With the continuous development of modern dental restoration technology, zirconium oxide has gradually become an important choice for dental restoration materials due to its excellent mechanical properties and superior biocompatibility.
As a high-performance ceramic material, zirconium oxide not only has strong performance, but also meets the aesthetic needs of patients. When evaluating the performance of zirconium oxide materials, fracture toughness is an important indicator that directly affects the clinical service life and reliability of the restoration.
What is fracture toughness?
Fracture toughness is defined as the ability of a material to resist fracture in the presence of a crack, usually in units of MPa·m1/2.
This indicator quantifies the ability of a material to prevent cracks from continuing to expand when subjected to stress. For dental restoration materials, higher fracture toughness means longer service life. In practical applications, fracture toughness directly affects the reliability and durability of the material, and the material’s properties, microstructure, preparation process, and surface treatment are all important factors affecting fracture toughness.
Fracture toughness of zirconia
Zirconium oxide has a unique phase transformation toughening mechanism, which is the key to excellent fracture toughness. When the material is affected by stress, the transformation of the tetragonal phase to the monoclinic phase will cause volume expansion and generate a compressive stress field at the tip of the crack, which can effectively inhibit the propagation of the crack. This stress-induced phase transformation is the essential reason why zirconia has high fracture toughness.
The microstructure has a decisive influence on fracture toughness. The size, distribution and orientation of the material grains will affect the phase transformation toughening effect. Smaller grain size is more conducive to improving the stability of the material, but too small will reduce the phase transformation toughening effect. Therefore, how to optimize the grain size plays an important role in improving fracture toughness.
Comparison of fracture toughness of different types of zirconia
Different types of zirconia have different fracture toughness. Traditional 3Y-TZP zirconia has a higher fracture toughness value of about 6.9 MPa·m1/2 due to its higher tetragonal phase content, so this material has excellent clinical performance and its transparency is also relatively high.
The latest research shows that adding cubic phase content will make zirconia more transparent, but the fracture toughness will decrease. The fracture toughness of some highly transparent zirconia can be as low as 5.0 MPa·m1/2, and the new multilayer zirconia material has a reasonable gradient structure design, while maintaining sufficient fracture toughness and achieving good aesthetic effects.
Effect of Surface Treatment on Fracture Toughness
Surface treatment processes have a complex effect on fracture toughness. Proper grinding can improve fracture toughness by inducing surface stress, but excessive grinding can have the opposite effect, resulting in surface defects.
Similarly, fine polishing processes can also improve surface quality and fracture toughness.
Glazing is also a type of surface treatment. This process can improve the aesthetic effect, but it may reduce the fracture toughness of zirconia. According to experimental data, the fracture toughness of zirconia samples treated with glaze is significantly lower than that of samples that have only been polished.
Although heat treatment can relieve processing stress, it may also cause phase changes, thereby affecting fracture toughness.
Therefore, the most suitable surface treatment process should be selected with caution.
Clinical application suggestions
In clinical application, the selection of zirconium oxide materials needs to consider many factors comprehensively. Based on the evaluation of fracture toughness, we recommend that you should give priority to the functional requirements of the restoration site when selecting restoration materials.
For areas such as the posterior teeth that often need to withstand greater chewing forces, we recommend that you should choose 3Y-TZP zirconium oxide with higher fracture toughness, while for the anterior teeth, you can choose high-transparent zirconium oxide to maintain a balance between beauty and performance.
Clinical operation process optimization is very important for maintaining the fracture toughness of the material. We recommend that you use CAD/CAM processing technology, which is a precision processing technology that can ensure the accuracy of your restoration. At the same time, the grinding force and time need to be strictly controlled during surface treatment. It is best to use a step-by-step polishing procedure to avoid excessive stress concentration.
Conclusion
Fracture toughness, as the core performance index of zirconia materials, plays a very important role in the long-term use and reliability of restorations. Thank you for reading this article and hope it can help you.
Further reading: Zirconia Material Guide