Thermal Shock of Porcelain Veneered Zirconia with Various Surface Treatments

Objective: Clinical reports indicate potential problems with veneer porcelain fracture on zirconia frameworks. The purpose of this study was to use thermal shock testing to evaluate the crack resistance of the different surface treatment of the zirconia veneered with different kind of porcelains

Methods: Yttria stabilized zirconia blocks (Vita) were sectioned into discs approximately 2 mm thick and sintered according to the manufacturer's instructions in the recommended furnace. A total of 144 specimens were randomly divided into three groups. For each group, a different surface treatment was applied:

(1) No treatment (2) Grinding using a 120 grit diamond disc with a 0.0034 kg/mm2 load (3) Grinding then heat treatment at 1000°C, 15 minutes. Four types of veneering porcelains were used: VM9, Lava Ceram, NobelRondo and Noritake CZR. Each type was fired according to the manufacturer's recommendations. Specimens were thermal shocked by heating to 90°C and then quenched in ice water. Specimens were dried at room-temperature and replaced in the furnace at 90°C until they reached equilibrium. Porcelain crazing was examined by:(1) Visual examination. (2) Microscope (10X) (3) UV- light inspection of fluorescent dye penetration

If failure was not observed the specimens were reheated to a temperature 10°C higher, quenched, and re-examined. This was repeated at 10°C higher each time until failure was observed in each specimen.

Results:

                     Table 1: Weighted Mean Temperature of  Failure. 

Porcelain Type	No Treatment	Grinding	Grinding and Heating	Total Mean
VM9	198°C	163°C	215°C	192°C
Lava Ceram	174°C	148°C	187°C	170°C
NobelRondo	156°C	140°C	145°C	147°C
Noritake CZR	160°C	144°C	172°C	158°C
Total Mean	172°C	148°C	180°C

ANOVA and Tukey showed there were significant differences among porcelain type and surface treatment (p < 0.05).

Conclusion: Grinding lowers crazing resistance. Grinding followed by a heat treatment improves crazing resistance.