الفهرس 
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Abstract
Aim of the study: Evaluation of the effect of air abrasion/sintering order and number of veneer firing cycles on shear bond strength of veneering ceramic to zirconia core. Materials and Methods: Forty zirconia blocks were milled from pre sintered zirconia disc (Ceramill Zolid ht white) and randomly divided into two main groups (n=20) according to airborne abrasion time whether before or after sintering of zirconia. Three specimens from each group were analyzed by x-ray diffraction (XRD) to determine any phase transformation on zirconia surface. All zirconia specimens were veneered with IPS e.max ceram and furtherly divided into 4 subgroups (n=10) according to number of veneer firing cycles three or five. Shear bond strength (SBS) was tested using a universal testing machine. All specimens of each group were examined for failure mode analysis using an optical microscope. XRD test was done again to evaluate the effect of firing cycles on phase transformation of zirconia. Data were collected and analyzed with one-way and two-way ANOVA test. Results: one-way ANOVA test revealed a statistically significant difference in SBS between the 4 study groups (p≤0.05). SBS was significantly higher in post sintered groups than pre sintered groups while there was no significant difference in SBS between 3 and 5 veneer firing cycles. Also, two-way ANOVA test results showed no significant difference regarding the interaction between the two variables in this study (time of air abrasion and number of firing cycles) p=0.247. XRD test revealed phase transformation after airborne abrasion and reverse transformation after sintering and different veneer firing cycles. Conclusion: Air abrasion performed at post sintering stage had a favorable effect on SBS and increasing the number of firing cycles from 3 to 5 had no significant effect on SBS. Key words: air borne particle abrasion, pre/post sintering, phase transformation, veneer firing cycles, shear bond strength.