Oral ceramic materials
Oral restorations made of specialized ceramic materials have excellent translucency, high color simulation, and good biocompatibility, making them currently the most aesthetically pleasing and promising restorations. However, due to the high brittleness and susceptibility to cracking of ceramic materials, they are currently mainly used in the production of front dental crowns, veneers, and other materials.
1、 Classification of Oral Ceramic Materials
（1） Temperature classification
1. The sintering temperature of high melting porcelain is 1290-1370 ℃.
2. The sintering temperature of medium melting porcelain is 1090-1260 ℃.
3. The sintering temperature of low melting porcelain is 870~1065 ℃.
High melting porcelain was once used to make porcelain crowns, but now it is usually used to make finished porcelain teeth. A typical high melting porcelain is composed of feldspar (70% -90%), quartz (11% -18%), and kaolin (1% -10%). The main component of feldspar is silicon oxide, manifested in the forms of NaOAl2O36SiO2 and K2OAl2O6SiO2. When melted, it forms a glassy substance, producing the translucency of ceramics. It serves as a matrix for high melting quartz (SiO2), while the latter forms a refractory skeleton structure. Fusion of other materials around it. This is very helpful for maintaining the shape of porcelain restorations during the sintering process. Kaolin, a clay like substance, is a sticky substance. When porcelain is not formed or sintered, it can combine particles together.
Low melting porcelain and medium melting porcelain are made through a so-called glass melting process. The crude raw materials of porcelain are melted, quenched, and crushed into very small powders. When sintered again to make the restoration, this powder melts at low temperatures and no longer undergoes high-temperature reactions.
Adding a certain amount of metal oxides (zirconia, titanium oxide, and tin oxide) can make the porcelain opaque. The opaque porcelain layer is used to cover the metal base crown of the metal ceramic restoration. Adding a certain amount of other metal substances during the sintering process can produce the color of porcelain, such as indium (yellow), chromium, tin (pink), iron oxide (black), and cobalt salts (blue).
（2） Component classification
1. Feldspar ceramics
The main raw material for feldspar ceramics is feldspar, while the feldspar used in oral feldspar ceramics is a mixture of natural sodium feldspar (Na2O. Al2O3.6SiO2) and potassium feldspar (K2O. Al2O3.6SiO2). The basic components of feldspar porcelain are feldspar, quartz, and white clay. Feldspar porcelain has good biological properties and can be used as a porcelain powder material and to make finished teeth and tooth surfaces. Due to its bending strength being only 50-80 MPa, it needs to be used in conjunction with high-strength alloys or ceramics. Currently, it is commonly used in clinical practice to make porcelain fused to metal restorations (PFMS) and porcelain fused to ceramic restorations (PFCS).
2. Glass ceramics
Glass ceramics are polycrystalline solids made from ordinary glass through microcrystalline treatment, with more crystalline phases than glass phases. It can be used to make crown and bridge restorations and serve as an implant material. The commonly used glass ceramics in clinical repair include cast glass ceramics, cut glass ceramics, injection glass ceramics, and implanted glass ceramics.
3. Aluminum oxide ceramics
The main crystal phase is a-Al2O3. It is made by dispersing a certain amount of alumina crystals in a glass matrix, and its essence is a type of glass ceramic. The bending strength of aluminum ceramic used for oral restoration increases with the increase of aluminum oxide content, which is above 50wt%.
4. Hydroxyapatite Ceramic Hydroxyapatite ceramic, with a molecular formula of Ca10 (PO4) 6 (OH) 2. Due to its structure being similar to the inorganic components of human dental bone tissue, artificially synthesized hydroxyapatite ceramic has good biocompatibility, can form a good structure with the cell membrane surface, and form a bony bond with bone tissue, making it a good substitute material for dental and bone defects.
（3） Classification of Forming Technologies
1. Casting glass ceramic technology
The bottom layer or all ceramic crown of composite porcelain crown is made by using traditional refractory material embedding wax mold, wax loss method ring less casting, and crystallization treatment processes to cast glass ceramics. Common cast glass ceramics can be roughly divided into two types: one is mainly composed of silica fluoromica, such as Dicor ceramics, and the other is mainly composed of apatite, such as Cerapearl ceramics, which have a bending strength of 125 MPa and a fracture toughness of 1.31 MPa. In addition to the aforementioned products, there are also domestically produced Liko glass ceramics, Plat ceramics, and Nissan Olympus ceramics. Among them, Liko ceramics belong to the K2O-Al2O3-MgO-SiO2-F system, with the main crystal phase being tetrafluoromica. Its physical and chemical properties are very similar to those of Dicor ceramics, with slightly lower strength than Dicor ceramics, but at a low price, it is very suitable for domestic promotion and use. The tensile strength of Plat ceramics is 145MPa, compressive strength is 250MPa, fracture strength is 165MPa, and elastic modulus is 93MPa, which basically meets clinical requirements. Olympus is currently the strongest among the reported cast ceramics, with a bending strength of 220-230MPa
2. Hot pressed ceramics
The basic composition of IPS Empress ceramics comes from feldspar ceramics, which are composed of 63% SiO2, 17.7% Al2O3, 11.2% K2O, 4.6% Na2O, 0.6% B2S3, 0.4% CeO2, 1.6% CaO, 0.7% BaO, 0.2% TiO2 by weight. The crystalline part of the ceramics is garnet crystals, with a content of 23.6-41.3%. The manufacturing process involves placing the restoration wax on a specially designed cylindrical furnace mold. Embed with phosphate embedding material and heat the model to 850 ℃. Open the top cover and place the porcelain block, cover it with aluminum oxide push rod, and press the automatic button. The furnace automatically heats to 1150 ℃ according to the given program and maintains it for 20 minutes. The viscous glass ceramic material is injected into the model under a pressure of 0.3-0.4 MPa, and the forming process of ceramic repair is completed. Subsequently, surface glaze porcelain with similar composition to the substrate material is used for coloring treatment, or the corresponding color is obtained through porcelain decoration technology. This technique has been widely applied in clinical practice due to its advantages of simplicity, time-saving, and low cost. The commonly used hot pressed ceramics include IPS Empress and OptecHSP. Due to the low strength of IPS Empress glass ceramic, which is only 133.5 ± 21.5MPa, it is currently limited to the production of inlays, veneers, and individual anterior crowns.
3. Powder coated aluminum porcelain
In ceram is a new type of powder coated all ceramic restoration high alumina ceramic product developed on the basis of Hi ceram, with an Al2O3 crystal content of over 99%. This technical method involves preparing high-purity Al2O3 powder into a high volume fraction slurry, coating it onto a specialized In Ceram refractory material mold, absorbing water through the capillary action of the mold to form a nucleus, and sintering it in an In Ceram furnace at 1100 ℃ for 2 hours to form a porous aluminum oxide framework with certain strength. Then, in the second process, a special component of glass material is coated and infiltrated at a high temperature of 1150 ℃. The glass material infiltrates between Al2O3 particles, As a result, cracks diffuse. The infiltration of glass material almost eliminates all pores, thus enhancing the bending strength. The experiment shows that the bending strength of In Ceram is about 10 times that of feldspar ceramics and 4 times that of glass ceramics, and its color is realistic with good suitability. The average edge lift is 39 μ m. Can be used to make all ceramic crowns and fixed bridges for anterior or posterior teeth. After glass infiltration, the alumina core is coated with Vita's specialized ceramic powder for surface decoration, resulting in an ideal restoration shape similar to natural teeth.
4. Machinable ceramics
There are currently four main types of machinable ceramics:
(1) Feldspar machinable ceramics, such as VitaMark I and VitaMark II;
(2) Feldspar ceramics with the main crystal phase being garnet crystals, such as ProCAD;
(3) Machinable glass ceramics, among which mica based ceramics include Macor-M, DicorMGC, MGC-F (containing tetrasilicate mica), photogel (also containing zirconia), etc. Apatite based ceramics include Bioram-M;
(4) Alumina ceramics, such as ProceraAllCeram and alumina glass composites.
The first three types of ceramics are restoration bodies that are directly milled and cut into the required size using CAD/CAM technology, and then glazed and colored to complete the restoration production. Due to their bending strength generally below 150MPa and fracture toughness less than 2.0MPa, they are only recommended for inlays, veneers, and anterior crowns. Aluminum oxide ceramics, on the other hand, are made by milling and cutting the partially sintered porous aluminum oxide body into the base core of the restoration using CAD/CAM technology, and then undergoing glass infiltration treatment to improve its strength. The decorative ceramics are processed, and their strength is similar to that of In ceram, which can be used to make crowns and bridges.
