Transient and residual stresses induced during the sintering of two dentin ceramics

To characterize the stress induced deformation of bi-axial flexure strength (BFS) test specimens during processing to provide an insight into sintering effects and associated BFS determination.

40 Vitadur-Alpha and 80 IPS e.max Ceram disc-shaped specimens were condensed and a sintered on a silicon nitride refractory tray under controlled firing and cooling parameters. The mean of the maximum deflection (μm) and Ra values (μm) were determined using a high resolution profilometer and were related to the orientation of the measured surface within the furnace. BFS testing of the subsequent groups (n = 20) was performed and the data related to the measured deformation of the sintered specimens. A two-way analysis of variance (ANOVA) where factors were identified as surface state and firing orientation with post hoc Tukey's tests was complemented by pair-wise comparisons with a Student's t-test for each measurement (P < 0.05).

The mean of the maximum deflection values and the mean BFS for Vitadur Alpha discs were not significantly influenced by firing orientation (P = 0.248 and P = 0.284, respectively). However, the Ra values were significantly different (P < 0.001). The two-way ANOVA revealed a significant impact on the mean of the maximum deflection measurements for surface state (P < 0.001) and firing orientation (P < 0.001) during sintering (P < 0.001). The mean Ra values were not significantly influenced. The BFS of sintered IPS e.max Ceram discs was sensitive to firing orientation (P < 0.001).

Conventional glass theory explains that residual thermal stress gradients induced during sintering can cause test specimen deformation which can alter the expected BFS data. The study demonstrates that variability such as firing orientation during sintering which is very rarely reported in the literature can have a significant impact on the reported BFS data and can confound its interpretation.