For ordinary materials, their performance will change due to long-term exposure to certain specific environments and the chemical or electrochemical effects of the surrounding medium. For example, steel structural parts exposed to the outdoor atmosphere for a long time are prone to corrosion . Therefore, in order to protect the surface of the material, it is often necessary to use thermal spraying technology to create a special working surface to achieve: anti-corrosion, wear resistance, friction reduction, high temperature resistance, oxidation resistance, heat insulation, insulation, conductivity, anti-microwave radiation, etc. A variety of functions.
The specific process of thermal spraying technology refers to the use of a certain heat source to heat powder, filament or rod-shaped metal or non-metallic coating materials to a molten or semi-molten state, and then use the power of the flame itself or an external high-speed airflow to heat It is atomized, sprayed and deposited on the surface of the pretreated base material at a certain speed, and combined with the base material. Compared with other surface engineering technologies, the outstanding features of thermal spraying technology are:
1. The temperature range of the heat source is very wide, so the coating materials available for spraying include almost all solid engineering materials, such as metals, alloys, ceramics, cermets, plastics, and composites composed of them;
2. During the spraying process, the degree of heating of the substrate is small and can be controlled, so spraying can be carried out on various materials, with almost no effect on the structure and performance of the substrate;
3. The equipment is simple, and the operation is flexible. It can spray large-scale components on a large area, or spray on designated parts; it can be sprayed indoors in the factory or on site outdoors.
Among the available spray materials, ceramic materials have become a commonly used spray material in thermal spray technology due to their high melting point, high hardness, and good chemical stability. Commonly used are aluminum oxide, titanium oxide, chromium oxide, Tungsten carbide, chromium carbide, silicon carbide, titanium nitride, silicon nitride, etc. are mainly used for corrosion, oxidation and wear protection of components. Among them, alumina is the most widely used high melting point oxide material. Its specific application in the field of thermal spraying can be seen below:
Alumina overview
Alumina is rich in natural resources, low in price, and has excellent properties in many aspects. Its melting point is 2050°C, it is white, and there are many homogeneous crystals. Common ones are γ-Al2O3, δ-Al2O3, θ-Al2O3 and α-Al2O3. γ-Al2O3 is a low-temperature alumina crystal. It is a cubic structure crystal with a density of 3.47g/cm3. It starts to transform into high temperature α-Al2O3 above 1200℃, and the transformation is one-way. The volume shrinks by 13%. α-Al2O3 is the most stable structure among various variants. Its stable temperature can reach the melting temperature, its density is 3.95g/cm3, and its crystal form is hexagonal.
Alumina has excellent mechanical and chemical properties under certain high temperature conditions. After being fully sintered, it is insoluble in inorganic acids and salts; it has strong resistance to hydrogen fluoride and good resistance to corrosion of sodium hydroxide, sodium carbonate, molten glass, etc. It has strong resistance to the corrosion of gases other than fluorine at high temperatures of 1700~1800℃; it can be used in an oxidizing atmosphere below 1900℃ or a strong reducing atmosphere, and it can be used for a short time at 1950℃.
Application of alumina powder in the field of thermal spraying
Using nano-sized powder as raw material cannot be directly used for spraying. In order to solve this problem, it is necessary to reprocess the nanoparticles to form spherical micron-sized particles with nanostructure characteristics and improve the fluidity of the particles. When plasma spraying, the molten particles experience impact on the substrate, spread, flatten, and solidify into quasi-circular small flakes. The droplets of the molten particles hit the substrate into a disc shape, and the specific shape is determined by the surface tension, density, viscosity and the speed of the droplets. This process takes a very short time, and a layered structure with small flakes is formed. Since the time from collision to solidification is very short, the molten particles cannot reach the corners of the previously spread small flakes, so pores must appear in the coating.
Prepare a single sprayed aluminum oxide coating, the resulting coating has a loose structure and poor bonding force, and the coating exhibits brittle peeling when worn. The size of the original powder particles has different effects on the structure of the coating after spraying. Studies have shown that the smaller the original powder size, the better the final performance of the coating. The coatings were analyzed by x-ray diffraction, and the coatings were all composed of α-Al2O3 and γ-Al2O3 phases. The difference in microstructure lies in the original powder size and the degree of melting of the particles during spraying. The full melting of the particles and the higher particle velocity before impacting the substrate can make the particles have good deformation during impact, and lead to good bonding between the layers and low porosity. Coatings made by powders of different particle sizes can explain to a certain extent that the more γ-Al2O3 phase in the coating, the more fully the powder will melt during the spraying process, and the resulting coating structure will be denser.
But after adding a certain amount of TiO2 powder to Al2O3 powder wood, preparing Al2O3/TiO2 composite coating can improve the overall performance of the coating. The melting point of Ti02 is 1840℃, and the melting point of Al2O3 is 2050℃. TiO2 has a good melting state during spraying and has strong adhesion. It can bond in the pores between Al2O3 coating particles during solidification, and it exists with Al2O3 The components diffuse and produce solid solution, which significantly improves the compactness and bonding strength of the coating, which is beneficial to the improvement of corrosion resistance and wear resistance. The wear resistance of the coating is the best when the content of TiO2 is between 13% and 20%. With the increase of TiO2 content, the compactness of the coating gradually increases, and the hardness gradually decreases. This is because the hardness of TiO2 is lower than that of Al2O3. The wear failure of the coating is transformed from brittle flaking to adhesive wear, fatigue wear and abrasive wear similar to metal materials.
The article is reproduced from the WeChat public platform: DT New Materials
