Sandblasting was identified as a key factor in

Sandblasting was identified as a key-factor in establishing a durable bond between the luting agent and the ceramic, when combined with 10-Methacryloyloxyde-cyl dihydrogen phosphate (MDP monomer), either contained in the adhesive primer (as in the present study) or in the cement itself [59,68,69]. Oyagüe et al. [42], and Blatz et al. [55], assumed that AL2O3 abrasive particles removed any organic contaminants, produced an activated micro-roughened zirconia surface, increased the bonding area, modifying the surface nucleoside transporters and wettability [68], so they improve the bond strength by allowing for micromechanical interlocking of the resin cement [70]. SEM evaluation showed that, AL2O3 sandblasting produced an increased roughness and irregularities with grooves and sharp edges [71,72] (Fig. 8) these are considered to be important for the interlocking of the composite resin cement to Y-TZP ceramic. Moreover, 3D-optical profilometer analysis of the sandblasted group (Table 8, Fig. 18) revealed higher mean value of irregularities 1.26 μm, compared to control group. Both silica plasma coated and oxygen plasma treated groups showed the statistically significantly highest mean shear bond strength values. Although there was no statistically significant difference between them, silica- plasma coated group in this study recorded higher mean value of shear bond strength (19.6 MPa), followed by oxygen plasma-treated group (18.1 MPa). Plasma deposition techniques could change the surface properties by attaching a film to the surface of the material [73], in a fast process that can be performed at low temperatures. Moreover the thickness and chemical composition of the film can be controlled [74,75] through the sputtered material from the target, the gas and the plasma source used [73,76]. In the present study silica deposition process using argon gas was performed, where Si-O bonds were available on the entire Y-TZP surface, promoting the chemical adhesion achieved by the silane coupling agent [42,77]. These results are consistent with those of Derand et al. [40], who showed that treating zirconia surface with plasma spraying increases shear bond strength to the resin cement, compared to untreated group. They are also consistent with those of Zhu et al. [67], who found that silica coating/salinization was most effective in improving the bond strengths of the resin cements to zirconia, compared with Al2O3 sandblasting group due to the chemical bond formed via the silica layer on ceramic surface, silane coupling agent, and resin cement. In contrast, Kern and Wegner [22], and Blatz et al. [59], concluded that air-abrasion combined with MDP containing resin composite provides superior long-term shear bond strength than silica-coated zirconia bonded to Bis-GMA resin cement. SEM evaluation of Si –coated plasma samples in this study (Fig. 10) support the shear results, where a typical honeycomb pattern and clusters of silica particles covered loosely the surface, creating chemically reactive islets on the surface of the samples and could thus have chemically modified the surface of zirconia to enable a better reaction with the primers. Microcracks, some roughness and porosities, were also supported by higher mean of irregularities, 1.6 μm, as indicated by 3-D profilometer analysis (Table 8, Fig. 20). These results are in agreement with those of Della Bona et al., [70] who showed that silica coating increase the surface roughness of zirconia. On the other hand Ozcan et al. [66], reported that surface roughness (Ra) of zirconia samples was the highest with 50 mm Al2O3 sandblasting than that of silica coated samples, also SEM images (×500) showed rougher surface of sandblasted group compared to silica coated one. Another application of plasma that was used in this study is the treatment of Y-TZP samples with oxygen gas, which allow physical etching that could improve the micro-roughness, increasing surface oxidation; allowing for modification of surface-bound chemical groups, and provide hydrophilic surfaces; improving wettability as well as the affinity toward the resin cement [26,78,79]. These results (Table 1) concur with those of Piascik et al. [80], who reported a more chemically reactive surface of Y-TZP samples when plasma treated by oxy-fluoride gas, thus allowing for covalent bonding between zirconia surface and resin cement, resulting in higher bond strength compared with control group. On the other hand plasma treated zirconia samples were found by Piascik et al. [74], to display the highest shear bond strengths and improved chemical bonding than control, polished, sandblasted and even Si-coated groups. SEM results (Fig. 9) showed roughness, irregularities, and both irregular and rounded pits and porosities, where the primer and adhesive resin can infiltrate and interlock, this was compatible with the highest mean of irregularities 1.8 μm showed by profilometer (Table 8, Fig. 19).