Aesthetic effects of zirconia are significantly affected by its opaque look. Hence, porcelain layering materials were suggested to improve the appearance. However, this issue was not entirely resolved despite changes in veneering methods. With yearly rates ranging from 0 to 54%, porcelain chipping has been identified as one of the most common technical issues in veneered zirconia crowns [29, 32].
Materials evaluated in this study were; Cercon Ultra-translucent, Cercon High-translucent and Cercon Translucent zirconias. Ultra-translucent and High-translucent zirconias were chosen as the manufacturer claims higher translucency, while, Translucent zirconia served as the control.
Er: YAG laser was employed in this study as multiple studies have shown that high-power lasers can improve bonding strength of adhesive materials to all-ceramic high crystalline restorations [33, 34].
In the current study, wettability test measured the contact angle (CA) using distilled water drop as it provides more hydrophilic measurement compared to viscous liquid.
Results of the present study revealed that the wettability was strongly influenced by laser surface treatment. Laser-treated groups exhibited significant decrease in the average contact angle compared to the control-untreated groups. These results showed that laser surface treatment might have increased the surface energy of the zirconia specimens which directly decreased the contact angles and therefore, increased their wettability, which comes in agreement with Moura et al. [21] and Hao et al. [28].
Moreover, Faria et al. [20] reported significant increase in wettability of textured zirconia samples because of changes induced by laser in the zirconia surface.
Measurements of Ra in the current study were performed by AFM according to the ISO 19606: 2017. Surface roughness measurements in nanometer order are widely performed to control quality of thin-film fabrications and material surface treatments [20].
As shown in Fig. (4), the average Ra was increased after laser surface treatment as evident by the ATM 3D topography images. This might be due to the used Er:YAG laser parameters. This is in accordance with Cavalcanti et al. [37] who reported that 200 mJ output energy intensity of Er:YAG laser was more reliable for zirconia surface treatment compared to 400 and 600 mJ intensities. Moreover, Akyıl et al. [36] reported that 400 and 600 mJ Er:YAG laser energy caused some deep cracks in zirconia surface and extreme loss of mass.
Er:YAG application period used in the current study was 10 sec which might be the reason of producing surface roughness with no surface cracks. This agrees with Çağlar et al. [38] who stated that application period of 15 sec. might result in formation of cracks on Y-TZP surface.
The used output power of 2W comes in concurrence with Alhaidary et al. [39] who concluded that the use of 1W Er:YAG laser did not alter zirconia disc surfaces, while 2 and 3W output power caused microscopic surface changes and that at 4 and 5 W melting and microstructures were observed. Ergun Kunt et al. [40] confirmed that perceptible loss of material was caused by Er: YAG laser of 3W and 4W output energy and application period of 45 sec.
Owing to simplicity of its testing procedures, shear-bond strength test is considered one of most used methods. In this study, specimens were subjected to 5000 thermal cycles to simulate 6 months of clinical performance in accordance with ISO 11405:2015 recommendations [41, 42].
The average SBS was significantly increased in Ultra-Translucent zirconia laser-treated group. This might be due to increased Ra and wettability as measured in the current study. Moreover, the used laser parameters might have played a role in increasing its SBS. This in agreement with Hou et al. [28] who stated that laser irradiation of zirconia with 200–400 mJ pulse energy, could render proper micromechanical retention and increases SBS. Moreover, Ghoveizi et al. [43] concluded that Er:YAG laser power of 2W results in significant increased SBS.
Using MDP based primers along with adhesive resin-cement, might have further increased the SBS after laser treatment. This agrees with Salem et al. [44], Xie et al. [45], Yagawa et al. [46] and Ruales et al. [47] who reported that laser treatment associated with application of hydrophobic phosphoric-acid monomer, such as 10-MDP, is considered one of most reliable adhesion bonding protocols for zirconia restorations. The MDP molecules enable chemical bonding to zirconia through zirconium phosphate formation.
In the current study, it is noteworthy that increased Ra does not imply increased resin-cement/zirconia bonding as observed in laser treated High-Translucent zirconia. In Tables (3 and 4), significant decrease in CA combined with significant increase in Ra after laser treatment of High-Translucent zirconia did not warrant significantly higher SBS compared to sandblasted group.
However, the highest Ra as observed in the laser-treated Ultra-Translucent zirconia group combined with the significantly low CA have resulted in improved resin-zirconia bonding. The highest roughness provided large surface area for resin-cement to flow and interlock with zirconia surface. The low CA suggest that resin-cement could effectively wet zirconia surface.
Since the results of the present study showed that there were differences in wettability, Ra and SBS between the three types of translucent zirconia after laser treatment, therefore, null hypothesis was rejected.
The effect of only one set Er:YAG parameters on translucent zirconia was used in the current study, which is considered a limitation. Further research should investigate the effect of different Er:YAG parameters on translucent zirconia. Moreover, mechanical cyclic fatigue should be done combined with thermal-aging before bond strength evaluation to best simulate the oral environment.