الفهرس 
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Abstract
Aim: To assess the effect of surface finish and acidic beverage (cola beverage) on hardness and fracture toughness of zirconia lithium silicate (Celtra press)
Methodology: Total of n 36 samples of zirconia reinforced lithium silicate (Celtra press) plates were divided into two main groups (n=18 glazed samples and n=18 polished samples) and then each group was subdivided according to immersion solution (n=9 samples in artificial saliva and n=9 samples in cola). group (A) subgroup (1) was auto glazed plates in cola beverage (acidic medium), group (A) subgroup (2) was auto glazed plates in artificial saliva, group (B) subgroup (1) was polished plates in cola beverage (acidic medium), and group (B) subgroup (2) was polished plates in artificial saliva. Surface hardness and facture toughness measurements were obtained for each specimen, before and after storage in the solutions. Surface micro hardness was measured by Vicker Micro-hardness tester. The fracture toughness was measured by using the indentation method. The indentation technique is based on sequences of cracks that develop around the indenter when brittle material subjected to certain load.
Results: for Vickers hardness (HV) results: For Glazed group, it was found that the highest mean ± SD values of Vickers hardness were recorded before immersion subgroup (285.65±5.50 Kgf/mm2) followed by AS immersed subgroup mean ± SD values (276.97±7.65 Kgf/mm2) meanwhile the lowest mean ± SD values were recorded with Cola immersed subgroup (250.35±14.73 Kgf/mm2). For Polished group, it was found that the highest mean ± SD values of
Vickers hardness were recorded before immersion subgroup (291.7±10.4 Kgf/mm2) followed by AS immersed subgroup mean ± SD values (278.52±10.39 Kgf/mm2) meanwhile the lowest mean
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± SD values were recorded with Cola immersed subgroup (275.83±5.55 Kgf/mm2). For fracture toughness (MPa.m1/2) results: For Glazed group, it was found that the highest mean ± SD values of fracture toughness were recorded before immersion subgroup (2.193±0.3403 MPa.m1/2) followed by AS immersed subgroup mean ± SD values (2.043±0.1155 MPa.m1/2) meanwhile the lowest mean ± SD values were recorded Cola immersed subgroup (1.6515±0.3989 MPa.m1/2). For Polished group, it was found that the highest mean ± SD values of fracture toughness were recorded before immersion subgroup (2.4501±0.3533 MPa.m1/2) followed by AS immersed subgroup mean ± SD values (2.3756±0.2129 MPa.m1/2) meanwhile the lowest mean ± SD values were recorded Cola immersed subgroup (2.3066±0.3141 MPa.m1/2).
Conclusion: Ageing in acidic medium (Cola beverage) negatively affected both the surface hardness and the fracture toughness for Celtra press (zirconia reinforced lithium silicate). Polished Celtra press (zirconia reinforced lithium silicate) had prominent hardness and fracture toughness and less crack length compared to auto-glazed Celtra press.
Keywords: Celtra press, hardness, fracture toughness, hydrothermal aging, acidic medium and artificial saliva.