Influence of Aging on Mechanical Properties of Translucent Zirconia

Objectives: The aim of this study was to evaluate the influence of aging and surface treatment on biaxial flexural strength (BFS), Vickers hardness and the roughness of translucent dental zirconia.

Methods: One surface of 80 disc-shaped zirconia specimens (1.2mm thickness and 12mm diameter, Katana HT10, Kuraray) was polished down to 1µm. Forty of these specimens were aged in an autoclave at 134°C, under 2bar (A) for 20 h, the other specimens were not aged (N). Afterwards the specimens of each group were randomly distributed into 4 subgroups according to the treatment of the second surface: no treatment (Sin), air-abrasion using 50 µm alumina particles at 1 bar (1b) and 2.5 bar (2.5b) and polishing down to 1 µm (1µm). After that BFS, Vickers hardness, surface roughness and phase changes were evaluated using a universal testing machine, an indentation tester, a laser scanning microscope and X-ray diffraction, respectively.

Results: Aging has no effect on roughness or hardness. Polishing groups either aged or non-aged showed the lowest mean value of roughness and hardness, while the other groups showed similar roughness. The average roughness (Ra) ranged from 0.002±0.001 µm (A-1µm) to 0.531±0.051 µm (N-1b). The Vickers hardness ranged from 1339±22 HV5 (N-1µm) to 1450±39 HV5 (N-Sin). No monoclinic phase was detected for three groups (N-Sin, N-1µm, and A-1µm). For the other groups the volume fraction of the monoclinic phase ranged from 9.8±0.6% (N-2.5b) to 41.5±0.3% (A-1b). BFS increased significantly with aging and air-abrasion. It ranged from 720±37 MPa (N-Sin) to 1153±92 MPa (N-1b).

Conclusions: A certain amount of monoclinic phase at the surface seems strengthen the high translucent zirconia ceramic, while hardness and roughness are not influenced. The used pressure during air-abrasion did not influence the investigated properties.