Evaluation of a high fracture toughness composite ceramic for dental applications

The introduction of yttrium partially stabilized zirconia polycrystals (Y-TZP) has pushed the application limits of all-ceramic restorations. The mechanical properties of these materials can be further improved by the addition of a secondary dopant phase. The aim of this work was to evaluate the properties of a new nano-composite ceramic used as a dental framework material.

The properties of a new ceria-stabilized tetragonal zirconia polycrystal co-doped with alumina (Ce-TZP-Al) were investigated. Y-TZP was used as control. Sixty bars (20 x 2.5 x 1.5 mm(3)) from each material were prepared by cutting CAD/CAM milling blocks. Twenty specimens were used to measure the 4-point flexural strength and the modulus of elasticity of the tested materials. The remaining specimens were used to measure the fracture toughness using indentation strength (IS), single edge notched beam (SENB), and fractography (FR). The thermal expansion coefficient (TEC) was measured using temperature expansion diagrams. The bond strength of the two framework materials to two esthetic veneer ceramics was tested using the microtensile bond strength test (MTBS). Finally, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX) were used to analyze the internal structure of the materials. One- and two-way analysis of variance (ANOVA) and Bonferroni post hoc tests were used to analyze the data (alpha= 0.5).

The flexural strength and modulus of elasticity of Ce-TZP-Al (856 MPa, 170 GPa) were significantly weaker (p < 0.001) than those of Y-TZP (1003 MPa, 215 GPa). The (IS) fracture toughness of the former (19.02 MPa m(1/2)) was significantly higher (p < 0.001) than SENB (12.6 MPa m(1/2)) or FR (12.8 MPa m(1/2)) values. These values were significantly higher (p < 0.001) than the fracture toughness of Y-TZP (7.4 MPa m(1/2)), which showed statistically similar values using the same three techniques. The measured TEC for the two materials was relatively similar, 10.1 microm/ degrees C and 10.4 mum/ degrees C, respectively. Regarding MTBS values, Ce-TZP-Al had significantly lower bond strength values (p < 0.001) and a higher percentage of interfacial failure than Y-TZP, which failed completely cohesively with the two used veneer ceramics. SEM analysis revealed zirconia grains pull out and structural defects at the core-veneer interface for Ce-TZP-Al material, which explained its weak bond to the two used veneers.

Despite the promising mechanical properties of Ce-TZP-Al nano-composite ceramic, its very low bond strength to esthetic veneers leaves such layered restorations highly susceptible to delamination and chipping under function. Further studies are needed to enhance the surface stability of this high fracture toughness ceramic.