Microstructural characteristics and oxidation behavior of the modified MCrAlX coatings: A critical review
Introduction
In recent decades, to achieve higher performance and efficiency, modern land- and marine-based gas turbines, as well as aero-engines which have been designed and made by superalloys, need to operate long-lasting in higher temperatures. Therefore, these critical operating conditions have imposed serious coatings restrictions concerning their higher resistance to hot-corrosion and oxidation attacks. So, the latest investigations have been carried out on the development of high-temperature materials and coatings to establish higher thermal and mechanical properties. So, the mentioned thermal limitations have been reduced through the introduction of high-temperature diffusion and overlay coatings [[1], [2], [3], [4], [5], [6], [7], [8]]. In this case, thermal protective coatings such as diffusion aluminides or Ni- or Co-based overlay coatings have been extensively employed to form a protective oxide layer during high-temperature service.
Among the variety of high-temperature coatings, MCrAlX coatings (M = Ni, Co, or NiCo; X = minor elements such as Y, Ce, Si, Ta) are widely used in some critical parts especially for gas turbine and aero-engine components, where they may be applied as oxidation and hot corrosion resistant overlays or as bond-coats for use with thermal barrier coatings (TBCs). Moreover, because of the wide range of using Y as an alloying element, the name of the MCrAlY coatings is often used for these types of coatings. Compared with the protective diffusion layer, MCrAlX coatings are more flexible in selecting a manner for attaining a relatively balanced combination of the coating properties and service conditions [9].
The state-of-the-art methods for applying MCrAlX coatings consist of electron beam-physical vapor deposition (EB-PVD) and thermal spray processes [9,10]. For all types of the MCrAlX coatings applied by various deposition techniques, the two-phase structure consisting of γ (Ni- or Co-rich) and intermetallic β (NiAl or CoAl) phases are often formed. These equiaxed phases can be stabilized in the MCrAlX coating after an appropriate post heat treatment process. The overall amount of microstructural difference from the equilibrium state in as-deposited MCrAlX coatings is related to the deposition technique used for applying the coating. Indeed, during high-temperature exposure of MCrAlX coatings, Al-ions become depleted in the near-surface layers of the coating, owing to the incorporation of Al-ions to the formation of the Al2O3 oxide scale [11].
Nowadays, the service temperature of the MCrAlX coating either as an overlay or as a bondcoat is more than 1000 °C. Therefore, the enhancement of thermal stability and service lifetime of the MCrAlX coatings are the key factors that need to be emphasized. Among them, the high-temperature oxidation resistance is one of the most important properties of the MCrAlX coatings. Al and Cr are essential components for the function of the MCrAlX coating as protection of the critical turbine components in contradiction of oxidation and hot-corrosion attacks. Nonetheless, excessive additions of Al and Cr can cause the creation of higher amounts of Ni3Al (γ′) and Ni-rich γ phases that deteriorate fatigue life. Therefore, the standard percentage of Cr and Al must be balanced for the precise requirements on the MCrAlX coating in high-temperature applications [12]. Although some reports indicated that thermal fatigue cracks may initiate from inclusions into γ′-phase in MCrAlY alloys [13,14].
During high-temperature exposure, transient oxides and spinel, as well as Al2O3 oxide scale, can develop on MCrAlX coating. Although, the transient/mixed oxides and spinels with a porous structure and higher growth rate do not have a large influence on high-temperature oxidation resistance. In contrast, the formation of a dense and continuous α-Al2O3 scale is beneficial for increasing thermal stability and oxidation resistance of the MCrAlX coatings. Furthermore, the addition of Co into the MCrAlX coating composition causes to improve the hot corrosion resistance and high-temperature fatigue life due to its ductility. In this case, a small percentage of active elements such as Y is beneficial for increasing oxide scale adhesion during high-temperature service.
The various modification techniques can be used to improve the structural properties, oxidation, and hot-corrosion behavior of MCrAlX coatings. These modification methods are usually based on the processes that change the microstructure and composition of the coatings. In this case, the optimization of the parameters for each modification technique is essential to achieve a coating with higher structural performance and thermal stability. In another point of view, however, due to the wide range of applications of the MCrAlX coatings, it is almost hard to compare the properties of the modified coatings by different investigations. Regretfully, the direct effect of some alloying elements or their oxides on both structural performance and thermal stability of MCrAlX coatings has not been clarified in detail. Furthermore, several viewpoints have been proposed in the literature to describe the role of various modification processes on MCrAlX.
The purpose of this article is to provide a comprehensive critical review of the research progress on structural, mechanical, and thermal characteristics of the modified MCrAlX coating applied by various deposition methods. Also, this critical review is complementary to other researches and reviews on the modified MCrAlY coatings such as Naumenko et al. [15], Sloof et al. [16], Evans et al. [17], and other existing publications [11,[18], [19], [20], [21], [22], [23]]. In the current critical review, we have also tried to present the overall characteristics of MCrAlX coatings and the latest results about the modification of MCrAlX coatings as well as the classical investigations in the related field. Besides, we have described the mechanisms related to the structural and oxidation improvements of MCrAlX coatings and also tried to provide advantages and disadvantages of the modification process on MCrAlX coatings.
In this case, we have highlighted the current state-of-art strategies to improve the overall properties of the modified MCrAlX coatings and have discussed approaches to present the mentioned coatings with a superior response. We have reviewed the production, deposition, characterization, and recent advances on the improved MCrAlX coatings by various modification methods, and have also tried to clarify the major drawbacks and technological challenges during modification of the MCrAlX coatings. This current critical review is organized as follows. The introduction and historical overview of the MCrAlX coatings are presented in sections 1 Introduction, 2 Historical overview of MCrAlX coatings. Various deposition methods used for MCrAlX coatings are reviewed in Section 3, and Section 4 describes modification techniques for the mentioned coatings. The characterization methods of the modified MCrAlX coatings consisting of structural and mechanical investigations as well as their oxidation behavior are covered in Sections 5 Characterization of the modified MCrAlX coatings, 6 Oxidation behavior of the modified MCrAlX coatings. Section 7 describes some major drawbacks and technical challenges related to the modification of MCrAlX coatings. The recent advances related to the improvement of MCrAlX coatings are presented in section 8 and finally, section 9 offers some final perspectives, summary, and concluding remarks.
Section snippets
Historical overview of MCrAlX coatings
In the early 1950s, the diffusion aluminides were introduced to protect hot sections of gas turbine engines [24,25] and were still actively used today due to their lower cost and their more availability [26]. In the first years of the 1970s, the NiCrAlY overlay coatings were introduced to protect hot sections of industrial parts (especially gas turbine blades) against hot corrosion and oxidation problems [27]. The oxide scale spallation and cracking were the main problems for the coated parts
Deposition methods of MCrAlX coatings
Typically, the various grades of the MCrAlX coatings are usually deposited using two major techniques. The first method is known as electron-beam physical vapor deposition (EB-PVD) [45,46] and the second one is the family of the thermal spraying process such s air plasma spraying (APS) [[47], [48], [49], [50], [51]], low-pressure plasma spraying (LPPS) [[52], [53], [54]], vacuum plasma spraying (VPS) [19,[55], [56], [57]], detonation spraying [58], high-velocity oxy-fuel spraying (HVOF) [5,19,41
Modification techniques for MCrAlX coatings
Owing to the higher operating temperatures for the modern turbine blades and combustion chambers, the key modification processes can be employed to improve the structural properties, oxidation, and hot-corrosion behavior of MCrAlX coatings. These methods mostly include techniques such as vacuum heat treatment, nano-crystallization, pre-oxidation, reactive element alloying, dispersion of oxide particles, laser treatment, spark plasma sintering, hot isostatic pressing, and using
Characterization of the modified MCrAlX coatings
The typical microstructure of the MCrAlX coatings mainly consists of two major γ and β phases. In addition, Cr-rich α and Ni3Al γ′ phases may appear into the coating structure depending on the composition of the MCrAlX coating. As mentioned before, the γ-phase is a (Ni,Co)-rich solid solution with a face-centered cubic (fcc) structure and a lattice parameter between 3.53 and 3.60 Å. Other metallic elements such as Fe, Cr, Al, and some refractory metals like Mo, Nb, and Re are often soluble in
Oxidation behavior of the modified MCrAlX coatings
The formation of a thermally-stabilized ceramic layer (e.g., Al2O3) on the coated parts is the main subject for oxidation and corrosion protection. Among various types of high-temperature coatings, MCrAlX coatings are good Al2O3 formers during exposure to high-temperature oxidation [80]. So, at temperatures above 950 °C, a dense and protective alumina layer can form at the surface of the MCrAlX surface, to protect the parts from extensive oxidation. Various types of oxides may form on the top
Some drawbacks in the way of the modification of MCrAlX coatings
It is noted that the standardization and planning for the large scale production for some cases of the modified MCrAlX coatings are still complicated. Owing to the application of the modified MCrAlX coatings, it is approximately hard to compare the overall performance of these coatings using various investigations. Moreover, the findings related to the different MCrAlX coatings were claimed by the different researchers together with the diverse viewpoints owing to the contradiction among the
Modern deposition techniques of MCrAlX coatings
Nowadays, special attention has been paid to the influence of new reactive elements and modern deposition techniques on the oxidation behavior as well as thermal-mechanical properties of MCrAlX coatings. The surface effect such as surface roughness, residual stress, and surface activation is one of the topics that researchers have recently focused on [147,200]. Furthermore, in recent years, the cold spraying deposition has emerged as a promising method to apply the different types of MCrAlX
Summary and conclusions
The key modification processes can be employed to improve the microstructural properties and high-temperature oxidation behavior of MCrAlX coatings. These methods mostly include techniques such as vacuum heat treatment, nano-crystallization, pre-oxidation, reactive element alloying, ODS modifying, laser treatment, SPS, HIP process, and development of multilayered/graded coating systems.
Recent investigations emphasized that nano-crystallization and/or adding rare earth elements)or their oxides(
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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