What is MCrAlY coating

       As early as the 1960s and 1970s, with the development of sputtering, low pressure plasma spraying and electron beam evaporation deposition technology, an ideal high temperature protective coating-MCrAIY coating was successfully developed. It not only has good resistance to high temperature oxidation and hot corrosion, but also has good toughness and thermal fatigue resistance.



The advantage of MCrAIY coating lies in the diversity of its composition selection, that is, the appropriate coating composition can be selected according to different working environments and different substrate materials. In order to prevent the degradation of the coating, a diffusion barrier can be added between the coating and the substrate to make Pt-MCrAIY and TiN-MCrAIY coatings.



MCrAIY coatings are usually multiphase. If the Al concentration is not high, NiCrAIY and CoCrAIY are mainly composed of γ solid solution and dispersed β-NiAl or β-CoAl phase. However, the structure of the MCrAIY coating actually used is more complicated. In addition to the γ and β phases, there may also be γ', σ and Y2O3 phases. Aluminum is added to these coatings to form a protective Al2O3 film. The addition of chromium can not only improve the hot corrosion resistance of the coating, but also promote the formation of Al2O, film. The addition of yttrium plays an active element effect and improves the adhesion of the oxide film.



The preparation of MCrAIY coatings mainly adopts two methods: physical vapor deposition and plasma spraying.



1. Physical vapor deposition



Physical vapor deposition methods include EB-PVD and sputtering. The MCrAlY coating prepared by EB-PVD method is carried out in vacuum, and the deposition rate is high, which can reach about 25 um/min. The coating prepared by this method has a columnar crystal structure perpendicular to the surface of the substrate, and there is a non-metallurgical interface between the columnar crystals. Heating the base alloy during the preparation of the coating can cause a certain mutual diffusion between the coating and the base, thereby increasing the bonding force of the coating. Sometimes in order to improve the coating properties, shot peening or subsequent heat treatment is performed on the MCrAIY coatings prepared by the EB-PVD method to eliminate the non-metallurgical bonding interface in the coating and improve the coating strength. However, when using EB-PVD technology, it is not easy to precisely control the coating composition according to the design requirements, because the vapor pressure of each component in the coating is quite different, especially when the coating contains more refractory metals, the coating The composition and target composition are significantly different. When the coating composition is relatively complex or the melting point is relatively high, the sputtering method has obvious advantages. This is because there is little difference between the coating composition and the target composition during sputtering. The microstructure of the sputtered coating is related to the temperature of the base material. If the temperature of the base material is very low, the sputter coating is tens to hundreds of nanometers



columnar crystallite structure. When the substrate temperature reaches 0.5Tm ~ 0.8Tm (melting point), the coating has a coarse columnar crystal structure. However, the deposition efficiency of this technique is low and it is difficult to be practical.



2. Plasma spraying



In the early days, the coatings prepared by atmospheric pressure plasma spraying had high porosity, many oxide inclusions, and poor coating adhesion. The performance of MCrAIY coatings prepared by low-pressure plasma spraying has been greatly improved. Low-pressure plasma spraying is carried out in a vacuum filled with a small amount of inert gas. In the low pressure vacuum chamber, the metal particles move at a high speed. Therefore, the deposition efficiency of plasma sprayed MCrAIY coating is high and the coating composition is easy to control. However, the high surface roughness of the low-pressure plasma spray coating requires surface polishing.



Both physical vapor deposition and plasma spraying have the disadvantages of expensive equipment and high cost, and it is difficult to coat workpieces with large sizes and irregular shapes. Therefore, other low-cost technologies have also been paid attention to, such as laser cladding electrophoretic reaction sintering and composite electroplating.