الفهرس 
Title : Influence of cooling rate on flexural strength of zirconia veneered with two different veneering ceramics
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Abstract
The recent introduction of zirconia based ceramics as restorative dental materials has generated considerable interest in the dental community. Zirconia holds a unique place amongst oxide ceramics due to its excellent mechanical properties. The mechanical properties of zirconia are the highest reported for any dental ceramic, this permit a substantial reduction in core thickness. But; due to the optical opacity of these materials, they are covered with veneering ceramics with esthetic characteristics similar to the natural tooth substance.The residual stresses developed in veneered zirconia restorations can be modified using different heat treatments, and these changes are a direct result of the viscoelastic behavior of the glass veneer during various cooling rates of veneering technique. The study of flexural strength of veneered zirconia restorations using different cooling protocols is an important parameter to improve the clinical behavior and performance of veneered zirconia prostheses. This study investigated the effect of veneering techniques and cooling rate of glaze firing cycle on flexural strength of veneered zirconia specimens. A total of twenty ICE Zirkonzahn Y-TZP zirconia disc samples of 1.5 mm thickness and 1.2 mm diameter were fabricated following manufacturer{u2019}s instructions using specially constructed mold. The disc samples were divided into two groups according to veneering techniques into: group A: Veneered with nano-fluorapatite glass-ceramic IPS e.max Ceram (no= 10). group B: Veneered with fluorapatite glass-ceramic ingot IPS e.max ZirPress ingots (no= 10). The samples of each group were further subdivided into two subgroups (five each) according to the cooling protocols. Slow cooling rate following the manufacturer recommendation of the glaze firing cycle (control group) (sub group 1). Rapid cooling rate; the muffle opened after the glaze firing cycle has been completed, and specimens were removed as soon as the muffle had opened (sub group 2)