To maintain their leading position in the field of aviation power in the 21st century, aero-engine companies around the world are seeking new ways to improve the performance of military and civil engines and maintain their competitiveness. Half of that will depend on material improvements, including low-temperature polymer composites and high-temperature ceramics; the other half relies on improving design guidelines, methods, and procedures.
As the key to the improvement of military engine materials is to rely on high-temperature ceramic materials, the military engine will be the primary verifier of ceramic technology. Why is it necessary to use chemical zirconia ceramics? The operating temperature of the existing engine is already very high, and the only way to increase the temperature again is through the fine design of the cooling air circuit or the increase of cooling air volume.
However, the effects of these methods follow the law of diminishing, and only by improving the working temperature of the material can the maximum effect be achieved. Because raising the operating temperature can improve working efficiency, reduce fuel consumption and obtain the maximum thrust, using the saved high-pressure air for cooling for circulation can also improve the thrust and efficiency. Another option is to reduce weight by choosing materials with greater specific strength and greater stiffness. At present, only ceramic materials have the potential in this respect.
The application of ceramics to aero-engines will be developed with new materials and manufacturing methods. Considering the brittleness of ceramic materials and the lack of design and use experience, the process will be very long, no less than 15-20 years of metal materials. The applications of chemical zirconia ceramics in aviation are as follows.
Chemical zirconia ceramics have high-temperature resistance, low density, good oxidation resistance, corrosion resistance and wear resistance. In the case of the cooling, the working temperature of chemical zirconia ceramics can reach 1600 ℃, the density is only 40% of that in the high-temperature alloy, and the same volume of parts can reduce the weight by about 60%, which can greatly reduce the centrifugal load of the high-speed rotor. The use of ceramics also simplifies the chemical zirconia by reducing or eliminating the cooling system, making the engine compact.
The increasing turbine inlet temperature is the key to improving the thrust-to-weight ratio of the aero-engine and reducing fuel consumption. Sages for example, when the ratio is 10 level, the temperature of the engine turbine can reach above 1500 ℃, while the use temperature of high-temperature alloys and intermetallic compounds highest is less than 1200 ℃. Therefore, the research of high-temperature chemical zirconia ceramics and their ceramic matrix composites becomes one of the key technologies for high thrust-weight ratio aero-engines.
Radar remains one of the most reliable means of detecting military targets in future wars. The essence of stealth technology is to reduce the target’s RCS（Radar Cross-Section）, that is, to select materials with good radar wave absorption to reduce its RCS.
Absorbing materials can be divided into coating type and structure type according to process and bearing capacity. The former has a poor bearing capacity and low strength, while the latter is a new functional composite chemical zirconia material, which has the characteristics of absorbing waves and can be directly used as the chemical zirconia material for aircraft.
We use the excellent mechanical and physical properties of chemical zirconia ceramics to carry out the research on absorbing materials, which can not only enhance the national defense force but also is an important aspect of expanding the application of chemical zirconia ceramics. Some new nano absorbents and their composites are being applied in this field, such as nano Silicon Carbide (SiC), nano nitride, carbon nanotube (CNT), and other nanocomposites.
In the mid-1980s, the United States developed an aerospace aircraft program that required both high-temperature tolerance and light mass. For this purpose, a variety of new high-temperature materials were developed, including advanced resin matrix composites, metal matrix composites, ceramic matrix composites, and carbon/carbon composites. Ceramic material is the preferred material for missile radome because of its excellent mechanical, thermal and electrical properties. The radome is the most widely used ceramic matrix composite material in missile structure.
The missile radome is located at the front end of the missile. Its function is to protect the navigation antenna from damage so that the missile can effectively hit the target. It is not only an important part of the aerodynamic shape of the missile but also the protection device of the antenna. During the flight of the missile, the radome should not only withstand aerodynamic heating and mechanical overload, resist the erosion of rain, sand, and other adverse working conditions, but also meet the stringent requirements of electrical performance proposed by the missile control loop. Therefore, the missile radome material should have the following properties:
Excellent dielectric properties
In the guidance system, the transmission efficiency and aiming error of the radome are very sensitive to the dielectric properties of the material and its relationship with temperature and frequency. It is required that the material has low dielectric constant (10) and dielectric loss, and the dielectric properties do not change obviously with temperature and frequency.
Good heat resistance and thermal shock resistance
The high Mach number of the missile can make the radome of instantaneous heating rate is as high as above 120 ℃ / s, so the material is required to have good thermal shock resistance, and the molecular structure of the material is required to be stable when the temperature is raised, and the material properties (such as dielectric properties and mechanical properties) change little to ensure that the radome can work normally when the temperature is raised.
High-strength structural properties
The strength of the radome material should be high and rigid enough to satisfy the mechanical stress and bending moment caused by the longitudinal or transverse acceleration of the aerodynamic forces in the spacetime of the missile flying at high speed.
Resistance to rain erosion
It plays a decisive role in the design allowable range of impact Angle and the sensitivity of aircraft in rain erosion.
The dielectric properties and strength properties of general materials change obviously when they work at high temperatures. Therefore, the properties of the radome material, especially the dielectric properties and strength, are affected by the temperature change as little as possible.
Ceramic-based missile radome
Ceramic-based missile radome materials mainly include silicon nitride-based, silicon oxide-based and phosphate-based materials. Silicon nitride ceramics have not only excellent mechanical properties and high thermal stability but also low dielectric constant. Its decomposition temperature is 1900 ℃, its erosion resistance is better than fused silica, and it can withstand 6 ~ 7 Ma rating of flight conditions. Silicon nitride ceramic composite radome is one of the main research targets in various countries, which has been identified as the most promising radome material by the test of the Georgia Institute of Technology. Yttria Stabilized Zirconia (YTZ), also known as yttria-zirconia, is the strongest ceramic material. This material offers the highest flexural strength of all zirconia-based materials, and the research on zirconia-based materials as missile radome is in progress.
Because of the high flying Mach number of the missile and the relatively long heating time, if the radome of the medium-range missile is made of a single quartz ceramic material, it cannot meet the bearing requirement of thermal stress. In order to meet the requirements of medium and long-range ground-to-ground tactical and strategic missile radome, quartz glass, high-silica puncture fabric and orthogonal tri-directional quartz fabric reinforced silica matrix composites have been developed and successfully applied.
Phosphate matrix composite material is a kind of Russian characteristic permeable material, which is made by impregnating cloth or fabric with a phosphate solution and then curing under pressure. Aluminum phosphate has stable performance in 1500 ~ 1800 ℃. At present, such materials have been used in cruise missiles, anti-missile missiles, tactical missiles and space shuttles. The most obvious disadvantage of phosphate is that it is highly hygroscopic, so the surface of the composite material needs to be coated with an organic coating for moisture-proof treatment.
Silicon carbide ceramic matrix composites
Silicon carbide ceramic matrix composites have a series of excellent properties, such as low density, high-temperature resistance, ablation resistance, erosion resistance, and oxidation resistance, and it has a wide application prospect in the field of aerospace. Since the late 1980s, the United States has successfully developed a series of C/SiC, SiC/SiC ceramic matrix composites, which can be applied to the re-entry nose cone of missiles, the front end of wings and other heat-resistant structures.
Zirconia is a new structural ceramic material developed in the 1970s. It is widely used in metallurgy, electronics, chemical industry, machinery and other fields due to its abrasion resistance, corrosion resistance, high strength, and high melting point.
Zirconia ceramic material, the most important material in advanced ceramics, is an important basic material for the development of the modern high-tech industry. In particular, nano-oxide ceramics with their special structure and performance has become the focus of the industry. The following is a brief introduction to the powder materials needed to prepare nano zirconia ceramics.
Ni-P coated nano zirconia composite powders
The preparation process of Ni-P coated nano zirconia (ZrO2) composite powders is firstly to prepare nano-ZrO2 powders by the chemical precipitation method, and then to prepare Ni-P nano-ZrO2 powders by an electroless plating method. Since ZrO2 has no autocatalytic activity in the electroless nickel plating solution, it is necessary to pretreat ZrO2 nanoparticles. Generally, Pd2+ is directly adsorbed on the surface of ZrO2 powder by the one-step palladium catalysis method, and then Pd2+ is reduced to palladium in a reducing solution so that the surface of nanopowder has the catalytic activity of electroless nickel plating. The two-step sensitization-activation method is usually used in the pretreatment of non – conductive powders. However, it is difficult to remove the residual nickel ions in the powder after two-step treatment, which often brings adverse effects on the activity of the powder. At present, one-step palladium catalysis and in-situ palladium pretreatment are used.
At present, Ni-P coated nano zirconia composite powders have been widely used and studied in semiconductor nanomaterials.
Zirconia toughened alumina ceramic is one of the most widely studied structural ceramic materials. The toughening mechanism of zirconia toughening alumina ceramics is the refinement of matrix grain, the toughening of phase change, the toughening of microcrack, and the turning and bifurcation of crack. The properties of zirconia toughened alumina ceramics are mainly determined by the microstructure formed during sintering, and the microstructure is mainly determined by the powder state of raw materials. Therefore, the preparation of high-quality Al2O3/ZrO2 nanocomposite ceramic powders is the prerequisite for the preparation of zirconia toughened alumina ceramics with excellent properties. The preparation methods of Al2O3/ZrO2 nanocomposite ceramic powders mainly include the mechanical mixing method, multi-phase suspension mixing method, the sol-gel method, chemical precipitation method, etc.
Alumina is a kind of high-strength matrix in the composite ceramic system of zirconia toughening alumina, and the zirconia in the intercalation provides a phase change toughening mechanism. The use of ZrO2 phase change properties to toughen ceramic materials is still one of the main research topics of ceramic toughening in the future.
Zirconia toughened alumina composite ceramics have excellent corrosion resistance, thermal shock resistance, high strength, and toughness, as well as wide application prospects. Zirconia toughened alumina composite ceramics can be used to make ceramic cutters for the processing of cast iron and alloy, and the interface structure of engineering ceramics can be made to extend the service life of engineering materials. Alumina toughened with zirconia can be used to make wear-resistant ceramic balls. Due to its good biocompatibility, alumina can also be used as a biomedical material for the reconstruction and repair of hard tissues (teeth).
Boron nitride-zirconia composite powder
Boron nitride-zirconia composite powders were prepared by mechanical mixing method. Boron nitride, zirconia, and additives were used as the main raw materials. After mixing, the powders were ball-ground and mixed in an alcohol medium, and then the zirconia composite ceramics were sintered in a hot press sintering furnace. Due to the poor sintering capacity of pure boron nitride and its difficulty in sintering densification, CaO, B2O3, Al2O3, and ZnO are generally added as sintering AIDS.
Boron nitride-zirconia composite ceramics are characterized by high strength, high toughness, high thermal conductivity, low expansion, and excellent physical and chemical properties, such as chemical inertness and chemical corrosion, which are present in molten metals. In addition, it also has excellent thermal shock resistance, erosion resistance, wear-resistance and easy processing and other properties, which make the material suitable for thin strip continuous casting side seal plate, jet forming liquid guide pipe, the nozzle for metal spinneret, continuous casting functional refractories and other fields.
Nano cerium-zirconium composite oxide powder
The preparation methods of nano cerium-zirconium composite oxide powders include high-temperature roasting, the sol-gel method, coprecipitation method, hydrothermal method, and solid-phase reaction method. High-temperature roasting was carried out in a water-ethanol solvent, and the suspension consisted of dry Al (NO3) 3•9H2 O•Ce (NO3) 3•6H2O and monocline phase zirconia nano-powder was pyrolyzed by high-temperature, Al2O3 doped CeO2 coated monoclinal zirconia powders with a particle size less than 100 m were prepared.
Nano cerium-zirconium composite oxide materials are used as auxiliary catalysts, mainly used in automobile exhaust treatment, with good high-temperature stability, high REDOX ability, high oxygen storage, and release capacity.
Zirconium dioxide (ZrO2) is a kind of metal oxide material with many excellent properties such as high melting point (2700 ℃) and high boiling point, small coefficient of thermal conductivity, thermal expansion coefficient, high-temperature resistance, good wear resistance, corrosion resistance. Nano zirconia powders have many important applications because of their nanometer properties. Fine ceramics made of nano zirconia have some special properties under different conditions, such as insulator at room temperature, conductivity, sensitivity, and toughness at high temperature.
In the zirconium industry chain, the most widely used composite zirconia is the stable/partially stable zirconia formed by doping the corresponding rare earth elements according to different uses. The variety and content of the added rare earth elements can be adjusted to produce composite zirconia that meets the requirements of different uses, such as yttrium stabilized zirconia used as structural parts and zirconium and cerium eutectic used as catalysts. Compared with common zirconia, nano-scale composite zirconia has a smaller particle size and reaches the nanometer level. Its higher additional use value and the market scale of over ten billion are being rapidly developed.
Here are 10 applications of nanocomposite zirconia.
Nano ZrO2 can obviously improve the room temperature strength and stress strength factor of ceramics, thus doubling the toughness of ceramics. The composite bioceramics prepared with nanometer ZrO2 have good mechanical properties, chemical stability, and biocompatibility. It is a promising composite bioceramics material, especially in the field of dental materials and artificial joints. Biomaterials refer to materials with natural organ tissue function or partial function, and they are the latest branch of biomedical science and have broad application prospects. Bioceramics have been widely used in the field of oral prosthodontics because of their excellent biocompatibility, stability, and aesthetics.
Zirconia toughened ceramic, as a new fine ceramic, has good mechanical properties (fracture toughness, strength, hardness, etc.), biocompatibility and stability, aesthetics, thermal conductivity, and formability, which can well solve the problem of insufficient strength and toughness of conventional all-ceramic crown materials. Secondly, as an excellent bioinert ceramic, it has excellent chemical stability both as an oral prosthesis and an implant, which fully meets the standard as an oral prosthesis material.
The initial ceramic artificial joint is not perfect and has undergone four generations of process improvement so far, gradually becoming perfect. The fourth generation of the artificial ceramic joint is composed of several kinds of oxidized crystal materials such as zirconia, with good toughness and strength its performance is much better than that of the third generation of the ceramic joint. When zirconia is compounded, the crystal particles become smaller. More importantly, zirconia disperses and absorbs the energy of the fracture, inhibiting crack growth. Zirconia is the best prosthesis material currently used in clinical hip replacement, the ceramic material with the best wear resistance is the most ideal especially for middle-aged and young patients with high exercise.
The sensor made of zirconia has good electrical conductivity, which plays an important role in controlling automobile exhaust and boiler combustion in power plants. In the automotive industry, oxygen sensors are essential for the use of three-way catalytic converters in engines to reduce emissions and pollution. The Zirconia oxygen sensor is one of the most mature oxygen sensors with the largest output. It is one of the key components of the automobile emission control system, and its signal output characteristics directly affect the engine fuel economy and emission control.
The catalyst for automobile exhaust purification
The catalyst for automobile exhaust purification: carrier (alumina), co-catalyst (nano-coating to increase the specific surface area, as a hydrogen storage material), catalyst (general gasoline parking space platinum, palladium, rhodium, etc., diesel vehicles for vanadium, tungsten, titanium, etc.). Zirconium-cerium solid solution composite oxide is used as a cocatalyst and important coating material. In addition, zirconium-cerium solid solution is also widely used in sensor materials, polishing materials, fuel cells, structural materials, high-strength ceramics, and other fields.
Catalysts for chemical synthesis of aromatic hydrocarbons
Zirconia has long been used in the study of isomeric synthesis. Isomeric synthesis is a process in which syngas is converted into isobutene and isobutane (i-C4) in high selectivity, and it is mainly composed of metal oxides such as zirconia, thorium oxide and cerium oxide. Since Pichler et al. studied isomeric synthesis for the first time, zirconia has become the core of isomeric synthesis catalysis research due to its high i-C4 selectivity and non-radioactivity. This highly selective formation of i-C4 has been attributed to the fact that zirconia surfaces are both acidic, alkaline, oxidizing and reductive. If a single zirconia catalyst can convert syngas into aromatics or high-octane products in one step, the problem of mismatching of active centers in the catalytic system doped by metal and molecular sieve can be avoided, which has far-reaching significance for future energy development.
Ceramic core for fiber optic connector
Due to the excellent mechanical properties, chemical stability and extremely high precision of nano-yttrium oxide stabilize zirconia (nano-YSZ) powders, it can be used to prepare rare earth structure ceramic fiber core (precision needle) and sleeve for optical fiber connectors. It is the optical fiber passive device with the widest application range and the largest demand in the optical fiber network and is an important part of the information network infrastructure construction.
Mobile terminal products
As 5G, wireless charging and other new transmission methods approach, wireless frequency band becomes more and more complex, and metal case shielding will become a major bottleneck. The strict layout of 5G antenna requires the transformation of the existing metal housing material, and both ceramic and glass will be optional. Metal is also unfriendly to wireless charging. Most of the previous wireless charging technologies used electromagnetic wave raw materials, and metal would cause interference to the electromagnetic wave, which greatly reduced the charging efficiency. There are alternative materials such as plastics, glass, and ceramics. Plastic surfaces are prone to scratches, while glass is brittle, so ceramic materials, with their excellent physical properties, are gradually penetrating the appearance of smartphones.
The mi MIX is equipped with an all-ceramic body, and the microcrystalline zirconium ceramics, second only to sapphire hardness, is selected as the blank. It has a Mohs hardness of 8.5. Keys, knives and so on do not cause any wear and tear.
In fingerprint unlock applications, zirconia’s dielectric constant is three times that of sapphire, making the signal more sensitive. Compared with the 0.3mm sapphire cover plate used in iPhone Touch ID, the zirconia has higher recognition when the same thickness is used. It is expected that fingerprint recognition will become the standard of smartphones in the next 5-10 years.
Zirconia ceramic crucible
In the smelting of rare and refractory precious metals and alloys, the general materials are difficult to meet the requirements due to the need to heat to a higher temperature. Crucible made of zirconium oxide can be heated to 2430 ℃, the zirconium oxide thus become the first choice under the condition of high-temperature crucible pot zirconia materials.
Zirconia ceramic cutter
Ceramic cutters were used in the early 20th century, but their brittleness limited their range of use. However, its toughness has been greatly improved with the development of nanocomposite zirconia composite in recent years. Zirconia can be processed into various cutting tools, while the zirconia ceramic blades are made of special ceramic materials belonging to non-metallic materials. Zirconia ceramic tool not only has the advantages of traditional metal tools but also has the characteristics of no rust, health, wear resistance and so on, so it is known as ceramic steel.
Zirconia is often used as a refractory due to its high melting point, low thermal conductivity, and stable chemical properties. The advantages of refractory materials prepared with nano zirconia are more obvious, such as high-temperature resistance, high strength, good thermal insulation performance, and excellent chemical stability.
As a new material, zirconium-containing material has been developed rapidly in the recent ten years. In the field of refractories, natural zirconium-containing mineral raw materials and artificial extraction or synthesis of zirconium oxide and composite oxide raw materials have also been widely used to produce a variety of excellent zirconium-containing refractories.
There are about 50 kinds of zirconium minerals known to us, among which more than 20 are common. Zirconium mineral raw materials for industrial use are mainly zirconium quartz, oblique zircon, hafnium zircon, and anisotropic zircon. With the development of science and technology, zirconium oxides and composite oxides have been extracted or synthesized by various processing methods and applied in various fields.
Zirconium-containing raw materials are widely used in the refractory industry, which is mainly because of their high melting temperature and strong chemical stability. They have good corrosion resistance to metal melt, slag, or glass fluid, as well as good thermal shock resistance, so they can be used as refractories for glass kiln, metallurgical industry refractories, and so on.
Zirconium-containing refractories are mainly used in the melting part, superstructure, side wall, and fluid hole of glass melting furnace. Refractories made from zirconium materials are widely used in the metallurgical industry and can be divided into zirconium quartz products, zirconia products, aluminum zirconia carbon products, zirconium carbon products, calcium zirconate products, zirconium boride products, zirconia modified refractories, etc.
Zirconium quartz products have the characteristics of high-temperature resistance, good resistance to acid slag, small erosion, slight viscosity of slag, small thermal expansion coefficient, good thermal shock stability, etc., which can be better used as the lining of steel drums, but also can be masonry in the direct impact of steel, slag line parts, around the nozzle and other key parts.
The main raw material for the production of zirconium quartz products is zirconium quartz concentrate, and some clay, pyrophyllite, chromium oxide, and zirconia can be also added as needed. In general, zirconium particles are small in size and are not suitable for direct brick production, which requires the raw materials of zirconium quartz and part of the combined clay to be mixed, semi-dry pressed, and made into the blank. There are a wide variety of zirconia products and many molding methods, such as mud pouring method, hot pressing method, machine pressing method, isostatic pressure method, etc.
Aluminum-zirconium carbonaceous product is developed on the basis of aluminum-carbonaceous product, and it can be used as sliding nozzle brick of ladle (or tundish), long nozzle, plug rod, immersed nozzle, and so on. Compared with the corresponding aluminum carbon material, aluminum zirconium carbon products have better oxidation resistance, thermal shock stability, erosion resistance, and higher strength, so the service life is longer. The addition of a certain amount of zirconia in refractory materials such as jade-quality, high-alumina, magnesium-calcium, aluminum-magnesium, magnesium-chromium and magnesium-carbon commonly used in the metallurgical industry can improve the chemical stability, thermal shock stability and strength of these materials. In these materials, zirconia is usually introduced in the form of zircon sand and zirconia.
The specific production process is usually the same or slightly changed before modification. Generally speaking, zirconium-containing raw materials have been widely used in the field of refractories due to their excellent properties, and their application scope will be more and more extensive.
Zirconia ceramics, with high strength, high toughness, wear resistance, corrosion resistance and other excellent properties, are widely used in mold, tools, ceramic bearings, electronic components, biomedical materials, and other fields. At present, with the wide application of zirconia ceramics in the field of electronic products, especially as the backboard of mobile phones, its single color has restricted its application and cannot meet people’s requirements on the appearance of structural devices. Therefore, the development of rich colors can greatly expand the application of zirconia ceramic materials in the field, which has broad prospects for development.
Overview of colored zirconia ceramics
With the development of technology, the synthesis methods of colored zirconia ceramics are becoming more and more diversified. The key to its preparation technology is that the color phase (such as CoO, Cr2O3, Fe2O3, etc.) can be evenly distributed in the ceramic matrix. The color zirconia ceramics must have a stable crystal structure, bright and uniform color, high temperature and good chemical stability without damaging its inherent properties.
For colored zirconia ceramics, the capillary force, electrostatic attraction and van der Waals force between particles are prominent due to the small size, large surface area and high surface energy of the particles forming the matrix and colorizing phase. In this environment, nano-powder particles are easily agglomerated into a larger particle body, which leads to a significant decrease in the relatively good physical and chemical properties of nano-complex phase ceramics. Therefore, the agglomeration phenomenon must be overcome to prepare zirconia ceramics with good properties and diverse colors, so that the color phase is evenly dispersed in the ceramic matrix material.
Preparation of colored zirconia ceramics
The preparation methods of color zirconia ceramics mainly include solid phase mixing, chemical co-precipitation, liquid phase impregnation, and high-temperature carburization.
Color zirconia powders were prepared by solid-phase mixing with ball milling technology. It mixes oxide particles such as the colorant and mineralization agent with stable zirconia nanometer powder in a certain chemical proportion and grinds them into balls. Solid particles are refined in this process, resulting in micro-cracks, lattice distortion, and surface energy increase that are conducive to the realization of the low-temperature chemical reaction.
After the solution of a zirconium salt, stabilizer salt and colorant ion salt is mixed, hydroxide or carbonate precipitation is generated by the reaction with alkali or carbonate, and then the zirconia composite powder is obtained by heating and decomposition. In coprecipitation, metal cations in a solution precipitate together to form a mixture due to an excess of precipitants. Under special circumstances, the composite oxides or their precursors that are required to be deposited must conform to a certain stoichiometric ratio, and cations are required to generate precipitation in a certain proportion.
Liquid phase impregnation
Liquid phase impregnation will firstly extract and degrease zirconia ceramic blank with connected pore structure after injection molding and then place it in a solution containing chromophore ions for impregnation. The colorized ions infiltrate into the surface of the billet through the pores of the solution, and the depth of infiltration is controlled by the length of infiltration time. In addition, the blank body obtained by water extraction and degreasing is directly used for infiltration, because the blank body after water extraction and degreasing will form a uniformly connected void structure, which facilitates the uniform distribution of chromophore ions in the blank body. Uniform color zirconia ceramics can be prepared only if they can be soaked completely.
High temperature carburizing/nitrogen
High-temperature carburizing is mainly used to prepare black zirconia ceramics. The technological process is to process zirconia ceramic into a blank, normal degreasing, dewaxing, at low temperature without protective atmosphere element burning treatment, and then the processed zirconia green blank under vacuum protection conditions for high-temperature sintering. Graphite crucible is used to place the workpiece during sintering, and graphite paper is placed on the workpiece surface. The black coloring of zirconia ceramics was realized by graphite infiltration into the zirconia surface at high temperatures.
Applications of color zirconia ceramics
The backplate of a mobile phone
Zirconia ceramic used in mobile phone backplate has no interference, no magnetic, strong reception signal, as well as color diversity, besides, it can also be used for fingerprint identification module ceramic cover plate.
Smart wearable appearance parts
Zirconia ceramic material has the advantages of scratch resistance, scratch resistance, no shielding, warm and moist hand texture, good corrosion resistance and bio-compatibility. It is applied to intelligent wearable appearance parts.
Zirconia ceramic knives have excellent characteristics such as ultra-high strength, abrasion resistance, sharp edge, no rust, no odor, and durability.
The main raw material of zirconia ceramics is high purity zirconia powder, and its performance and content have a great impact on zirconia ceramics. Besides that, the properties of zirconia ceramics are affected by other factors. In order to prepare high-performance zirconia ceramics, we should control the main influencing factors, including raw material size, molding method, and sintering.
Zirconia ceramics with low porosity and high density have excellent jointing properties. High density means that the grains in the ceramic body are closely arranged, and it is not easy to form a destructive breakthrough point when subjected to external loads or corrosive substances.
The forming method is the key to obtaining the calcium density of the ceramic embryo body. Zirconia ceramics are usually formed by means of dry pressing, isostatic pressing, and hot die-casting. Different methods have different characteristics and have different effects on sintering properties as well as the microstructure of curing rate ceramics. Generally, grouting and hot die-casting are the main technologies for products with complex shapes, while dry compression molding can be adopted for products with simple shapes. Generally speaking, the density of dry-pressed products is better than that of hot-die-cast products.
The particle size of the raw material
The particle size of raw material has a great influence on the properties of products. Only when the raw material is fine enough can the final finished product be fired into a microstructure, which makes it have good wear resistance. The finer the zirconia powder particles are, the more active they are and the sintering can be promoted.
Due to the difference between corundum and glass phase linear expansion coefficient, the stress concentration at the grain boundary can reduce the risk of cracking. The fine grain can also hinder the development of micro-cracks, and it is not easy to break into transgranular, which is conducive to improving fracture toughness and abrasion resistance.
Sintering of ceramic is the densification process of raw ceramic at high temperatures. With the increase in temperature and time, the adhesion between powder particles and the strength of sintered body increase, the aggregation of powder particles becomes a strong polycrystalline sintered body with a certain microstructure, and the required physical/mechanical properties of products or materials are obtained. The densification rate and the final structure of the sample often reflect what kind of heat treatment process it has gone through.
Zirconium and zirconium alloys have excellent corrosion resistance to acid and alkali, and even surpass niobium, titanium and other metals in some media. Therefore, zirconium and zirconium alloys are gradually used as structural materials such as equipment and pipelines in the chemical industry with strong corrosion resistance due to their good corrosion resistance in recent years.
Due to the high-temperature chemical activity, zirconium and zirconium alloys can react with various elements in the air at high temperature, thus damaging their mechanical properties. Therefore, in the process of zirconium and zirconium alloy welding, the key to ensuring the quality of welding is to select a clean operating environment and strengthen the isolation and protection of welding seams and parts in the heat-affected zone.
Basic properties of zirconium and zirconium alloys
Zirconium and zirconium alloy materials mainly include R60702, R60704, and R60705. Zirconium and zirconium alloys have good welding properties and stable chemical properties at room temperature. However, its high-temperature chemical properties are very active, and it has a strong affinity for the pollution of oxygen, nitrogen, hydrogen and dust and humidity in the operating environment.
The excellent corrosion resistance of zirconium and zirconium alloys comes from the oxide film formed on the surface and depends on the integrity and firmness of the oxide film. When zirconium and zirconium alloy absorb a certain amount of oxygen, nitrogen, hydrogen, and other gas impurities, their mechanical properties and corrosion resistance will decrease sharply. Therefore, strengthening the protection of the surface of environmental dust, humidity and heat affected area and the back of the welding seam is the key element of quality control in the welding process.
Factors influencing the welding quality of zirconium and zirconium alloy
The tendency of weld cracks
Due to the low thermal expansion coefficient of zirconium and zirconium alloy, the volume change caused by thermal deformation and phase change is very small, and the content of sulfur, phosphorus, carbon and other impurities is very low, there is no obvious trend of cracks in the welding process. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the welding seam, cold cracks will occur.
At the same time, hydrogen atoms have the property of diffusing and aggregating to the high-stress parts in the heat-affected zone with lower temperature, which leads to the formation of relatively weak links in these parts, which may lead to the generation of welding delay cracks.
Selection of welding materials
The filler wire for zirconium and zirconium alloy welding should be selected according to the principle of matching the base material composition. The surface of welding wire shall not have heavy skin, crack, the oxidation phenomenon and metal or non-metal inclusion defects. Besides, the welding wire should be cleaned and dried before use.
Selection of protective gas
Argon arc welding with tungsten electrode of zirconium and zirconium alloy shall adopt high purity argon with 99.999% purity and the impurity content shall meet the requirements of GB/T4842 current standards.
Due to the extremely high requirements on the purity of welding protective gas, continuous gas charging is required during the welding process, and the gas cannot be interrupted in the process; otherwise, argon charging needs to be replaced again. Therefore, the direct gas supply method using ordinary argon in a single bottle cannot meet the protection requirements. It is necessary to increase the gas supply capacity of multiple argon bottles in series and satisfy the simultaneous operation of multiple welders through the air separation cylinder.
Zirconia ceramics are characterized by unique physical and chemical properties such as high hardness, low thermal conductivity, high melting point, resistance to high temperature and corrosion, chemical inertia and amphoteric properties. As a special ceramic material, zirconia has a broad application prospect in electronics, aerospace, aviation, and nuclear industries.
At present, the toughening methods of ceramics mainly include phase transformation toughening, particle toughening, fiber toughening, self-toughening, diffusion toughening, co-toughening and nano toughening.
Phase transformation toughening
The toughening of the phase transition refers to a phase transition of t-ZrO2 of the metastable quadrilateral phase under the stress field at the crack tip, then the compressive stress is formed on the crack, which hinders the crack growth and plays a role of toughening.
Besides, external conditions (such as laser shock, fatigue fracture toughness, low temperature, grain size and content, critical transformation energy, etc.) have great influence on the toughening of zirconia ceramics. If the stress and volume produced by the phase transition are large, the product is prone to fracture. Therefore, the influence of external factors on the toughening of zirconia ceramics should be avoided in the production process.
Particle toughening refers to adding ZrO2 ceramic powder as a toughening agent. Although the effect is not as good as whisker and fiber, there is still a certain toughening effect if the type, size, content and matrix materials are selected properly. The advantages of particle toughening are simple and feasible, and the toughening will bring about the improvement of high-temperature strength and high-temperature creep property. The toughening mechanism of particle toughening mainly includes grain refinement and crack turning to the bifurcation.
The toughening principle of fiber and whisker is that the closed stress is applied to the crack surface due to the deformation of crystal close to the crack tip, the external stress of the crack tip is offset, and the passivating crack growth is achieved, so as to play a toughening role. In addition, when the crack grows, the pulling out of the column crystal also overcomes the friction force, which also plays a role in toughening.
Due to the existence of columnar crystals, the fracture process of zirconia ceramics can cause the crack to deflect, change and increase the crack growth path, thus passivating the crack to increase the crack growth resistance, thus achieving the purpose of toughening.
Diffusion toughening mainly refers to the toughening of tetragonal ZrO2 particles to ceramic matrix. Besides the phase change toughening mechanism, there is also the diffusion toughening mechanism of the second phase particle. Before the crack propagation, the internal residual strain energy of the ceramics must be overcome, so as to achieve the purpose of toughening.
Microcrack toughening refers to adding ductile materials to the stress tip of the crack to generate microcracks to disperse the stress, reduce the force of the crack forward, and thus increase the toughness of the material. When phase transition occurs, residual strain energy effects and microcracks often occur. Therefore, the effect of phase transition and toughening is remarkable.
Composite toughening refers to the application of several toughening mechanisms in the actual toughening process of ZrO2 ceramics, so as to improve the toughening effect of ZrO2 ceramics. In the practical application process, the specific toughening mechanism is selected according to the different properties of zirconia ceramic materials to be prepared.
As we all know, a car is a product with an extremely demanding operating environment and working conditions, so the various components that make up this giant must have very superior functions. The zirconia ceramic structure has been widely used in auto parts.
Zirconia ceramic structure parts with excellent performance just make up for the lack of metal materials, so they began to be accepted by the automobile. For example, it has a long vibration tolerance of 20 grams; as parts used in combustion and exhaust systems, it can endure 50 ~ 60 ℃/S of thermal shock for a long time; due to the strong mechanical reliability of the zirconia ceramic, the failure rate is usually between 10 and 5; it can also be mass-produced and low in price, which is convenient for the formation of industrial management.
In recent years, scientists in the international special ceramics field have developed a large number of automobile special ceramics through hard research, and experiments and industrial applications have proved that the superior mechanical properties and high-temperature chemical properties of ceramic materials have far surpassed those of metal materials or other materials. At present, the applications of zirconia ceramic structure parts in auto parts industry are as follows.
Zirconia ceramic oxygen sensor
The zirconia ceramic oxygen sensor has high mechanical properties and reliability. As a component of clean exhaust, O2 concentration in automobile exhaust is measured, and the measured value is fed back to the gas and fuel supply system of the engine to keep the fuel always in full combustion state. Since all phases of the ceramic material are partially stabilized zirconia mixed with fully cubic, tetragonal and monoclinic crystals, the mechanical properties are superior during use and the heat generated by friction can be reduced.
Zirconia ceramic valve heater
In order to make the engine burn completely when starting, a heating device, the valve heater, is installed on the suction side of the engine, which is used to heat the air so that the fuel vaporizes and mixes completely. In order to control the temperature and improve the reliability of the device, the barium titanate ceramic PTC (thermistor) is used as the valve heater. After adopting the ceramic valve heater, the engine is in full combustion state when it starts, so as to improve thermal efficiency, energy saving, and purification and exhaust efficiency.
Zirconia ceramic engine
The application of special ceramics in the automobile has been popularized by the piston engine, and there will also be an auxiliary combustion chamber, piston head, cylinder liner, cylinder head, pressurized rotor, etc. Special ceramic materials such as silicon nitride, silicon carbide, and partially stabilized zirconia are also being considered for these parts.
Zirconia ceramic sensor
The shock absorber of the high-class car is a smart shock absorber that is developed by using the positive piezoelectric effect, inverse piezoelectric effect and electrostrictive effect of sensitive ceramics. The smart shock absorber, with its ability to recognize and self-regulate the road, minimizes the vibration of cars on rough roads, making them comfortable for the passenger.
Intelligent ceramic wipers
The intelligent ceramic windshield wiper is made of barium titanate, which can automatically sense rainfall and adjust the windshield wiper to the best speed. Some other ceramic sensing elements, such as thermal, pressure, humidity and magnetic ceramic materials, can also be sensitive to temperature, humidity, condensation, anti-freezing, etc. with automatic control and adjustment.
In addition, many parts, and small devices used in automobiles are made of special ceramic materials, such as the electronic buzzer, ultrasonic vibrator, heat-absorbing glass, photocell, oil plug ring, oil seal, etc. These kinds of automobile products made of new special ceramic materials generally have high physical and chemical properties, such as anti-seismic, wear-resisting, anti-corrosion, high-temperature resistant, lightweight and easy to process and produce.