ABSTRACT
OBJECTIVES: To investigate the influence of the zirconia and sintering parameters on the optical and mechanical properties. METHODS: Three zirconia materials (3/4Y-TZP, 4Y-TZP, 3Y-TZP) were high-speed (HSS), speed (SS) or conventionally (CS) sintered. Disc-shaped specimens nested in 4 vertical layers of the blank were examined for grain size (GS), crystal phases (c/t'/t/m-phase), translucency (T), and biaxial flexural strength. Fracture load (FL) of three-unit fixed dental prostheses was determined initially and after thermomechanical aging. Fracture types were classified, and data statistically analyzed. RESULTS: 4Y-TZP showed a higher amount of c + t'-phase and lower amount of t-phase, and higher optical and lower mechanical properties than 3Y-TZP. In all materials, T declined from Layer 1 to 4. 3/4Y-TZP showed the highest FL, followed by 3Y-TZP, while 4Y-TZP showed the lowest. In 4Y-TZP, the sintering parameters exercised a direct impact on GS and T, while mechanical properties were largely unaffected. The sintering parameters showed a varying influence on 3Y-TZP. Thermomechanical aging resulted in comparable or higher FL. CONCLUSION: 3/4Y-TZP presenting the highest FL underscores the principle of using strength-gradient multi-layer blanks to profit from high optical properties in the incisal area, while ensuring high mechanical properties in the lower areas subject to tensile forces. With all groups exceeding maximum bite forces, the examined three-unit FDPs showed promising long-term mechanical properties.
Subject(s)
Flexural Strength , Materials Testing , Zirconium , Zirconium/chemistry , Mechanical Phenomena , Particle Size , Stress, Mechanical , Optical PhenomenaABSTRACT
STATEMENT OF PROBLEM: The development of zirconia materials with optimized properties has been rapid, and studies comparing the mechanical and optical properties of recently introduced zirconia with lithium disilicate materials are limited. PURPOSE: The purpose of this in vitro study was to compare the mechanical and optical properties of cubic/tetragonal zirconia materials with those of a lithium disilicate ceramic. MATERIAL AND METHODS: Specimens were fabricated from 6 different noncolored zirconia materials: Ceramill Zolid FX (CZ), CopraSmile (CS), DD cubeX2 (DD), NOVAZIR MaxT (NZ), priti multidisc ZrO2 (PD), and StarCeram Z-Smile (SC), and 1 lithium disilicate ceramic as a control, IPS e.max Press LT A2 (CG). Four-point flexural strength (N=105/n=15) and fracture toughness using the single-edge V-notched beam (N=105/n=15) were examined according to International Organization for Standardization standard 6872:2015. Translucency (N=70/n=10) was evaluated with an ultraviolet spectrophotometer. Grain size (N=6/n=1) of zirconia was investigated by using scanning electron microscopy. Data were analyzed using the Kolmogorov-Smirnov test, multivariate analysis, 1-way analysis of variance, followed by the post hoc Scheffé test and Kruskal-Wallis and Mann-Whitney U tests, and Weibull analysis, using the maximum likelihood estimation method at 95% confidence level (α=.05). RESULTS: Zirconia materials showed higher mechanical and lower optical properties than CG (P<.001). No differences were observed among the zirconia materials with respect to flexural strength (P=.259) or fracture toughness (P=.408). CG and CS showed significantly higher Weibull modulus than SC and PD. The lowest translucency values were measured for NZ and SC, followed by CS, DD, and PD (P<.001). CZ showed the highest translucency values (P<.001). The lowest grain sizes were found for NZ, DD, and SC; the largest were shown for CS (P<.001). CONCLUSIONS: Cubic/tetragonal zirconia showed better mechanical properties than lithium disilicate ceramic. However, the optical properties and the reliability of zirconia are lower than those of lithium disilicate ceramic.
