How to prepare zirconia ceramics and the common problems and causes in the sintering of zirconia ceramics
Time:2019-09-12
Views:5160
Zirconium oxide ceramics have excellent properties such as high strength, high hardness, good toughness and corrosion resistance, and are widely used in various fields such as tools, molds, valve materials, oxygen sensors, and solid fuel cells. In recent years, with the continuous improvement of people‘s demand for decorations, color zirconia ceramics have become the new favorite of high-end decorative materials such as bracelets, bracelets and buttons with its excellent performance, bright color, metallic luster and no allergic effects. So how is the color zirconia ceramic prepared? Solid phase mixing Process for preparing colored zirconia ceramics by solid phase mixing method Advantages: The process is simple, the cost is low, the operation is convenient, and the industrialization is easy. At present, the synthesis of colored zirconia is basically carried out by this method. Disadvantages: It is impossible to overcome the agglomeration of nanoparticles, the mixing of the colored phase and the matrix nanoparticles is uneven, and the ball milling time is long, and the ball milling medium or the atmosphere may cause serious pollution to the powder. Chemical coprecipitation Process for preparing colored zirconia ceramics by chemical coprecipitation Advantages: The obtained powder has high purity and excellent performance. Disadvantages: Due to the complex ion precipitation of colored zirconia, the reaction in the post-sintering process is complicated, and the zirconia stabilizer may react unpredictably with the colored ions, thereby consuming a large amount. Eventually it will affect the performance of the colored zirconia article, as well as the color rendering optical properties of the colored ions. Liquid phase infiltration Process for preparing colored zirconia ceramics by liquid phase infiltration Features: Compared with the traditional preparation process, the liquid phase infiltration method is simpler in process, and the prepared color zirconia ceramic has obvious advantages in color uniformity and physical properties. In addition, the liquid phase infiltration method can make full use of injection molding to obtain various shapes of complex zirconia blanks, thereby producing various complex shapes of colored zirconia ceramics. Other methods Zhong Wei and others have developed a new process to process zirconia ceramics into slabs, perform normal degreasing and dewaxing, and treat the zirconia greens under vacuum conditions in a low temperature unprotected atmosphere. High temperature sintering is carried out. A graphite crucible is placed on the workpiece during sintering, and graphite paper is placed on the surface of the workpiece. Black coloration of zirconia ceramics is achieved by the infiltration of graphite into the zirconia surface at elevated temperatures. Disadvantages: This method requires high equipment and requires a vacuum environment; the coloring only stays on the surface of the zirconia product, which is not conducive to the deep processing in the later stage; the black is not bright enough, and it may gradually fade in the process of use. The ceramic green body is composed of many individual solid particles before sintering. There are a large number of pores in the green body, and the porosity is generally 35%~60% (that is, the relative density of the green body is 40%~65%). The values depend on the characteristics of the powder itself and the molding methods and techniques used. When the solid solid is heated at a high temperature, the particles in the green body migrate, and after reaching a certain temperature, the green body shrinks, grain growth occurs, and the pores are removed, and finally at a temperature lower than the melting point (generally The melting point is 0.5 to 0.7 times. The green body becomes a dense polycrystalline ceramic material. This process is called sintering. Sintering is the last process of forming ceramic blanks. The performance of ceramic products is largely determined by sintering. The zirconia ceramics are sintered to a high density and uniformity. Not only is the degreasing step of the previous processing step critical, but also affected by factors such as powder, additives, sintering temperature and time, pressure and sintering atmosphere. During the specific sintering process, the zirconia ceramic body may have problems such as deformation, cracking, and abnormal grain growth. What are the reasons? Below we will analyze the specific issues according to specific issues. First, the deformation The zirconia ceramic is deformed during the sintering process, which may be due to the excessive distribution of the powder particle size; the selection and addition of additives in the powder are unreasonable; the shrinkage of the ceramic is inconsistent. The shrinkage of ceramics is inconsistent, for three reasons: 1 The furnace temperature is not uniform, and the ceramic green body undergoes inconsistent shrinkage; 2 The heating rate is fast, the temperature conduction produces a gradient, and the closer the ceramic body shrinks to the surface layer, the slower the center shrinks. 3 There is a density gradient. During the molding, the internal shrinkage ratio of the blank is inconsistent due to factors such as pressure and filler. Second, rupture The main reason for the cracking of the sintered ceramic body is that there are defects inside the ceramic body, and it is also related to the shrinkage of the green body. The reason for the inconsistent shrinkage of the green body is analyzed by the cause of the sintering deformation. When the shrinkage is inconsistent, if there is a defect (hole, crack, etc.), the defect becomes a source of the fracture, and the crack diffusion causes the body to crack. Third, the grain grows abnormally When the grains grow abnormally, there are often a large number of pores in these oversized grains which are difficult to be discharged from the grains to the grain boundaries, which makes it difficult to achieve a high density of ceramic materials, and many properties of the materials. Deterioration, especially mechanical properties (fracture toughness, flexural strength, etc.). There are three main reasons for the abnormal growth of crystal grains: 1 The original powder particle size distribution range is too wide, that is, the largest particle in the powder is greater than or much larger than twice the average grain size; 2 When the forming body density is uneven (powder agglomeration, gradient of the pressing pressure of the green body, unevenness of the additive), uneven densification occurs in the sintering; 3 too high sintering temperature and too long holding time.
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