Zirconia ceramics are widely used in clinical practice due to their excellent mechanical properties, biocompatibility, and aesthetic advantages. However, when exposed to temperatures in the range of 150–400 °C and a humid environment over extended periods, zirconia ceramics undergo a transformation from the tetragonal to the monoclinic phase. This phase transition is accompanied by volume expansion, which generates numerous microcracks and severely reduces the mechanical strength of the implant. This phenomenon is known as low‑temperature degradation (LTD). The microcracks produced by this process provide pathways for water vapor in the air to further penetrate the ceramic, thereby accelerating further phase transformation and generating more microcracks. These microcracks intersect, overlap, and propagate into macroscopic cracks, ultimately leading to fracture and failure of the dental implant.
In addition to the problem of low‑temperature degradation, zirconia ceramics also suffer from a lack of bioactivity. When zirconia materials are implanted into the oral cavity as implants, they cannot form a stable chemical bond with biological tissues. Over time, a layer of biofilm forms around the implant and envelops it, eventually causing loosening and failure of the implant. The bioinert nature of zirconia ceramics greatly limits their application in the field of dental implantation.
The inability of bioinert zirconia ceramic surfaces to integrate with surrounding bone tissue and other tissues is a primary cause of in vivo implant failure. Bioactive ceramic materials, in contrast, form a bone‑like hydroxyapatite layer in the body and chemically bond with adjacent bone components. Therefore, enhancing the bioactivity of zirconia can improve the success rate of implantation. Bioactivity can also be verified through in vivo experiments, in which the material is implanted into animals for a certain period. In addition to the absence of inflammatory reactions, bioactivity is assessed by analyzing the integration between the implant and bone tissue-for example, evaluating new bone formation, morphological changes, and removal torque tests.
To improve the bioactivity of inert materials, surface treatment or the incorporation of bioactive substances is generally employed to generate organic functional groups (Zr‑OH) or deposit bioactive calcium‑phosphate (Ca‑P) coatings on the surface, thereby enhancing bioactivity.
(1) Incorporation of Bioactive Substances
Incorporating bioactive substances is the simplest and most direct method. Bioactive components are mechanically mixed with zirconia powder to improve the bioactivity of the zirconia ceramic material.
For instance, Kishi S. et al. added nano‑HA to ZrO₂ ceramics to form a composite material, and in SBF mineralization experiments, a bone‑like apatite layer formed on the surface. Cortes D.A. et al. added wollastonite to Mg‑PSZ ceramics and sintered the composite at 1550 °C. After soaking in SBF and 1.5×SBF for 7 days each (with wollastonite powder embedded in the SBF solution during the first immersion), the surface mineralized into HA. Nogiwa‑Valdez A.A. et al. incorporated CaO‑SiO₂ bioactive glass into ZrO₂ to produce a composite material, and HA formed on the sample surface in SBF mineralization experiments.
(2) Surface Modification
Sandblasting is a well‑established surface treatment process. It uses compressed air to propel abrasive particles at high speed onto the object surface, increasing surface roughness and cleanliness. In the era of zirconia ceramic restorations, alumina sandblasting is the most commonly used surface treatment in production. Currently, researchers employ varying sandblasting parameters, mostly using alumina particles of 50–100 μm, blasting pressures of 0.1–0.5 MPa, and blasting times of 10–30 s.
Thermal acid etching can enhance the bond strength between zirconia and veneering porcelain or resin. However, this process involves hazardous industrial chemicals such as methanol and hydrochloric acid, demanding high safety standards and complex procedures. At present, there is no mature and reliable thermal acid etching equipment available, which limits its promotion in production.
Laser treatment also enhances the bond strength between zirconia and veneering porcelain or resin through surface roughening.
Zirconia ceramic surfaces lack a glass phase and contain little or no silica, making it difficult for them to chemically react with the veneering porcelain layer. Various methods have been used to prepare silicon coatings on zirconia ceramic surfaces, making their surface chemical composition similar to that of glass‑based veneering porcelain and thereby facilitating chemical reactions that increase the zirconia‑porcelain bond strength. However, the optimal silicon coating thickness has not yet been determined; current silicon coating techniques are numerous, and there is a lack of standardized sample preparation and testing criteria, resulting in poor comparability between different studies. Moreover, some equipment used for preparing silicon coatings is still in the laboratory stage, and the processes are complex, so they are not yet suitable for large‑scale production.
Low‑temperature plasma can enhance the bond strength between zirconia and veneering porcelain or resin. However, treatment parameters vary across studies, making comparisons difficult. It remains unclear which plasma treatment yields the best results, and whether low‑temperature plasma can indirectly induce chemical bonding between zirconia and veneering porcelain or resin has not been elucidated.