Classification and function of high temperature protective coatings

       Superalloys generally require two performance requirements at the same time, namely excellent high temperature mechanical properties and high temperature corrosion resistance. But in fact, for the same alloy, the properties of these two aspects are sometimes contradictory, and it is impossible to achieve the optimization at the same time. To solve the contradiction between the two, the process improvement of the superalloy base material itself cannot meet the performance requirements of the development of modern aerospace aircraft. It is necessary to deposit an alloy coating on the surface of the alloy through a high-temperature protective coating, and deposit the alloy coating on the alloy surface. The surface of the coating is then applied with an oxide ceramic coating to solve the problem. Usually the coating is thin, mainly to protect the base metal from high temperature corrosion. The requirements for high temperature strength are mainly borne by the base alloy itself. Since the base alloy and the protective coating can be designed separately, the alloy parts with the protective coating can not only maintain sufficient high temperature strength of the alloy, but also have excellent high temperature corrosion resistance on the surface. Generally, high temperature coatings are divided into three types, namely diffusion coatings, cover coatings and thermal barrier coatings developed later.



Coatings that change the outer layer of the substrate by contacting the substrate and reacting with certain elements within it are called diffusion coatings. A typical representative of this type of coating is thermal diffusion aluminizing on nickel-based and cobalt-based alloys to obtain NiAl and CoAl coatings, respectively. Diffusion coating preparation technology is currently the most widely used high temperature protective coating technology. It is based on the formation of intermetallic compounds on the surface of nickel, cobalt, and iron-based alloys through thermal diffusion infiltration, thereby improving the adhesion of the coating and the oxidation resistance of the base alloy. The most common diffusing elements are aluminum, chromium, silicon, etc. Taking the aluminized coating as an example, intermetallic compounds such as β-NiAl, β-CoAl, FeAl are formed on the surface of nickel, cobalt, and iron-based alloys during thermal diffusion infiltration. The coating and the base alloy are metallurgically combined, which not only improves the coating performance. layer adhesion, and these intermetallic compounds can form dense films during the oxidation process, thereby improving the oxidation resistance of the substrate.



Diffusion aluminized coatings have been widely used on various aviation gas turbines. For land and marine turbines, it is only used in parts with severe thermal corrosion such as wings and blades. And in the study of reducing the corrosion tendency, the corrosion resistance of sulfate can be improved by adding Cr and Pt to the diffusion aluminum coating.



The diffusion aluminum coating on the surface of superalloy sensitive to carbide grain boundaries can significantly reduce the creep strength. Catell and Willett et al. found that forged Udimet 520 had a severe decrease in creep strength, which was not improved by heat treatment. related. In the diffusion zone, the contents of Cr, Mo, and W are relatively high, which usually exist in the form of stable carbides. A similar phenomenon has been reported in as-cast polycrystalline alloys.



For solidified alloys and single-crystal superalloys where low-cycle fatigue cracking does not originate at grain boundaries, diffusion aluminum coatings and other coatings are expected to not cause low-cycle fatigue and thermal fatigue performance degradation. In fact, in the actual use process of the non-plated coating alloy, there is also a serious hot corrosion phenomenon.



In the United States, a large number of modern coatings suitable for industrial and military engines are CoCrAlY coatings applied to spacecraft auxiliary gas turbines. Aviation blades with CoCrAlY (containing 15% ~ 20% Cr, 12% Al) coating have a longer service life than those with diffusion aluminum coating when the metal temperature reaches 800 ° C ~ 1100 ° C. But when the metal temperature reaches 700°C ~ 750°C, severe sulfuric acid hot corrosion will also occur in engines operating in a low-pressure atmosphere, which is different from Type I hot corrosion that occurs at high temperatures, which is called "low temperature" hot corrosion or Type I hot corrosion. Type II hot corrosion, the existence of Type II hot corrosion has been clarified in engine tests and drilling rig tests in the UK.



With the development of aviation gas turbines in the direction of high flow ratio, high thrust-to-weight ratio, and high inlet temperature, the gas temperature and pressure in the combustion chamber continue to increase. To adapt to this harsh working environment, thermal barrier coatings were developed.



Thermal Barrier Coatings are coating systems consisting of a ceramic insulating surface layer and a metal bonding primer. The purpose of thermal barrier coating using metal bonding bottom layer is to improve the physical compatibility between the ceramic surface layer and the base alloy and to protect the base from oxidation. The thickness of the bonding bottom layer is generally 0.1mm ~ 0.2mm, and its composition is mostly MCrAIY, because MCrAIY has good high temperature corrosion resistance. Due to the poor thermal conductivity of the ceramic layer, a temperature gradient is formed in the ceramic layer, which reduces the temperature of the substrate surface. The thickness of the ceramic layer is about 0.1mm ~ 0.4mm. Depending on the structure and thickness of the coating, the temperature of the substrate surface with a thermal barrier coating can be reduced by 50 to 170 °C compared with a substrate without thermal barrier coating. Simple, strong heat resistance and other advantages.



The main functions of the thermal barrier coating are: 1) reduce the temperature of the metal while reducing the creep performance of the cooling device and prolong its service life; 2) greatly reduce the fatigue strain and prolong its service life; 3) reduce the cooling requirements of the engine blades Air volume, thereby improving its use effect and prolonging its service life.



Following the diffusion aluminum coatings and CoCrAIY coatings that appeared in the 1970s, various ZrO2-based coatings began to appear, among which MCrAIY (M = Ni, NiCo) and ZrO2 bilayer coatings showed the best performance, while more Stable MgO, Y2O3 also began to appear. Although various thermal barrier coatings have appeared, their thermal corrosion resistance is generally poor. This is because the SO3 in the sulfate of the corrosion solution reacts with the oxide in the coating to form the corresponding sulfate, which loses the corrosion resistance, such as MgO - ZrO2. 

与此原文有关的更多信息要查看其他翻译信息，您必须输入相应原文