Korean J Dent Mater.  2021 Mar;48(1):61-70. 10.14815/kjdm.2021.48.1.61.

Effect of the cooling rate on characteristics of zirconia core ceramics during porcelain firing

  • 1Department of Dental Lab, Wonkwang Health Science University, Iksan-si, Jeonbuk, Republic of Korea
  • 2Department of Dental Materials, School of dentistry, Pusan National University, Yangsan-si, Gyeongsangnam-do, Republic of Korea


The effect of the cooling rate on changes in hardness, flexural strength, and microstructure of zirconia core ceramics was investigated during simulated porcelain firing without layering porcelain on the zirconia core ceramic. Three cooling rates were tested: 227.5 ℃/min, which is the rate suggested by the manufacturer, Stage 0 (taking the ceramic out of the firing chamber immediately after firing and bench cooling to room temperature), and Stage 3 (cooling to 600 ℃ with the firing chamber closed and then bench cooling to room temperature (33 ℃/min)). In the Stage 0 group and the group cooled at the rate suggested by the manufacturer, the hardness increased compared to the group before firing (p<0.001). The hardness of the Stage 3 group was not different from that of the group before firing (p>0.05). The grain size of the specimen groups whose hardness increased after firing was reduced by recrystallization, but the Stage 3 group had coarsened grains. In all test groups before and after firing, only the tetragonal phase was observed. In particular, a metastable phase (T’) in which the axial ratio (c/a ratio = c/√ 2a) was closer to 1 coexisted with the tetragonal phase. The flexural strength of the zirconia core did not exhibit a significant difference with respect to the cooling rate (p>0.05).


Zirconia core ceramic; Simulated firing; Cooling rate
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