Fracture Toughness & Strength of Zirconia Blanks
Since the start of digitalization, the quality-relevant criteria for the production and processing of zirconium oxide have been addressed and discussed in numerous publications.
The understanding and knowledge of zirconium oxide & zirconia blanks have undergone an evolution in recent years—an evolution towards an aesthetically sophisticated dental prosthesis material (zirconia blanks) that is absolutely stable and safe for the end customer.
The understanding and knowledge of zirconium oxide with regard to the material behaviour and its processing and machining has undergone an evolution in recent years – an evolution towards an aesthetically highly sophisticated dental prosthesis material that is absolutely stable and safe for the end customer.
Dental Direkt GmbH already entered the discussion in 2008 and has published the first compendium with information about the production of the high-performance ceramic zirconium oxide under the above title (Fig.1).
Directly To Our Zirconia Blanks
The last four years have seen the introduction of many new, innovative zirconia blank types and the proven 3Y TZP types (high-strength, stabilized with 3 mol% yttrium oxide). It is becoming increasingly difficult to maintain an overview of the composition and suitability for the indication.
A clear typification and differentiation of the different qualities and material concepts are therefore currently difficult to achieve.
Especially the zirconium oxide ingots, whose translucency has been increased, have recently enjoyed growing popularity. In this context, the relationship between increasing translucency on the one hand, and decreasing strength on the other hand has been repeatedly discussed in technical articles and at congresses.
Super high translucent 5Y TZP (with 5 mol% yttrium oxide approx. 50% tetragonal and 50% cubic crystals are stabilized) have been on the market since about 2014, and indications are limited to 3-units molar bridges in most cases due to reduced strength.
However, zirconium oxide has been developed several times – it is undergoing rapid development.
Not all zirconium blank is the same!
Materials of varying opacity allow the choice for aesthetic lighting management, as this monolithic work in zirconium dioxide shows. This enables attractive solutions for the patient even under different ambient conditions.
Picture: © Anton Sawizki
Let us turn our attention to two relevant quality criteria for Zirconia blanks and blocks
1. Bending strength
The bending strength describes how much force is required to deform and ultimately break a test piece with a defined diameter. High values in this discipline are essential for multi-unit restorations.
For bridges larger than four elements, a value of 800 MPa must be achieved in a test according to the essential standard for dental ceramics (DIN EN ISO 6872). There can be no doubt that flexural strength is an important quality criterion for all dental ceramics.
(sources: DentalDirekt Germany)
2. Fracture Toughness
Fracture toughness describes the resistance of a material to crack propagation. The higher the value of KIc, the better the prognosis for long-term clinical behavior. The ideal combination of flexural strength and fracture toughness is based on a zirconia blank material‘s ability to transform its crystalline phase when stress occurs in such a way that the progress of a crack is stopped and the crack itself has closed again.
This physical-mechanical property is reminiscent of the self-healing abilities on the organic level.
One might be led to believe that the inorganic dental material zirconium oxide has copied from its “organic neighbours” by virtue of its self-healing ability. For example, a similar self-healing ability has been observed in bones as well as in some types of Swiss cheese.
This analogy shows that more attention should be paid to fracture toughness as a relevant quality criterion. The KIc – value provides information on how robust a material is, such as during processing in the laboratory or during insertion by the practitioner.
The classic high-strength 3Y TZP zirconium oxide grades generally have fracture toughness greater than 5 MPa√m, an essential factor for their clinical success. However, as with the flexural strength tests, there are different test methods for fracture toughness in DIN EN ISO 6872.
The phase transition enhancement property of a material is also relevant for the aging behavior of a ceramic. The super high translucent 5Y TZP has a lower initial KIc, which is recommended for 3-unit molar bridges with at least >3 MPa√m. This material’s crystalline structure thus offers little potential for phase transformation, which may have led to stronger aging effects in earlier research.
How to choose best zirconia crown
In choosing a zirconia blank, it is important to consider both flexural strength and fracture toughness.
The DD cube ONE® ML zirconia blank (4Y TZP) has a hybrid crystal structure of about 30% cubic and 70% tetragonal crystals. The KIc is available at ≈10 MPa√m SEVNB, and aluminum oxide (Al2O3) was added in a balanced concentration to positively influence the aging behavior.
Large cubic crystals carry out their role as the most effective means of increasing translucency so that they can be seen from all sides without having to rely on straight edges.
Adding Al2O3 provides an even higher fracture toughness in the incisal area of multilayer zirconia blanks variants, which provides the best possible security even in case of corrections to the occlusal surfaces.