الفهرس 
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Abstract
This study was conducted to evaluate the effect of sintering time and thermocycling on properties of zirconium dioxide core materials.Eighty zirconia discs were divided into two groups. Discs of the first group were sintered in sintering oven (ZirkonZahn) at 20–1500◦C temperature using rise time of 3h and kept at 1500◦C for 2h. Discs of the second group were sintered at 20–1500◦C using rise time of 1h 40 min and kept at 1500◦C for 1h. Half of the discs from both groups were thermocycled in distilled water for 5000 cycles (5–55◦C). Crystalline structure, surface roughness, fracture toughness and shear bond strength between zirconia cores and porcelain veneers were studied. The results were then statistically analyzed and compared. Based on the results and within the limitations of this study, the following conclusions can be made. 1. Variation in sintering time from 2h and 40 m to 5 h had no influence on mechanical properties of Y-TZP zirconia. 2. Thermocycling of zirconia sintered at normal sintering time has no apparent detrimental effect on their properties. 3. Thermocycling of zirconia sintered at short sintering time could have detrimental effect on their properties. 4. Different types of veneering porcelains can be used successfully with Y-TZP zirconia core provided that they are compatible with it, as they have similar coefficient of thermal expansion (CTE) and elastic modulus.