Effect of aging on tzp on hardness and flexural strength – an in vitro study

Objective: The purpose of this study was to evaluate the hardness and flexural strength of yttrium stabilised zirconia ceramics after various low temperature aging using computerised micro hardness tester and universal testing machine respectively. Materials and methods: The study included blocks of yttrium stabilized tetragonal zirconia from two manufacturers (n=15) Group A-AIDITE (Qinhuangdao Technology Co., Ltd, Hebei, China) and Group B-UPCERA (Shenzhen Upcera Dental Technology Co., Ltd, Guangdong, China), for each brand were machined, sintered and glazed according to manufacturer’s specifications to mimic a final polished crown. The sintered blocks were of 40mm (length) ×5mm (width) ×3mm (height) in dimensions approximately. Specimens were artificially aged in distilled water by heat treatment at temperatures of 100°, 150° and 200°C for 10 hours each in order to induce phase transformation. These specimens after aging were individually evaluated for hardness using computerised microhardness tester (Reichert Austria microhardess tester) and flexural strength using universal testing machine (Unitest-10). Results: The results from hardness testing showed that the hardness was highest without aging. There was a considerable decrease in hardness in group B when compared to group A. The results from flexural strength evaluation showed a decrease in the mean flexural strength with aging in Group A. On the contrary, flexural strength was higher without aging. Furthermore, the flexural strength in group B kept fluctuating and was the highest when aged at 100°C. A sudden decrease in the flexural strength was noted when aged at 150°C. Conclusion: The in vitro tests conducted reveal that when low temperature aging was carried out at a temperature of 100°C for 10 hours, the hardness of Y-TZP ceramics decreased whereas the flexural strength increased. However, beyond this temperature of 100°C especially at 150°C and above, the flexural strength of both the materials started to decrease.