The internal reinforcement of dental ceramics is achieved by dispersing second phase crystals, particles, or fibers in the matrix. These second phase substances are dispersed into the matrix, which can cause the generated cracks to shift, branch, blunt or stop, thereby increasing the strength and toughness of the material. The commonly used second phase substances include Al2O3 particles, ZrO2 particles, titanium particles, tetrafluorosilicon mother crystals, garnet crystals, MgO crystals, magnesium aluminum pointed crystals, apatite crystals, wollastonite crystals, fluorogold mother crystals, stainless steel fibers, etc. Research has found that Al2O3 can improve the strength and toughness of the matrix by inhibiting the growth of ZrO2 particles. When the Al2O3 content reaches 30% (mass fraction), the flexural strength of the composite ceramic is 986 MPa, and the fracture toughness is 13.7 Mpa · m1/2.
(2) Surface treatment
The brittle fracture of ceramic restorations is often caused by the generation and expansion of surface microcracks. Therefore, certain surface treatment of the restoration can heal the surface microcracks generated during preparation. Surface treatment includes polishing, glazing, chemical enhancement and thermal enhancement. There are two methods of glazing: glaze-ceramic glazing and self-glazing. Glaze-ceramic glazing is to apply sintered glaze porcelain on the surface of a properly ground ceramic restoration to form a uniform glass thin layer. Self-glazing is to put the ceramic restoration into a porcelain furnace again, raise the temperature above the glass transition temperature, so that a glassy flow layer is generated on the ceramic surface to repair the surface microcracks. In addition, laser treatment of the ceramic surface can also be used to improve its strength. After the dental ceramic surface was treated with a 308rm XeCl laser, the roughness was significantly reduced, and the surface roughness was lower when the power was 6.28J/cm2 compared with the power of 1.57J/cm2 and 3.14J/cm2 for the same time as the same ceramic material. However, due to the presence of some microcracks and bubbles on its surface, dental ceramics treated with XeCl laser need further treatment. In studying the effects of surface treatment and heat treatment on the strength of dental ceramics, they found that due to the compressive stress layer caused by phase change, the polished and sandblasted specimens after heat treatment have higher strength. In addition, the specimens with the grinding direction parallel to the bending axis of the specimen are stronger than the specimens with the grinding direction perpendicular to the bending axis of the specimen.
Chemical strengthening mainly uses ion exchange technology, which is also called ion filling. Usually, sodium ions with smaller diameters are used to exchange feldspar porcelain. The mechanism of ion exchange toughening mainly includes the following two points: ① Replace ions with larger ions with smaller ions at a temperature below the glass softening temperature. The rigidity of the material prevents the introduced stress from being released, forming a pressure layer on the surface; ② Replace sodium ions with lithium ions to reduce the thermal expansion coefficient of the surface layer of the material, so that the surface layer of the ceramic is in a compressed state during the cooling process, which increases the energy required for crack propagation. The effect of ion exchange is affected by factors such as exchange time, temperature, and ion concentration. The most commonly used is a paste whose main component is K2HPO4 or potassium nitrate, which is applied to the porcelain surface and heated in a standard dental laboratory furnace to complete the ion exchange reaction.
Research conclusion Since ceramic materials entered the field of dental restoration, they have been widely used due to their good biocompatibility and beautiful and realistic effects, but their inherent defects such as insufficient strength and high brittleness have greatly limited their application in dental restoration. Therefore, material workers have done a lot of work to toughen and reinforce dental ceramics, such as internal reinforcement, surface treatment, particle toughening, phase change toughening, etc., and developed a series of toughened and reinforced ceramic materials based on theoretical research. The related products developed have also achieved good clinical application results, such as the InCeram system launched by Germany's VITA company and the GLⅡ type Al2O3 glass infiltration ceramics developed by the Fourth Military Medical University of my country. Among them, the In-Ceram system has better short-term and long-term clinical effects, and its clinical application has expanded from anterior crowns to posterior crowns and bridges.
As high-strength crown and bridge restoration materials, people have high hopes for Al2O3 ceramics, ZrO2 ceramics, Al2O3-ZrO2 composite ceramics and hydroxyapatite coated ceramics. As we all know, the structure of a material determines its performance. How to increase the energy absorption mechanism in the ceramic microstructure and increase the path of crack propagation is the core issue of improving the toughness of ceramics. With the improvement of the mechanical properties of ceramic materials and the increase in reliability, the application of ceramic materials in dental restoration will surely make great progress and development.