Third-generation zirconia was developed to overcome the low translucency issues of the first and second generations (3 mol% yttrium-stabilized tetragonal zirconia polycrystal – 3Y-TZP). The 5 mol% yttrium-partially stabilized zirconia (5Y-PSZ) was developed with increased amounts of non-birefringent cubic phase, which noticeably improved its translucency [1]. Cubic crystal structures have higher volume and are more isotropic than tetragonal ones. This isotropic structure allows the incident light to be transmitted in all directions [2].

Despite the increased translucency, the mechanical behavior of third-generation zirconia is a concern. Strength and toughness of 5Y-PSZ were diminished when compared to tetragonal zirconia, since it does not undergo stress-induced transformation [3]. Hence, 5Y-PSZ is indicated for use in anterior or posterior single crowns, onlays, inlays, and veneers. Even with higher initial strength than other materials within the same clinical indication range (i.e. glass-ceramics), defects introduced by clinical procedures or mastication can easily decrease the strength of translucent zirconia restorations [4], [5].

Monolithic zirconia restorations are usually finished with glaze to provide natural and esthetic appearance. However, glaze application has been reported to decrease the flexural strength of third-generation zirconia [6]. Moreover, clinical data has reported that the glaze layer over zirconia crowns tends to wear off within one year of clinical usage [7]. According to Callister, glass has an annealing point, which is a temperature where the residual stresses are eliminated within ∼15 min [8]. Glass loses its viscosity at this temperature, enabling molecular arrangement and stress relaxation [8]. Previous investigations have applied glass materials over zirconia for an extended time and reported promising results regarding mechanical behavior improvement [9], [10]. However, there is a lack of information regarding the effect of these treatments on 5Y-PSZ.

Soda-lime glass is widely used in materials science since it is a prototypical brittle material which has been used for direct observation of damage modes due to its transparency [11], [12]. Variations of soda-lime glass have been developed and tested as coating materials for 3Y-TZP [13], [14]. Boric oxide is usually added to glass because boron can work as an important glass former and flux material which forms in a glass network [15]. The addition of boron has the potential to reduce the softening point and the coefficient of thermal expansion, as well as improve the mechanical and chemical resistance of glass [15], [16]. A smooth and natural appearance could be achieved in third-generation zirconia restorations by varying the composition of soda-lime glass. Once the glass coating can also improve the mechanical behavior of third-generation zirconia, the potential strength degradation over time could be hindered. Moreover, applying a more resistant glassy layer may extend the longevity of the coating layer (i.e. glaze) and even suggest broadening the application scope for these materials due to the enhanced strength.

In this context, an experimental boron-doped soda-lime glass (borosilicate glass) was developed with the purpose of providing an alternative finishing material for third-generation zirconia. Therefore, this study aimed to evaluate the effect of the borosilicate glass on the mechanical behavior of 5Y-PSZ zirconia. This experimental glass treatment was compared to a commercial glaze and no additional glass application (as-sintered 5Y-PSZ). The effect of the experimental glass application on color alteration and translucency were also investigated. Complementary microstructural and residual stress analyses were performed. The tested hypothesis was that the experimental borosilicate glass would improve the mechanical behavior (flexural strength and toughness) of 5Y-PSZ without jeopardizing its optical properties.