Can annealing improve the chemical strengthening of thin borosilicate glass?

doi:10.1016/j.jnoncrysol.2017.03.038

Journal of Non-Crystalline Solids

Volume 465, 1 June 2017, Pages 1-7

https://doi.org/10.1016/j.jnoncrysol.2017.03.038 Get rights and content

Highlights

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Annealing for 16 h at 425 °C densifies thin borosilicate glass.
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Densification of glass increases the residual compressive stress build up by ion exchange.
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Densification has no significant impact on the final strength of ion-exchanged glass.

Abstract

In this work, we try to point out the influence of annealing prior to chemical strengthening on the mechanical strength of thin ion exchangeable alkali borosilicate glass. The effect of annealing at 425 °C on density, hardness and cracking behaviour were investigated. Then, as-received and annealed samples were subjected to ion exchange in a molten potassium nitrate bath at the same temperature for 4 h and the generation of compressive stress in glass was analyzed as well as the bending strength.

Annealing makes the glass denser, improves hardness and enhances the compressive stress build-up. However, bending strength of as-received and annealed glass after ion-exchange is substantially the same, this being probably attributed to the limited case depth if compared to surface flaws. Annealing before ion exchange does not appear to be a crucial factor in improving chemical strengthening efficiency in thin borosilicate glass.

Introduction

Ion exchange strengthening is nowadays a conventional approach to improve the mechanical properties of thin glasses such as those used in touchscreen devices and flexible electronics [1], [2], [3]. This technique, which is also known as chemical strengthening, is usually conducted by immersing an alkali (lithium or sodium) - containing glass in a molten potassium nitrate bath for several hours where the larger potassium ions diffuse into the glass and partially replace the alkalis. Stuffing larger potassium ions into the smaller alkali sites produces a compressive stress [1], [2], [4], [5], [6], [7], [8] and the generated surface compression is beneficial to neutralise the detrimental effect of surface defects on strength [4], [8], [9].

Glass composition and structure can influence sodium/potassium exchange and stress build up. The glass structure, in turn, depends on chemical composition as well as on thermal history [5], [10]. Hyper-quenched (rapidly cooled) glass, such as thin glass or glass fibres, has lower fictive temperature and, consequently, different structure and physical properties compared to normally cooled one [11], [12]. Interestingly, Svenson et al. reported that the densification of glass before ion exchange improves the compressive stress generation, while decreases the case depth [13], i.e. the depth at which the compressive stress is zero; these changes make the influence of such treatments on glass strength equivocal. In addition, it is known that structural transformations are reversible at elevated temperature and that thermal history effect is distorted if the glass is held for several hours at high temperatures below Tg, as it is required for chemical strengthening [8], [14], [15], [16].

In this work, an attempt has been done to answer the question whether annealing can influence the chemical strengthening of thin sodium borosilicate glass.

Section snippets

Experimental procedure

Ion-exchangeable thin borosilicate glass sheets (D 263® T eco, with nominal thickness of 200 μm and provided by Schott AG) were used in the present work. Samples of different size were manually cut from the original sheets, washed with distilled water under ultrasonic agitation and with acetone and then air dried.

A portion of the specimens was annealed for 16 h in a muffle kept at 425 °C where they were placed in a stainless steel basket; then, they were removed immediately from the furnace and

Results

The density of glass before and after annealing for 16 h at 425 °C is reported in Table 1 and we see that it increases after annealing. The mass change during the treatment is, in practice, zero and it can be ignored; therefore, the volume shrinkage upon annealing can be estimated as: $\frac{∆ V}{V_{0}} = \frac{ρ_{Ann} - ρ_{As - rec}}{ρ_{Ann}}$ where ΔV and V₀ are the volume change and the initial volume of glass, respectively, ρ_Ann and ρ_As-rec being the density of annealed and as-received glass, respectively. By using the data in Table

Discussion

The annealing of hyper-quenched glass, such as a thin sheet, at elevated temperature below Tg causes relaxations towards a denser structure associated with a slowly cooled glass [29], [30]. The annealing for 16 h at 425 °C is responsible for a density increase and reduction of the glass transition temperature as reported before for other silicate glasses and silicates structures [13], [31], [32], [33], [34], [35]. Moreover, Tg change reveals that glass densification upon annealing is reversible.

Conclusions

Annealing at 425 °C for 16 h increases glass density and decreases the glass-transition temperature of thin borosilicate glass. Though the hardness is slightly increased by annealing, the glass becomes more brittle. Ion-exchange produces larger compressive stress in annealed glass compared to the as-received one; however, bending strength shows no meaningful improvement, which is probably related to the limited case depth produced in annealed glass by ion-exchange. Annealing and densification of

Acknowledgement

The authors are grateful to Schott AG, Advanced Optics, for providing D 263® T eco.

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View full text

Can annealing improve the chemical strengthening of thin borosilicate glass?

Highlights

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Conclusions

Acknowledgement

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