Overlay Coatings Research Articles

High-temperature-driven degradation analysis and modelling of an industrial gas turbine applicable γ/β NiCoCrAlYRe coating– Part II: Diffusion modelling and experimental validation under isothermal conditions

In a two-part research, the degradation process of a commercially utilised NiCoCrAlYRe coating on an IN738LC substrate was both simulated and experimentally validated. The first part of the study delved into the microstructural characteristics of the overlay coating system and analysed the oxidation and interdiffusion mechanisms. This second part extends the analysis to simulations of the coating degradation at temperatures of 900 °C and 1000 °C.The interdiffusion between the substrate and coating was modelled utilising thermodynamic and kinetic mobility data, while the oxidation aspect was addressed using two different established models from Whittle and Meier et al., along with an adapted approach specifically tailored for MCrAlY overlay coatings devised in this research. The simulation results were validated experimentally for up to 7000 h, with the degradation state evaluated based on the depletion of β-NiAl in the coating.Good agreement was found between simulated and experimental data. The initial coating and substrate microstructures were sufficiently predicted based on thermodynamic data, and the simulations closely matched the observed degradation pattern. The most precise alignment between simulated and experimental β-NiAl depletion was achieved with the newly proposed oxidation modelling approach for MCrAlY systems. While more optimisation is required to accurately predict the precipitation of chromium phases and carbides and to account for interdiffusion-obstructing effects, particularly at 1000 °C, the results demonstrate the potential of thermodynamic-kinetic modelling for estimating the lifespan of MCrAlY systems under diffusion-driven degradation conditions.

A review of research into chromium and chromium-based coatings for superalloys

This review examines recent research on chromium and chromium-based coatings for superalloys, focusing on their application methods and protective mechanisms in extreme environments. The study explores diffusion coatings, particularly the pack cementation method, and overlay coatings applied through thermal spraying techniques. The formation of protective Cr2O3 oxide scales and the role of chromium in enhancing oxidation and corrosion resistance are discussed. The review also addresses the challenges and limitations of chromium coatings, including the need for precise control of chromium content. Finally, it highlights the importance of selecting appropriate coating methods based on specific application requirements and environmental conditions.

Chromium–Aluminum Coatings for Oxidation Protection of Titanium–Aluminum Intermetallic Alloys

This article explores the utilization of cathodic-arc deposition Cr-Al overlay coatings as oxidation protection for Ti-Al-Nb intermetallic alloys. The primary objective is to investigate PVD Al-Cr coatings deposited via cathodic-arc deposition without subsequent vacuum annealing. The microstructure, phase, and chemical composition of the coatings were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. Isothermal exposure of samples in a laboratory air furnace was conducted, revealing the efficacy of Cr-Al coatings in protecting the Ti49-11Al-40Nb-1.5Zr-0.75V-0.75Mo-0.2Si (mass%) intermetallic alloy VTI-4 against oxidation. The findings highlight that the as-deposited coatings possess a layered structure and contain Al-Cr intermetallics. Post-exposure to the furnace without prior vacuum annealing results in coatings exhibiting a porous microstructure, raising concerns regarding oxidation protection. This investigation of Cr-Al coatings on a VTI-4 alloy substrate yields valuable insights into their nanolaminate structure and challenges associated with aluminum droplet fractions. The proposed additional vacuum heat treatment at 650 °C for 500 h effectively homogenizes the coating, leading to predominant Cr2Al and Ti-Al phases. Additionally, the formation of diffusion layers at the “coating–substrate” interface and the presence of oxide barriers contribute to the coatings’ heat resistance. Our research introduces possibilities for tailoring coating properties for specific high-temperature applications in aerospace, energy, or industrial contexts. Further refinement of the heat treatment process offers the potential for developing advanced coatings with enhanced performance characteristics.

A COMPREHENSIVE REVIEW ON COMBATING THE ELEVATED-TEMPERATURE SURFACE DEGRADATION BY MCrAlX COATINGS

Surface degradation (oxidation/corrosion/erosion) at elevated temperature is encountered commonly in engineering industries like gas turbines and thermal power plants. MCrAlX coatings which came into the picture in 1960s were used widely for surface protection in the elevated-temperature section of the gas turbine engines and in boilers to combat oxidation/corrosion/erosion. Among them, MCrAlY ([Formula: see text], Co or [Formula: see text]) were developed to be used as the overlay coatings and bond coat (BC), which offer a combination of multiple features such as oxidation, corrosion and ductility. MCrAlY coatings form a second layer of aluminum oxide beneath the chromium oxide layer at elevated temperatures which minimizes the oxidation/corrosion/erosive wear rates. But the desire to increase combustion efficiencies of power plants and gas turbine engines along with lower CO2 emissions poses a significant challenge for coating design. As the temperature surpasses 900∘C, NiCrAlY coating degrades quickly due to nonregeneration of chromia or alumina. The research and development (R&D) efforts are focusing continuously on improving the existing MCrAlX coatings or developing new sustainable MCrAlX coatings with improved oxidation performance. In this review, the roles of the alloying elements, microstructures, post-deposition treatment techniques and different deposition processes in the elevated-temperature oxidation/corrosion performance of MCrAlX-based alloys have been explored in detail.

Erosion Response of High-Velocity Oxy-Fuel Sprayed Marble Dust-NiCr Coatings

In the present research, raw marble dust (an ornamental stone waste) with and without pre-mixing of nickel chrome (NiCr) powder are used as the feedstock material to deposit overlay coatings on the mild steel substrates through HVOF spraying technique. The physical as well as mechanical characterizations of the deposited coatings in terms of deposition efficiency, adhesion strength, coating thickness and micro-hardness are done at controlled laboratory conditions. To get an insight to the erosion behavior of the deposited coatings, high temperature solid particle erosion trials on them are carried out as per ASTM G76 considering the test parameter combinations obtained from Taguchi’s L16 orthogonal array. On analyzing the experimental outcomes, impact velocity, NiCr content in the feedstock, erodent temperature and impingement angle are found to be sequentially significant in terms of affecting the wear loss of the coatings. Based on the findings from the analysis, further experimentations are conducted at different levels of the significant control factors to ascertain their effects on the wear rate more precisely. The micrographs of the worn surfaces taken using a scanning electron microscope are studied to identify the possible damage mechanisms causing the erosion loss.

Thermal Spraying of MCrAlY Overlay Coating Using New Ethanol-Fueled HVOF Gun

The MCrAlY overlay coatings are widely used for high-temperature protection of hot section part of gas turbines and jet engines. This type of coatings are usually thermally sprayed using APS (Atmospheric Plasma Spraying), LPPS (Low Pressure Plasma Spraying) as well as HVOF (High Velocity Oxygen Fuel) methods. In present article the newly developed ethanol based HVOF gun was used for production of this type of coatings. The stainless steel 18-8 type was used as a base material. The AMDRY 386 (Oerlikon-Metco) NiCrAlY powder was used for coatings production. In the research different oxygen (400, 500, 600 NLPM) and ethanol (16.5, 18.3, 21.3, 23.6 and 26.6 dm3/h) flow ratio were selected for experimental processes. The powder feed ratio was also changed during process. After deposition the microstructural assessment using Scanning Electron Microscopy and chemical composition analysis using EDS method were conducted. The obtained results showed that coating was above 100 μm thick depending on the process parameters. The low concentration of pores and oxides was also observed on coatings cross-section. Using of ethanol HVOF gun enables to form good quality MCrAlY coatings with 50% reduction of oxygen consumption in comparison with conventional HP/HVOF torch using kerosene such as JP 5000. The other benefit of its using is lower CO2 emission and lower concentration of carbon in coating in comparison with classic JP 5000 HVOF gun. The ethanol HVOF is a promising technology and might be considered as an replacement of LPPS and HVOF process for production of MCrAlY type of coatings.

Assessment of CrFeCoNi and AlCrFeCoNi High-Entropy Alloys as Bond Coats for Thermal Barrier Coatings

High-entropy alloys (HEAs) represent a relatively new group of multicomponent alloys that have shown great potential for applications requiring tribological and oxidation resistant properties. Consequently, thermally sprayed coatings of different HEA chemistries have received increasing research attention. In this paper, atomized equimolar CrFeCoNi and AlCrFeCoNi feedstocks were used for high velocity air-fuel spraying (HVAF) to produce overlay coatings using two different nozzle configurations. The microstructure, phase constitution and hardness of the coatings were analyzed along with the primary aim of testing the coatings for their oxidation behavior. The performance of the two HEA chemistries was compared with two commercial MCrAlY coatings that are well-established bond coat materials for thermal barrier coatings (TBCs). An investigation was conducted to test the coatings’ performance as bond coats by applying suspension plasma sprayed yttria-stabilized zirconia top coats and evaluating the thermal cycling behavior of the TBCs. The AlCrFeCoNi-coating was found to demonstrate a lower oxidation rate than the CrFeCoNi-coating. However, the AlCrFeCoNi-coating was found to form more rapid oxide scales compared with the commercial bond coat material that also contained reactive elements.

Laser innovation of YSZ electrophoretic deposition overlay on plasma sprayed thermal barrier coating system

In this study, laser surface treatment was used on electrophoretically deposited YSZ overlay coatings on APS YSZ to enhance and improve the TBCs system. First, preliminary preparation of TBC YSZ samples were produced by APS over superalloy substrates and the co-deposition to protecting of TBCs by applying overlay EPD coating followed by sintering was characterized. An electrophoretic deposition process was carried out on the TBC YSZ samples at the optimum coating parameters 40 V in 5 min with a solid concentration of 10 g/L of YSZ. After deposition the samples were dried at room temperature for 24 h and Subsequently, the electrophoretic deposition was followed by sintering in a vacuum furnace at 1200 °C for 2 h. coating samples after sintering were further treated by using Pulsed Nd: YAG laser beam. The surface morphology and microstructure of the samples were studied by a field emission scanning electron microscope (FESEM), Optical microscope, microroughness, and phase purity of the coating material were investigated by XRD. Results indicated that the YSZ oxide overlay produced by electrophoretic deposition (EPD) was successfully performed as an environmental barrier overlay in protecting the APS YSZ TBCs against environmental degradation. The analysis of the XRD pattern of the sintered coating showed the presence of only equilibrium tetragonal phase (t) while in laser state the phase was only nontransformable tetragonal ZrO2 phase (t -ZrO2). Which is regarded to be a desirable ZrO2 phase to beneficial for TBC. It is found from microroughness inspection, decreasing surface roughness after laser treatment. Which indicated improvement in coating surface properties.

Influence of Plasma Transferred Arc Remelting on Microstructure and Properties of PTAW-Deposited Ni-Based Overlay Coating

Although overlay coatings deposited via plasma transferred arc welding (PTAW) can effectively prolong the service life and improve the performance of components, cracks and pores tend to occur in the high-hardness and wear-resistant coating during the deposition process. Further treatment is thus necessary to improve the coating quality. Herein, PTAW-deposited Ni-based overlay coating was remelted using plasma transferred arc (PTA) under different remelting currents (RCs). The microstructure and phase composition of the coatings were investigated via metallographic microscopy and x-ray diffraction. The hardness, surface quality, and wear resistance of the coatings were also tested. The results show that PTA remelting could effectively eliminate the defects and improve the coating properties when the RC was controlled within a given interval (100-120 A). The type and content of the hard phase in the remelted coating were increased to enhance the microhardness and wear resistance. However, remelting with excessive currents resulted in the agglomeration and growth of the hard phases, which negatively affected the coating hardness and wear resistance. The proper use of PTA remelting technology to treat PTAW-deposited overlay coating is an effective approach for eliminating the coating defects and enhancing the coating properties.

Sea-salt-induced moderate-temperature degradation of thermally-sprayed MCrAlY bond-coats

Metallic NiCoCrAlY bond-coats or overlay coatings are commonly used to protect superalloy components in the hot-section of shipboard gas-turbine engines. However, the accelerated degradation (or hot-corrosion) of such bond-coats at modest temperatures (~850 °C) in the presence of impurities such as sea salt from the marine environment is a serious concern. Here, the interplay between the microstructure, presence/absence of sea salt or pure NaCl, heat-treatment duration, atmosphere and pre-oxidation, and the resulting degradation of thermally-sprayed NiCoCrAlY bond-coats is studied systematically. The degradation in the presence of sea salt or pure NaCl entails the formation of a two-layer oxide, where Cr-rich oxide is on top and Al-rich oxide is at the bottom. The mechanisms behind this two-layer oxide formation are investigated, which appear to be a combination of: (i) preferential formation of Al-rich oxide first, (ii) initial fluxing of the Al-rich TGO by the molten sea salt, and (iii) upward diffusion of Cr from the bond-coat through the Al-rich oxide layer. It is also shown that a modest pre-oxidation heat-treatment can be an effective way to mitigate sea-salt-induced degradation of NiCoCrAlY bond-coats.

Effect of thermal energy on the deposition behaviour, wear and corrosion resistance of cold sprayed Ni-WC cermet coatings

Cold spraying is emerging as one of the versatile coating techniques applicable in vast variety of applications ranging from overlay coatings to structural repairs. Though variety of metals and their alloys have been successfully deposited using this technique, deposition of cermet with high volume percentage of carbide particulates is still challenging. Existing reports reveal that the deposition of cermet coatings was obtained from blend of metal (binder) and ceramic (particulates) powders. Understanding the deposition behaviour of commercially available cemented carbide in cold spraying has technical significance since these cermets have huge industrial potential to protect components from wear and corrosion. Deposition of cermet using cold spray has a challenge since increase in particle velocity not only increases the plastic deformability of binder phase but also reduces the particulate phase in coatings since the supersonic velocity leads to fracturing and shattering of particulates. State of binder and particulate in the feedstock plays a crucial role in successful deposition. In this work, two different Ni-WC cemented carbides were deposited and comprehensively characterized for inter-splat bonding state and powder state and the same is correlated with the mechanical and corrosion performances of the coatings. The role of thermal energy in the coating formation is studied and discussed in detail. Along with a proper feedstock, engineered nozzle design to enhance the thermal energy input to the feedstock is required to obtain dense cemented carbide coatings with high particulate retainment.

Effects of iron and platinum on the isothermal oxidation of β-NiAl overlay coatings fabricated by spark plasma sintering

The Fe and Pt modified β-NiAl overlay coatings with controlled concentrations were firstly fabricated on Hastelloy substrates using spark plasma sintering. The effects of Fe, Pt and their concentrations on the isothermal oxidation of the β-NiAl coatings were investigated. The unmodified NiAl coatings showed the worst spallation resistance, both Fe and Pt addition could significantly improve the oxidation performance of β-NiAl coatings. The mechanisms of Fe, Pt and their concentrations on the NiAl oxidation behavior were further discussed in related to the transformation of θ-Al2O3 to α-Al2O3, void formation at scale/coating interface and impurities (S and C) segregations. It is proposed that Fe could be a cheap alternative for Pt, though it was less effective than Pt to improve the oxidation resistance of β-NiAl coatings.

Effect of welding energy on the corrosion and tribological properties of duplex stainless steel weld overlay deposited by GMAW/CMT process

Duplex stainless steels (DSS) show good mechanical properties, wear, corrosion resistance and fatigue strength. In order to decrease operational costs in paper and pulp industry, DSS is employed through weld overlay coatings on mild or low carbon steel components. In this work, the aim was to evaluate the DSS weld overlay properties obtained through deposition of ER 2209 duplex stainless steel alloy on a mild steel plate (ASTM A 516 Gr 60). The deposition was performed through GMAW/CMT welding process. The coatings were deposited using four heat inputs: 0.53, 1.01, 1.27 and 1.74 kJ/mm. Moreover, microstructural characterization by optical microscopy showed that heat input influences ferrite/austenite phase balance. Secondary austenite that precipitates due to the welding process in DSSs are Widmanstätten austenite (WA), grain boundary austenite (GBA), partially transformed austenite (PTA) and intragranular austenite (IGA). The amount of secondary austenite increases with welding energy input. The secondary austenite improves abrasive wear resistance while corrosion resistance is the lowest when welding energy is 1.74 kJ/mm.

Simulating the effect of aluminizing on a CoNiCrAlY-coated Ni-base superalloy

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. The present work focuses on the effect of the aluminizing process on the CoNiCrAlY coating microstructure. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al with interspersed precipitates of Cr-rich carbide and Cr-rich boride precipitates. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY microstructure. Scanning electron microscopy (SEM) combined with energy and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterize the aluminized CoNiCrAlY coating. Phases were then identified by electron backscatter diffraction (EBSD). Detailed microstructural studies of the coating were corroborated with the help of coupled thermodynamic-kinetic calculations to model the aluminizing process. The calculations were performed with the in-house developed code employing the commercially available thermodynamic and kinetic databases (ThermoCalc). The mechanisms of the observed microstructural changes were elucidated with the help of the modelling results.

A new approach to manufacture oxidation-resistant NiCrAl overlay coatings by electrodeposition

NiCrAl overlay coatings have been manufactured by means of conventional electrodeposition of Ni into a porous electrophoretic deposition layer of CrAl nanoparticles. The NiCrAl coatings are compact with a uniform distribution of Cr and Al elements in nano-length scale. They can thermally grow alumina scales in air at 1000 °C, with parabolic oxidation constants as low as that of the ternary coating counterparts prepared by means of physical vapor deposition technologies such as sputtering and arc ion plating.

Corrosion protection of low-cost carbon steel with SS-309Mo and Inconel-625 bimetallic weld overlay

The dissimilar weld overlay coatings of components can lead to corrosion. In the present work, two different weld overlay layers (SS-309Mo and Inconel-625) are deposited on IS2062 grade B carbon steel, in order to avoid distortions and improve corrosion resistance. The microstructural investigations revealed that the intermediate layer SS-309Mo on carbon steel prevents the dilution by aiding crack-free weld overlay of Inconel-625 on the top. The corrosion behaviour of the coatings is studied using Linear Polarization Resistance and the Electrochemical Impedance Spectroscopy. The corrosion rate of the coatings decreased by two orders of magnitude compared to that of low carbon steel substrate. The corrosion rate of the deposited Inconel-625 along with intermediate SS-309Mo is 1.16 × 10−3 and is comparable to that of bare Inconel-625 (1.12 × 10−3 mmpy). Further, the immersion tests for 14 days show that, the depositions are stable with the formation of the passive film.

Weld overlay coating of Inconel 617 M on type 316 L stainless steel by cold metal transfer process

A study of microstructure and volumetric dilution of Cold Metal Transfer (CMT) clad of Nickel based super alloy, Inconel 617M on type 316L stainless steel (SS) substrate was carried out. Overlay coatings were deposited at three different current inputs (80, 90 and 100 A) to synergetic CMT welding set while the wire feed rate and pulse frequency were pre-programmed. The ability of CMT to produce very low dilution and low heat input with the minimal heat affected zone (HAZ), when compared to conventional Tungsten Inert Gas welding (TIG) process was successfully demonstrated. Detailed microstructural evaluation of the cross sections revealed porosity and crack free Inconel 617M overlay with complete fusion, making a metallurgical bonding at the interface. Transverse micro Vickers hardness evaluation from clad to substrate showed the absence of significant HAZ for overlay produced by CMT. Calculation of percentage volumetric dilution, based on elemental Energy Dispersive Spectroscopy (EDS) of Fe-K counts, revealed that the cladding deposited at 100 A current exhibited low dilution value of 6.3% at 100 μm away from the interface while clad produced by TIG show significantly higher dilution value of ~50% at 100 μm from interface. Electron Backscattered Diffraction (EBSD) studies revealed morphology similar to epitaxial growth, with same crystallographic orientation of grains across the interface for CMT overlaid coatings.

Fracture Toughness of Thermal Spray Ceramics: Measurement Techniques and Processing Dependence

Fracture toughness measurements are critical for materials design and characterization but can be difficult to perform on overlay coatings due to a range of geometric factors and substrate constraints. Thermal spray (TS) coatings bring additional complications to measurement interpretation due to their defected, anisotropic structures. Toughness of free-standing coatings has been studied in the past, and literature results indicate promise in measurement with a range of methods. One of these, single-edge, notched beam (SENB) method offers a straight forward approach for measuring fracture toughness and lends itself well for use with TS coatings. In this work, SENB method is used with deliberate modifications to specific parameters of the test specimens, namely free-standing thickness, notch depth, notch sharpness, and heat treatment state, to ascertain the impact of these modifications on the measurement results for air plasma spray Al2O3. Additionally, two methods adapted from the literature, a modified adhesion method and a tensile elongation method, are evaluated for use with three different Al2O3 coatings, including by air plasma spray, flame spray, and high velocity oxy-fuel processes. Results indicate good correlation between SENB and the modified methods for all three coating variants and give insight into the orientation-dependent toughness properties of TS coatings.

Study of the cavitation resistance of Sn-based coatings

In this study, the resistance to cavitation erosion of different Sn-based journal bearing overlay coatings were studied and compared. An ultrasonic horn set-up was used to generate cavitation over the surface of samples coated with the evaluated coatings, the volume loss during the tests was estimated and the surface morphology and structure of the overlay coatings was investigated after tests. The results show how the overlay coating structure has an effect on the cavitation resistance of the Sn-based coatings studied, as the multilayered Sn-based overlay coating exhibited a significantly enhanced resistance to cavitation erosion compared with monolayered Sn-based overlay coatings.

Modified Bond Coatings Improve Service Life of Plasma Sprayed Thermal Barrier Coatings and Protective Performance of Overlay Coatings

Abstract New thermal and cold spray technologies are being developed to extend the lifespan of thermal barrier coating (TBC) systems at elevated temperatures. This article describes the relevant attributes of coating technologies such as high velocity air fuel, high velocity air plasma spray, and high pressure cold spray for use in TBC systems.