The COVID-19 pandemic highlighted the critical need for reliable pharmaceutical packaging, with borosilicate glasses being widely used for their chemical durability. However, the demand for enhanced stability and longevity of pharmaceuticals necessitates improved understanding of glass structure-property relationships. While ion-exchange and surface coating methods enhance mechanical properties, they are costly. This study focuses on the often-overlooked impact of water corrosion on the mechanical properties of pharmaceutical glasses. Water molecules interact with glass surfaces via hydration, hydrolysis, and ion-exchange, affecting topography, surface chemistry, and nanomechanical properties. Understanding this water-induced surface modification is crucial for designing next-generation pharmaceutical glasses with superior mechanical properties. This research systematically examines the mechanical property evolution of a low-boron BAS glass under various corrosion conditions using nanoindentation, nanoscratch, and Vickers indentation tests. The surface and subsurface changes are characterized using atomic force microscopy (AFM), Raman spectroscopy, time-of-flight secondary-ion mass spectrometry (TOF-SIMS), and sum frequency generation (SFG) spectroscopy to correlate the changes in structure and composition with changes in mechanical properties. The aim is to elucidate the mechanisms behind the altered mechanical properties and contribute to the development of more durable and robust pharmaceutical glass vials.

Numerous studies demonstrate the effectiveness of ion-exchange and surface coating in strengthening glass vials and improving their mechanical and chemical properties. However, limited research exists on the effect of water corrosion-induced surface modification on the mechanical properties of pharmaceutical glasses. Existing literature highlights the significant impact of water-glass interactions on glass topography, surface chemistry, and nanomechanical properties, suggesting that aqueous corrosion plays a crucial role in the overall performance of pharmaceutical glass vials. This study aims to bridge this gap in the literature by focusing on the effects of corrosion-induced alteration layers on the mechanical behavior of borosilicate pharmaceutical glasses.

A low-boron BAS glass (68SiO2-2.2Al2O3-2.8B2O3-11.6Na2O-6.7CaO-8.6MgO (mol%)) was prepared via melt-quenching. Samples were subjected to static corrosion in deionized water at 121 °C for varying durations (0, 10, 30, and 90 min). Surface characterization employed TOF-SIMS (to analyze the distribution of network modifiers), Raman spectroscopy (to identify OH groups and water content), and SFG spectroscopy (to assess hydrogen bonding interactions). Mechanical testing included nanoindentation (measuring nanohardness and reduced modulus), nanoscratch (evaluating nanowear resistance), and Vickers indentation (determining Vickers hardness and indentation fracture toughness). AFM was used to analyze surface topography and subsurface changes before and after annealing treatments to quantify densification and plastic flow volumes. The ICP-MS technique was used to analyze the concentration of leached ions in the solution after the corrosion tests.

TOF-SIMS analysis revealed a decrease in network modifier ions (Na, Ca, Mg) near the surface with increasing corrosion time, forming an AL. The AL thickness increased with corrosion time, reaching ~1.6 µm after 90 min. Raman spectroscopy showed a corresponding increase in OH groups and total water content within the AL. Nanoindentation revealed a decrease in nanohardness (from ~7.5 to ~6.4 GPa) and reduced modulus (from ~83 to ~75 GPa) with increasing corrosion time and water content. Nanoscratch tests showed a significant increase in nanowear volume and depth, also correlated with water content. Analysis of nanoindentation and nanowear imprints before and after annealing revealed that the corrosion-induced "silica-rich" AL facilitated subsurface densification, increasing the recovery ratio (Va/Vi). Vickers indentation tests showed a decrease in Vickers hardness (from ~6.5 to ~5.8 GPa) but a surprising increase in indentation fracture toughness (from 0.70 to 0.78 MPa·m1/2) with increasing corrosion time and AL thickness. The increase in fracture toughness was attributed to the densification of the "silica-rich" AL, hindering crack propagation. The SFG measurements confirmed stronger hydrogen-bonding interactions between the adsorbed water molecules and the corroded glass surface. The findings show an almost linear relationship between water content in the AL and the decrease in hardness, modulus, and wear resistance; while the increase in fracture toughness is also correlated to the water content. A clear increase in densification volume and recovery ratio from both nanoindentation and nanowear tests were observed with increasing alteration layer thickness and water content. The increase in indentation fracture toughness was explained by the higher propensity of the silica-rich AL to undergo densification, reducing residual stress around the indentation and hindering water penetration at crack tips.

The results directly link aqueous corrosion-induced AL formation with changes in the mechanical properties of BAS glass. The decreased hardness and modulus are attributed to the incorporation of water-related species within the AL, which weakens the glass network. The increased fracture toughness, despite the reduced hardness, is attributed to the densification of the "silica-like" AL, which enhances crack resistance. This counterintuitive finding highlights the complex interplay between surface modification and bulk mechanical response. The study reveals a critical point for the design of pharmaceutical glasses; while the hydration weakens the surface, the silica-like structure enhances fracture toughness. Further research could investigate the optimization of glass composition to balance these effects, leading to glass vials with both high chemical durability and superior mechanical resistance.

This research demonstrates a direct correlation between aqueous corrosion-induced alteration layers and the mechanical properties of pharmaceutical BAS glasses. Increased corrosion time leads to thicker alteration layers with higher water content, resulting in reduced hardness and wear resistance. Conversely, the "silica-like" structure within the alteration layer enhances subsurface densification and fracture toughness. This study provides crucial insights for optimizing glass compositions to improve the mechanical performance and extend the service lifetime of pharmaceutical glass vials. Future work could explore the effects of different corrosion environments and glass compositions on the AL formation and resulting mechanical properties.

The study focused on a specific BAS glass composition and corrosion conditions. Extrapolating these findings to other glass compositions or different corrosion environments requires further investigation. The accelerated corrosion conditions used may not perfectly represent real-world storage and handling conditions. The analysis mainly focused on the near-surface region; further investigation of deeper subsurface changes could provide additional insights.