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High temperature protective coating can provide effective anti-oxidation and corrosion protection for metal materials used at high temperature, which has been widely used in aerospace, energy, petrochemical and other fields. Among them, the representative applications are in various kinds of gas turbine engines for aircraft, ship and ground power generation. The development of high temperature protective coating mainly experienced three stages: the first generation of thermal diffusion coating, the second generation of M (M = Fe, Ni or CO) craly coating, and the third generation of thermal barrier coating.


In order to further improve the working efficiency of turbine engine and achieve the purpose of energy conservation and emission reduction, it is necessary to increase the imported temperature of the engine. Therefore, scientists are constantly committed to developing more advanced materials, coating systems and preparation technologies. For example, the temperature bearing capacity of the fourth generation nickel base single crystal superalloy has reached 1180 ℃. Accordingly, higher requirements are put forward for high temperature protective coating, and a variety of new high temperature protective coatings with unique design concept emerge. In this paper, the structure, preparation method and application characteristics of common high-temperature protective coatings are described, several characteristic high-temperature protective coatings are introduced, and the latest research progress of high-temperature coatings at home and abroad is summarized. The development trend of high temperature protective coating is prospected.

02 common high temperature protective coating


2.1 diffusion coating
The coating formed by contacting some antioxidant elements, such as Al, Cr, Si, etc., with the metal matrix and entering the surface of the matrix is a diffusion coating. During the formation of the diffusion coating, the matrix participates in the formation of the coating, the elements in the matrix enter into the coating, and the diffusion layer is formed in the matrix under the coating. Diffusion coating includes aluminizing coating, chromizing coating, siliconizing coating, and improved aluminizing coating, and representative aluminizing coating and improved aluminizing coating.

2.1.1 aluminizing coating
Aluminizing coatings were first described by Van aller in the US patent in 1911, and were prepared by the powder embedding method. Later, hot dip aluminizing, slurry aluminizing, and non-contact aluminizing (above the base material) and chemical vapor deposition (CVD) appeared ）Aluminizing and other preparation methods. In the 1950s, the powder embedded aluminizing coating began to be used for cobalt based guide vanes. In the 1970s, most of the nickel based and cobalt based turbines and guide vanes used powder embedded aluminizing coating and non-contact aluminizing coating.

In the method of powder embedded aluminizing, the sample embedded aluminizing agent powder consists of aluminum source powder, halide activator and filler. The aluminum source powder can be metal Al or suitable alloy powder, and the filler is usually inert Al2O3. The penetrant generally contains 2% - 5% activator, such as ammonium chloride, 25% aluminum source, and the rest is filler. When heated, the activator volatilizes in the penetrating agent and reacts with the aluminum source to form volatile coating metal compounds. The volatile material diffuses to the surface of the substrate, where the deposition reaction takes place. When aluminizing, protective gas such as argon must be introduced to avoid oxidation of aluminum source and metal substrate.

The structure and deposition rate of aluminized coating depend on the activity of Al in the aluminizing agent, aluminizing temperature, base material composition and post-treatment process. Taking the aluminizing coating of nickel base superalloy as an example, in a relatively low temperature range, such as 700 ~ 800 ℃, the activity of Al is higher than that of Ni. During the aluminizing process, the growth of the coating mainly depends on the inner diffusion of Al through the initial Ni2Al3 surface layer, forming the inner diffusion coating, also known as high activity aluminizing (HALT), which needs to be annealed twice to form the NiAl phase. In a relatively high temperature range, such as 980 ~ 1090 ℃, the activity of Al is lower than that of Ni. The growth of the coating is mainly by the combination of Ni diffusion and surface deposited al to form NiAl phase, which is also called low activity aluminizing (LAHT).

There are several advantages compared with other methods of preparing diffusion coating. One is that the aluminizing agent has the function of supporting the infiltrated material and preventing large device from bending down. The commercial aluminizing process can be used to prepare aluminum rich coating for several meter long pipes; the other is that the aluminizing agent contacts with the base material to make the composition of the aluminizing layer more uniform and the deposition rate faster, but there are also disadvantages. In the aluminizing agent The material will wrap into the coating. This situation can be avoided by the non-contact aluminizing process of "base material on the aluminizing agent" and the chemical vapor deposition aluminizing process. The former is to fix the workpiece on the top of the aluminizing agent, and the coating reaction gas is generated from the aluminizing agent and flows upward to the surface of the base material, while the latter is generated from the outside during the deposition process, and then filled into a vacuum container filled with the aluminized material, so the reaction The composition of the gas is adjustable, and the reaction gas can be delivered to the inner cavity, such as the inner cooling hole of the gas turbine blade.

Simple aluminizing coating has good oxidation resistance, simple process, stable performance and low cost. However, there are many disadvantages, such as poor thermal corrosion resistance, high brittleness and fast degradation of coating, etc. in the 1970s, the improved aluminide coating developed.