Towards Ultra-Tough Oxide Glasses with Heterogeneities by Consolidation of Nanoparticles

We prepared heterogeneous alumina-silicate glasses by consolidating nanoparticles using molecular dynamics simulations. Consolidated glasses from either low alumina content alumina-silicate glasses or high alumina content alumina-silicate glasses show significantly improved ductility around consolidation pressure of ~3 GPa. The introduced structural heterogeneities, namely over-coordinated network formers and their neighboring oxygen atoms, are identified as plasticity carriers due to their high rearrangement propensity. In addition, consolidated oxide glass from both 23.4Al2O376.6SiO2 and 73.1Al2O326.9SiO2 nanoparticles show improved flow strength (up to 1GPa) due to the introduction of chemical heterogeneities. Last but not least, apparent hardening behavior appears upon cold work in consolidated glasses, with an increase of yield strength from ~3.3 GPa to ~6.4 GPa. This method is a big advancement toward ultra-strong and ultra-tough glasses by breaking the structure, composition and size limitations in traditional melt-quench process.


INTRODUCTION
Oxide glasses are widely used in energy, electronic and engineering applications thanks to their intriguing optical, thermal and chemical properties. [1][2][3] However, glasses are generally brittle due to their low shear-flow propensity and poor crackpropagation resistance. 4,5 Such brittleness is the stumbling block of current commercial glass products since it will decrease the damage tolerance and practical strength 6 . Hence, there is a strong drive to develop ultra-tough oxide glasses.
Many efforts have been made to toughen amorphous oxides in the past. The most widely used strategy is surface strengthening, which can inhibit crack propagation with the pre-built compressive stress near surface layer. However, such surface treatments are not universally applicable. For example, thermal tempering 7 can't be applied to low-thermal-expansion oxide glasses; chemical tempering 8,9 is restricted to glasses with mobile ions. More importantly, surface strengthening is exclusively effective in thin-slab sample, as the internal cracks will cause catastrophic failure in bulk.
Apart from developing compressive stress, tough oxide glasses can also be obtained by increasing ductility. On one hand, new glass systems like cesium aluminoborate, binary aluminosilicate and pure alumina glasses [10][11][12][13] have shown potential to possess intrinsically high ductility. For instance, amorphous alumina was found to exhibit viscous creep at room temperature, which leads to exceptional ductility up to 100%. However, the glass forming ability of ductile amorphous oxides is usually very low, hence we cannot easily synthesize them by traditional melt-quenching processes. Even with special levitation synthetic techniques, the size of the obtained amorphous alumina is still restricted to submicron. On the other hand, ductility of existing amorphous oxides can be enhanced using hydrostatical pressure treatments, either at room temperature or during meltquenching process [14][15][16] . The key of this technique is to introduce structural heterogeneities as plasticity carriers, such as five-fold silicon atoms in silica glasses 17,18 . But the plastic carriers are very challenging to create, since it usually requires extremely high pressure (>10 GPa for silica glass) to display substantial ductility. In the meanwhile, considerable strength reduction is commonly observed after applying hydrostatical pressure, hence limits the toughening effects. Therefore, besides introducing structural heterogeneities, the bonding topology should also be optimized to maintain the high strength.
Based on the discussion above, to satisfy the escalating need in stronger and more damage-resistant oxide glasses, we should develop more powerful, feasible and universal toughening strategies. To this end, Zhang et al 19 prepared tough silica glass with significantly reduced pressure by consolidating glassy nanoparticles. This consolidation technique may be a universal route to improve both the intrinsic ductility and strength simultaneously.
Herein, we systematically investigated the toughening effects of consolidation technique with nanoparticles on various typical oxide glasses using molecular dynamics (MD) simulations. We found that the brittle to ductile transition in intrinsically brittle glass (with low alumina content) requires ~3 GPa by consolidating nanoparticles, much lower than that for compressing bulk sample (over 7 GPa). For intrinsically ductile yet low-formable glasses (high-alumina content and pure alumina), ~3 GPa is already sufficient to obtain considerable ductility in bulk samples by consolidating nanoparticles. Interestingly, up to 1 GPa enhancement of flow strength is obtained in consolidated heterogenous glasses owing to the unique and stable bonding among atoms in glassy nanoparticles. Last but not least, apparent work hardening ability is achieved in the consolidate glasses, with yield strength increasing from ~3.3 GPa to ~6.4 GPa after 40% cold work. We believe that the method delineated here can break the structure, composition and size limitation in traditional glass preparation route, and will be regarded as a universal and feasible technique toward ultra-tough oxide glasses.

SIMULATION SETUP
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 August 2020 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 August 2020 doi:10.20944/preprints202008.0200.v1 Classical molecular dynamic simulations were carried out in LAMMPS package 24 (https://lammps.sandia.gov/). The equations of motion were integrated by Velocity-Verlet algorithm with a time step of 0.8 fs. To control the pressure and temperature of the system, Nose-Hoover barostat and thermostat 25,26 were used. To mimic the behavior of binary aluminosilicate glasses, a modified version of the Buckingham potential 27 is used, which can accurately reproduce the structure and mechanical properties over a wide composition range (as in Figure S4). The shortrange cutoff is set as 0.8 nm. The long-range Columbic interaction which cut at 1 nm was calculated via the Wolf truncation method 28 to increase the computational efficiency.
Bulk samples with varying compositions were quenched from wellequilibrated high temperature (3600 K) melt to room temperature (300 K) with a rate about 4 K/ps under zero pressure. Nanoparticles with uniform size are then carved from this bulk glass. Charge balance of the entire samples are maintained by deleting surface atoms of nanoparticles. The size of nanoparticles might be slightly adjusted to maintain the same nominal concentration. Proceeding consolidation, relaxation of nanoparticles is conducted at room temperature under constant volume for ~1 ns to avoid any artificial periodicity. The entire simulation box is then subjected to consolidation following a temperature and pressure profile in Figure S5. The 1 st neighbor cutoff for Si-O and Al-O is 0.22, 0.24 nm, respectively as in the RDF curves in Figure S6. For starting nanoparticle size ranging from 1 to 8 nm, the resulting glassy sample after consolidation is around 10 nm by 10 nm by 20 nm (atoms population ~200000). The sample size is around 20 nm by 20 nm by 20 nm when nanoparticle size is above 8 nm (atoms population ~1 million). The sample is sufficiently long and not slender enough to trigger the known size/shape effect. 29,30 Uniaxial tension testes with an engineering strain rate of 1.25 ns -1 were carried out under room temperature to investigate the mechanical properties. For ductile oxide glasses, the flow stress is averaged from 30% to 65%. Periodic boundary conditions were applied in all three directions. The visualization software OVITO 31 was used to generate simulation snapshots, animations and local atomic strain/stress was calculated based on method proposed by Falk and Langer 32 .

RESULTS AND DISCUSSIONS
To prepare bulk samples, the nanoparticles are consolidated at different temperature and pressure followed the profile in Method section. Figure 1 illustrates the sample morphology consolidated under different pressure after returning back to zero pressure and room temperature (300 K). To introduce nanoscale chemical heterogeneities inside the oxide glasses, a mixture of nanoparticles with two different concentrations are used for consolidation. In this regard, the volume fraction of starting nanoparticles is carefully adjusted to maintain the same nominal composition. The degree and distribution of chemical heterogeneities can be tuned in a systematic way by changing the size and composition of the starting nanoparticles. In contrast, if only one type of nanoparticles were used for consolidation, the obtained glasses are thought to be free of chemical heterogeneities. For convenience, the consolidated glasses with chemical heterogeneities, the consolidated glasses without chemical heterogeneities and the compressed as-quenched glasses are termed binary consolidated glasses (BCS), mono consolidated glasses (MCS), compressed glasses (CP), respectively. The mechanical properties of obtained oxide glassed are investigated by conducting uniaxial tension tests with an engineering strain rate of 1.25 ns -1 . First, we systematically investigated the mechanical properties of consolidated glasses with a nominal composition of 23.4Al 2 O 3 76.6SiO 2 . The as-quenched glass is free of pores inside, yet still exhibits generic brittle behavior as in Figure 2 (b). To prepare glasses with chemical heterogeneities, both 8-nm amorphous alumina and silica are consolidated at room temperature using various pressure. With low applied pressure (~5 GPa), large pores are observable in the consolidated glasses. The densities of consolidated glasses are provided in Figure S1. As the applied consolidation pressure increases, the BCS exhibit apparent brittle to ductile (BTD) transition with a critical pressure of ~10 GPa. Interestingly, by introducing chemical heterogeneities, the flow strength of consolidated glass is enhanced from 6.4 GPa to 7.2 GPa. Such substantial improvement in strength will not sacrifice ductility, which is of vital importance for toughness optimization. We then reduced the size of both silica and alumina starting glassy nanoparticles from 8 nm to 1 nm, Figure 2 (c). Despite the existence of pores insides, improved ductility can be achieved in BCS samples when applying ~3 GPa pressure, much lower than over 7 GPa needed for compressing bulk samples albeit the bulk sample is indeed defectsfree ( Figure S2). The critical pressure observed here is slightly higher than that of pure silica (2.4 GPa), which might originate from the inhibition of pore healing by the high elastic moduli of amorphous alumina. Note that the flow strength of 1-nm-BCS is lower than that of the 8-nm-BCS, but still higher than CP sample. The mechanism for enhanced strength in the heterogeneous consolidated glasses will be discussed later. When subjecting to higher consolidation temperature, CS samples display both improved ductility and strength as shown in Figure 2(d). Specifically, the BCS consolidated under 1500 K and 3.2 GPa possesses comparable strength as the as-quenched glass, but over 2 times enhancement in ductility. Finally, different compositions of starting nanoparticles are consolidated at 3.2 GPa as in Figure 2(e), where subtle difference in critical pressure is observable as the starting nanoparticle composition changes. Besides, BCS prepared from a mixture of pure glassy alumina and silica are found to be slightly stronger than those made from 23.4Al 2 O 3 76.6SiO 2 nanoparticles.
Using this consolidation technique, we then prepared two types of low-formable ductile oxide glasses with different starting materials, where final morphologies can be found in Figure 3(a). For high alumina content binary aluminosilicate glass with nominal composition of 73.1Al 2 O 3 26.9SiO 2 , the as-quenched sample exhibits generic ductile behavior, Figure 3(b). When preparing glass with the same nominal composition by consolidating a mixture of 2-nm glassy alumina and silica upon different pressure at room temperature, we found ~3 GPa to be the critical pressure to obtain sufficient ductility for BCS. Meanwhile, the BCS exhibits slightly enhanced flow strength as compared to both CP and as-quenched glass. We also made the sample using 2-nm glassy 73.1Al 2 O 3 26.9SiO 2 nanoparticles at room temperature under various pressure. Similarly, as applied pressure increases, enhancement in ductility is observed, where the critical pressure is again ~3 GPa. For pure alumina, the as-quenched sample also exhibits superior intrinsic ductility as in Figure 3(c). To prepare amorphous alumina, either 2 nm glassy nanoparticles or crystalline nanoparticles are consolidated at room temperature. Due to the high intrinsic ductility of amorphous alumina, the improvement in ductility as applied pressure increases for consolidated alumina is mainly due to the closing of pores, as proved by the enhancement in density ( Figure S3). Note that the critical pressure to obtain sufficient ductility (~40%) in those two consolidated glasses is less than 3 GPa. However, the glasses prepared with glassy nanoparticles show obviously improved ductility at the same applied pressure.
Apart from the obtained ductile behavior for both low and high aluminosilicate glasses, we also investigated the effects of chemical heterogeneities on the strength of consolidated glass. When consolidating with glassy alumina and silica, the flow strength of 23.4Al 2 O 3 76.6SiO 2 first increases with the starting nanoparticle size, then reaches plateau as the size exceeds 8 nm, see Figure 4(a). This plateau value (~ 7.2 GPa) is very close to the theoretical flow strength of glass composited with densified amorphous silica and alumina (~7.3 GPa). Besides pure amorphous alumina, we also prepared the sample using vitreous silica and various binary aluminosilicate nanoparticles as starting materials; we maintained the same nominal composition by carefully adjusting the volume fraction of silica nanoparticles. The flow strength is found to increase with volume fraction of silica nanoparticles, since silica glass display higher flow strength (~8 GPa), Figure 4(b). Finally, we prepared glasses over a wide range of nominal composition using 8nm-glassy alumina and silica under 300 K and 19.2 GPa. Compared with compressing the as-quenched glasses, enhancement in flow strength is obtained for glasses consolidated from nanoparticles over a large nominal composition range, reaching 1 GPa in 50Al 2 O 3 50SiO 2 , Figure 4(c). We attribute the enhanced strength to the preserved unique stable bonding in starting nanoparticles through introducing chemical heterogeneities. This stable bonding is confirmed by the low potential energy of consolidated glasses as in Figure 4(d). Note that such bonding cannot be obtained in traditional melt-quenching process since it is considerably deteriorated due to entropic effect, especially for 50Al 2 O 3 50SiO 2 . Consequently, by introducing both structural and chemical heterogeneities across the samples, we realized simultaneous enhancement of strength and ductility in consolidated glasses.
Furthermore, we observed more apparent hardening ability in the consolidated glasses by conducting loading-reloading-unloading mechanical tests, Figure 5. When compressing as-quenched glass (23.4Al 2 O 3 76.6SiO 2 ), the yield strength of the original samples is ~3 GPa, which increases to ~4.2, 5.4, 5.8 GPa after cold work corresponding with 8%, 20%, 40% strain. In contrast, consolidated glass prepared with 8-nm amorphous alumina and silica nanoparticles originally yields at ~3.3 GPa, which is significantly enhanced to 4.9, 6.2, 6.4 GPa when cold working at 8%, 20% and 40% strain, respectively. Such more obvious hardening ability is due to their superior low strength introduced by chemical heterogeneities.
It is well known that over-coordinated atoms appear after pressure treatment 20,21 and are found to be responsible for enhancement in ductility for silica glass 18,19,22 . Therefore, to understand the structural origin of superior ductility in the binary aluminosilicate glasses, we first analyzed the correlation between plasticity and over-coordinated atoms. We found that the plasticity of binary oxide glasses is increased with the population of oxygen atoms bonded with over-coordinated cations, regardless of composition and processing routes, as shown in Figure 6(a). Theoretically, plastic deformation is realized by the incremental bond switching events; hence, we further examine the rearrangement propensity of oxygen atoms with different coordination environment in the remaining plastic deformation after unloading. As shown in Figure 6 (b), the oxygen atoms bonded with those overcoordinated cations (CN>=5) display relatively high rearrangement propensity of 40%, which renders them acting as plasticity carriers during plastic deformation. By consolidating nanoparticles, a larger population of plasticity carriers is generated, hence the BTD transition is triggered with low pressure, Figure 6(c). The work hardening behavior can also be understood by the relaxation of plasticity carriers as in Figure 7. Before loading, the population is ~25% for compressed bulk sample. The plasticity carrier population is reduced to 21%, 13%, 11% as cold worked at 8%, 20% and 40%, respectively. Similarly, the population of plasticity carriers is 28% for consolidated glasses proceeding cold work. By cold work with 8%, 20%, 40% strain, the population of plasticity carriers is reduced to 25%, 22%, 20%, respectively. Therefore, the stress facilitated relaxation of plasticity carriers is the origin for the observed hardening behavior, similar behavior is observed in consolidated silica glass 19 and rejuvenated metallic glasses 23 .
In conclusion, the consolidated glasses with nanoparticles enables synergistic optimization of structural and chemical heterogeneities, which leads to simultaneous enhancement in strength and ductility. For binary aluminosilicate and pure alumina glass, ductile glasses in bulk size can be obtained by consolidating nanoparticles with relatively low pressure (~3 GPa). Over-coordinated cations and their neighboring oxygen atoms are identified as the plasticity carriers in the binary glasses due to their high rearrangement propensity. The introduction of chemical heterogeneities results in higher flow strength due to preserved stable bonding. Ductile binary glasses exhibit apparent hardening ability, especially for oxide glasses with chemical heterogeneities. The proposed consolidation technique will shed light on designing stronger and more damage-resistant oxide glasses, paving the way for future applications.        Figure S1 (a) ambient-condition density as a function of pressure for 23.4Al2O376.6SiO2 glass prepared with different starting materials. The consolidation temperature is fixed at 300 K. (b) ambient-condition density as a function of temperature for 23.4Al2O376.6SiO2 glass prepared with different starting materials. The consolidation pressure is fixed at 300 K.