Among oxides with a melting point of more than 2000°C, alumina ceramics is the most flexible and cheap material. Alumina ceramics is a ceramic material with alumina (alumina) as the main body. Alumina ceramics have high mechanical strength, high hardness, low high-frequency dielectric loss, and because of its wide source of raw materials, relatively cheap prices, and mature processing technology, it is widely used in electronics, electrical appliances, machinery, textiles and Aerospace and other fields.
This also established its high position in the field of ceramic materials. It is reported that alumina ceramics is currently the largest amount of oxide ceramic materials in the world. In this article, the editor of Xianji.com will give you a detailed introduction to alumina ceramic materials.
The structure of alumina ceramic material belongs to the corundum type, which has the characteristics of ionic bonds, which makes the sliding system far less than that of metal, which leads to its lack of certain toughness and plasticity. Therefore, the fracture toughness exhibited is low, which greatly limits the wide application of alumina ceramics. So what are the main toughening methods for alumina ceramics?
1. Toughening of layered structure
Natural materials such as bamboo, shells, etc., have good overall performance because their structure is distributed in layers. People get inspiration from these natural structures and use bionic structures to improve the brittleness and toughness of ceramic materials.
The layered composite ceramic material is composed of multiple layers of materials. The elastic modulus and linear expansion coefficient of each layer are different, which in turn causes macroscopic stress between the layers and compressive stress on the surface. When subjected to external force, the strain energy can be absorbed to the maximum, and the crack can be repeatedly deflected and bent along the interface. In order to achieve the purpose of improving surface properties and overall toughness.
For example: Alumina/Ni layered ceramics, the linear expansion coefficient of nickel is about) times that of alumina, which produces stress in the alumina layer and has a large crack deflection ability, so the material has better toughness.
Layered ceramics are a new type of material with broad prospects, but the main disadvantage is that weak interlayer will reduce the strength of the material, and the properties of parallel and perpendicular to the interlayer are quite different and present anisotropy. Therefore, experts in the industry put forward the idea of using a strong interlayer to prepare a ZTA/alumina strong interlayer with an impact toughness of more than 10 Mpa.m1/2, which is 2.8 times that of ZTA materials and 5.6 times that of alumina ceramics. Some scientists have simulated layered composite ceramics through computers, and found that if the strength of the soft layer material is too high or too low, the overall toughness will be reduced, and the ratio of the thickness of the hard layer and the soft layer to the elastic modulus will be increased, and the uniformity of the hard layer will be uniform. Can improve the toughness of ceramics. This provides certain research ideas and optimization methods for layered toughened ceramics.
2. Fiber composite toughening
Studies have shown that the toughening efficiency of continuous fiber on ceramics is greater than other toughening methods, and it is the highest toughness that ceramic series can achieve so far, which can reach about 20Mpa.m1/2, so it is a very effective way to improve the brittleness of ceramic materials.
This method disperses fibers with higher strength and elastic modulus in a ceramic matrix. Under the action of external force, a part of the load of the composite material is borne by the fiber, so as to reduce the load of the matrix itself. Moreover, when the fiber in the matrix breaks when the bearing force is greater than its strength, the fiber will have a pull-out mechanism. In addition, these fibers also have crack bridges and deflection in the matrix to prevent crack propagation. These three toughening mechanisms work together to greatly improve the toughness of ceramic materials.
At present, the fibers used for alumina ceramics mainly include carbon fiber, silicon carbide fiber, aluminum silicate fiber and so on. Studies have found that increasing the length-to-diameter ratio of the fiber can improve the toughening effect. In the form of fiber use, the three-dimensional braid with fiber has a better toughening effect. Similar to fiber, there are more whiskers to toughen alumina porcelain, and the effect is also very good. Because whiskers are short fibers with a single crystal structure and a very small diameter (usually less than 3 um). It has few crystal defects, highly ordered atomic arrangement, and its strength is close to the theoretical value of the bonding force between adjacent atoms. Theory and practice have proved that applying it to the toughening of ceramics has a certain effect on improving toughness. If silicon carbide whiskers (volume fraction up to 20%-30%) are introduced into alumina-based ceramics, the segment toughness can reach 8-8.5 Mpa.m1/2.
In addition to the mechanisms of whisker toughening, such as pull-out, crack deflection, crack bridging, and pinning, its own high strength is also a reason. Therefore, in theory, increasing the strength of the whisker, reducing its elastic modulus, and increasing the aspect ratio can improve the toughening effect. The disadvantage of fiber and whisker toughened alumina porcelain is that it is difficult to ensure the uniformity of mixing.
The so-called self-toughening refers to the growth of toughened and reinforced phases under certain technological conditions. It eliminates the physical or chemical incompatibility between the matrix phase and the toughening phase to a certain extent, and ensures the thermodynamic stability of the matrix phase and the toughening phase.
For alumina ceramics, anisotropically grown grains toughened alumina has become a research hotspot in overcoming the brittleness of alumina ceramics. The main mechanism is to control the growth direction of alumina crystal grains through technological measures, so that it grows into rods and long columns along certain crystal planes, which has a toughening effect similar to whiskers. When subjected to an external load, a bridging method occurs at the tail of the crack; and these anisotropically grown alumina will also produce toughening mechanisms such as pull-out and crack deflection, which improves the toughness of the entire alumina ceramic.
4, phase change toughening
This is a toughening formula that has been studied relatively early and commonly. It artificially creates a large number of very fine cracks in the material to absorb energy and prevent crack propagation. Among them, the main focus is on the martensitic transformation of ZrO2, and the more successful ceramic materials such as ZTA and ZTM. ZrO2 is dispersed in the alumina matrix. Due to the different linear expansion coefficients of the two, during cooling, the ZrO2 particles are subjected to compressive stress and the phase change is hindered. Then, when the material is subjected to an external force, the pressure on the ZrO2 particles is relaxed, the tetragonal phase is transformed into a monoclinic phase, and microcracks are generated in the matrix after volume expansion, and the energy of the main crack is absorbed to achieve the toughening effect. This is the stress-induced phase transformation toughening mechanism.
In the toughening mechanism, in addition to the induced phase transition mechanism of ZrO2, the phase transition produces volume expansion, and the phenomenon of squeezing from the crack area to the non-phase transition area makes the crack closed, difficult to propagate, and can also improve toughness. Some researchers used ZrO2 with a volume fraction of 10% to 30% to prepare ZTA ceramics and found that when the volume fraction of ZrO2 was 20%, the toughening effect was the best.
Ceramic toughening technology will be a hot technology in the material industry for a long time in the future. If the inherent high strength, high temperature resistance, low expansion coefficient and other characteristics of ceramic materials can be combined with high toughness, it will be a high-performance material coveted by the material industry, and it will be used in a wide range of fields. Here is a brief introduction to some applications of alumina ceramics.
(1) Mechanical aspects
The flexural strength of alumina ceramic sintered products can reach 250MPa, and the hot-pressed products can reach 500MPa. The Mohs hardness of alumina ceramics can reach 9, plus excellent wear resistance, so it is widely used in the manufacture of tools, ball valves, grinding wheels, ceramic nails, bearings, etc., among which alumina ceramic tools and industrial valves are used The widest.
Alumina Ceramic Tool
The optimal cutting speed of alumina ceramic tools is higher than that of ordinary cemented carbide tools, which can greatly improve the cutting efficiency of different materials. With a lot of research by scientists, other components are added to form two phases or exist in the form of solid solution in alumina-based composite ceramics and whisker-reinforced ceramics. These technologies make up for the shortcomings of pure alumina ceramics, thereby improving its cutting performance and durability.
In terms of electronics and electricity, there are various alumina ceramic base plates, substrates, ceramic membranes, transparent ceramics, and various alumina ceramic electrical insulating ceramics, electronic materials, magnetic materials, etc., among which alumina transparent ceramics and substrates are used The widest.
Alumina transparent ceramics
At present, transparent ceramics is an important frontier in the research and application of materials. As an emerging material, transparent ceramics, in addition to its wide range of light transmittance, also has high thermal conductivity, low electrical conductivity, high hardness, high strength, low dielectric constant and dielectric loss, wear resistance and corrosion resistance A series of advantages such as good sex.
Alumina ceramic substrate
Alumina ceramic substrates have excellent properties such as high mechanical strength, good insulation, and high light resistance, and are widely used in multilayer wiring ceramic substrates, electronic packaging and high-density packaging substrates.
(3) Chemical industry
In chemical applications, alumina ceramics also have a wide range of uses, such as alumina ceramic chemical filler balls, inorganic microfiltration membranes, and corrosion-resistant coatings. Among them, alumina ceramic membranes and coatings have been studied and applied the most.
(4) Medical aspects
In medicine, alumina is more used to make artificial bones, artificial joints, artificial teeth and so on. Alumina ceramics have excellent biocompatibility, biological inertness, physical and chemical stability, high hardness and high wear resistance, and are ideal materials for the preparation of artificial bones and artificial joints. However, it has the same shortcomings as other ceramic materials, such as high brittleness, low fracture toughness, high machining technology difficulty, and complex technology, so further research and application are needed.
In building sanitary ceramics, products can be seen everywhere, such as alumina ceramic lining bricks, grinding media, rollers, ceramic protection tubes, and alumina refractory materials. Among them, alumina ball milling media is the most widely used.
The charm of materials science is to learn from each other’s strengths and create 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. In the continuous development of toughening technology, alumina ceramic materials will surely have better performance, and the application field will be more extensive.
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