Improvement of Surface Chemical Stability of Glass-Ceramics by Zinc Chloride Solution Immersion

In recent years, glass-ceramics have become a preferred material for new-generation glass covers due to their superior mechanical properties, excellent optical characteristics, and stable chemical performance. However, their inherent brittleness makes them prone to catastrophic fracture during crack propagation. To solve this problem, ion exchange technology as a strengthening method designed for aluminosilicate glass systems is widely adopted. This process significantly enhances mechanical properties such as hardness and flexural strength, while maintaining the substrate optical transparency. However, this process introduces a large amount of sodium and potassium ions onto the glass surface, which simultaneously instigates the degradation of chemical stability. This degradation primarily arises from water-induced corrosion, which occurs due to two sequential stages, i.e., leaching, where alkali metal ions (e.g., K⁺, Na⁺) undergo ion exchange with H₃O⁺/H⁺ in aqueous environments, generating an alkaline surface; and network dissolution, where hydroxyl ions attack and depolymerize the silicate network. Crucially, condensation droplets concentrate OH^-ions while atmospheric CO₂ forms highly corrosive mixed alkali carbonates under humid conditions, dramatically accelerating degradation. This fundamentally restricts the applicability of ion-exchanged glass-ceramics as an advanced display material, which a sustained surface integrity under environmental exposure is essential. It is thus necessary to improve the surface chemical stability of chemically strengthened microcrystalline glass in industry. To address this critical industrial challenge, this study was to introduce an immersion treatment using zinc chloride (ZnCl₂) solution designed to enhance chemical stability and impact resistance in lithium aluminosilicate (LAS) glass-ceramics simultaneously.

LAS glass-ceramic specimens (composition: 69.43% SiO₂, 5.62% Al₂O₃, 12.13% Li₂O, 6.21% ZrO₂, 5.27% P₂O₅, 1.34% Na₂O; primary crystalline phases: lithium disilicate and β-spodumene) were processed through sequential cutting, grinding, and polishing procedures to obtain glass samples with the sizes of 147.0 mm×72.0 mm×0.6 mm. The specimens underwent sequential chemical strengthening, i.e., immersion in molten NaNO₃ at 440 ℃ for 4 h, followed by molten KNO₃ at 410 ℃ for 0.5 h. Afterwards, an immersion treatment was performed in both distilled water and ZnCl₂ solutions at different contents (i.e., 0.05, 0.10, 0.20 mol/L) at 98 ℃ for 3.5 h, respectively. The treated and untreated specimens underwent accelerated weathering at 85 ℃ /85% RH for 1, 4, and 6 d. The tests evaluated post-treatment efficacy for enhancing surface chemical stability and analyzed the underlying mechanisms. The elemental distributions (i.e., Na, K) on the surfaces and cross-sections of samples before and after treatment under different conditions were characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The surface roughness of samples was characterized by atomic force microscopy (AFM) across 30 μm×30 μm scan areas. The impact resistance of samples before and after treatment under different conditions was evaluated by drop-ball testing (132 g steel ball, maximum drop height of 1700 mm), and the results represented the average values from 10 samples per group.

Immersion in ZnCl₂ solutions significantly modifies the surface composition and properties of the chemically strengthened glass-ceramic samples. The SEM-EDS analyses indicates that this immersion process reduces the concentration of K⁺ on the surface, while leading to an increase in Na⁺ content. Particularly, ZnCl₂ solutions with higher concentrations enhance the outward migration of K⁺ from the subsurface region. The reason is since a weak acidity of ZnCl₂ solution promotes H⁺/OH^- neutralization, thereby enhancing the ion exchange process. In parallel, Na⁺ migration appears in the samples during immersed progress, particularly in distilled water. This is due to the reduced cross-linking density is caused by the formation of Si-OH and the fracture of Si–O–Si bonds after K⁺ ions on the surface are displaced. This induces a relaxation in the surface network structure, forming diffusion channels. Consequently, the weakly bound Na⁺ migrates outward under the synergistic effects of concentration gradients and structural defects. However, Na⁺ migrates in the samples treated with ZnCl₂ solutions, as the smaller, higher-charge Zn²⁺ ions fill in the surface pores.

For immersion, the surface roughness of the samples increases. The most severe damage, measured at 3.2 nm occus in samples treated with distilled water due to the sustained corrosion of the glass network. In contrast, ZnCl₂ solutions cause less severe, and concentration-dependence increases in the roughness (i.e., 1.5–2.2 nm). This is because H⁺ neutralization reduces the availability of alkaline media, thereby inhibiting silicate regeneration and limiting network degradation.

The results of accelerated weathering tests conducted at 85 ℃ and 85% relative humidity (RH) for 1, 4, and 6 ddemonstrate an enhancement in the chemical stability of samples treated with ZnCl₂. The untreated samples exhibit dense white spots after just 1 d and develop severe haze at 6 d. The samples immersed in distilled water show minor spots rich in Na⁺, while those treated with ZnCl₂ exhibit a minimal visible degradation. This improvement is attributed to two mechanisms, i.e., 1) a reduction in surface K⁺ concentration, which suppresses ion exchange driven by leaching, and 2) the diffusion of Zn²⁺, which blocks further alkali ion leaching and H⁺ adsorption.

The SEM-EDS analysis shows some distinct corrosion patterns. The untreated samples display severe dendritic K₂CO₃ aggregates and needle-like corrosion zones. The samples treated with distilled water show fine Na⁺-rich surface spots with limited corrosion. The samples treated with 0.05 mol/L ZnCl₂ have sparse deposits without significant elemental anomalies, indicating the blocking effect of Zn²⁺ against alkali migration.

The samples treated with 0.05 mol/L ZnCl₂ exhibit a maximum average impact height of approximately 1600 mm, which is 1.5 times greater than that of the untreated samples. This impact resistance improvement is attributed to the blunting of microcracks on the glass surface during mild corrosion.

This study investigated the effect of ZnCl₂ immersion on the surface chemical stability of chemically strengthened glass-ceramics. The results revealed that ZnCl₂ immersion significantly reduced surface alkali metal ion concentrations and suppressed their outward diffusion, substantially improving the chemical stability. The treated specimens exhibited superior corrosion resistance and surface integrity under accelerated aging (i.e., 85 ℃/85% RH). Also, ZnCl₂ treatment simultaneously enhanced the impact resistance. The samples immersed in 0.05 mol/L ZnCl₂ at 98 ℃ for 3.5 h achieved a maximum average impact height of 1600 mm (i.e., 132 g steel ball), which was 1.5 times greater than that of the untreated samples. These results indicated that ZnCl₂ immersion treatment could improve surface chemical stability and mechanical durability, having an effective surface-modification technology for glass-ceramics.