Influence of firing temperature and duration on the hardness of dental zirconia for optimum selection of sintering conditions.

Abstract

Objective: In order to optimize the properties of dental zirconia, the sintering process involves firing zirconia to elevated temperatures for an extended time that can take several hours. The aim of this study is to investigate the effect of firing temperature and firing duration on the hardness of dental zirconia to indicate the optimum sintering conditions.

Methods: Thirty-six zirconia specimens in shape of bars were randomly assigned to nine groups. The zirconia specimen groups were sintered using a sintering furnace with different firing temperatures (900°C, 1200°C, and 1800°C) and firing durations (6, 9, and 12 h). A total of 108 hardness measurements were conducted for all specimens (12 hardness readings per group). For each of the specimen groups, micro Vickers hardness test was performed using a load of 1 Kgf (9.807 N) and the Vickers hardness number was computed. Statistical analysis using Kruskal-Wallis test was conducted to examine the significant differences on Vickers hardness number HV among the specimen groups according to the firing parameters with 0.005 p-value used as an indicator.

Results: Results suggest that there is an association between the increase in the hardness number and the increase in firing duration at a given firing temperature. The results also indicate that there is an association between the increase in the hardness number and increase in firing temperature at a given firing duration.

Conclusions: The greatest rate of hardness increase with time is associated with groups of firing temperature 1200°C. The highest rate of hardness increase with temperature happened during the first 6 h of sintering process. On the other hand, there is no significant increase in the hardness number when increasing the firing temperature beyond 1200°C.