Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks

The purpose of this study is to evaluate the effects of multiple firings on the mechanical properties and microstructure of veneering ceramics used with zirconia frameworks.Five different veneering ceramics for zirconia frameworks were used: Vintage ZR (ZR), Cerabien ZR (CZR), Vita VM9 (VM9), Cercon ceram KISS (KISS), IPS e.max ceram (e.max), and one veneering ceramic used for PFM frameworks: Vintage MP (MP). Twenty specimens were fabricated of each veneering ceramic. Ten specimens were fired twice and another ten specimens were fired ten times. Three-point flexural strength following the ISO 6872 and Vickers hardness were measured, and fracture toughness (K(IC)) was calculated. Density and porosity were determined. Specimens were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM).For all materials, density increased and porosity decreased after 10 firings. Significant differences in density and porosity were observed between 2 and 10 firings, with the exception of VM9 (P0.05). There were no significant differences in flexural strength between 2 and 10 firings except for MP. The Vickers hardness of ZR, VM9, KISS and MP increased significantly after 10 firings (P0.001). There were no significant differences in fracture toughness for ZR, CZR, VM9 and MP between 2 and 10 firings. However, e.max underwent a significant increase in fracture toughness (P=0.000), and there was a significant decrease in the toughness of KISS after 10 firings (P=0.007).Multiple firings could be effective for improving the densification and the hardness of veneering ceramics for zirconia restorations.By 10 firings, the density and hardness of the veneering ceramics used with zirconia frameworks were raised, and porosity was reduced. However, no significant changes occurred in flexural strength, fracture toughness or microstructure.