Effect of Artificial Aging on Hardness and Surface Roughness of Two Types of Zirconia (In-Vitro Study)
EL-Sherif, Ramy Hammed
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Zirconia has been one of the most important ceramic materials for well over a century. It has other intrinsic physical and chemical properties, including its strength, high toughness, low wear resistance, high elastic modulus, ionic conductivity, chemical inertness as well as high melting temperature that make it attractive as an engineering material and the focus of continued effort to understand and improve its mechanical properties (133) . The increase in its industrial applicability has been brought about by the formation of the solid solution to prevent deleterious t-m phase transformation during cooling after sintering. It is well known that a suitable percentage of MgO, CeO, and Y are most commonly used for this purpose, to inhibit crack propagation, controlling the extent of stress induced by t-m transformation, and exhibited high mechanical properties A total of forty discs of two brands of CAD/CAM zirconia ceramics were used in the current in-vitro study. The specimens were divided into two main groups according to the type of zirconia; Group 1 (n=20): Tetragonal zirconia (BioZX 2 color) and Group 2 (n=20): Cubic zirconia (DD Cube X 2 98color). The required shape of the specimens was designed using digital software system in order to accurately design a cylinder shape (15mm diameter×25mm thickness) from the zirconia blanks (98mm diameter×25 mm thickness). Isomet was used to cut forty discs from their respective cylinder with dimensions (15 mm diameter × 1.5 mm thickness) which is approximately 20-25% oversize to compensate for sintering shrinkage. Then all discs were sintered, finished and polished according to manufacturer instructions. Low thermal degradation (LTD) aging was performed using an autoclave. The autoclave was programmed at 134°C, 2 bar pressure for 5 hours (10 cycles) which is equivalent to the standard aging protocol to simulate oral conditions for 15years 66 (126) . The autoclave cycle starts from zero pressure and increased to the desired pressure (2 bars) in 15 minutes so the autoclave cycle (45 minutes) was calculated only as 30 minutes. All the forty specimens were tested for surface roughness using optical profilometer and hardness using Vickers micro-hardness tester before and after aging. One specimen of each group was examined using ESEM before and after aging to determine any change in the surface topography due to aging. The results of surface hardness revealed that regarding both tetragonal and cubic zirconia; there was a statistically significant decrease in mean hardness after aging (P-value = 0.002) and (P-value = 0.027) respectively. However, regardless of aging; there was no statistically significant difference between mean hardness of the two zirconia types (P-value = 0.930). Moreover, regardless of zirconia type; there was a statistically significant decrease in mean hardness after aging (P-value <0.001). The result of surface roughness revealed that regarding both tetragonal and cubic zirconia; there was a statistically significant increase in mean Ra after aging (P-value = 0.003) and (P-value <0.001) respectively. Meanwhile regardless of aging; tetragonal zirconia showed statistically significantly higher mean Ra than cubic zirconia (P-value = 0.023). Furthermore regardless of zirconia type; there was a statistically significant increase in mean Ra after aging (P-value <0.001). Moreover before aging; tetragonal zirconia showed statistically significantly higher mean Ra than cubic zirconia (P-value = 0.042) while, after aging; there was no statistically significant difference between mean Ra of the two zirconia types (P-value = 0.221).
CONCLUSIONS Within the limitations of the present study, the following could be concluded: 1. Artificial aging (low thermal degradation) negatively affects the hardness of both tetragonal and cubic zirconia. 2. Artificial aging causes surface roughness increase for both tetragonal and cubic zirconia.