**1. Introduction**

During the ion exchange process on glass, some ions in the materials are replaced by new ions from the liquid that can modify the physical and chemical properties locally [1-3]. The ion

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exchange process has been widely used to change the reflective index in a selected area of glass for producing waveguide devices [4, 5]. Starting about 50 years ago, ion exchange has been employed also to improve the mechanical properties of silicate glass [1].

Surface defects are responsible for the limited glass resistance and its large scatter [6]. The creation of a compressive stress layer in the surface of the material can limit the formation or propagation of flaws and improve the mechanical properties; thermal and chemical tempering of glass are two main methods for producing a compressive stress in the glass surface. The thermal method is widely used to make windows and other transparent flat structural components [2, 7].

The ion exchange or chemical strengthening of glass was almost abandoned for many years because of the high processing cost and long-duration process. In recent years, this method has been reconsidered because of the possibility of mechanical treatment after strengthening, the applicability to complicated shape and limited thickness components, and the absence of optical distortion [8].

The ion exchange process is typically carried out by immersing the components made of a glass containing lithium or sodium in molten potassium nitride salt. The process can be carried out at a temperature between the melting point of the salt and the transforming temperature of the glass and takes times in excess to 4 h, depending on the required depth for the com‐ pressive stress layer. After finishing the process, the samples are removed from the bath and the salt on the surface is simply washed out by water [2, 9].

The ion exchange process can be considered as an inter-diffusion reaction between the mobile ions in glass and the cations in the molten salt while the other glass components are considered as an immobile matrix of negative groups [10-13]. An external electric field can be the source of an extra driving force for the inter-diffusion of the mobile ions. This process is known as electric field-assisted ion exchange (EF-IOX) and it has been used especially for manufacturing waveguides [4, 5, 14, 15]. Three different procedures have been proposed. In the first one, a thin metal film as a source for ions is applied on the glass surface and the application of an electric field induces the oxidation/reduction at the interface with glass that generates cations that move into the glass on the anode side, creating the chemical concentration profile [5, 16, 17]. Alternatively, a molten salt can be used as the ion source at only the anode side of the sample, the cations at the anode side penetrates into the glass under the field [5]. In the last approach, the sample acts as a wall, separates two molten salt baths and each bath is connected to an electrode; by applying the electric field the cations in the salt start moving in the direction of the electric field [4, 5, 14].

By considering the ions flow and the electrical charge neutrality balance, different mathemat‐ ical modelling have been proposed for the ion diffusion during the exchange process [18]. For a large enough field, the concentration of exchanged ions, at distance x from the surface and at time t, can be defined as:

$$C\left(\mathbf{x},t\right) = \frac{C\_0}{2}\sigma f c\left(\frac{\mathbf{x}-\mu Et}{2\sqrt{Dt}}\right) \tag{1}$$

where C0 is the surface concentration of the ions, E the applied electric field, μ the ion mobility, and D the inter-diffusion coefficient [8]. Thermal diffusion does not play an active role and the migration of ions is governed by the electric field, only. Consequently, a step-like profile is generated in a short period of time for the ions concentration [9]. The long range migration of cations in the glass allow to produce a deeper compressive layer [18, 19].

Field-assisted ion exchange has been limitedly studied with the aim of improving the me‐ chanical properties during the 1970s [20]. The main problem was the unbalanced residual stress that caused the deformation of the samples.

In this work, we performed electric field-assisted ion exchange on commercial borosilicate glass tubes, thus avoiding the problems associated with the deformation of the samples. The aim was to analyze the possibility of using an external field for speeding up the ion exchange process and obtaining improved mechanical properties.
