Bio-inert ceramics

Bio-inert ceramic materials mainly refer to ceramic materials with stable chemical properties and good biocompatibility. The structure of this kind of ceramic materials is relatively stable, the bonding force in the molecule is relatively strong, and it has high mechanical strength, wear resistance and chemical stability.

Biologically inert ceramics include: alumina ceramics, alumina single crystals, zirconia ceramics, glass ceramics and so on.

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1. Alumina ceramics

(1) The structure and performance of alumina ceramics

①Crystal structure

Alumina ceramics refers to ceramic materials whose main crystal phase is corundum (α-Al2O3). α-Al2O3 has the most stable structure. Because the crystal form of natural corundum is α-Al2O3, it is also called corundum structure. α-Al2O3 belongs to the hexagonal crystal system. The oxygen ions are closely packed in hexagons, and an octahedron is formed by 6 O2- ions. The center of the octahedral gap is filled with an Al3+ ion with a smaller radius.

Corundum has a compact structure, high internal ionic bond strength and uniform bond force distribution, so corundum ceramics have high mechanical strength,excellent electrical insulation, high temperature resistance, chemical corrosion resistance and good biocompatibility.

(2) Microstructure

From the microstructure point of view, alumina ceramics are mainly composed of alumina grains of different orientations assembled through grain boundaries body.

The crystal grain is the existence form and constituent unit of the crystal phase in the ceramic polycrystalline material, that is, the crystal grain is the non-definite geometric shape in the polycrystalline material shaped small single crystal. In the process of forming and growing, each kind of crystal grows into a regular geometric polyhedron according to its own crystalline habits. This is a basis for understanding and identifying crystals. Differences and changes in the physical and chemical conditions and the external environment during crystal growth will seriously affect the morphology of the crystal. For ceramic materials, it will cause huge differences in the microstructure. For example, if the crystal grows freely in a better environment, the crystal can develop into a complete crystal form according to its own crystalline habits, called euhedral crystal; but when the growth environment is more Poor or inhibited during growth, its crystal shape can only be partially complete or completely incomplete, which are called semi-automorphic crystals and other-shaped crystals, respectively.

Practice has proved that the main crystal phase of the same composition, such as alumina ceramics, is α-Al2O3. Due to the difference in crystal grain size, the mechanical properties of the material will be very different, and the flexural strength of the material will be very different.

Grain boundaries are a very important part of ceramic polycrystalline materials. It has a significant impact on many physical properties of the material, which is discussed here in conjunction with the mechanical strength.

Experiments show that most of the damage of ceramic materials is fracture along the grain boundary. For small crystal materials, the proportion of grain boundaries is large. When the cracks are broken along the grain boundaries, the expansion of cracks will take a tortuous path, and the grains become finer. The longer the journey. For brittle materials such as ceramics, the initial crack size is equivalent to the grain size, so the finer the grain, the smaller the initial crack size and the higher the mechanical strength. Therefore, in order to obtain good mechanical properties, it is necessary to study and control the grain size. In addition, due to the irregular arrangement of the particles on the grain boundary, the uneven distribution of the particles causes the formation of microscopic grain boundary stress. For single-phase polycrystalline materials, due to the different orientations of the crystal grains, the thermal expansion coefficient and elastic modulus of adjacent crystal grains in a certain direction are different; for multi-phase polycrystalline materials, there are more performance differences among the phases; For the solid solution, the fluctuations in the chemical composition between the crystal grains will also produce a large grain boundary stress on the grain boundary. The larger the grain size, the greater the grain boundary stress. This kind of grain boundary stress can even cause transgranular fractures in large grains, which may be one reason for the poor mechanical strength of ceramic materials with coarse-grained structures. For this reason, in order to control the excessive growth of crystal grains in the production process of alumina ceramics, especially to prevent secondary recrystallization, a small amount of MgO is often mixed in the process of raw material processing to make the crystals between the α-Al2O3 grains. A thin layer of magnesium aluminum spinel is formed on the boundary, which surrounds the α-Al2O3 grains to prevent the grains from growing and becoming a fine-grained structure. Secondly, due to the large amount of impurities in the raw materials, when the number of additives is large, the second phase substance is often precipitated on the grain boundary, which will also have a very important impact on the material properties. In short, how to control the microstructure of alumina ceramics through a certain process is an important way to improve its performance.


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