What Factors Can Affect The Hardness of Dental Zirconia

The hardness of dental zirconia is influenced by multiple factors, which can be summarized into several aspects such as material composition, microstructure, processing technology and environmental conditions. The following is a detailed analysis of these influencing factors:

First, material composition

Zirconia purity: The higher the purity of zirconia, the higher its hardness is often. High-purity zirconia can offer better physical properties, including hardness.

Additives: To improve the processing performance and mechanical properties of zirconia, a certain amount of auxiliary materials, such as alumina, yttrium oxide and other stabilizers, are usually added. The types and proportions of these additives will affect the hardness of zirconia. For instance, aluminium oxide plays a certain toughening role in zirconia, but if its content is too high, it will reduce transparency and may have a certain impact on hardness. The content of the stabilizer yttrium oxide is closely related to transparency and hardness.

Second, microstructure

Crystal phase composition: Zirconia has multiple crystal phases, such as tetragonal phase, monoclinic phase and cubic phase, etc. Zirconia with different crystal phases has different hardness. Tetragonal zirconia usually has a higher hardness, while cubic zirconia, although having good light transmittance, has relatively lower hardness.

Grain size: The grain size of zirconia also affects its hardness. Generally speaking, the smaller the grain size, the higher the hardness of zirconia. This is because small grains can resist external pressure more effectively, thereby enhancing the hardness of the material.

Third, processing technology

Sintering temperature: Sintering temperature is one of the key factors affecting the hardness of zirconia. The increase of sintering temperature can enhance the density of zirconia and reduce the porosity, thereby improving its hardness. However, excessively high sintering temperatures may also cause the growth of zirconia grains, thereby reducing hardness instead.

Polishing treatment: Polishing treatment can remove micro-cracks and defects on the surface of zirconia, thereby enhancing its surface hardness and smoothness. However, it should be noted that the polishing treatment has a limited effect on enhancing the overall hardness of zirconia.

Fourth, environmental conditions

Hydrothermal aging: Under the influence of the oral environment and repeated chewing, zirconia undergoes hydrothermal aging, leading to a transformation from the tetragonal phase to the monoclinic phase. During this transformation process, phenomena such as volume expansion and fracture intrusion will occur, thereby reducing the hardness and flexural strength of zirconia.

Corrosive environment: Although zirconia itself has excellent corrosion resistance, it may still be affected by corrosion in certain specific environments (such as strong acid and strong alkali environments), leading to a decrease in its hardness.