Basic principles of selection of high temperature resistant coating materials

       1. Has a sufficiently high melting point



The higher the melting point of the coating material, the higher the maximum temperature that can be used.



2. Good chemical stability at high temperature



The material itself does not decompose, sublime or detrimentally transform the microstructure of the material at high temperatures.



3. Has the required thermal fatigue resistance



Under the thermal fatigue condition of alternating cold and heat, the thermal physical properties such as thermal expansion coefficient and thermal conductivity of the base material and the coating material should be matched. If the difference is too large, a gradient coating design should be adopted for transition, otherwise the peeling failure of the coating will occur. During the high temperature thermal cycle, a phase change occurs inside the matrix material or coating material. If the phase change causes a volume change, the volume change stress will result in cracking or peeling of the coating. For example, zirconia crystals undergo a phase transition with a 7% volume change at elevated temperatures. Therefore, the material should be stabilized when used as a high temperature resistant coating.



4. High temperature oxidation resistant alloys should contain alloying elements with high oxygen affinity



Elements with high affinity for oxygen include chromium, aluminum, silicon, titanium, yttrium, etc., which combine with oxygen to form very dense and chemically stable oxides. In addition, the volume of the generated oxide is larger than that of the metal atoms, so the metal matrix can be effectively



Coated to prevent further oxidation. The lower the decomposition pressure of the metal oxide, the greater the affinity of the metal element for oxygen, and the more stable the metal oxide film.



(5) There are certain requirements for the microstructure of superalloys



Superalloys generally use a metal parent phase with a face-centered cubic lattice, which can be strengthened by the atomic solid-melting of high-melting refractory metal elements, or react between alloy elements to form a phase with a coherent structure with the parent phase. The phase produces precipitation strengthening, or can form intermetallic compounds with high melting point, which can strengthen the grain boundary and dispersion strengthening of the metal parent phase.