Evaluation of surface treatments of monolithic zirconia in different sintering stages

Purpose: This study evaluated the effect of surface treatment of zirconia in pre-sintered and post-sintered stages on its surface roughness, phase transformation, and biaxial flexural strength (BFS). Methods: Zirconia ceramic discs (n = 40) having a final dimensions of 12 mm diameter and 1.2 mm thickness were milled then divided into three main groups according to the type of surface treatment performed (Group 1 (n = 16); air-abrasion using Al2O3 particles, Group 2 (n = 16); silica coating using Rocatec soft, and Group 3 (n = 8); a control group receiving no surface treatment). Groups 1 and 2 were divided into two subgroups each according to the stage in which the surface treatment was performed (Subgroup A; surface treatment performed in the pre-sintered stage and subgroup B; surface treatment performed in the post-sintered stage). Surface roughness, phase transformation, and biaxial flexural strength (BFS) were later assessed. Data was then analyzed using two-way ANOVA and Tukey post-hoc tests at a significance level of p ≤ 0.05. Results: Subgroups treated in the pre-sintered stage showed higher mean Ra in μm (1.81 ± 0.36) when compared to the subgroups treated in the post-sintered (0.68 ± 0.07) stage and the control group (0.51 ± 0.10) (p ≤ 0.05). The pre-sintered treated group and the control showed no monoclinic phase while the post-sintered group showed significantly higher portions of monoclinic phase. Regarding BFS the post-sintered treated group had statistically significant higher values in MPa (1228 ± 81) when compared to the pre-sintered treated group (940 ± 101) and the control (1019 ± 82) (p ≤ 0.05). Conclusions: Air abrasion in the pre-sintered stage might be a promising surface treatment method to produce promising surface roughness values of zirconia without subjecting it to early degradation.