الفهرس 
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Abstract
This in vitro study was conducted to reveal the effect of surface treatment of zirconia cores with nanosilica slurry and dentin porcelain slurry before sintering and the effect of aging in terms of thermocycling and cyclic loading on chipping resistance of veneers.
A model acrylic simulates first molar tooth was prepared for zirconia crown and 60 silicon impressions were recorded. Epoxy resin was mixed and poured in impressions. After 24 hours epoxy dies were transferred to scanning and designing machine for optical impression recording and zirconia cores milling. Zirconia cores were randomly divided into 6 groups (10 specimens in each group). group I and II cores were sintered and veneered according to the manufacturer instructions. group III and IV cores were treated by nanosilica slurry before sintering and then they were veneered as previous groups. group V and VI cores were treated by dentin porcelain slurry and then they were sintered according to the manufacturer instructions but these groups failed just after sintering by noticing obvious maroscopic cracking of the porcelain layer and zirconia cores; so these groups were cancelled.
All crowns were cemented to their corresponding epoxy dies by dual activated resin cement. Specimens of group II and IV were subjected to 10000 thermal cycles of 5°C-55°C intermittently (300 cycles per day) then they were stored in distilled water for 24 hours. The thermocycled specimens were transferred to chewing simulator to receive 10000 compressive cyclic loads in the center of crowns by 200N.
After that, all crowns of all groups were transferred to universal testing machine for chipping resistance test. The crowns were subjected to compression by steel ball with diameter of 5mm and with rate of loading 0.5 mm/min until chipping occurred.
Loads were recorded, area of loading was calculated and the chipping resistance stress was calculated.
Chipping resistance test showed that group III has the highest statistically significant mean of chipping resistance while group IV showed the lowest one. The analysis by SEM and EDX revealed that mode of failure in group I and II was mainly cohesive within the veneer layer. group III showed predominantly adhesive failure between nanosilica layer and zirconia core. group IV showed predominantly adhesive mode of failure between nanosilica layer and porcelain veneer.
Conclusions:
Within the limitations of this study it was concluded that:
1- The treatment of pre-sintered zirconia cores with nanosilica without aging led to high chipping resistance.
2- Aging decreased this resistance to chipping but it is still within the accepted range of ISO standards for ceramic coping and veneering porcelain.
3- The treatment of zirconia cores with dentin porcelain slurry before sintering causes obvious failure of zirconia after sintering.