As a kind of ceramic material, zirconium has high strength, high hardness, good wear resistance, acid and alkali resistance, high temperature resistance and other excellent properties. In addition to being widely used in the industrial field, with the vigorous development of the denture industry in recent years, zirconia ceramics have become the most potential denture materials and attracted the attention of many researchers.
Sintering method
The traditional sintering method is to heat the body through heat radiation, heat conduction, heat convection, so that the heat is from the surface of zirconia to the interior, but the thermal conductivity of zirconia is worse than that of alumina and other ceramic materials. In order to prevent cracking caused by thermal stress, the traditional heating speed is slow and the time is long, which makes the production cycle of zirconia long and the production cost is high. In recent years, improving the processing technology of zirconia, shortening the processing time, reducing the production cost, and providing high performance dental zirconia ceramic materials have become the focus of research, and microwave sintering is undoubtedly a promising sintering method.
It is found that microwave sintering and atmospheric pressure sintering have no significant difference on the influence of semi-permeability and wear resistance. The reason is that the density of zirconia obtained by microwave sintering is similar to that of conventional sintering, and both are dense sintering, but the advantages of microwave sintering are low sintering temperature, fast speed and short sintering time. However, the temperature rise rate of atmospheric pressure sintering is slow, the sintering time is longer, and the whole sintering time is roughly 6-11h. Compared with normal pressure sintering, microwave sintering is a new sintering method, which has the advantages of short sintering time, high efficiency and energy saving, and can improve the microstructure of ceramics.
Some scholars also believe that zirconia after microwave sintering can maintain more metastable tequartet phase, possibly because microwave rapid heating can achieve rapid densification of the material at a lower temperature, the grain size is smaller and more uniform than that of normal pressure sintering, lower than the critical phase transformation size of t-ZrO2, which is conducive to maintaining as much as possible in metastable state at room temperature, improving the strength and toughness of ceramic materials.
Double sintering process
Compact sintered zirconia ceramics can only be processed with emery cutting tools due to high hardness and strength, and the processing cost is high and the time is long. In order to solve the above problems, sometimes zirconia ceramics will be used twice sintering process, after the formation of the ceramic body and initial sintering, the CAD/CAM amplification machining to the desired shape, and then sintering to the final sintering temperature to make the material completely dense.
It is found that two sintering processes will change the sintering kinetics of zirconia ceramics, and will have certain effects on the sintering density, mechanical properties and microstructure of zirconia ceramics. The mechanical properties of the machinable zirconia ceramics sintered once dense are better than those sintered twice. The biaxial bending strength and fracture toughness of the machinable zirconia ceramics sintered once compact are higher than those sintered twice. The fracture mode of primary sintered zirconia ceramics is transgranular/intergranular, and the crack strike is relatively straight. The fracture mode of twice sintered zirconia ceramics is mainly intergranular fracture, and the crack trend is more tortuous. The properties of composite fracture mode are better than simple intergranular fracture mode.
Sintering vacuum
Zirconia must be sintered in a vacuum environment, in the sintering process will produce a large number of bubbles, and in a vacuum environment, bubbles are easy to discharge from the molten state of the porcelain body, improve the density of zirconia, thereby increasing the semi-permeability and mechanical properties of zirconia.
Heating rate
In the sintering process of zirconia, in order to obtain good performance and expected results, a lower heating rate should be adopted. The high heating rate makes the internal temperature of zirconia uneven when reaching the final sintering temperature, leading to the appearance of cracks and the formation of pores. The results show that with the increase of heating rate, the crystallization time of zirconia crystals is shortened, the gas between crystals cannot be discharged, and the porosity inside the zirconia crystals increases slightly. With the increase of heating rate, a small amount of monoclinic crystal phase begins to exist in the tetragonal phase of zirconia, which will affect the mechanical properties. At the same time, with the increase of the heating rate, the grains will be polarized, that is, the coexistence of larger and smaller grains is easy. The slower heating rate is conducive to the formation of more uniform grains, which increases the semipermeability of zirconia.
Post time: Aug-15-2023