Views: 1 Author: Site Editor Publish Time: 2021-05-26 Origin: Site
As a kind of structural ceramics, zirconia has excellent properties such as high compressive strength, high strength, good wear resistance, corrosion resistance and heat resistance. In addition to being widely used in industrial production, along with the rapid development of denture field in recent years, zirconia ceramics has become the most potential denture raw material, which has been concerned by many scholars
The properties of zirconia ceramics will suffer from many factors. Today, let's talk about the harm of sintering to some properties of zirconia ceramics.
The traditional sintering method is based on radiant heat, heat conduction and convective heat transfer, so that the calorific value is from the surface of zirconia to the inside, but the heat transfer performance of zirconia is worse than that of aluminum oxide and other structural ceramics. In order to avoid cracking caused by internal stress, the traditional heating is slow and takes a long time, which leads to a longer production cycle and higher production cost. In recent years, improving the manufacturing process of zirconia, reducing the production and processing time, reducing the cost of manufacturing, clinical medicine to give high-performance dental zirconia ceramic raw materials gradually become the hot spot of scientific research network, and microwave heating sintering is undoubtedly a promising sintering method.
It is found that the harm of microwave heating sintering and superheated steam sintering to its semi permeability and wear resistance is not very different. In the final analysis, the relative density of zirconia obtained by microwave heating sintering is similar to that of basic sintering. All of them are high density sintering. However, the advantages of microwave heating sintering depend on low sintering temperature, faster sintering speed and less sintering time. However, the heating rate of superheated steam sintering is relatively slow, and the sintering time is relatively long, and the total sintering time is about 6-11h. Compared with superheated steam sintering, microwave heating sintering is a new sintering method, which has the advantages of shorter sintering time, high efficiency, environmental protection and energy saving, and can improve the external economic structure of porcelain.
Some experts and scholars also think that after microwave heating sintering, zirconia can maintain more metastable tetragonal phase, which may be because rapid heating by microwave heating can complete the rapid high density of raw materials at lower temperature, and the crystal is more finely symmetrical than that of superheated steam sintering, which is less than the critical value of t-ZrO2 to change the specification, which is beneficial to maintain as much metastable state as possible at indoor temperature, Improve the compressive strength and ductility of structural ceramics.
Secondary sintering process
Because of its high strength and high compressive strength, high density sintered zirconia ceramics can only be produced and processed with emery numerical control blade, which has high production cost and long production time. In order to deal with the above problems, in some cases, the secondary sintering process of zirconia ceramics will be selected. After the ceramic body is formed and basically sintered, it will be processed to the required shape by CAD / CAM, and then sintered to the final sintering temperature, so that the raw materials are completely high-density.
It is found that the two-step sintering process can change the sintering kinetic model of zirconia ceramics, and cause certain harm to the sintering relative density, mechanical equipment performance and microstructure of zirconia ceramics. The mechanical properties of drillable zirconia ceramics sintered once with high density are better than those sintered twice. The biaxial bending strength and impact toughness of drillable zirconia ceramics sintered once are higher than those sintered twice. The fracture mode of once sintered zirconia ceramics is transgranular / intergranular composite, and the crack tends to be straight; The results show that intergranular fracture is the key to the fracture mode of twice sintered zirconia ceramics, and the fracture direction is rough. The properties of raw materials with composite fracture mode are better than those with pure intergranular fracture mode.
Sintering vacuum value
Zirconia must be sintered in vacuum, which will cause a lot of bubbles in the whole process of sintering. In the natural environment of vacuum pump, bubbles are very easy to be discharged from the molten ceramic body, which improves the relative density of zirconia, and further improves the semi permeability and structural mechanical properties of zirconia.
In the whole sintering process, in order to get better performance and expected target, the lower heating rate should be selected. The high heating rate makes the internal temperature of zirconia uneven when it reaches the final sintering temperature, resulting in cracks and air holes. It has been confirmed by scientific research that with the increase of temperature increasing rate, the vapor between the crystals can not be discharged and the internal porosity increases slightly due to the decrease of crystallization time; With the increase of heating rate, a small amount of monoclinic phase gradually exists in tetragonal zirconia, which will harm the mechanical properties of the structure. In addition, with the increase of heating rate, the crystal will polarize, that is, it is very easy to coexist large and small crystals. A relatively slow heating rate is beneficial to the formation of more uniform crystals, which improves the semi permeability of zirconia.