Effect of Processing History on Phases and Strength of 3Y-TZP

Objectives: It is well established that thermal and surface treatments dictate phase composition and mechanical properties of 3Y-TZP. A detailed analysis of the surface crystalline phase composition and its effect on strength of 3Y-TZP is needed.
Objective: to investigate the relationship between surface crystalline phases and mechanical properties of 3Y-TZP sintered at various temperatures, with or without air abrasion.
Methods: 3Y-TZP blanks were prepared by cold isostatic pressing, sectioned into discs, randomly divided into ten groups (n=20), and sintered at 1350, 1450, 1550, 1600 or 1650°C for 2h. For each temperature, one group was left as sintered as control (AS) and one group was air abraded (AA). The microstructure was investigated by SEM. Surface crystalline phases were analyzed by grazing incidence x-ray diffraction (GIXRD). The mean biaxial flexural strength (BFS) was measured according to ISO 6872.
Results: The mean grain size increased with sintering temperature from 0.35±0.03μm (AS1350) to 2.24±0.52μm (AS1650). GIXRD confirmed the sole presence of tetragonal phase for AS groups. A coexistence of monoclinic, rhombohedral and tetragonal phases was found for all AA groups. The amount of rhombohedral phase decreased linearly with increasing sintering temperature, while the amount of monoclinic phase increased. The mean BFS of AS1650 (886±148MPa) was significantly lower than that of AS1450 (1036±114MPa), AS1550 (1147±89MPa) and AS1600 (1015±143MPa) but comparable to that of AS1350 (1000±146MPa). BFS for AA groups was significantly higher than that of AS controls. AA1350 had the highest BFS (1553±201MPa), followed by AA1450 (1502±102MPa) and AA1550 (1391±108MPa). The BFS of AA1600 (1258±115MPa) and AA1650 (1024±191MPa) was significantly lower than that of all other groups (p≤0.0466). There was a linear relationship between the amount of rhombohedral phase and the mean BFS (R²=0.99).
Conclusions: Lower sintering temperatures (1350 and 1450°C) and air abrasion favored the formation of rhombohedral phase, which was associated with significantly higher BFS.