Alumina ceramics are the most flexible and cheap materials among oxides with a melting point of more than 2000 ℃.Alumina ceramic is a kind of ceramic material with alumina as the main body. Alumina ceramics are widely used in the fields of electronics, electrical appliances, machinery, textile, aerospace and so on because of its high mechanical strength, high hardness and low dielectric loss.This also established its high position in the field of ceramic materials.It is reported that alumina ceramic is the largest oxide ceramic material in the world.
The structure of alumina ceramics belongs to corundum type, which has the characteristics of ionic bond, making the slip system far less than that of metals, which leads to the lack of certain toughness and plasticity.Therefore, the fracture toughness is low, which greatly limits the wide application of alumina ceramics.So what are the main toughening methods of alumina ceramics?
Lamellar structure toughening
Natural materials, such as bamboo and shell, have good comprehensive properties because of their layered structure.People get inspiration from these natural structures, using bionic structure to improve the brittleness and toughness of ceramic materials.
Layered composite ceramic materials are composed of multilayer materials.The elastic modulus and linear expansion coefficient of each layer are different, which leads to macro stress between layers and compressive stress on the surface.Under the action of external force, the strain energy can be absorbed to the maximum extent, and the crack can deflect and turn repeatedly along the interface.So as to improve the surface properties and overall toughness.
For example, for alumina / Ni layered ceramics, the linear expansion coefficient of nickel is about) times of that of alumina, which produces stress in the alumina layer and has great crack deflection ability, so the material has better toughness.
Lamellar ceramics is a new type of material with a broad prospect, but its main disadvantage is that the weak interlayer will reduce the strength of the material, and the properties parallel to and perpendicular to the direction of the interlayer are quite different, showing anisotropy.So experts in the industry put forward the idea of using strong interlayer to prepare ZTA / alumina strong interlayer. The impact toughness is more than 10mpa.m1/2, which is 2.8 times of ZTA material and 5.6 times of alumina ceramic.Some scientists have simulated the laminated composite ceramics by computer, and found that if the strength of the soft layer material is too high or too low, the overall toughness will be reduced, and if the ratio of the hard layer thickness to the soft layer thickness and the elastic modulus is increased, the uniformity of the hard layer can improve the toughness of the ceramics.This provides some research ideas and optimization approaches for the lamellar toughened ceramics.
Fiber Composite Toughening
The results show that the toughening efficiency of continuous fiber is higher than that of other toughening methods, and it is the highest toughness of ceramic series so far, which can reach about 20mpa. m1/2, so it is a very effective way to improve the brittleness of ceramic materials.
In this method, fibers with high strength and elastic modulus are dispersed in ceramic matrix.Under the action of external force, part of the load of the composite is borne by the fiber, so as to reduce the load of the matrix itself. Moreover, when the strength of the fiber in the matrix is greater than its strength, the fiber will pull out. In addition, these fibers also have crack bridging and deflection in the matrix to prevent crack growth. The three toughening mechanisms together improve the toughness of ceramic materials.
At present, the fibers used in alumina ceramics are mainly carbon fiber, silicon carbide fiber, aluminum silicate fiber and so on. It is found that the toughening effect can be improved by increasing the ratio of length to diameter. In the form of fiber, the three-dimensional woven fabric with fiber has better toughening effect. Similar to the fiber, there are many alumina ceramics toughened by whiskers, and the effect is very good. Because whiskers are short fibers with single crystal structure and very small diameter (usually less than 3 UM). It has few crystal defects, highly ordered arrangement of atoms, and its strength is close to the theoretical value of bonding force between adjacent atoms. The theory and practice prove that it can improve the toughness of ceramics.If silicon carbide whiskers (volume fraction up to 20% - 30%) are introduced into alumina based ceramics, the segment toughness can reach 8-8.5mpa.m1/2.
The mechanism of whisker toughening is not only pull-out, crack deflection, crack bridging and pinning, but also high strength.Therefore, in theory, increasing the strength, reducing the elastic modulus and increasing the aspect ratio of whiskers can improve the toughening effect.The disadvantage of fiber and whisker toughened alumina ceramics is that it is difficult to ensure the uniformity of mixing.
Self toughening
The so-called self toughening is to grow toughened and reinforced phases under certain technological conditions.To a certain extent, it eliminates the physical or chemical incompatibility between the matrix phase and the toughened phase, and ensures the thermodynamic stability of the matrix phase and the toughened phase.
As for alumina ceramics, the research focus of overcoming the brittleness of alumina ceramics is to toughen alumina with different growth grains.The main mechanism is to control the growth direction of alumina grains through technological measures, so that it can grow into rod-shaped and long column shaped along some crystal surfaces, which plays a similar role in toughening whiskers.In the case of external load, the crack tail will form bridging mode, and the anisotropic growth of alumina will also produce pullout, crack deflection and other toughening mechanisms, which will improve the toughness of the whole alumina ceramics.
Phase transformation toughening
This is a kind of toughening formula which has been studied earlier and widely.It artificially creates a large number of extremely fine cracks in the material to absorb energy and prevent crack growth.Most of them focus on the martensitic transformation of ZrO2, and ZTA, ZTM and other ceramic materials are successful.ZrO2 dispersed in alumina matrix, because of their different linear expansion coefficient, ZrO2 particles are subject to compressive stress and phase transformation is blocked during cooling.Then, when the material is subjected to external force, the pressure on the ZrO2 particles is relaxed, the tetragonal phase changes into monoclinic phase, and microcracks are produced in the matrix after volume expansion, and the energy of the main cracks is absorbed to achieve the toughening effect.This is the toughening mechanism of stress-induced transformation.
In the toughening mechanism, in addition to the induced transformation mechanism of ZrO2, the transformation produces volume expansion, and the phenomenon of extrusion from the crack area to the non transformation area makes the crack close, difficult to expand, and also improves the toughness.Some researchers found that the best toughening effect was obtained when the volume fraction of ZrO2 was 20%.
Ceramic toughening technology will be a hot technology in material field for a long time in the future.If the inherent characteristics of ceramic materials, such as high strength, high temperature resistance and low expansion coefficient, can be combined with high toughness, it will be the high-performance materials that the material field is dreaming of, and the application field is very extensive.Let's briefly introduce some applications of alumina ceramics.
Mechanical
The bending strength of sintered alumina ceramic products can reach 250Mpa, and that of hot pressed products can reach 500MPa.The Mohs hardness of alumina ceramics is up to 9. In addition, it has excellent wear resistance, so it is widely used in the manufacture of cutting tools, ball valves, grinding wheels, ceramic nails, bearings, etc., among which alumina ceramic cutting tools and industrial valves are the most widely used.
Alumina ceramic cutter
The optimal cutting speed of alumina ceramic tool is higher than that of common cemented carbide tool, which can greatly improve the cutting efficiency of different materials.With a great deal of research by scientists, alumina matrix composite ceramics and whisker reinforced ceramics which are composed of two phases by adding other components or exist in the matrix in the form of solid solution.These technologies make up for the shortcomings of pure alumina ceramics, thus improving its cutting performance and durability.
Electronic / power
In terms of electronics and electric power, there are various alumina ceramic soleplates, substrates, ceramic films, transparent ceramics and various alumina ceramic electrical insulating ceramics, electronic materials, magnetic materials, etc., of which alumina transparent ceramics and substrates are the most widely used.
Alumina transparent ceramics
At present, transparent ceramics are an important frontier in the field of materials research and application.As a new material, transparent ceramics not only has wide range of light transmittance, but also has a series of advantages, such as high thermal conductivity, low conductivity, high hardness, high strength, low dielectric constant and dielectric loss, good wear resistance and corrosion resistance.
Alumina ceramic substrate
Alumina ceramic substrate has the advantages of high mechanical strength, good insulation and high light resistance, which is widely used in multilayer wiring ceramic substrate, electronic packaging and high-density packaging substrate.
Chemical industry
In chemical application, alumina ceramic has a wide range of uses, such as alumina ceramic chemical filler ball, inorganic microfiltration membrane, corrosion-resistant coating, etc., among which alumina ceramic membrane and coating are the most studied and applied.
Medical
In medicine, alumina is more used to make artificial bone, artificial joints, artificial teeth and so on.Alumina ceramics have excellent biocompatibility, biological inertia, physical and chemical stability, high hardness and wear resistance, and are ideal materials for the preparation of artificial bone and artificial joint.However, it has the same disadvantages as other ceramic materials, such as high brittleness, low fracture toughness, high machining technology difficulty, complex technology, etc., so it needs further research and application.
Architecture / sanitation / ceramics
In the field of building sanitary ceramics, products can be seen everywhere, such as alumina ceramic lining brick, grinding medium, roller, ceramic protective tube and alumina refractory.Among them, alumina ball milling medium is the most widely used.
The charm of material science lies in learning from each other's strong points to make up for each other's weak points and creating ideal materials.In addition to the above applications, alumina ceramics are also widely used in other high-tech fields, such as aerospace, high-temperature industrial furnaces, composite reinforcement and other fields.With the continuous development of toughening technology, alumina ceramic materials will have better properties and more extensive application fields.
