Mechanical and optical properties of machinable and pressable glass ceramic
Bukhari, Asmaa Faisal
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OBJECTIVES: To investigate the effect of various finishing procedures and repeated firings on color stability and translucency of different ceramics. Also, to examine the mechanical properties after finishing procedures. MATERIALS & METHODS: Four glass-ceramics were investigated; IPS e.max CAD, IPS e.max Press, Celtra Duo, and Celtra Press. For each material 87 disc-shaped specimens, 2 mm in thickness and 12 mm in diameter were fabricated. Specimens were divided into 3 groups (n=29) as no treatment, polishing, and glazing group. Five samples from each group were submitted to 1, 3, and 5 simulated “stain” firings using a ceramic furnace according to the manufacturer’s instructions. Color coordinates were recorded in the CIE L*a*b* system using an X-rite Ci7600 spectrophotometer and Vita EasyShade. Color differences relative to the control and translucency by contrast ratio were recorded. The 3-point flexural strength test was performed using an Instron universal testing machine. Hardness test was performed using Vickers microhardness and surface roughness was measured using a contact profilometer. Data were analyzed using multiple ANOVA (α=0.05). RESULTS: In general, glazed materials showed higher flexural strength than polished and no treatment groups. Celtra Duo showed the highest mean Vickers hardness followed by e.max CAD, e.max Press then Celtra Press. Surface roughness is lowered significantly when the glaze was applied to all materials. The contrast ratio of Celtra Duo is significantly lower than e.max CAD, e.max Press, and Celtra Press. Firing cycles displayed a significant effect in lowering the translucency of e.max CAD, and in raising the translucency of Celtra Duo and Celtra press, but no significant effect on Emax press. Surface treatment showed a non-significant effect on contrast ratio in all materials tested. Repeated firing cycles significantly affected color change for all materials tested, except for Celtra Duo. Also, polishing and glazing significantly affected color change for all tested materials. CONCLUSION: Type of material, surface treatments and firing cycles have a significant effect on color change and translucency. Surface treatment have a significant effect on the mechanical properties of glass ceramics; glazing can improve the flexural strength and reduce surface roughness of the ceramic materials.