Evaluation of Different Zirconia Surface Treatments on their Microhardness ISSN : 1812 – 1217

Aims: To investigate the effect of mechanical and chemical surface treatments including (AL2O3 —110μm) and citric acid solution (30%) on the microhardness of zirconia ceramic. Materials and Methods: Twenty four cubic shaped specimens were prepared from partial sintering zirconia (IPS e.max ZirCAD MT).Specimens were randomly assigned into three main groups (GI, GII, and GIII) according to the surface treatment methods (n=8). Group I: zirconia surface left without surface treatment (control), Group II: In this group all specimens were subjected to sandblasting with 110 μm Aluminum oxide particles (Al2O3), while Group III: The surface was treated with citric acid solution 30% for 10 minutes. Force of Vickers microhardness was applied on the intaglio surface of the zirconia specimen at 0.5 kg for 30 seconds. Data were analyzed with one way ANOVA at 5% level of significance. Results: Sandblasting with AL2O3 110μm significantly increased the Vickers microhardness value of the zirconia surface. Chemical treatment with 30% citric acid did not exhibit any significant changes in comparison to the control group. Conclusions: Within the limitation of this in vitro study, the mechanical surface treatment of the zirconia surface with AL2O3 improves the surface microhardness.


INTRODUCTION
In the last decades, the increased requirements in esthetic dentistry have reached to the overcoming of the metalceramic prosthesis with a focus on nonmetal restorations. Zirconia restoration gains an effective role in dentistry due to its superior mechanical characteristics with good biocompatibility (1) . The mechanical behavior of zirconia restoration was documented to be better than other ceramics (2) . For example, the fracture toughness is double than aluminum oxide ceramics thus restoration fabricated from zirconia is expected to be more clinically durable (3,4) . Due to the resistance of the material to many treatments, the adhesion of zirconia to tooth substrate or luting material becomes difficult subject (5,6). Different methods have been proposed to promote adequate adhesion between the resin cement and zirconia. Zirconia surface can be treated either by micromechanical process or by chemical modification of the surface or a combination of both (7). The abrasion with aluminum oxide particles at different sizes ranged from 50-125 µm is considered and proved as an efficient treatment of zirconia surface before their delivery (8 (13) . Such Acidic solution can't be really applied for glass-free zirconia. A substitute acidic solution for zirconia was invented by different studies (14) . A Recent study evaluated the citric acid solution of 30% applied for 10min. They claimed that surface roughness was possible with this type of acid and protocol (15). Some studies showed an increase in the strength of zirconia after alumina particle abrasion (16,17

Specimen preparation:
Twenty four specimens of cubic shaped   Then the specimens were gently washed with distilled water for one minute and dried.

Microhardness test:
Zirconia specimen was fixed on the    Figure (4).

DISCUSSION
Surface hardness is one of the paramount physical property that affects the clinical performance of zirconia restorations. It contributes to resistance of the material to the external force (22). Zirconia-based dental restorations are subjected to continuous loads with contacts during functional chewing and mastication therefore; the mechanical properties such as hardness influence the long-term performance of the restorations (23) .
In this study, microhardness property was investigated after mechanical sandblasting of zirconia restorations with (AL 2 O 3 110µm) and  (24,25,26). Several studies reported an increase in strength after sandblasting as compared to other treatment techniques and related such finding to increase in tetragonal to a monolithic transformation of the surface crystals with an associated increase in strength (5,27) .  (28). Zirconia ceramic has a polycrystalline structure, which is chemically inert and may exhibit a potential resistance to aggressive chemical agents such as (strong acid, alkalis, organic, and inorganic dissolving agents). A zirconia has property dissimilar to ceramic because zirconia is a glass-free material (16). In this study when the surface of the zirconia exposed to 30% of citric acid for 10 min and evaluated in response to external force represented by microhardness test, the result shows no difference from the untreated surface. Xie et al (29) showed that when zirconia specimens  (15,18) demonstrated topographical alteration within surface exposed to citric acid when examined under SEM. Further study may be necessary to confirm this alteration.

CONCLUSIONS
Within the limitation of this study mechanical surface treatment of zirconia via sandblasting with AL 2 O 3 (110µm) has increased the microhardness of zirconia meanwhile treatment with 30% of citric acid solution applied for 10 min has no effect on the microhardness.