الفهرس 
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Abstract
This study was conducted to investigate the effect of silica coating techniques which were tribiochemical coating and application of two silica containing materials before and after sintering of translucent zirconia on the surface characterization and the bond strength to resin cements.
Translucent Zirconia plates (15.4 mm width × 19 mm length × 4 mm thickness) were prepared by cutting inCoris TZI blocks using a diamond disc mounted on Computer Numerical Control (CNC) milling machine. The plates were finished using silicon carbide paper 400 grit.
Three different techniques were used to apply silica to the surface of the zirconia plates in this study. These techniques were tribochemical silica coating and the application of two silica containing materials. In Tribochemical silica coating the surface of zirconia plate was bombarded with silicatized sand particles resulting in coating the surface with silica. Two silica containing materials were also applied which were: sodium silicate solution and filtek Z350 XT flowable composite that contains silica and zirconia nanofillers and nanoclusters. The three techniques were applied to presintered zirconia plates i.e before sintering of the zirconia and also to sintered zirconia plates i.e after sintering of the zirconia.
One sample from each group was examined for surface morphology using SEM and elemental surface composition using EDX.
Silane coupling agent was applied on all zirconia plates for one minute. Then, the plates were dried for 10 seconds with oil/water free compressed air. Small transparent microtubules were cut from polyvinyl tube with inner diameter 0.9 mm and height 2 mm. The microtubules were attached to the zirconia plate using a bonding agent. Each microtubule was held by tweezer and the bonding agent was coated at one of its ends by a microbrush. Then the microtubule was put on to the zirconia plate. A light emitting diode curing uniti of high intensity 1500 mW/cm2 was used to cure the bonding agent for 20 seconds. Five microtubules were bonded to each plate.
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The dispensing tip of the resin cement was inserted inside the microtubule and the resin cement was slowly injected till complete filling of the microtubule. The excess cement was removed using a microbrush. The resin cement was then light cured for 40 seconds. A circle was drawn around each microtubule by a pencil inorder to locate the area of bonding easily while microscopic examination after the microshear bond strength testing. The microtubules were sectioned to expose the resin cement microcylinders (0.9 mm diameter and 2 mm height) using sharp scalpel blade number 11. The resin cement microcylinders were examined by a magnifying lens for any defects at the interface.
Microshear bond strength testing was carried out immediately after the bonding with the resin cement for half of the samples without water storage. The other half of the samples were tested after water storage for one month in distilled water. The samples were stored at 37º C in an incubator
Each Zirconia sample with its own bonded composite micro-cylinders was secured with tightening screws to the lower fixed compartment of a materials testing machine with a load cell of 5 KN and data were recorded using computer software. A loop prepared from an orthodontic wire (0.014” in diameter) was wrapped around the bonded micro-cylinder assembly as close as possible to the base of the microcylinder and aligned with the loading axis of the upper movable compartment of the testing machine.
A shearing load with tensile mode of force was applied via materials testing machine at a crosshead speed of 0.5 mm/min. The relatively slow crosshead speed was selected in order to produce a shearing force that resulted in debonding of the microcylinder along the substrate-adhesive interface. The load required to debonding was recorded in Newton. The zirconia plates were examined under stereomicroscope and SEM to detect the failure mode.
The results of the present study showed that: The SEM images of Zirconia plate treated with Tribochemical silica coating before sintering of zirconia: The surface was covered with fine particles. Prominent micro pores 116
Summary and conclusions
and irregularities were also observed. The irregularities were more prominent when the treatment was applied after sintering of zirconia. Surface of Zirconia plate treated with application of sodium silicate solution, either applied before or after sintering, were covered with fine particles. The surface of zirconia treated with the application of flowable composite before sintering of the zirconia: The surface was covered with highly bound agglomerated clusters. While when applied after sintering separate clusters are observed and some areas were uncovered.
The EDX analysis showed: For the uncoated group the zirconia peak was the highest in addition to lower peaks of oxygen, yttrium and hafnium respectively. For all silica coated groups, the zirconia peak was the highest in addition to lower peaks of silica except for the application of flowable composite before sintering of zirconia group as the silica was the highest peak followed by the zirconia peak. The silica peaks varied among all the silica coated groups. Low peaks of alumina appeared in the groups of tribochemical silica coated zirconia plates either applied before or after the sintering of zirconia.
Concerning the effect of the application of silica coating technique before as well as after sintering of zirconia: for the non aged samples, there was no statistically significant difference in microshear bond strength values between tribochemical silica coating (16.14 Mpa for before sintering & 20.02 Mpa for after sintering) and application of flowable composite (17.4 Mpa for before sintering &17.9 Mpa for after sintering); both showed the statistically significantly highest bond strength.
Also for the samples aged in distilled water for one month: when the techniques were applied before sintering, there was no statistically significant difference in mean micro shear bond strength values between tribochemical silica coating (3.34 Mpa) and application of Flowable composite (8.46 Mpa); both showed the statistically significantly highest bond strength. There was no statistically significant difference in micro shear bond strength values between
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control (1.85 Mpa) and Sodium Silicate groups (2.02 Mpa) both showed the statistically significantly lowest bond strength values.
Concerning the effect of the application of silica coating technique for the samples aged in water: when the techniques were applied after sintering of zirconia, tribochemical silica coating showed the statistically significantly highest mean micro-shear bond strength (5.09 Mpa). There was no statistically significant difference between control, Sodium Silicate and Flowable composite groups; all showed the statistically significantly lowest mean micro-shear bond strength values (1.85, 1.22 & 1.85 Mpa respectively). The flowable composite applied after sintering with aging showed the statistically significantly lowest mean micro-shear bond strength values which may be due to the uneven distribution of silica clusters on the surface where loosening of silica in areas of low concentration during storage in water took place.
Concerning the effect of the application of the silica coating technique whether before or after sintering of zirconia: For non-aged samples, there was no statistically significant difference between micro-shear bond strength before and after sintering for tribochemical silica coating, Sodium Silicate as well as Flowable composite. While for aged samples, there was no statistically significant difference between micro-shear bond strength before and after sintering for the tribochemical silica coating.
Concerning the Effect of aging: for all samples silica coated before and after sintering of zirconia, control group, Cojet, Sodium Silicate as well as Flowable composite, there was a statistically significant decrease in micro-shear bond strength with aging. The thickness of the coating resulted from the application of all the tested techniques were below 60 μm.
After debonding the microscopic images revealed that the failure in samples tribochemical silica coated before sintering of zirconia was mainly adhesive as no remnants of resin were left on zirconia. While tribochemical silica coated samples after sintering of zirconia showed adhesive, mixed and cohesive failures with the resin cement. While for samples coated with sodium 118
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silicate solution, the resin cement was totally removed from all samples and the failure was mainly adhesive.
For samples coated witth flowable composite after debonding the SEM images revealed mainly mixed failures (remnants of resin cement remained on some areas and small areas of coating were detached) and few adhesive failures (no resin cement on zirconia). While for samples coated with flowable composite after sintering mainly adhesive failures.