Fracture toughness of heat-pressed and layered ceramics.

STATEMENT OF PROBLEM: Veneering ceramic materials designed to be used with high noble alloy and zirconia-based restorations have been reported to be susceptible to chipping in vivo. PURPOSE: The purpose of this study was to evaluate and compare the fracture toughness of heat-pressed and layered ceramics intended for zirconia and high-noble alloy substrates. MATERIAL AND METHODS: Bar specimens were fabricated from 8 different ceramics (Ivoclar-Vivadent [I] and Noritake [N]) intended for pressing (P) and layering (L) to high noble alloy (M) and zirconia (Z) substrates, following the ISO 6872 protocol. The single edge notch beam test method was used to create a notch in the center of each specimen, which was then tested with a universal testing machine (n=6, cross-head speed=0.5 mm/min) and the fracture force values recorded. These values were used to calculate the fracture toughness (K1c) for each specimen. Fracture surfaces were examined with a scanning electron microscope, and the basic components of the tested ceramics were determined by using energy dispersive x-ray (EDX) spectroscopy. Data were analyzed with 3-way ANOVA, followed by multiple comparisons using the Holm method (α=.05). RESULTS: The mean (SD) of the calculated fracture toughness values obtained ranged from 1.20 (0.04) MPa·m(1/2) (group NZL) to 1.74 (0.04) MPa·m(1/2) (group IZL). Fracture toughness was significantly higher in group IZL (1.74) than group IZP (1.41), but lower in group NZL (1.20) than group NZP (1.36) (P<.001). Fracture toughness was somewhat lower in group IML (1.36) than group IMP (1.47) (P=.018), and no significant difference was found between group NML and group NMP (P=.14). Veneering ceramics used with a metal substrate showed a crystalline structure mixed with a glassy phase pattern on the fracture surface. The results of EDX analysis on the fracture surfaces indicated that the tested ceramics were composed of Si, Al, K, Na, Mg, and oxygen elements. CONCLUSIONS: Ceramics used for veneering zirconia substrate may have various fracture toughness values that relate primarily to the processing technique.

Shear bond strengths of pressed and layered veneering ceramics to high-noble alloy and zirconia cores.

STATEMENT OF PROBLEM: Heat-pressed ceramics to metal alloys and zirconia have been available for some time. However, information regarding their shear bond strengths is limited. PURPOSE: The purpose of this study was to evaluate the shear bond strengths of heat-pressed and layered ceramics with regard to their corresponding high-noble alloy and zirconia cores. MATERIAL AND METHODS: Forty cylinders (approx. 5 mm in diameter) of high-noble alloy (Olympia) were cast and divided into 4 groups (n=10). Metal cylinders were veneered with ceramics to produce shear test specimens: Group PMI with IPS InLine POM; Group LMI with IPS InLine; Group PMC with Pulse press-to-metal; and Group LMC with Authentic Pulse Metal ceramic. Forty cylinders (approx. 5 mm in diameter) of zirconia (Lava) were obtained and divided into 4 groups (n=10). These cylinders were veneered with ceramics to produce shear test specimens: Group PZI with IPS e.max ZirPress; Group LZI with IPS e.max. Ceram; Group PZV with VITA PM9; and Group LZV with VITA VM9. The veneering ceramics, 3 mm in thickness, were either pressed or layered to their corresponding cylinders. Thermal cycling was performed at 5°C and 55°C for 20,000 cycles with a 20 second dwell time. Shear bond strength testing was conducted in a universal testing machine, and the failure strengths were recorded. Fracture surfaces were characterized visually, under a stereomicroscope, and with a scanning electron microscope (SEM). Data were analyzed using rank-based Kruskal-Wallis and Mann-Whitney tests with Bonferroni correction to adjust for multiple comparisons (α=.05). RESULTS: For metal ceramic specimens, the mean (SD) shear bond strengths ranged from 37.8 (20.6) MPa to 66.4 (22.1) MPa. There were significant differences between Groups PMI and PMC and between Groups LMI and PMC, in which Groups PMI and LMI had significantly higher strength values than Group PMC (P=.041). For zirconia ceramic specimens, the mean (SD) shear bond strengths ranged from 30.03 (9.49) MPa to 47.2 (13.0) MPa, with Group LZV having a significantly higher shear bond strength value than Group LZI (P=.012). Half of the Group PZV specimens failed during thermal cycling, and Group PZV was, therefore, excluded from statistical analysis. For all shear bond strength testing specimens, cohesive failures in the veneering ceramics were observed. CONCLUSIONS: For shear bond strength of veneering ceramics to high-noble alloy, there was no significant difference between pressing and layering with the same manufacturer. For shear bond strength of veneering ceramics to zirconia, there was no significant difference between the pressed and layered groups.