Dear Dr. Ravindra Kumar ,
a superalloy is a metal specially designed for applications requiring a high level of resistance to elevated temperatures, tensile strength, and resistance to corrosion. These alloys are usually categorized under three possible types: a cobalt base, a nickel base, and a nickel-iron base. Superalloys are often used in chemical conversion plants, industrial turbines, and aerospace turbines. Although they have high resistance properties, some superalloys may need additional protective coatings to maintain performance levels, particularly if they are employed under temperatures near their incipient melting points or under extreme load-bearing conditions.
Most superalloy coatings are used to shield components from potentially harmful environmental effects and stresses, with an emphasis on increasing heat tolerance and structural integrity for materials functioning at temperatures up to 1,850 degrees Fahrenheit (F) and higher. A protective coating is a layer of material that blocks or inhibits interaction between a substrate and damaging environmental conditions. This damage can take the form of metal wastage from oxidation and corrosion, or a loss of mechanical properties from high-temperature diffusion of contaminants into the substrate. Most protective coatings are designed to shield the superalloy substrate from these effects.
For more details, please see the source:
Principles of Superalloy Protective Coating by THOMAS
Available at: https://www.thomasnet.com/articles/chemicals/superalloy-coating-principles/
My best regards, Pierluigi Traverso.
Conventional corrosion protection coatings on nickel superalloys are not used, as there is no point in them. However, there are two types of coatings that make sense for superalloys - hard coatings for wear protection and scale resistant coatings for oxidation protection at temperatures of 900-1100°C.
Hard coatings such as hard chromium, nickel phosphorus, titanium nitride and others are used at normal temperatures and only when nickel superalloys are used in friction pairs.
Scale resistant coatings consisting of MCrAlY (M=Ni, Co, Fe) or Pt-Al alloys serve to protect against oxidation at high temperatures. In addition, for gas turbine engine applications, a layer of YSZ ceramics is applied to the scale-resistant coating layer in some cases (combustion chamber parts and first-stage blades of gas turbines) to create a thermal barrier.
hi ravindra kumar,
Superalloy engine components are often coated to prevent environmental degra-dation and more recently to provide thermal barriers which allow higher oper-ating temperatures[Coating Technology]. Environmental coatings are used to prevent environmentalattack (i.e., oxidation) of the substrate for the maximum possible time with themaximum degree of reliability. It should be noted that these coatings are not inertbut react with the atmosphere to provide protection by forming a dense, tightlyadherent oxide scale chromium oxide and aluminium oxide through the interaction of chromium and aluminum with oxygen in the atmosphere. All oxidation resistant coatingswill eventually fail, due to interdiffusion between the coating and substrate, i.e.the coating chemistry changes with time so that it is no longer protective. It isalso important that coatings are compatible with the superalloy substrate, toprevent cracking and spalling of the coating.
Suggested Reading:Manufacturing Technology for Aerospace Structural Materials by Samuel Gerardo Varela Angles
thanks & regards,
g.sudhakar,phd(materials engineering),HCU.
Vadim Verlotski
thank you Vadim, am wondering why we should protect the steel with plasma sprayed MCrAlY, whilst this coating does not have any bonding with the substructure. which bonding do you get when you perform plasma spray? chemical bond or mechanical interlocking bond? in the case of the latter, detachment of the coating is probable over the process. Do you agree?
Dear Arezoo,
First of all, a small correction: the MCrAlY coatings are not suitable for steel, only for nickel-based superalloys. In the case of steel, permanent protection against oxidation is not possible at temperatures above 800°C, since there is strong interdiffusion between the iron-based substrate and the MCrAlY layer.
Now to your question. Bonding between the plasma-sprayed MCrAlY layer and the nickel-based substrate is actually not entirely metallurgical. In order to get a 100% metallurgical connection between the layer and the substrate, the component is subjected to a vacuum heat treatment at 1000-1100°C after coating. Only after such a heat treatment can the coating function as an oxidation protection.
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