-prepared and compressed samples, X-ray diffraction spectrum of as-prepared glass, differential scanning calorimetry upscan of asprepared glass, ATR-FTIR spectrum of as-prepared glass, homogeneity check using micro-Raman spectroscopy, Brillouin scattering spectra, nanoindentation curves, details of Raman deconvolution, 6Li and 7Li MAS NMR spectra, 11B and 27Al 3QMAS NMR spectra, and overview of crack resistance data for various oxide glasses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Morten M. Smedskjaer: 0000-0003-0476-2021 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Vladimir Popok (Aalborg University) for access to the AFM instrument and Yuanwei Chang (University of California − Los Angeles) for technical assistance with nanoindentation measurements. K.J. and M.M.S. acknowledge financial support from VILLUM FONDEN under research grant no. 13253. A.G. and M.B. acknowledge financial support from the National Science Foundation (NSF) under Grant Nos. 1507131 and 1562066, respectively. S.J.R. acknowledges financial support from the National Science Center of Poland under Grant No. UMO-2011/03/B/ST3/02352.

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REFERENCES

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