Views:2 Author:Site Editor Publish Time: 2021-05-24 Origin:Site
Damage, erosion and fracture are the key reasons for the invalidity of machine parts, which are commonly found in metallurgical industry, engineering construction, power engineering, mechanical equipment and other industries, among which the damage problem is more serious. Especially for conveying equipment, about 80% of the invalid parts are due to the damage of raw materials. As a result, the problem of damage has been attached great importance by the raw material academia.
In order to keep up with the development trend of processing and manufacturing industry, surface engineering design comes into being. The technical performance is greatly improved with low capital input, and has obvious economic benefits. In recent years, the preparation of wear-resistant ceramic coating on the surface of the substrate has become a hot topic in the world
Characteristics of wear resistant ceramic coating
Ceramic has high melting point, high toughness, high toughness, high organic chemical reliability, high insulating layer working ability, low thermal conductivity, low coefficient of linear expansion and other characteristics. As a coating, it can reasonably improve the wear resistance, high burning resistance, corrosion resistance and high temperature air oxidation resistance of raw materials. Ceramics are widely used to prepare various kinds of ceramic coatings because of the characteristics that metal composites are difficult to achieve. Wear resistant ceramics combine the advantages of ceramics and the ductility of metal composite materials. Spraying paint on the surface of raw materials can make raw materials take into account the strong ductility and processability of metal materials, as well as the wear resistance, heat resistance, corrosion resistance and dielectric strength of ceramics. It has a key practical significance for improving the economic benefits of social development and increasing the service life of parts.
Classification of wear resistant ceramics
Wear resistant ceramics are typical structural ceramics, including metal oxide ceramics, cementite ceramics and nitride ceramics.
Metal oxide ceramic is a kind of ceramic raw material with early development trend and application. It is the crystallization of inorganic compounds. Its ions are fused according to the strong ionic bond, so it has the characteristics of heat resistance, high toughness and oxidation resistance. Generally, it contains simple metal oxide ceramics or composite metal oxide ceramics with the melting point higher than 1730 ℃, such as aluminum oxide, magnesium oxide, silicon oxide, silicon oxide, etc Zirconia, corundum, spinel, etc. Among them, some metal oxide ceramics, such as zirconia ceramics and aluminum oxide, are widely used in human joints, bone screws, ear bones and other industries for their organic chemical reliability and wear resistance.
The development of nitride ceramics is relatively late, and it has been developing rapidly since the 1970s. It is basically based on man-made materials. Apart from high toughness and high toughness, it also has high-quality electrical and thermodynamic properties. After many years of scientific research, the ductility and reliability of nitride ceramics have been improved significantly. After decades of development trend, silicon nitride, aluminum nitride, boron carbide and other nitride ceramics as high toughness mechanical parts, corrosion-resistant components, and wear-resistant components, have been widely used in aerospace, mechanical equipment, metallurgical industry and other industries.
Cementite ceramics are widely used in carbon carbon composites (SIC), boron carbide (B4C), titanium carbide (TIC), cobalt alloy (WC), etc., such as SiC used in turbocharged motor rotor, rolling bearing, high temperature exchanger of automobile engine, etc; Boron carbide as exo armor, etc; Titanium carbide is used to make NC blade. The key advantages of cementite ceramics are high melting point and high strength. For example, the strength of B4C is only inferior to that of diamond and CBN.
Preparation method of ceramic coating
01 liquid phase stacking method
The liquid deposition method is divided into physical liquid deposition (PVD) and organic chemical liquid deposition (CVD). PVD technology took place at the end of 1970s. It vaporizes raw materials into positive ions, molecules or molecular structures by physical methods under the standard of vacuum pump, and accumulates on the surface of substrate to produce a protective film. The whole process of PVD includes volatilization, transmission, reflection and accumulation. PVD film has strong adhesion, wear resistance, corrosion resistance and strong oxidation resistance. CVD is a technology that uses vapor chemical substances to produce solid plastic film on the surface of substrate according to chemical changes. It essentially belongs to the whole process of vapor convective heat transfer in the molecular range, and has the following characteristics: it can be carried out under the standard of superheated steam or vacuum pump; The coating can be prepared at ultra-low temperature; The structure and purity of the coating can be controlled; Good winding property.
02 sol gel method
The preparation of ceramic coatings by sol-gel method (Sol-Gel) is based on the reaction of metal salts, alkoxides or carbonates, which are easy to hydrolyze and react with water in certain organic solvents. The colloidal solution is produced by condensation reaction of hydrolytic reaction, and the colloidal solution is applied on the surface of metal material substrate, and then the coating is produced after drying and heat treatment. This kind of method can be carried out at ultra-low temperature, and the quality of the coating is high, but the processing technology is complex and time-consuming, and the film is easy to crack.
03 self diffusion high temperature formation method
Self diffusion high temperature generation (SHS) is a kind of new technology application, which is based on the external kinetic energy to cause a small range of reactions, uses the high chemical change heat caused by the middle of the product to promote the continuous spontaneous chemical change of the product, and generates the overall target material in a short time. It has the advantages of low cost, clean energy, easy production and manufacturing
04 thermal spraying method
Thermal spraying was first invented by sehoop in France in 1910. It is a coating method that melts and atomizes the raw material of coating at high temperature to produce particle flow in molten or semi molten state and spray paint on the surface of metal substrate at a very high rate. Thermal spraying has many advantages, such as wide application field, good coating effect, simple equipment and high efficiency. It is not only used to prepare ceramic coating, but also has competitive power in the repair industry of parts.
05 high temperature melting method
The high temperature melting sintering method is to make the raw material of coating into slurry at room temperature, then evenly coat it on the surface of metal, and then make the raw material of coating by high temperature melting sintering. This method has the characteristics of simple equipment and easy operation, and is one of the key ways to prepare ceramic coating. In addition, this method can also repair the damaged coating, which can be solved by using high temperature to make it self compensation, and can be completed by making up the slurry and solving at moderate high temperature.
06 laser cladding
Laser cladding is a kind of surface modified material, which spray the powder with unique properties (such as wear resistance, corrosion resistance, oxidation resistance, etc.) on the surface of metal or send the powder in the same step with the laser, then make it melt, expand and condense rapidly under the effect of laser, and produce a metallurgical industry bonding layer without cracks and blowholes on the surface of plate. Laser cladding has many advantages: easy to use, low energy consumption, low heat output, small thermal deformation, no need for post production or small output, reducing ecological crisis, etc.
Laser cladding ceramic coating also has some shortcomings. Because in the whole process of laser cladding, the cladding layer is heated and melted rapidly, and then cooled greatly, which belongs to non-equilibrium condensation, and the raw materials of coating and substrate are very different. In addition, there are many influencing factors in the laser processing mode, which leads to the quality of cladding layer is not easy to control. As a result, some defects often occur in the coating, such as blowholes, cracks, ablation and so on.
Wear resistant ceramic coating has a significant practical effect on improving the wear resistance and corrosion resistance of the substrate, which is very important in the whole process of the application of machine parts. It is widely used to increase the service life of large and medium-sized components working in the relatively seriously damaged natural environment, and reduce the energy consumption of large and medium-sized damaged components, which is in line with the sustainable development strategy of environmental protection and energy conservation, energy conservation and emission reduction measures and environmental protection in China.