The preparation of ceramic coating on metal substrate can organically combine the characteristics of ceramic materials with those of metal materials to obtain composite material structures and products, which is becoming an important branch of the high-tech field of contemporary composite materials and products. Due to the flexibility of the process and the wide range of sprayable materials, thermal spraying technology has been widely used in more and more fields such as aerospace, petrochemical, electronics and electrical, automobile, medical, marine, mining, etc.
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Thermal spraying technology is a method of using a heat source to heat the spraying material to only melt or semi-melt state, and spray and deposit it on the pre-treated substrate at a certain speed to form a coating, with the purpose of giving the substrate surface special functions.
Application of thermal spraying ceramic coating
Ceramic is a crystalline or amorphous compound composed of metal elements and non-metallic elements. Together with metal materials and high molecular polymer materials, it constitutes the three pillars of solid engineering materials. Modern materials have classified metal ceramics and other inorganic non-metallic materials into the category of ceramics, making it a large family of materials with many varieties and functions. Generally speaking, any material that can be molten or plastic after heating can be used as a coating material for thermal spraying. In addition to metal materials, ceramics can also be used for thermal spraying anti-corrosion coatings. Commonly used ceramic materials for thermal spraying are mainly oxide ceramic materials such as Al2O3, TiO2, Cr2O3, and ZrO2. Non-oxide ceramics such as carbides are usually prepared into metal-based ceramic composite materials using metal alloys as binders. Since nitrides are very brittle and have poor oxidation resistance, they are rarely used.
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Spraying ceramic coatings can be divided into several categories according to their application areas: wear-resistant ceramic coatings, corrosion-resistant ceramic coatings, thermal barrier ceramic coatings, bioceramic coatings, and piezoelectric ceramic coatings.
1. Wear-resistant coatings
Statistics show that the losses caused by wear and corrosion account for about 3%-5% of the GDP of developed countries, while in developing countries, it is as high as 10%, which causes the performance of parts to decline and eventually fail during processing and production. Thermal spraying technology has always been used as the most important method of effective wear reduction and corrosion protection technology. The energy and material losses caused by friction and wear are all derived from the surface of the material. Therefore, various surface engineering technologies have become an important method to improve the friction reduction and wear resistance of component materials. Thermal spraying technology is one of the most widely used surface engineering technologies. At present, coatings prepared by thermal spraying technology have been widely used in mechanical equipment. The wear resistance of thermal spray coatings mainly depends on the coating composition, phase composition, particle size and content, and is also related to factors such as the coating's bonding strength, hardness, porosity, and coating particle size. Coatings with high bonding strength, high hardness, and low porosity have better erosion and wear resistance. Currently, commonly used wear-resistant coatings include: Al2O3 layer, Al2O3+TiO2 coating, Cr2O3 coating, and WC-Co coating.
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Only by selecting appropriate coating materials and corresponding thermal spraying processes can anti-friction and anti-wear coatings that meet the working conditions be prepared. Based on the rapid development of industry and technology, the working conditions of mechanical equipment are becoming more and more demanding, and it is necessary to develop anti-friction and anti-wear coatings with better performance. On the one hand, starting from the components of the coating material, a new material system with high strength, hardness or self-lubricating function can be designed, such as adding a lubricating phase with self-lubricating effect (such as graphite, molybdenum disulfide, polymer materials, etc.) to a hard ceramic coating or a metal-ceramic coating to improve the friction reduction performance of the coating. At present, most studies only propose to add a lubricating phase to improve the friction reduction and wear resistance of the coating, and have not explored the optimal raw material ratio and other conditions in depth. On the other hand, process parameters are one of the important factors affecting the quality of thermal spray coatings. Starting from optimizing the thermal spray process, the optimal process parameters of coatings of different systems can be studied to lay the foundation for subsequent research work.
2. Corrosion-resistant ceramic coating
Steel used in harsh environments (such as marine salt spray environments, etc.) is prone to corrosion and wear and loss of component precision. The traditional surface treatment process is mainly electroplating hard chromium, that is, hexavalent chromate is deposited at the cathode to form a wear-resistant and corrosion-resistant coating with high hardness, which is characterized by simple process and low cost. However, the electroplating chromium process will cause serious environmental pollution and is restricted in many industrial fields. In addition, hydrogen embrittlement of the substrate will occur during the electroplating process of chromium, which will significantly reduce the mechanical properties of the substrate. Therefore, it is urgent to find an effective alternative process. Thermal spray coatings have begun to be used in the field of corrosion protection due to their high efficiency, environmental protection and good chemical stability. Anti-corrosion coating materials are mainly divided into aluminum, zinc and their alloy materials, nickel-based alloy materials, stainless steel materials and composite ceramic materials.
3. Thermal barrier ceramic coatings
Thermal barrier coatings (TBCs) are often used on the surface of aircraft engine turbine blades to separate the high-temperature components of the jet engine from the high-temperature gas to improve the service conditions of the high-temperature components. On the one hand, it can increase the service life of the turbine blades, and on the other hand, it can save fuel. In order to make the ceramic thermal barrier coating and the metal substrate have good thermal insulation performance, the ceramic thermal barrier coating must have the following properties:
① Low thermal conductivity;
② The thermal expansion coefficient matches the metal substrate;
③ Maintain good phase stability at high temperature. Low thermal conductivity ZrO2-based ceramic materials are usually selected as the working layer materials of thermal barrier coatings. The main preparation technologies of ceramic thermal barrier coatings include plasma spraying, electron beam physical vapor deposition, laser cladding, high-speed flame spraying and explosion spraying. The two most commonly used preparation methods are plasma spraying (PS) and electron beam physical vapor deposition (EB-PVD). The organizational characteristics of plasma spray coatings are a large number of pores and microcracks, which can relieve thermal stress and improve the thermal fatigue life of the coating.
4. Bioceramic coatings
Titanium and its alloys, cobalt-chromium-molybdenum alloys and stainless steel are all commonly used metal bone implant materials in clinical practice. The metal materials used in clinical practice are not biologically active and need to be improved in combination with bone tissue. Surface modification of metal implants is a necessary way to improve their biological properties. Thermal spraying technology is one of the commonly used surface modification methods. The biomedical coatings prepared by it mainly include metal oxide coatings (Al2O3, ZrO2, TiO2, etc.), bioactive hydroxyapatite (HA) coatings, etc. Titanium and hydroxyapatite coatings have been widely used in clinical practice. In recent years, the research on calcium silicate bioceramic coatings has also attracted people's attention.
5. Piezoelectric ceramic coatings
Piezoelectric ceramics are a type of ferroelectric with excellent piezoelectricity. They are information functional ceramic materials that convert mechanical properties and electrical energy into each other. In addition to the piezoelectric effect, piezoelectric ceramics also have dielectric properties and ferroelectric properties. They have been widely used in various fields such as medical imaging, acoustic sensors, acoustic transducers, ultrasonic motors, and display devices. In recent years, the preparation of piezoelectric ceramic coatings by applying spraying technology is gradually gaining attention.