Temperature-resistant 3000 metal anti-oxidation coating has high melting point, oxidation resistance, good thermal shock stability, low coefficient of thermal expansion, low volatilization at high temperatures, and excellent ductility and fatigue resistance. However, ceramic coatings are brittle and easy to crack, so how does this high-temperature anti-oxidation coating solve the problem of brittleness and cracking?
Second-phase toughening is a technique that enhances the toughness and strength of a material by adding a second phase. This technique is to control the generation conditions and reaction process so that the second phase is evenly distributed in the material to form a reinforcement. This technology has many advantages, such as improving the strength, toughness, fatigue resistance, and high temperature resistance of the material.
The principle of second phase toughening is based on the control of the microstructure of the material and the phase transformation process. By adding a second phase, the stress field and dislocation motion in the material can be altered, thereby enhancing the toughness of the material. In addition, the second phase can also improve the fatigue resistance and high temperature resistance of the material, and extend the service life of the material.
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1. Phase change toughening.
The phase transformation of the second phase consumes a large amount of energy required for crack propagation, which relaxes the stress at the crack tip and hinders the further propagation of the crack. At the same time, the volume expansion generated by the phase transformation compresses the surrounding matrix, prompting the other cracks to close, thereby improving the fracture toughness and strength. This phase change toughening is also known as stress-induced phase change, phase change-induced toughness.
Phase change toughening is an effective method of strengthening and toughening materials, which consumes a large amount of energy required for crack propagation through the phase transformation of the second phase, which relaxes the stress at the crack tip and hinders the further propagation of the crack. At the same time, the volume expansion generated by the phase transformation compresses the surrounding matrix, prompting the other cracks to close, thereby improving the fracture toughness and strength. This phase change toughening is also known as stress-induced phase change, phase change-induced toughness.
For example, yttrium oxide (Y2O3) has a hexagonal crystal structure at high temperature, which is called hexagonal yttrium oxide (Hexagonal Y2O3). As the temperature decreases, yttrium oxide undergoes a crystal structure transformation into a cubic crystal structure, called cubic yttrium oxide (cubic Y2O3). The transition between these two crystal structures is a solid-state phase transition behavior.
Between hexagonal yttrium oxide and cubic yttrium oxide, there is a high-temperature metastable hexagonal-cubic mixed phase state called hexagonal-cubic mixed phase Y2O3. This mixed phase state can exist stably under certain conditions, and has excellent physical and chemical properties.
At lower temperatures, titanium oxide appears as a monoclinic phase, while at higher temperatures, it appears as a tetragonal phase. In the monoclinic phase, the crystal structure of titanium oxide is made up of two titanium atoms and four oxygen atoms. The distance between the titanium and oxygen atoms in this structure is shorter, so the chemical bonds between them are strong. This gives monoclinic phase titanium oxide a higher hardness, lower electronic conductivity, and a higher dielectric constant.
Whereas, in the tetragonal phase, the crystal structure of titanium oxide is made up of four titanium atoms and eight oxygen atoms. The distance between the titanium and oxygen atoms in this structure is longer, so the chemical bonds between them are weaker. This gives tetragonal titanium oxide a lower hardness, higher electronic conductivity, and a lower dielectric constant.
Other toughening materials in the metal anti-oxidation coating with temperature resistance 3000 are: SiC, TiO2, ZRO2 and so on.
2. Fiber toughening.
The addition of fiber materials to the ceramic coating can significantly improve the strength and toughness of the material, which is also the most commonly used toughening method for high-temperature ceramic coatings. The incorporation of high-strength, high-toughness fibers into the ceramic coating can prevent macroscopic cracks from passing through the fibers, thereby improving the strength and toughness of ceramic materials. The toughening mechanism is: fiber pull-out and breaking, fiber breakage, fiber bending and deflection, fiber dispersion and orientation, fiber reinforcement damping. In order to obtain better material properties, it is necessary to select the appropriate fiber type, content, arrangement, etc. according to the application needs.
3. In-situ toughening.
In-situ toughening is also called self-toughening, that is, adding raw materials that can generate the second phase in the ceramic matrix, controlling the generation conditions and reaction process, and directly growing uniformly distributed whiskers, grains with high length-diameter ratio and wafer morphology reinforcements in the matrix through high-temperature chemical reaction or phase transformation process to form ceramic composite materials. The toughening mechanism of in-situ toughening is similar to that of whisker fiber toughening, mainly by the deflection mechanism of pull-out, bridging and cracking of autogenous reinforcements. This method can overcome the problems of incompatibility and uneven distribution of the two phases in the addition of the second phase toughening, so the strength and toughness of the obtained composite materials are higher than those of the same material with the second phase toughening.
The in-situ self-toughening components include Si3N4, SiC, Al2O3, Zrb2 and so on.
4. Crack bridging.
Crack bridging refers to the process of crack propagation, in which two crack surfaces are connected by a bridging unit and a closed stress is generated between the two interfaces. This closure stress can dissipate the energy of crack propagation and increase the toughness of the ceramic coating. Crack bridging usually occurs at the crack tip, and the smaller the particle size of the bridging unit, the more significant the toughening effect.
In ceramic coatings, the dispersion or movement of high-strength and high-toughness second-phase particles has a significant inhibitory effect on crack propagation. When the crack spreads to dispersed phase particles, the crack tip deviates from the original propagation direction and bends due to the obstruction of the particles. This bending effect can dissipate the energy of crack propagation, thereby improving the toughness of the ceramic coating.
In addition, residual compressive stresses are generated at the junction of the dispersed phase particles and the matrix, which can change the direction of crack propagation. When a crack encounters dispersed particles, the original forward direction may be reversed, increasing the toughness of the ceramic coating.
The molybdenum oxide, aluminum powder, phosphorus nitride and other components in this coating are used as crack bridging agents.