Sprayed MCrAlY Research Articles

Cold spray for production of in-situ nanocrystalline MCrAlY coatings – Part II: Isothermal oxidation performance

Cold spray (CS) MCrAlY coatings have been widely explored as potential bond coats for thermal barrier coatings used on the hot-section components of modern gas turbines. In this study, NiCoCrAlTaY coatings were applied on CMSX-4 single-crystal Ni-base superalloy substrates using the CS technique. Nitrogen was used as process gas with low spray parameters to allow the production of in-situ nanocrystalline NiCoCrAlTaY coatings while using conventional powders of large grain size, as demonstrated in Part I of the study using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). In this part of the study (Part II), the coated samples were examined under isothermal oxidation conditions at 1100 °C for 1 h to 500 h. The as-deposited NiCoCrAlTaY coatings, characterized by dense microstructure, low surface roughness, nanoscale grains, and a large number of crystallographic defects such as grain boundaries and dislocations, promoted the formation of a protective α-Al2O3 scale during short-term oxidation. Fast diffusion via these crystallographic defects facilitated Al diffusion in the coatings. This provides sufficient Al replenishment to the coating subsurface region, sustaining the growth of the α-Al2O3 scale during prolonged oxidation. The relatively low scale growth kinetics found in the study demonstrated enhanced oxidation performance of the CS nanocrystalline NiCoCrAlTaY coatings.

Tensile Behavior of Air Plasma Spray MCrAlY Coatings: Role of High Temperature Agings and Process Defects

$$\beta $$-$$\gamma $$ MCrAlY coatings generally exhibit a brittle mechanical behavior below 600 °C. When exposed at elevated temperatures, the microstructure of such coatings evolves, leading to an increasing content of $$\gamma $$ phase, i.e., the ductile phase, and a decreasing content of $$\beta $$ phase, i.e., the brittle phase. Therefore, the evolution of the mechanical properties of such environment-protective materials is worth of investigation. In the present study, a $$\beta $$-$$\gamma $$ NiCoCrAlY coating was processed by air plasma spray (APS) technology. $$150\,\mu \text {m}$$-thin freestanding specimens were prepared then aged in an oxidative atmosphere at high temperatures (950 ºC up to 1150 ºC) for different durations to simulate in-service degradation of the coatings. Microtensile testings were conducted at room temperature for all the aging variants and the mechanical properties of the aged specimens were found to evolve as follows: (i) an increase in both Young’s modulus and tensile strength and a loss in ductility for aging temperatures below 1050 °C, (ii) a decrease in Young’s modulus and a gain in ductility for aging temperatures above 1050 °C, and (iii) a significant scatter in mechanical properties for high temperature agings. The low ductility observed for high temperature agings was related to intruded oxides developing during the aging treatment, heterogeneously distributed in the volume of the coating. The gain in ductility was mainly attributed to the $$\beta $$-phase decrease, the loss in interconnection between $$\beta $$ phases compared to the as-received microstructure and a topological inversion of the $$\beta $$-$$\gamma $$ microstructure.

Influence of Different Annealing Atmospheres on the Mechanical Properties of Freestanding MCrAlY Bond Coats Investigated by Micro-Tensile Creep Tests

The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar–O2, Ar–H2O, and Ar–H2–H2O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900–950 °C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the β-NiAl phase. Additionally, its creep properties at 900 °C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar–H2O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar–O2 and Ar–H2–H2O atmospheres.

Hydrophobic/icephobic coatings based on thermal sprayed metallic layers with subsequent surface functionalization

Hydrophobic/icephobic coatings have been fabricated using a combination of thermal sprayed metallic MCrAlY (M = Ni, Co) coatings with a subsequent deposition process using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS). The MCrAlY coatings provide the desirable surface roughness feature for hydrophobicity, and water contact angle of 135° was directly obtained after aged in the atmosphere for 1 week. However, it was found that the hydrophobicity of MCrAlY was not stable under water impinging due to unstable hydrocarbon absorption. Better hydrophobicity with water contact angle of 154° and improved durability have been achieved by further modification using POTS vapour on the rough MCrAlY coatings. X-ray photoelectron spectroscopy results revealed that replacement of absorption of hydrocarbon by functional C-F groups played important role in the improvement of hydrophobicity and durability. The ice adhesion test confirmed that lower ice adhesion strength of MCrAlY based coatings have been obtained compared with the threshold for icephobicity which is desirable to be applied as icephobic coatings for aircraft. The electro-thermal heating de-icing test showed an energy saving of 28.6% for de-icing with the two-step MCrAlY based coatings. The combination of strong metallic MCrAlY rough layers and the subsequent functionalization enables a new approach for the fabrication of durable hydrophobic/icephobic coatings.

Thermal Cycling Behavior of Thermal Barrier Coatings with MCrAlY Bond Coat Irradiated by High-Current Pulsed Electron Beam.

Microstructural modifications of a thermally sprayed MCrAlY bond coat subjected to high-current pulsed electron beam (HCPEB) and their relationships with thermal cycling behavior of thermal barrier coatings (TBCs) were investigated. Microstructural observations revealed that the rough surface of air plasma spraying (APS) samples was significantly remelted and replaced by many interconnected bulged nodules after HCPEB irradiation. Meanwhile, the parallel columnar grains with growth direction perpendicular to the coating surface were observed inside these bulged nodules. Substantial Y-rich Al2O3 bubbles and varieties of nanocrystallines were distributed evenly on the top of the modified layer. A physical model was proposed to describe the evaporation-condensation mechanism taking place at the irradiated surface for generating such surface morphologies. The results of thermal cycling test showed that HCPEB-TBCs presented higher thermal cycling resistance, the spalling area of which after 200 cycles accounted for only 1% of its total area, while it was about 34% for APS-TBCs. The resulting failure mode, i.e., in particular, a mixed delamination crack path, was shown and discussed. The irradiated effects including compact remelted surface, abundant nanoparticles, refined columnar grains, Y-rich alumina bubbles, and deformation structures contributed to the formation of a stable, continuous, slow-growing, and uniform thermally grown oxide with strong adherent ability. It appeared to be responsible for releasing stress and changing the cracking paths, and ultimately greatly improving the thermal cycling behavior of HCPEB-TBCs.

Impact of processing conditions and feedstock characteristics on thermally sprayed MCrAlY bondcoat properties

One of the options to manufacture MCrAlY bondcoats (M=Co, Ni) for thermal barrier coating systems is High Velocity Oxy-Fuel spraying (HVOF). In this work, particle diagnostics were applied to investigate the impact of processing conditions and feedstock characteristics on the relevant bondcoat properties. The results showed that compromises must be made on the oxygen/fuel ratio, spray distance, and particle size distribution to strike a balance between low oxidation and dense microstructures.These limitations initiated the development of the High Velocity Atmospheric Plasma Spray process (HV-APS) as a further alternative process. In this work, HV-APS process parameters were developed for a three cathode torch in combination with a 5mm diameter high speed nozzle. A one-dimensional calculation of the expansion through this nozzle to atmospheric pressure yielded supersonic conditions with a Mach number of 1.84. The calculated plasma temperatures at the nozzle exit and in the expanded jet are 8400K and slightly above 5200K, respectively, which is low compared to conventional APS processes.A very fine powder with a median particle size of 18μm was identified to be most suitable. Although the spray conditions were relatively cold, reasonable deposition efficiencies up to 61% and rather dense coatings were achieved using this feedstock. The as-sprayed porosity was ≈2% which was reduced by the subsequent vacuum heat treatment to <1%. The oxygen content determined by chemical analysis for a sample sprayed at a spray distance of 100mm was 0.41±0.04wt%.Moreover, reference samples were manufactured by Low Pressure Plasma Spraying (LPPS). The oxidation behavior was compared in isothermal and cyclic oxidation tests. The oxidation rates of the HV-APS coatings were found to be significantly lower than those of LPPS coatings. The thermally grown oxide scale showed less yttrium incorporation and better adherence in case of HV-APS. The latter is suggested to be related to a unique new distribution of Y-rich nano-sized oxide precipitates. The cyclic oxidation test confirmed the better oxidation resistance of the HV-APS coatings.

Potential of New-Generation Electron Beam Technology in Interface Modification of Cold and HVOF Sprayed MCrAlY Bond Coats

Electron beam (EB) technology treatment was carried out on CoNiCrAlY bond coats deposited on Inconel substrates via cold spray and HVOF techniques in dissimilar thicknesses. Such treatment was carried out with regard to the final materials microstructure, composition, surface roughness, and the quality of the coating-substrate interface. Following a multiple-step optimization of the processing parameters (such as beam pattern configuration, accelerating voltage, longitudinal speed, and multiple beam incidence), two final EB modifications were carried out on both coating types. It was found that the optimized EB treatment could lead to a significant alteration of the interface from a distinctive divide into smooth chemical and structural transition between the materials, significant decrease in surface roughness and porosity, and changes in mechanical properties (increase in Young’s modulus and decrease in hardness of the coating).

Healing of the Interface Between Splashed Particles and Underlying Bulk Coating and Its Influence on Isothermal Oxidation Behavior of LPPS MCrAlY Bond Coat

The thermally grown oxide formed on the bond coat surface plays an important role in determining the lifetime of thermal barrier coatings (TBCs). The splashed particles on the thermally sprayed MCrAlY bond coat surface are weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides and contributing to the TBC failure. In this study, the healing behavior of the weakly bonded interface between splashed particles and underlying MCrAlY bulk coating deposited by low pressure plasma spraying was examined, and the influence of interface healing on the isothermal oxidation behavior of the bond coat was discussed. Results show that the granular particles resulting from splashing of molten droplets were exposed on smooth splats which make up the surface of bulk coating. After the pre-diffusion treatment in vacuum, the small granular splashed particles are immersed into the bulk coating resulting from the element diffusion on the interface between splashed particles and underlying bulk coating. After the vacuum heat treatment, the formation of mixed oxides was effectively restrained due to the healing of the splashed particle/underlying bulk coating interface.

Effect of surface conditions on internal oxidation and nitridation of HVOF MCrAlY coatings

This study deals with the isothermal oxidation behaviour of high velocity oxy-fuel sprayed MCrAlY coatings. Both free-standing coatings and coatings attached to IN738 and CMSX4 substrates underwent isothermal oxidation at 1100°C for up to 100 h. The effect of surface conditions (as sprayed and polished) has been investigated. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterise the details of oxidation behaviour. The results have shown that both surface conditions and substrate type have a pronounced effect on oxidation behaviour. Extensive internal oxidation and nitridation was observed for polished coatings on Inconel 738 and can be attributed to the combined effects of titanium diffusion from the substrate and enhanced surface diffusion due to polishing.

Changes in Microstructure of Air Plasma Sprayed M-CrAlY Coatings After Short Thermal Exposure in Argon Atmosphere

the microstructure of resulting coatings. The oxide scale was formed by internal oxidation in a coating region primarily at higher temperatures. In this case, aluminium was depleted from the NiAl phase within the coating region and the Ni, Co, Cr solid solutions and the aluminium oxide started to form rapidly. No interaction was observed after the short thermal exposure below the substrate surface. The microstructure of coatings was recorded by scanning electron microscope. Coating thickness, amount of voids and oxide scale were measured by means of image analysis. The concentration of phases was estimated by energy dispersive microanalysis. PACS: 81.15.Rs

Nano-Composites NiCrAl (MCrAl) Ternary Alloy Powder Synthesized by Mechanical Alloying

The purpose of this study was to explore an innovative method for producing nano-crystalline Ni, Cr and Al powders using high energy ball milling and brief secondary processes. Nano-crystalline powders were produced by using a planetary ball mill. The contaminations of the milled metal/alloy powders were carefully avoided by choosing appropriate lining of the vial and the alloy balls. Two approaches were investigated, namely attrition milling of pure metal powders (e.g. Al, Cr and Ni) and mechanical alloying of the elemental powders to form a ternary alloy of NiCrAl. Currently MCrAlY (M=Ni, Co or Fe) powders, up to 40µm in diameter are used in the production of thermal barrier coatings (TBC) on industrial gas turbine components. The powder samples synthesized were characterized by scanning electron microscope (SEM) and transmission microscope (TEM), the crystallite size was found to be less than 20nm; the powder particle size was in the range of 250-300nm; no undesirable particle contamination was detected. It is anticipated that these very fine particles will increase high temperature properties of the thermally sprayed MCrAlY overlay coatings.

Synthesis of advanced nano-composites &ndash; MCrAlY powders for high temperature applications

The developmental work contributing to the synthesis of nano-Ni powders by chemical reduction is presented; nano-crystalline Ni, Cr and Al have also been produced by attrition milling in a liquid medium using a Planetary Ball Mill. Binary and ternary alloys of these elements i.e., NiAl, AlCr, NiCr NiCrAl were then synthesised by mechanical alloying. These NiCrAl powders will be used to synthesise MCrAlY powders after doping with yttrium. Commercially used MCrAlY powders investigated had been found to be larger than 100 μm in diameter. The focus of this study is in the application of nano-composites in industrial turbines for aero-engine/power generation industry. Finer particles are expected to increase high temperature properties of the thermally sprayed MCrAlY overlay coatings. These particles were characterised using SEM; XRD measurements show the crystallite size to be less than 20 nm; the powder particle size was in the range of 200-270 nm; no undesirable particle contamination was detected.

Effect of HVOF Sprayed MCrAlY Coating on Thermomechanical and Isothermal Fatigue Life of Superalloy M963

A cast nickel based superalloy M963 was coated by high-velocity oxy-fuel (HVOF) spraying process. The effect of HVOF MCrAlY coating on thermo-mechanical fatigue (TMF) and isothermal fatigue (IF) in M963 was studied to understand fatigue life and failure mechanisms in coated and uncoated M963 alloy. Two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed in temperature range of 450~900°C, and IF tests were conducted at 900°C. It was found that the coating had a detrimental effect on fatigue life under OP TMF, while a beneficial effect of the coating existed under IP TMF and IF. Crack initiation time in the coated specimen was shorter than that in the uncoated specimen and the former’s crack density was higher than the latter’s one under OP TMF. The relationship of deformation and fracture response with fatigue life was discussed based on microscopic analysis.

Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD: A Review

The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.

Pulsed electron beam treatment of MCrAlY bondcoats for EB-PVD TBC systems part 1 of 2: Coating production

This is the first part of a two part paper discussing the influence of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Part one describes the production of MCrAlY bondcoats via HVOF and VPS thermal spray methods. Some of these coatings were then modified by using a pulsed electron beam treatment before all the samples were coated with an EB-PVD 7–8 wt.% yttria partially stabilized zirconia topcoat. Part two describes the results obtained under cyclic oxidation conditions at 1150 °C with 1 h at temperature and 15 min cooling. The MCrAlY bondcoats manufactured with the two thermal spray methods show differences in microstructure, pore evaluation and surface roughness, these factors will be discussed along with details of the PEB treatment method in this paper. After PEB treatment the typical two phase (γ, β)structure of MCrAlY was no longer visible in the microsection near the surface of the bondcoat. The surface roughness of the HVOF sprayed sample was reduced by the treatment to an RZ below 4 μm and the porosity in the modified layer was completely removed. Some inclusions containing Y, Al and oxygen were found in and at the surface of the PEB treated layer. Their influence on the cyclic oxidation performance is further discussed in part 2 of this study.

Pulsed electron beam treatment of MCrAlY bondcoats for EB PVD TBC systems part 2 of 2: Cyclic oxidation of the coatings

This paper discusses the effect of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Standard MCrAlY bondcoats were produced via HVOF, VPS and LPPS thermal spray methods. Some of the HVOF and VPS coatings were then given a PEB treatment before all the samples were coated with an EB PVD 7-8 wt.% yttria partially stabilized zirconia topcoat. The samples were all tested under cyclic oxidation conditions at 1150 °C with 1 h at temperature and 15 min cooling, samples were removed after 20% coating spallation and prepared for cross sectional analyses. Cyclic testing revealed that although the PEB treatment had no measurable effect on the VPS sprayed samples, the HVOF coatings showed a significant increase in the cyclic life after the PEB treatment. The effect of the PEB treatment on the various samples is discussed as well as its effect on TGO growth morphology. PEB treatment of the HVOF bondcoat was found to reduce the rate of alumina growth and to suppress the formation of oxide pegs resulting in a smoother bondcoat interface.

Measurement of Young's Modulus and Poisson's Ratio of Thermal Barrier Coatings

In gas turbines, thermal barrier coatings (TBCs) applied by air plasma spraying are widely used to lower the temperature of hot components. To analyze the characteristics of TBCs such as residual stress, bond strength, fracture toughness, and crack propagation ratio, the Young's modulus and Poisson's ratio are important parameters. For TBC is a brittle and thin film, it is desirable to evaluate those properties while the coatings are bonded to a substrate. An atmospheric plasma spray MCrAlY bond coat and Yttria stabilized zirconia (YSZ) top coat are deposited onto a nickel-base super alloy GH150 substrate. The Young's modulus and Poisson's ratio are measured by cantilever beam bending with NDI. The method will be developed to test the Young's modulus and Poisson ratio of other multilayer systems.

Improvement of the oxidation behaviour of electron beam remelted MCrAlY coatings

The efficiency of gas turbine engines can be significantly improved by increasing their operating temperatures. The engine components, especially the hot-end components, should maintain their mechanical integrity at high-temperatures. The complex environment (thermal, chemical and mechanical) of the gas turbine engine makes it extremely difficult for one material to meet all the different requirements imposed on the various engine components. Coatings are thus required to provide increased corrosion protection at high temperatures in order to assure durability and field performance of the base alloys. Problems include: substrate compatibility, interdiffusion, adhesion, porosity of the coating, possibilities of repairing or recoating, effect of thermal cycling, wear and corrosion resistance and costs. Ni-base turbine blades are generally corrosion protected by an MCrAlY overlay and a ceramic thermal barrier top-layer, both thermally sprayed. A new technique used for these coatings is high-velocity oxygen-fuel (HVOF) spraying. A subsequent remelting of the HVOF-coatings by electron beam (EB) irradiation has proven to minimize the oxide content and the porosity of the thermally sprayed MCrAlY coatings. The surface was remelted up to a depth of approximately 30 μm. The high cooling rates produce modifications in the morphology and the phase composition of the MCrAlY coating. In this paper the influences of EB-remelting process on the characteristics of the thermally grown oxide (TGO) scale formed during oxidation in air at 950 °C in the MCrAlY coating will be discussed.

Aluminizing Behaviors of Vacuum Plasma Sprayed MCrAlY Coatings

AbstractThe objective of this study is aluminide overlay coatings of MCrAlY sprayed by a vacuum plasma spraying (VPS) process for the protection against high-temperature corrosion and oxidation of gas turbine components. Diffusion coating processes have been applied for many years to improve similarly the environmental resistance by enriching the surface of nickel-based superalloys with chromium, aluminum, or silicon element. Recently, aluminizing of MCrAlY coatings is used for improving further the high-temperature oxidation resistance. However, the aluminizing properties of plasma-sprayed MCrAlY coatings, which have an important effect on the coating performance, have not been clarified. In this study, five kinds of plasma-sprayed MCrAlY (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, and NiCoCrAlY) coating were selected for pack-aluminizing tests. The as sprayed and the heat-treated (1393 K, 2 h, argon cooled and 1116 K, 24 h, argon cooled) MCrAlY specimens were Al-Cr-Al2O3-NH4Cl pack-aluminized at 1173, 1223, and 1273 K for 5, 10, and 20 h, respectively. The experimental results showed that the aluminizing process formed the aluminum rich layers of NiAl or CoAl phase. It also indicated that the thickness of the aluminum rich layer showed a parabolic time-dependence in all MCrAlY coatings. The order of reaction diffusion rate was NiCoCrAlY=NiCrAlY&amp;gt;CoNiCrAlY&amp;gt;CoNiCrAlY+Ta&amp;gt;CoCrAlY. There was a tendency that the reaction diffusion rate by aluminizing increased with increasing nickel content in the MCrAlY coatings and the reaction diffusion rate of as sprayed MCrAlY coatings is faster than that of the heat-treated MCrAlY coatings.

The effect of heat treatment on the stiffness of zirconia top coats in plasma-sprayed TBCs

Thermal barrier coating (TBC) specimens have been prepared by plasma spraying. A vacuum plasma spray (VPS) MCrAlY bond coat and atmospheric plasma spray (APS) zirconia top coat were deposited onto a nickel superalloy substrate. The stiffness of detached top coats was measured by cantilever bending and also by nanoindentation procedures. Measurements were made on specimens in the as-sprayed state and after various heat treatments. Significant changes were detected in the Young's modulus of the top coat as a result of the heat treatments. The rate of sintering was found to be a function not only of the temperature but also of whether or not the coating was attached to the substrate during the heat treatment. This influences the stress state in the coating. A previously-developed numerical model has been modified in order to incorporate the effects of top-coat stiffening on the development of stress within the TBC system during exposure to high temperature. It is expected that sintering of the top coat will lead to increases in the driving force for debonding at the interface between the top coat and the bond coat. This effect may be at least partly responsible for the spallation of top coats which commonly afflicts TBCs after periods under service conditions.