Effect of Thermocycling on Vickers Hardness of Zirconia

Objectives: This study evaluated the effect of thermal cycling on Vickers hardness of different zirconia core materials. Methods: Disc shaped zirconia (ZirkonZahn, Cercon, Ceramill) specimens (15 mm x 1.15±0.02 mm) were fabricated according to manufacturers' instructions. The specimens from each system (N=20) divided into 4 groups (n=5/gp): control, 1000, 3000, 5000 cycles between 5 and 55ºC in water. The specimens were tested for Vickers microhardness, either with or without being subjected to thermal cycling, by applying a load of 9.8 N (1 kgf) with a velocity of 0,015-0,070 mm/sec on the indenter. Three indentations were performed for each specimen and both diagonal lengths of each indentation were measured using a microscope that is connected to the hardness tester. The results are stated as Vickers hardness number (VHN). Data were statistically analyzed using by one-way ANOVA and Tukey's test (p=0.05). Results: There was no statistically significant difference in the Vickers microhardness of ZirkonZahn (1378±51 VHN), Cercon (1414±27 VHN) and Ceramill specimens (1357±24 VHN)) specimens. There was no statistically significant decrease in the mean Vickers micro hardness value of ZirkonZahn and Ceramill specimens after thermal cycling whereas a statistically significant (p<0.05) decrease was seen in microhardness of Cercon specimens. There was no statistically significant change in mean microhardness of Cercon specimens at 1000 and 3000 thermal cycles. However, 5000 thermal cycles significantly decreased mean Vickers microhardness value (p<0,05). Conclusions: There was no statistically significant decrease in the mean Vickers micro hardness value of ZirkonZahn and Ceramill specimens after thermal cycling whereas a statistically significant decrease was seen in microhardness of Cercon specimens.

Keywords: Zirconia. Vickers Microhardness. Thermal Cycling.