What influence do different corundum blasting and aging methods have on the flexural strength and phase composition of zirconium oxide? 1st and 2nd generation? This question was examined in more detail as part of a study at the LMU Munich.
Matthias Kelch, Munich
Metal-free prosthetic restorations have become an essential part of modern treatment concepts. The rapid development of various zirconium oxides requires in-depth knowledge of the material properties and their processing processes.
Zirconia exists in three different phases. The proportion of phases determines the properties of the material. When processing a zirconium oxide surface, the behavior of the phase composition under stress plays a central role. This can lead to a transformation from the tetragonal to the monoclinic phase. This process is associated with an increase in volume of 3-5%.
Different phases of zirconium oxide:
- monoclinic
- tetragonal
- cubic
Temperature-dependent crystal phases of zirconium oxide. Volume increase of 3-5% during the transition from the tetragonal to the monoclinic phase. [Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N: Materials science update: Zirconium oxide and its generations - from veneered to monolithic. Quintessenz Zahntechnik 2016, 42(6):740-765].
The effects provoked by this behavior are controversially discussed in the literature. On the one hand, crack growth should be prevented and the resulting compression layer should lead to an increase in strength. On the other hand, strength-reducing effects due to the formation of microcracks have also been described.
Pre-treatment of the surface by corundum blasting...
- for example before blending or
- as part of the fastening
...can represent a potential cause of a phase transformation. The in-vitro study from the LMU Munich summarized below illustrates why aging plays a key role here.
Aim of the investigation
The aim of the study was to examine the influence of different corundum blasting and aging methods on the flexural strength of 3Y-TZP0.25 to analyze and investigate the effect of a possible monoclinic phase. Furthermore, the influence of grain size and applied pressure on the formation of monoclinic phases in 3Y-TZP was examined0.25 and 3Y-TZP0.05 examined.
material and methods
Investigation of bending strength
A total of 3 test specimens from the material group 180Y-TZP were used for the 3-point bending test0.25 (Ceramill Zi, AmannGirrbach, Austria). In the first step, these were randomly divided into four pretreatment groups (n=45):
- Corundum blasting with 50 µm aluminum oxide powder (Hasenfratz, Germany)
- Corundum blasting with 105 µm aluminum oxide powder (Hasenfratz)
- Corundum blasting with 30 µm SiO2-coated aluminum oxide powder (Rocatec soft powder, 3M, Germany)
- No pretreatment
Device for ensuring constant conditions during corundum blasting with a fixed angle of 45° and a constant nozzle distance of 10 mm.
Analysis of crystalline structures
To analyze the crystalline phase structure, additional test specimens made of 3Y-TZP were used0.25 (Ceramill Zi, AmannGirrbach, n=12) and 3Y-TZP0.05 (Ceramill Zolid, Amann Girrbach, n=8). These were pretreated according to the methods already described and aged in an autoclave. In order to investigate any influence of the blasting pressure, corundum blasting was carried out with 50 µm and 105 µm aluminum oxide powder with different blasting pressure parameters of 0.05, 0.25 and 0.4 MPa. To investigate possible changes in the phase structure of 3Y-TZP0.25 After 5 years of dry storage at room temperature (23°C), the fragments of the test specimens from the 3-point bending test were also analyzed. Phase transformation was demonstrated using a Raman spectroscope (inVia Qontor, Renishaw plc, UK).
Results
Mechanical pretreatment by corundum blasting increased the flexural strength of unaged 3Y-TZP0.25 with increasing grain size. The aging of test specimens in the autoclave and in the chewing simulator, which were corundum blasted with 50 and 105 µm aluminum oxide powder, had more influence on their flexural strength than those that had previously been blasted with 30 µm SiO2-coated aluminum oxide powder were corundum blasted. Larger corundum particles and increasing jet pressure increased the monoclinic phase fraction. All pretreatment groups showed a higher monoclinic phase content after aging in an autoclave and/or 5 years of aging at room temperature. The exception is 30 µm SiO2-coated aluminum oxide powder corundum blasted test specimens.
Graphical representation of the results
Pretreatment/Aging Zirconia: Conclusion
Corundum blasting of a zirconium oxide surface with 30 µm SiO2-coated aluminum oxide powder can be recommended as a pretreatment method. This is regardless of whether it is 3Y-TZP0.25 or 3Y-TZP0.05 acts. When pre-treating with aluminum oxide, you should always ensure that the blasting pressure is as low as possible.