Pretreatment and aging of zirconia: Effect on different material properties

Zirconia: What influence do different air-abrasion and aging methods have on the flexural strength and phase structure of first- and second-generation zirconia? This question was examined in detail in an investigation conducted by LMU Munich.

Metal-free prosthetic restorations have become an essential part of modern treatment concepts. The rapid development of different zirconia ceramics requires in-depth knowledge of their material properties and processing procedures. The behavior of the phase structure when subjected to stress plays an important role in treating a zirconia surface because this can trigger a transformation from the tetragonal to monoclinic phase. This process is associated with a volumetric expansion of 3% to 5% (Fig. 1).

Fig. 1 Temperature-dependent crystal phases of zirconia. Volume increase of 3-5% during transition from the tetragonal to the monoclinic phase. Source: Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N: Werkstoffkunde-Update: Zirkonoxid und seine Generationen – von verblendet bis monolithisch. Quintessenz Zahntechnik 2016, 42(6):740-765.

The question of which effects are induced by this behavior is a matter of controversy in the literature. On the one hand, crack propagation is supposed to be inhibited and the resulting compression layer is supposed to lead to increased strength. On the other hand, weakening effects due to the formation of microcracks have been described. Surface pretreatment by air-abrasion, performed prior to veneering or during cementation, could potentially cause a phase transformation. The reason why aging plays a crucial role in this process is shown in the in vitro investigation by LMU Munich summarized below.

Objective

The aim of the investigation was to analyze the impact of different air-abrasion and aging methods on the flexural strength of 3Y-TZP_0.25 and investigate the effect of the possible formation of a monoclinic phase. Furthermore, the effect of air-particle size and applied pressure on the monoclinic phase formation in 3Y-TZP_0.25 and 3Y-TZP_0.05 were examined.

Materials and methods

Investigation of flexural strength

A total of 180 specimens of material group 3Y-TZP_0.25 (Ceramill ZI, Amann Girrbach, Koblach, Austria) were fabricated in order to perform a three-point flexural strength test. In the first step, the specimens were randomly divided into four pretreatment groups (n=45):

1. Air-abrasion with 50 µm alumina powder (Hasenfratz Sandstrahltechnik, Assling, Germany) (Fig. 2).
2. Air-abrasion with 105 µm alumina powder (Hasenfratz Sandstrahltechnik).
3. Air-abrasion with 30 µm silica-coated alumina powder (Rocatec Soft, 3M, Seefeld, Germany).
4. No pretreatment

Fig. 2 Device used to ensure constant conditions during air-abrasion at a fixed angle of 45 degrees and a consistent nozzle distance of 10 mm.

Analysis of crystalline structures

For analysis of the crystalline phase structure, additional specimens were fabricated from 3Y-TZP_0.25 (n=12) and 3Y-TZP_0.05 (Ceramill Zolid, Amann Girrbach, n=8). Pretreatment was carried out according to the methods already described and aging was performed in the autoclave. To investigate the potential effect of the air-abrading pressure, air-abrasion with 50 µm and 105 µm alumina powder particles was carried out at various air-abrading pressure parameters (0.05, 0.25 and 0.4 MPa). The fractured specimens from the three-point flexural test were also analyzed to show possible changes in the phase structure of 3Y-TZP_0.25 after 5 years of dry storage at room temperature (23°C). Evidence of phase transformation was verified using Raman spectroscopy (inVia Qontor, Renishaw plc, Wotton-under-Edge, UK).

Results

Mechanical pretreatment by air-abrasion increased the flexural strength of non-aged 3Y-TZP_0.25 with increasing particle size. Aging of specimens in the autoclave and chewing simulator air-abraded with 50 µm and 105 µm alumina powder had a greater effect on their flexural strength compared to prior air-abrasion using 30 µm silica-coated alumina powder. A larger alumina particle size and increasing blasting pressure increased the monoclinic phase fraction. All pretreatment groups showed a higher monoclinic phase fraction after autoclave aging and/or aging for 5 years at room temperature, with the exception of the specimens air-abraded with 30 µm silica-coated alumina powder.

Results

Conclusion

Air-abrasion of a zirconia surface with 30 µm silica-coated alumina powder can be recommended as a pretreatment method regardless of whether 3Y-TZP0.25 or 3Y-TZP0.05 is used. For pretreatment using alumina oxide, the air-abrading pressure should be kept as low as possible.