الفهرس 
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Abstract
This in-vitro study was designed to evaluate the effect of heat tempering on the biaxial flexural strength of heat pressed lithium silicate glass ceramics using four materials: Lithium disilicate glass ceramic (E), High density micronization lithium disilicate glass ceramic (L), Reinforced lithium silicate glass ceramic (C) and Reinforced lithium disilicate glass ceramic (A).
Sixty heat pressed glass ceramic discs of 15mm diameter, 1 mm thickness were fabricated using heat pressed technique. The discs were divided into four groups according to type of heat pressed glass ceramic (n=15), each group was subdivided into three subgroups (n=5) according to suggested thermal tempering temperature. Subgroup (T0): Samples with no thermal tempering. Subgroup (T1): Samples were subjected to thermal tempering at a temperature 9% below the recommended pressing temperature. Subgroup (T2): Samples were subjected to thermal tempering at a temperature 5% below the recommended pressing temperature.
All the discs of each material were tested for biaxial flexural strength (MPa) according to the guidelines of ISO standard 6872 for dental ceramics. The test was done using piston-on-three-ball technique in a universal testing machine Instron-3345. Then two fractured fragments of each material were tested for micro hardness that was measured using Tukon 1102 Wilson Hardness tester. The microstructure analyses then were performed using a FieldEmission Scanning Electron Microscopy (FESEM, Carl ZeissUltra, Oberkochen, Germany), the mineralogy of the specimens was studied by X-ray diffraction technique (Bruker, AXS D8 Advance) using monochromatic Cu K˛1 X-rays (= 1.5406 Å) and Energy dispersive X-ray Analysis (EDAXwas done to identify the elemental composition of materials.
Regarding the biaxial flexural strength, the results showed that ceramic type (regardless of tempering) had a statistically significant effect on mean biaxial flexural strength (P-value <0.001, Effect size = 0.586). Tempering (regardless of ceramic type) had a statistically significant effect on mean biaxial flexural strength (P-value <0.001, Effect size = 0.647). The interaction between the two variables had a statistically significant effect on mean biaxial flexural strength (P-value = 0.013, Effect size = 0.277). Since the interaction between the variables is statistically significant, so the variables are dependent upon each other.
Regarding the hardness, the results showed that ceramic type (regardless of tempering) had a statistically significant effect on mean hardness (P-value <0.001, Effect size = 0.981). Tempering (regardless of ceramic type) had a statistically significant effect on mean hardness (P-value <0.001, Effect size = 0.967). The interaction between the two variables had a statistically significant effect on mean hardness (P-value <0.001, Effect size = 0.875). Since the interaction between the variables is statistically significant, so the variables are dependent upon each other.
Regarding the biaxial flexural strength, IPS e.max Press showed the highest mean strength. There was no difference in strength between Celtra Press and VITA Ambria; both showed the lowest mean strength values. Regardless of ceramic type 5% below pressing temperature showed the highest mean strength and no thermal tempering showed the lowest mean strength. Regarding the hardness, LiSi press showed the highest mean hardness. Regardless of ceramic type 9% below pressing temperature showed the highest mean hardness and no thermal tempering showed the lowest mean hardness.