Nanostructured alumina coating zirconia pretreatment prepared under different firing conditions

Objectives: Bond between resin-cement and yttria partially stabilized tetragonal zirconia ceramics (Y-TZP) can be enhanced by Y-TZP surface modification with a nanostructured alumina coating (NSAC), an alternative to airborne-particle abrasion (APA). For enhancing NSAC integration to Y-TZP surface, subsequent firing after its application is mandatory. In order to integrate this preparative step, the aim of our study was to examine the effect of various firing procedures during laboratory Y-TZP restoration fabrication on the phase evolution of NSAC and to characterize the bond between resin-cement and NSAC-coated Y-TZP depending on the firing protocol employed.
Methods: 120 sintered Y-TZP ceramic discs divided into six groups (n=20) received different surface treatments: AS – as-sintered; APA – airborne-particle abraded. Other four groups were treated with NSAC (VALLBOND) subjected to different laboratory firing protocols (end temperature/holding time): C-725(1min) – 725°C/1min (IPS e.max glaze-firing), C-900(1min) – 900°C/1min (Vita VM9 glaze-firing), C-900(30min) – 900°C/30min (reference), C1050(15min) – 1050°C/15min (IPS ZirCAD regeneration-firing). Zirconia-resin shear bond strength (SBS) was tested before and after thermocycling (TC). NSAC`s morphology was analyzed using scanning electron microscopy (SEM), while X-ray diffraction (XRD) coupled with in-situ heating stage was used to assess NSAC phase evolution during firing procedures. Two-way ANOVA with Bonfferoni post-hoc test was used for data analysis.
Results: TC only affected AS group, which provided the lowest SBS (P<0,05). Groups APA and C-725(1min) provided comparable (P>0,05) and durable bond-strength, while only groups C-900(1min), C-900(30min), C-1050(15min) provided higher (P<0,05) and clinically acceptable strength (>20Mpa). NSAC morphology remained unaffected (SEM), while crystal phase and adhesion to Y-TZP surface varied during different firing procedures (XRD). Namely, specimens treated at higher temperatures exhibited more residual NSAC areas after SBS testing, indicating stronger NSAC-Y-TZP surface integration.
Conclusions: NSAC offers zirconia pretreatment method superior to conventional APA with a beneficial role in clinical practice especially when subjected to higher temperature firings such as glaze- or zirconia-regeneration-firing procedures.