Bio inert ceramic materials mainly refer to ceramic materials with stable chemical properties and good biocompatibility. The structure of these ceramic materials is relatively stable, the bond force in the molecules is relatively strong, and they have high mechanical strength, wear resistance and chemical stability.
Bio inert ceramics include alumina ceramics, alumina single crystal, zirconia ceramics, glass ceramics, etc
(1) Structure and properties of alumina ceramics
① Crystal structure
Alumina ceramics refer to corundum as the main crystalline phase（ α- Al2O3). α- Al2O3 has the most stable structure because the crystal form of natural corundum is α- Al2O3, so it is also called corundum structure. α- Al2O3 belongs to hexagonal crystal system. Oxygen ions are closely packed in hexagonal, and an octahedron is formed by six O2 ions. The center of the octahedral gap is filled with a small radius Al3 + ion.
Corundum has compact structure, large internal ionic bond strength and uniform bond force distribution. Therefore, corundum ceramics have the characteristics of high mechanical strength, excellent electrical insulation, high temperature resistance, chemical corrosion resistance and good biocompatibility.
In terms of microstructure, alumina ceramics are mainly composed of alumina grains with different orientations gathered through grain boundaries.
Grain is the existing form and constituent unit of crystalline phase in ceramic polycrystalline materials, that is, grain is a small single crystal without certain geometric shape in polycrystalline materials. In the process of formation and growth, each crystal grows into a regular geometric polyhedron according to its own crystallization habit.
This is a basis for understanding and identifying crystals. The differences and changes of physicochemical conditions and external environment during crystal growth will seriously affect the morphology of crystals. For ceramic materials, the microstructure will be very different. If they grow freely in a better environment, the crystal can develop into a complete crystal form according to its own crystallization habit, which is called automorphic crystal; However, when the growth environment is poor or the growth is inhibited, the crystal form can only be partially complete or completely incomplete, which are called subhedral crystal and heteromorphic crystal respectively.
Practice has proved that the main crystalline phase of the same composition, such as alumina ceramics, is α- Due to different grain sizes, the mechanical properties of Al2O3 will be very different, and its flexural strength varies greatly.
Grain boundary is a very important component of ceramic polycrystalline materials. It has a significant impact on many physical properties of materials, which is discussed here in combination with mechanical strength.
The experimental results show that the failure of ceramic materials is mostly along the grain boundary. For fine crystalline materials, the proportion of grain boundary is large. When the crack is destroyed along the grain boundary, the crack propagation should take a tortuous path, and the finer the grain is. 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, we should study and control the grain size. In addition, due to the irregular arrangement and uneven distribution of particles on the grain boundary, micro grain boundary stress is formed. For single-phase polycrystalline materials, the thermal expansion coefficient and elastic modulus of adjacent grains in the same direction are different due to different grain orientation; For multiphase polycrystals,
There are more performance differences between phases; For solid solution, the fluctuation of chemical composition between grains will also produce great grain boundary stress on the grain boundary. The larger the grain size, the greater the grain boundary stress. This grain boundary stress can even cause transgranular fracture of large grains, which may be one of the reasons for the poor mechanical strength of coarse-grained ceramic materials. Therefore, in the production process of alumina ceramics, in order to control excessive grain growth, especially to prevent secondary recrystallization, a small amount of MgO is often added in the process of raw material treatment α- A thin layer of magnesium aluminum spinel is formed on the grain boundary between Al2O3 grains α- Al2O3 grains are surrounded to prevent grain growth and become a fine grain structure. Secondly, due to the large amount of impurities in the raw materials and the large amount of additives, the second phase substances are 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 properties.