الفهرس 
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Abstract
Statement of Problem: In order to achieve better contour, color, shape and esthetics, multiple firing procedures are essential for the fabrication of all-ceramic restorations. However, subjecting it for multiple firing cycles and different temperatures consequently affect the mechanical properties and the restoration’s durability as well as restoration’s esthetics.
Aim of the Study: The aim of this in vitro study is to assess the effect of multiple firing cycles with different firing temperatures on the Flexural strength and Micro shear bond strength of the lithium silicate.
Materials and Methods: Blocks of Specimens of Lithium Silicate (Obsidian) were divided into 2 groups according to the measurements done. First group was sectioned into 36 bar-shaped samples with the following dimensions 14mm *2mm* 2mm for the three point bending test, according to the ISO 6872 standard using using IsoMet 5000 microsaw with cutting speed 2500 rpm. While for Second group the Obsidian Lithium silicate blocks were used to prepare 32 slices with the following dimensions: 14mm x 12mm x 1mm Using IsoMet 4000 micro saw with cooling water system, by a dimond disk 0.6 mm thickness with cutting speed 2500 rpm.
All Obsidian samples were crystallized according to manufacture recommendations in ceramic furnace, manually polished and each group was divided into four subgroups of according to the number of the firing cycles which were 1, 2, 3 and 5 fire cycles respectively.
For group F, three point bending tests were carried out on the specimens using a universal testing machine (ElectroPuls E3000, Instron, USA) with 12 mm distance between supporting rollers. It was then uniaxially loaded and Loading was increased gradually with the crosshead speed of 1 mm/min until fracture occurred.
For group S, For easier handling and fixation during the micro shear test, the whole group of 32 slices of Obsidian Lithium Silicate material were embedded in an acrylic blocks. They were etched using IPS ceramic hydrofluoric etching gel according to the specified etching duration 10 seconds according to manufacturer’s instructions and then washed with an air water spray for 30 seconds and air-dried with oil-free air/water syringe. Silane coupling agent was applied for 60 seconds. Then air drying was done for the specimens using oil free air way syringe.
Each ceramic specimen received 5 resin micro cylinders. Irises of polyethylene tube having 1mm diameter and 1mm height were positioned over the disc surface, then cement was injected into the tubes through the mixing tip, light curing was done through the tube for 40 seconds.
Polyethylene tube irises were not removed in order not to subject the resin micro cylinders to shear stress at the interface, and to eliminate any cause of failures.
At the end Micro-shear bond strength test was performed on all 160 micro-tubes.
Results: For group F, it was found that there was no significant difference between vales of different groups (p=0.166). The highest flexural strength (MPa) value was found in control samples followed by samples subjected to 5 cycles then samples subjected to 2 cycles, while the lowest value was found in 3 cycles’ samples.
As for group S, There was no significant difference between vales of different groups (p=0.363). The highest shear bond strength (MPa) value was found in samples subjected to 3 cycles, followed by control samples then samples subjected to 5 cycles, while the lowest value was found in 2 cycles’ samples.
There was no significant correlation between flexural and shear bond strength.