Research on Failure Cause and Mechanism of High Temperature Protective Coatings

In the aerospace industry, engines are exposed to extreme stress during long-term operation in high-temperature oxidation atmospheres and gas-corrosive environments. To extend the engine’s service life, it is often necessary to apply high-temperature protective coatings to enhance their resistance to high-temperature oxidation and thermal corrosion. However, in harsh, highly corrosive, and high-temperature environments, such as medium environments, as well as high vibrations and flow rates, these protective coatings can often suffer from damage and failure. This research work aims to summarize the causes of failure in high-temperature protective coatings, discuss the failure mechanisms in detail, and address issues such as interdiffusion, matching of thermal expansion coefficients, wear resistance, and erosion resistance in the design of these coatings. The findings will provide theoretical support for failure analysis, life prediction, and new material design of high-temperature protective coatings.


Introduction
The engine is the "heart" of airplanes and rockets.High-performance engines are characterized by their ability to operate at high turbine inlet temperatures.To withstand the demanding conditions, hightemperature alloys are indispensable as metal materials for engines.These alloys exhibit excellent resistance to stress in high-temperature oxidation atmospheres and gas-corrosive environments.Hightemperature alloys typically require superior mechanical properties at elevated temperatures as well as high resistance to corrosion.However, in practice, these two aspects of performance can sometimes conflict with each other for the same alloy.For instance, the initial Ni-20Cr heat-resistant alloy contains a high chromium (Cr) content, enabling it to form a continuous protective Cr2O3 film on its surface.This film effectively defends against hot corrosion caused by oxygen and sulfate.However, this alloy has limitations in terms of strength [1] .To achieve high-temperature and high-strength performance in alloys, designers need to reduce the chromium (Cr) content while increasing the formation rate of the γ phase by incorporating aluminum (Al).Additionally, the titanium (Ti) content needs to be minimized.Based on these requirements, the alloy should ideally have a minimum chromium content of 8wt.% and a maximum aluminum content of 6wt.%.These adjustments help optimize the alloy's properties for hightemperature applications, balancing both strength and corrosion resistance [2] .While the improved alloys were expected to possess good thermal corrosion resistance, they have proven to be susceptible to sulfate thermal corrosion even in aviation engines.In attempts to enhance the solid solution strength of these alloys, elements such as molybdenum (Mo) and tungsten (W) were added.However, this resulted in a decrease in their corrosion resistance.With advancements in engine efficiency, the service temperature of high-temperature alloys has also increased.Therefore, it is evident that solely improving hightemperature alloy materials cannot fully meet the performance requirements for modern aerospace development.To protect hot end components from oxidation corrosion in high-temperature environments and extend their service life, the application of high-temperature protective coatings becomes necessary.These coatings can provide an additional layer of defense against high-temperature oxidation and thermal corrosion, addressing the limitations of the alloys themselves and ensuring the desired performance in aerospace applications [3,4] .
In harsh high-temperature and highly corrosive environments, such as medium environments, as well as under high flow rates and high vibrations, high-temperature protective coatings often experience damage and failure.The present work summarizes the failure causes of high-temperature protective coatings and delves into the failure mechanisms in order to provide theoretical support for failure analysis, life prediction, and the design of new materials for high-temperature protective coatings.

Failure cause of coatings
High temperature protective coatings are primarily utilized to safeguard the base alloy against high temperature corrosion.As a thin film material, coatings adhere to the surface of the alloy.The key distinction between the coating and the matrix alloy lies in the fact that the coating undergoes degradation.This is due to the mutual diffusion between the coating and the matrix alloy at the interface, which results in the rapid consumption of antioxidant elements within the coating.Additionally, the repeated cycle of destruction and regeneration of the oxide film on the coating's surface leads to the depletion of antioxidant elements, eventually causing coating failure.There are various causes of failure for protective coatings in high temperature environments, including high-temperature oxidation, hot corrosion, thermal fatigue, and mechanical damage.The subsequent discussion will delve into the failure mechanisms of high-temperature protective coatings based on these four aspects.

High-temperature oxidation of coatings
MCrAlY coating is a commonly used high-temperature protective coating, which is primarily composed of metal elements such as M (usually Ni, Co), Cr, Al, and Y.During its service life, it is subject to various types of damage, such as thermal fatigue, hot corrosion, and oxidation.MCrAlY generally forms a continuous and dense protective oxide film (Al2O3/Cr2O3) during the oxidation process, which can be divided into three stages [5] .
(1) The transition period refers to the stage before the formation of a continuous layer of protective oxides (Al2O3/Cr2O3), during which the alloy surface reacts with oxygen to form multiphase oxides of various components.At this stage, the protective performance is poor, and the oxide film grows rapidly.Therefore, for coated alloys, the transition period should be minimized as much as possible.This requires improving the formation ability of protective oxide films.
(2) The stable period refers to the stage after the formation of a protective oxide film, where diffusioncontrolled selective oxidation (formation of Al2O3/Cr2O3) takes place.This is characterized by the thickening of the oxide layer according to parabolic kinetics.The resistance of coatings and alloys to high-temperature corrosion depends on this stage.The progress of the oxidation reaction is determined by the diffusion rates of anions O 2-and cations Al 3+ or Cr 3+ through the protective oxide layer.Al2O3 typically undergoes inward diffusion growth of O2 or simultaneous outward diffusion growth of Al 3+ .The oxidation reaction mainly occurs at the interface between the alumina layer and the coating, weakening the interfacial bonding.Oxidation reactions occurring within the inner layer can also generate compressive stress within the oxide layer, leading to arching, cracking, and even delamination of the oxide layer.
(3) Instability period.During the instability period, after the oxide layer thickens to a certain extent, cracking and peeling may occur due to internal stress.The newly exposed surface of the coated alloy begins to oxidize again from the transitional period.In the initial stage, timely supply of antioxidant elements such as Al or Cr may lead to the healing of the oxide film.However, as oxidation progresses, the re-exposed areas of the coated alloy surface will become depleted of Al or Cr, causing the entire oxidation process to accelerate and deviate from the steady-state parabolic kinetics, gradually losing the protective properties of the coating surface.

Hot corrosion of coatings
Hot corrosion refers to the phenomenon of accelerated material corrosion in high-temperature environments due to the deposition of molten or semi-molten salt films.It is widely prevalent in important sectors of the national economy and military industry, such as energy, metallurgy, and chemical industry.In the operating environments of aviation engines and gas turbines, hot corrosion can be categorized into chloride salt, carbonate salt, sulfate salt, vanadium salt hot corrosion, as well as multiple salt interaction hot corrosion, depending on the types of deposited salts.The corresponding mechanisms of typical sodium chloride salt hot corrosion and sulfate hot corrosion will be described below.

The hot Corrosion mechanism of sodium chloride salts
In NaCl salts, the Cr2O3 film does not have a protective effect, and solid, liquid, or gaseous sodium chloride has a destructive effect on the Cr2O3 film [6] .Although NaCl salts have less corrosiveness to Cr2O3 films, the adhesion of Cr2O3 films generated by aluminum coatings is poor, and molten salts penetrate the matrix or coating through cracks to accelerate corrosion [7] .Due to the higher temperature, the higher reaction rate, and the higher vapor pressure of NaCl, the destructive effect of NaCl on the oxide film is generally more severe in the high-temperature region than in the low-temperature region [7]   .The reaction mechanism is as follows: On the one hand, NaCl reacts directly with the protective Cr2O3 film on the surface, resulting in volatile CrCl3.
On the other hand, vaporized sodium chloride can penetrate the interface between the oxide film and the alloy matrix through cracks in the oxide film, leading to the generation of gaseous chlorides.At a temperature of 800°C, the vapor pressure of sodium chloride can reach 1.9×10 3 [8] .The pressure resulting from the formation of gaseous chlorides or chloride oxides is significant enough to cause extensive damage to the oxide film [7,9] .
The reaction equation is [10] : Gaseous CrO2Cl2 exists for a very short time and is quickly converted into Chromate and dichromate: Chlorides in the environment can also react with aluminum, tungsten, and titanium in alloys to form highly volatile metal chlorides [11][12][13] .This is the cause of crack formation in hot corrosion experiments.Due to the abundant deposition of salts within the cracks, there is insufficient oxygen supply.Therefore, in a high-temperature corrosive environment containing chloride salts, the oxygen partial pressure is low and inadequate to naturally form a protective oxide film [9] .In reactions involving NaCl, it is commonly observed that chlorine tends to accumulate near or at the interface between the protective oxide film and the metal.The presence of NaCl affects the mechanical integrity of the oxide film, resulting in accelerated corrosion after the film experiences blistering, cracking, and delamination.The mechanism of chlorine transport through the oxide film towards the inner surface is not yet fully understood.However, one proposed mechanism suggests that chlorine diffuses inward in the form of atoms or molecules through microscopic pathways such as grain boundaries in the oxide film.

The hot Corrosion mechanism of sulfate salts
In various fuel and coal-fired power plants, impurities present in the fuel, such as sulfur, undergo combustion reactions to form compounds like SO2, SO3, H2S, and more.These compounds then react with atmospheric oxygen (O2) and sodium chloride (NaCl) to generate sulfate deposits on the surface of materials.Possible reactions that can occur are as follows: According to the different temperatures at which hot corrosion occurs, the corrosion of sodium sulfate salts can be divided into two categories: low-temperature hot corrosion and high-temperature hot corrosion.Low-temperature hot corrosion refers to the accelerated corrosion of materials caused by the formation of a low-melting-point eutectic during the corrosion process.This occurs at temperatures lower than the melting point of the deposited salt, while the deposited salt remains in a solid state.On the other hand, high-temperature hot corrosion occurs at temperatures higher than the melting point of the deposited salt, with the deposited salt being in a molten state.The high-temperature hot corrosion mechanism of sodium sulfate salts can be explained by three main models: the sulfide model, salt melting model, and electrochemical mechanism model.The sulfide model suggests that sulfur species released from the decomposition of sodium sulfate react with the metal surface, forming metal sulfides.These metal sulfides are less protective than the original oxide layer, contributing to accelerated corrosion.The salt melting model proposes that the molten sodium sulfate salt infiltrates the protective oxide layer, disrupting its integrity.This leads to the dissolution of the oxide layer and exposes the underlying metal to further corrosion.The electrochemical mechanism model states that the molten sodium sulfate salt acts as an electrolyte, creating an electrochemical cell between the metal surface and the oxide layer.This results in localized corrosion and accelerated material degradation.These models provide insights into the high-temperature hot corrosion mechanism of sodium sulfate salts, illustrating the complexity of this type of corrosion.

Sulfide model
The sulfide model of the hot corrosion mechanism was first proposed by Siomons et al, which describes the accelerated oxidation process caused by sulfides in hot corrosion.According to this model, during the corrosion process of metals, sulfides react with metals to form low-melting-point metal sulfide eutectics.These eutectics are then oxidized by oxygen passing through the salt film, forming oxides and sulfides.The sulfides react again with the components of the metal matrix, forming a self-sustaining eutectic reaction.Additionally, the presence of sulfide phases in the oxide film provides a pathway for rapid outward diffusion.
In the sulfide model, the formation of low-melting-point metal sulfide eutectics is crucial.Once liquid eutectics are formed, the corrosion medium can pass through the melt and react with the metal rapidly, greatly increasing the corrosion rate.The vulcanization model must satisfy two conditions: firstly, the formation of metal• MS eutectics within the metal matrix; secondly, the ability of the metal• MS eutectics to be preferentially oxidized over the metal matrix.The first condition has been confirmed, but the second condition is not entirely consistent with experimental results.For example, Spengler et al. [14,15] found that pre-vulcanization treatment can accelerate the oxidation of Ni-15Cr alloy, while Goebel et al. [16] found similar oxidation rates for Ni-Cr alloy with and without CrS coverage.Tedmon et al. [17] also discovered no difference in the oxidation rate between metallic Cr and solid Cr7S8.Furthermore, many experiments have shown that hot corrosion is not solely an accelerated oxidation process caused by sulfides but rather a dissolution and destruction process of the oxide film due to the presence of Na2O in the Na2SO4 salt film.

Acid and alkali melting model
Based on the observation that solid sulfides do not trigger accelerated oxidation and that sulfur-free molten deposition can also cause accelerated oxidation of certain alloys, Bornstein [18,19] proposed an acid-base melting model for hot corrosion.Later, Goebel et al. [20] further supplemented and improved this model, establishing a comprehensive acid-alkali melting model for hot corrosion.According to this model, protective oxides on the surfaces of metals and alloys dissolve at the oxide/molten salt interface, and subsequently deposit as loose, unprotected particles at the molten salt/gas phase interface.The loose oxide layer deposited on the surface does not provide effective protection to the substrate.Based on the dissolution mechanism of the oxide film, this model can be further categorized into an acidic melting model and an alkaline melting model.In the alkaline melting model, when oxygen ions in the molten salt react with oxides to form soluble substances, alkaline melting occurs.On the other hand, in the acidic melting model, an oxide dissolves by transferring its oxygen ions to the molten salt, thereby leading to acidic corrosion.
(1) Alkaline melting corrosion Sodium sulfate can be considered as composed of the alkaline component Na2O and the acidic component SO3 [15] : As the oxide film on the surface of an alloy covered with Na2SO4 continuously forms and grows, the partial pressure of oxygen in Na2SO4 in contact with the oxide skin on the alloy surface decreases, while the partial pressure of SO3 relatively increases.In order to maintain the equilibrium relationship, SO3 must be decomposed: If the oxide film ruptures due to thermal stress or other factors, sulfur will diffuse into the inner layer through the oxide film, forming a sulfide phase.It is worth noting that the diffusion coefficient of sulfur is very high [21] .As an impurity element, sulfur precipitates and accumulates at the interface between the oxide and metal, which reduces the adhesion of the alumina membrane [22][23][24][25][26] .The significant consumption of sulfur in sodium sulfate increases the oxygen partial pressure and the content of Na2O relatively increases, leading to an increase in the alkalinity of Na2SO4.As a result, the following reaction occurs: Ultimately, the oxide film is destroyed.
(2) Acid melt corrosion Acid melting can be further divided into two categories: alloy-induced acidic corrosion and gas-phaseinduced acidic corrosion.The acidity in the former molten salt originates from the dissolution of alloy substances, which react strongly with Na2O or O 2-.The latter acidity is derived from the reaction with the gas phase.
In the process of alloy-induced acid melting, when the alloy contains a certain amount of refractory metals such as tungsten, molybdenum, vanadium, and other Refractory metals elements, the following reactions occur [26] : V enter the sedimentary salt and preferentially react with Na2SO4: Increasing PSO3 in sodium sulfate results in a relative decrease in oxygen ion activity, causing molten sodium sulfate to become acidic, resulting in: The formed cation (M 2+ ) combines with oxygen in the outer layer to form a loose oxide film without a protective effect.Consequently, the previously continuous protective oxide film (MO) continues to decompose, and the newly formed oxide film develops into a loose sponge structure.This leads to catastrophic damage to the alloy coating.
Regarding gas-induced acidic melting, the acidic components enter the sedimentary salt from the gas phase based on the following equation: A clear characteristic of this type of hot corrosion is that the corrosion rate is higher at low temperatures (such as 605-750 °C) than at high temperatures (such as 950-1000 °C), commonly known as "low-temperature" hot corrosion.An obvious feature of this type of hot corrosion is the need for sulfate generation, such as CoSO4 and NiSO4.As the temperature increases, the partial pressure of SO3 required for sulfate generation also increases.In most combustion environments, if the sulfur content of the fuel remains constant, the equilibrium partial pressure of SO3 decreases with increasing temperature [29]   .Therefore, as the temperature increases, this type of thermal corrosion diminishes.The resistance to hot corrosion of alloys or coatings is mainly determined by their ability to generate a continuous and complete oxide film and the ability of the oxide film to resist stripping [7]  .Almost all alloys susceptible to corrosion undergo two stages of hot corrosion: an initial incubation stage and an accelerated corrosion stage.During the initial incubation period, the components that can form a protective oxide film in the alloy gradually deplete, and the oxide dissolves into the salt, leading to cracks or channels in the oxide film.The components in the deposited salt (such as S) enter the coating or matrix alloy, causing internal sulfidation or oxidation.These changes often alter the composition of the sediment, making it more alkaline or acidic.These changing sediments begin to affect the corrosion product layer, causing damage to it.The liquid deposited salt permeates through the product layer and comes into contact with the alloy, initiating the accelerated corrosion stage.
Under oceanic conditions, seawater may be drawn into gas turbines, resulting in an environment where sulfates and chlorides coexist.In a mixed salt of sulfur and chloride salts, insoluble metal elements in the matrix diffuse outward, reacting with corrosive substances in the environment, which accelerates the formation of SO2.The concentration of sulfur at the interface between the oxide film and the alloy increases, and sulfur diffuses into the alloy, reacting with alloy elements to form sulfides, leading to a decrease in Al and Cr elements in the alloy.The presence of chlorine increases the tendency of the oxide film to crack and peel off.The increasing trend of peeling of the protective oxide film causes the alloy to enter the accelerated hot corrosion stage after a short period of exposure.

Thermal fatigue and mechanical damage
High temperature oxidation resistance and hot corrosion resistance are two important aspects of the performance requirements for high-temperature structural materials.In actual use or in special industrial environments, the factors affecting the material are very complex.For example, alloy coatings can be subjected to high-temperature oxidation, thermal cycling, and mechanical impact.These factors often cause the protective oxide layer on the alloy to peel off and become damaged long before the expected period of stable temperature oxidation and thermal corrosion.
During repeated heating and cooling cycles, the component undergoes temperature variations, leading to alternating thermal stress within the material.This results in cycles of elastic-plastic deformation.Over time, plastic deformation accumulates and eventually leads to fracture.This phenomenon is known as thermal fatigue.At high temperatures, when the oxide film on the surface of the alloy coating undergoes deformation, the film can crack under high strain rates and exhibit brittle behavior even at low strains.However, at lower strain rates, the oxide film remains intact even at higher strains.Therefore, thermal fatigue often causes brittle fracture with minimal or no significant plastic deformation near the fracture point.Research by Yu et al. [30] has shown that an increase in the frequency of thermal cycling accelerates the degradation rate of the coating.
The erosion of solid particles and the effect of external loads often cause serious damage to the protective coating.When there is an external load, when the strain rate of the metal is high, the oxide film grown on the surface cracks and cannot self-heal, and the antioxidant alloy also becomes non antioxidant.In high-temperature oxidation environments, the presence of erosion may lead to the destruction or removal of protective oxide films, making the process of metal surface occurrence more complex.The combined effect of erosion and corrosion in high-speed gas flow has significant damage to turbine engine materials.Therefore, for the wear-resistant packaging components of turbine engines, the applied protective coating should not only have good oxidation resistance, but also have good erosion resistance and wear resistance [31,32] .Link et al. [33] studied the degradation process of pure Ni, Ni -20% (mass fraction) Cr, and Ni-30% (mass fraction) Cr alloys exposed to erosion corrosion environments at 700 °C and 800 °C.The pure oxidation results at two different temperatures indicate that both alloys can form protective oxide films, and the oxidation rate is very low.However, in erosion corrosion environments, the degradation rates of the two alloys are comparable to those of pure nickel.This is because the rate of oxide removal by surface particle erosion flow is higher than the rate of oxide spreading growth, thus preventing the formation of continuous chromium oxide film.

Outlook
The actual service conditions of key components, such as aircraft and space shuttle jet engines, are complex and variable.When operating in very harsh environments, their surface protective coatings are prone to high-temperature oxidation, thermal corrosion, thermal fatigue, and mechanical damage.
High-temperature protective coatings need to address three issues.Firstly, the formation ability and continuous integrity of the Al2O3/Cr2O3 oxide layer require a reasonable design of coating composition and diffusion barriers.Secondly, the bonding ability between the protective oxide layer and the coating substrate alloy needs to solve the problem of mismatch between the thermal expansion coefficient of the oxide film and the alloy coating.Thirdly, the protective coating needs to have good wear resistance and erosion resistance.
In recent years, cermet coatings have been developed, which can serve as bonding layers for thermal barrier coatings or as standalone high-temperature protective coatings.Cermet coatings may have better thermal shock resistance than traditional MCrAlY coatings [34][35][36][37][38][39][40][41] .This is because the mismatch between the thermal expansion coefficient of cermet coatings and thermal barrier coatings is much lower, effectively reducing the thermal stress between the thermal growth oxide film (TGO) and the thermal barrier coating, thus improving the coating's service life.