Pt-modified Aluminide Coatings Research Articles

Cyclic oxidation-induced deleterious effects of Ru on the surface rumpling of a Pt-modified aluminide coating

Generally, surface rumpling is detrimental to high temperature protective coatings as it leads to adhesion losses and Ru can suppress rumpling behaviour in most Pt-modified aluminide (Pt-Al) coatings during the cyclic oxidation. However, in this study, we found that this suppression is related to the cyclic oxidation cycles of Pt-Al-coated superalloys: Ru suppresses rumpling behaviour with cyclic oxidation for more than 200 cycles at 1100 °C but aggravates it with cyclic oxidation from 50 to 150 cycles. The aggravation of rumpling behaviour was turned out to be associated with coating oxidation. Microstructure observation indicated that Ru participates in oxidation and deteriorates the oxidation resistance via destroying the integrity of the Al2O3 oxide film. More Al atoms are depleted owing to oxidation degradation, attributing to partial β-NiAl phases transformation towards the L10 martensitic phases, which exacerbates volume shrinkage and the driving force of rumpling behaviour. The Ru diffusion process during cyclic oxidation has also been studied. Furthermore, the relationship among martensite transformation, rumpling behaviour and cyclic oxidation in Pt-Al coated Ru-containing superalloy was ascertained to provide a guideline for integrated design of Pt-Al coating/high-generation SX superalloys and propose an investigation strategy for rumpling behaviour of the Pt-Al coating.

(Invited) Pt-Modified Aluminide Coatings for Ni-Base Single Crystal Superalloy

Single phase (Ni,Pt)Al coating has been widely employed as state-of-the-art bondcoat for protecting hot-sectional components in commercial aero engines, especially for the parts made of Ni-base single crystal (SX) superalloys. For high temperature protective coating serving on SX superalloy, two interfaces are vitally essential for acquiring reliably long service life: the interface growing thermally-grown oxide (TGO) and the interface connecting coating and substrate alloy. Normally, the excellent performance of PtAl coating derives from high Al content and the incorporation of Pt, which ensures formation of exclusively adherent oxide scale, α-Al2O3. To further improve the performance of Pt-modified aluminide coating, in present study reactive elements are introduced using co-electroplating method. Compared with singular doping, co-doping of Hf-Y demonstrated superior oxidation resistance. The oxide scale growth rate and spallation tendency of Hf-Y co-doped (Ni,Pt)Al coating were much smaller, as well as the surface rumpling extent. Besides, the grain size of Hf-Y co-doped (Ni,Pt)Al coating was finer than that of Hf- or Y- doped coatings in as-received state, indicating the capability of grain refinement. With the benefits of RE co-doping, the desirably thin/smooth TGO is thus accessible for Pt-modified aluminide coatings. The other issue is the element interdiffusion at high temperature, which would affect the mechanical property of SX superalloy. According to Fick′s law of diffusion, J Al = - D Al · δμ Al / δ x ,to reduce diffusion coefficient ( D Al ) or chemical potential ( δμ Al / δ x ) would both decrease the diffusion extent ( J Al ). Therefore, two strategies are employed: adding a diffusion barrier between coating and substrate, or designing a γʹ-PtAl coating. The performance of these modified aluminide coatings are evaluated in high temperature oxidation test. At last, doping behavior of some elements were explained by First-Principle Calculations.

High-Temperature Corrosion Behaviour of Pt-Modified Aluminide Coating Under the Effect of Nacl Plus Water Vapour and the Influence of Pre-Oxidation Treatment

High-Temperature Corrosion Behaviour of Pt-Modified Aluminide Coating Under the Effect of Nacl Plus Water Vapour and the Influence of Pre-Oxidation Treatment

Oxidation Behavior and Oxide Transformation of a Pt-Modified Aluminide Coating at Moderate High Temperature

The oxidation performance of a single-phase Pt-modified aluminide coating was assessed in oxidation test at 980 °C in comparison with the single crystal superalloy. The results suggested that the Pt-modified aluminide coating exhibited superior oxidation resistance. During oxidation, the oxide scale formed on bare alloy changed constantly followed by the constitution of the multi-layer scale structure: An outer scale mainly consisted of Cr2O3 + NiCr2O4 + TiO2 with scarce protection, and an internal scale mainly consisted of Al2O3. The thickness of the outer oxide scale increased with time, where the scale became looser and more porous. Meanwhile, the internal scale was discontinuous. Oxygen and nitrogen inwardly diffused into substrate, forming Ta2O5 and TiN particles. In contrast to the complex constitution of oxide scale, a uniform and continuous Al2O3 scale formed on Pt-modified aluminide-coated samples after oxidation at 980 °C for 1500 h, which showed no spallation and cracking. Interestingly, θ-Al2O3 and α-Al2O3 phases remained after such a long oxidation time. It is the relatively lower temperature and the presence of Pt retarded θ-α transformation. The degradation rate from β-NiAl to γ′-Ni3Al was very slow in the coating. The various development of oxide scale on the coating and substrate was individually studied.

High-temperature performance of (Ni,Pt)Al coatings on second-generation Ni-base single-crystal superalloy at 1100 °C: Effect of excess S impurities

Acidic and basic single-phase β-(Ni,Pt)Al coatings were prepared by Pt electroplating (employing acidic and alkaline Pt-plating baths, respectively) and gaseous phase aluminizing. In both isothermal and cyclic oxidation tests at 1100 °C, the acidic (Ni,Pt)Al coating showed inferior oxidation resistance compared to the basic coating. The poorer oxidation performance and higher scale spallation tendency of the acidic (Ni,Pt)Al coating was attributed to the presence of excess S, which deteriorated oxide scale adherence by promoting the formation of pores and cracks under Al2O3 scale. The void formation mechanism and detrimental role of excess S in state-of-the-art Pt-modified aluminide coatings are discussed.

Oxidation Performance and Interdiffusion Behavior of a Pt-Modified Aluminide Coating with Pre-deposition of Ni

To refrain the interdiffusion of elements while holding good oxidation resistance, a (Ni,Pt)Al/Ni composite coating was prepared by sequential treatments of electroplating Ni and Pt and successive gaseous aluminization. In comparison with normal (Ni,Pt)Al coating, high-temperature performance of the composite coating was evaluated in isothermal oxidation test at 1100 °C. Both the two coatings exhibited good resistance against high-temperature oxidation, but the interdiffusion of elements between composite coating and single-crystal (SC) superalloy substrate was greatly relieved, in which the thickness of secondary reaction zone (SRZ) and the amount of precipitated topologically close-packed phase in the SC alloy matrix were significantly decreased. Mechanisms responsible for delaying rate of coating degradation and SRZ growth/propagation are discussed.

Preparation and Isothermal Oxidation Behavior of Zr-Doped, Pt-Modified Aluminide Coating Prepared by a Hybrid Process

To take advantage of the synergistic effects of Pt and Zr, a kind of Zr-doped, Pt-modified aluminide coating has been prepared by a hybrid process, first electroplating a Pt layer and then co-depositing Zr and Al elements by an above-the-pack process. The microstructure and isothermal oxidation behavior of the coating has been studied, using a Pt-modified aluminide coating as a reference. Results showed that the Zr-doped, Pt-modified aluminide coating was primarily composed of β-(Ni,Pt)Al phase, with small amounts of PtAl2- and Zr-rich phases dispersed in it. The addition of Zr diminished voids on the coating surface since Zr could hinder the growth of β-NiAl grains. It also helped to increase the spalling resistance of the oxide scale and reduce the oxidation rate, which made the Zr-doped, Pt-modified aluminide coating possess better oxidation resistance than the reference Pt-modified aluminide coating at the temperature of 1100 °C.

Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation

Hot corrosion behaviour of nickel-based aluminide coatings with different thicknesses of initial Pt plating was evaluated in the Na2SO4/NaCl (75:25, wt./wt.) salt mixture at 900°C. Compared to plain aluminide, the Pt-modified coatings showed better hot corrosion resistance by hindering the outward diffusion of undesirable elements from the substrate alloy. Pre-oxidation was confirmed to be capable of preventing molten salt from accessing the alloy surface and reversibly dissolving the oxide scale. Oxide growth and dissolution, acting as two competitive processes, occurred simultaneously and had an important impact on the entire performance of the Pt-modified aluminide coatings in hot corrosion.

Evolution of Oxide Scale on Aluminide and Pt-Aluminide Coatings Exposed to Type I (870 °C) Hot Corrosion

Simple aluminide (NiAl) and Pt-modified aluminide (NiPtAl) coatings were produced by a low-temperature high-activity aluminizing (LTHA) technique on a Rene-80 substrate. A Pt layer (6-8 lm) was deposited on the substrate before the LTHA process to prepare the NiPtAl coating. Hot corrosion testing was con- ducted by a furnace method using Na2SO4-30 wt% NaCl molten salt at 870 C (i.e., type I hot-corrosion conditions) for 700 h. Corrosion kinetics were determined by measuring the weight change of the specimens at regular intervals of 20 h. Mor- phology of corroded surfaces and scale chemistry were determined as a function of exposure time. Results indicated that the addition of Pt to aluminide coatings sig- nificantly enhances the Type I hot corrosion by promoting the slow growth of a highly adherent oxide on the sample surface. In contrast to the NiAl coating, the NiPtAl coating showed no significant sample weight increase or subsequent catastrophic failure up to 700 h of the hot corrosion exposure. The improved life- time of NiPtAl coating was attributed in large part to the excellent oxide adherence and the role of Pt in promoting the formation of slow-growing a-Al2O3. In addition, Pt limits diffusion of substrate refractory elements from substrate into the oxide/ metal interface or the external alumina scale. As a result, the Al2O3 scale on the NiPtAl coating remained adherent and virtually no spallation was observed even after prolonged Type I hot corrosion.

The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl2) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.

Microstructure degradation of EB-PVD TBCs on Pd–Pt-modified aluminide coatings under cyclic oxidation conditions

The study concerns Pd/Pt-modified aluminide bond coatings for EB-PVD TBCs on N5 superalloy in terms of microstructural evolution during cyclic oxidation tests.The coating was deposited by Pd+Pt electroplating, followed by high activity vapor phase aluminizing at 1050°C and pre-oxidation heat treatment in order to form an α-alumina TGO. The 7wt.% yttria stabilized zirconia TBC was deposited using electron beam physical vapor deposition. Cyclic oxidation tests of the coating were performed at 1100°C in 1hcycles in laboratory air. The microstructure of the coatings in the as-deposited state as well as after cyclic oxidation test was studied using SEM. The evolution and phenomena occurring at the interface between the bond coating, thermally grown oxide and TBC during high temperature exposure were studied in detail using high resolution FEG-S/TEM. The samples for S/TEM analysis were prepared using FIB (Focused Ion Beam). The study is mostly focused on the growth of TGO during high temperature exposure and the overall performance of the Pd/Pt-modified aluminide coatings as a cost effective alternative to Pt-modified aluminide coatings commonly used as EB-PVD bond coats.

Zirconia Modified Aluminide Coatings Deposited by VPA and CVD Methods

The article presents the experimental trial to connect aluminizing methods: VPA and CVD. Inconel 713 alloy was used as the base material. The purpose of the process was to obtain a layer modified with more Zr than the coatings deposited by CVD. The conducted tests revealed intercrystalline corrosion in the area of β-NiAl phase granules, which indicates that precise selection of processing conditions is the key factor for Zr-modified aluminizing. The resultant microstructural faults of the layer, however, didn’t impair the properties PS-PVD-deposited ceramic layer. The zirconium content in the outer zone equalled 2-3 at.%. in the second variety of the process. In future the described coating may form an alternative for Pt-modified aluminide coatings.

Effect of Polishing Treatment on Rumpling of Oxide Scales on NiPtAl Coatings with Different Pt-Content

Pt-modified nickel aluminide coatings have been more widely used for protection of jet-engine components against high-temperature oxidation. The coating rumpling of two Pt-content NiPtAl coatings was studied in this paper during high temperature exposure. The results indicated that the NiPtAl coating grains size made a great contribution to the oxide surface morphologies, especially rumpling. Smaller grain size within high-Pt coating indicated a denser rumpling compared to low-Pt coating due to PtAl2 formation in the earlier coating. The failed local alumina at the ridges was also found on the low-Pt coating after cyclic oxidatioin. It was found that polished treatment resulted a comparatively flat and homogeneous oxide layer compared to as-received coatings. The temperature cycling could promote the aluminide coating rumpling, however, the polished treatment could not completely eliminate the roughening.

Microstructure degradation of simple, Pt- and Pt + Pd-modified aluminide coatings on CMSX-4 superalloy under cyclic oxidation conditions

The paper presents results of simple, Pt/Pd- and Pt-modified aluminide coatings' cyclic oxidation-induced degradation analysis. The coatings were deposited by Pt and Pt+Pd electroplating, followed by vapor phase aluminizing at 1050°C. Cyclic oxidation tests were performed at 1100°C in 23hcycles. Microstructural and phase analysis conducted using XRD, SEM, EDS and EBSD methods revealed that the oxide layers that formed on the coatings were composed of three distinctive types of oxides growing according to a specific pattern which is described. The oxide layer that formed on the simple aluminide coating exhibited low adhesion in comparison to Pt- and Pt,Pd-modified aluminide coatings which managed to maintain an adherent oxide layer that contained higher amount of desirable α-alumina. The Al-depleted βNiAl grains remained much larger in the modified aluminide coatings, even after failure. What is more, stripes characteristic for martensitic transformation were discovered in the β phase grains in all coatings.

Microstructure of Thermal Barrier Coatings (TBC's) Obtained by Using Plasma Spraying and VPA Methods

Thermal Barrier Coatings are the main type of coatings used for protecting turbine blade surfaces and the surface of modern jet engines combustion chamber parts. Depending on the type of engine element, the coatings are produced as plasma-sprayed MeCrAlY bond-coats with a ceramic outer layer or as Pt-modified aluminide coatings with a ceramic EB-PVD-deposited layer. Currently, research is being conducted on the deposition of a new type of coatings consisting of bond-coats with mulitlayer structure. In the article, the results of the study on the obtainment of TBC's with multilayer structure are presented. To obtain the metallic bond-coat, the process of atmospheric plasma spraying and the out of pack aluminizing (VPA) method were combined. The coatings were deposited on the surface of Rene 80 nickel superalloy. The first layer of the coating was a plasma-sprayed MCrAlY bond-coat, on which a diffusion aluminide layer was deposited with out of pack method. On the bond-coat, a standard ceramic zirconium oxide (ZrO2*20Y2O3) layer was deposited. The microstructure analysis, was conducted, using light and SEM microscopy. The phase and chemical composition analyses were done using EDS and XRD methods.

Synthesis of Hf-modified aluminide coatings on Co-base superalloys

Hf-modified and Hf–Pt-modified aluminide coatings on Haynes 188 Co-base alloy coupons were developed and furthermore, Hf-modified aluminide coatings on MAR-M-509 Co-base aircraft turbine engine vane segments were produced for potential industrial applications. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and Glow-Discharge Mass Spectrometry (GDMS). Experimental results showed that Hf was successfully incorporated and localized Hf-rich precipitated phase particles were observed on the coating surface and within the coating additive layer. The Hf-rich particles and the effect of heat treatment on the particles are also discussed.

Synthesis of Hf-modified aluminide coatings on Ni-base superalloys

With small additions of Reactive Elements (RE), such as hafnium (Hf), to aluminide coatings applied on aircraft turbine engine components, the adherence of the protective oxide scale to the coatings and the oxidation resistance of the coatings at high temperatures can be significantly improved. At SIFCO, Hf-modified aluminide coatings, Hf–Pt-modified aluminide coatings on Ni-base superalloys and the industrial-scale production of Hf-modified aluminide coatings on different aircraft turbine engine components were successfully developed. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using a Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and Glow-Discharge Mass Spectrometry (GDMS). Experimental results showed that Hf was successfully incorporated and precipitated Hf-rich particles were observed on the coating surface, along grain boundaries within the additive layer, and at the interface between the additive layer and the interdiffusion layer. Finally, Hf-rich particles as well as Hf solubility are discussed.

Effect of increased water vapor levels on TBC lifetime with Pt-containing bond coatings

To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycle tests were performed at 1150°C in air with 10vol.% water vapor (similar to natural gas combustion) and 90vol.%. Either Pt diffusion or Pt-modified aluminide bond coatings were applied to specimens from the same batch of a commercial second-generation single-crystal superalloy and commercial vapor-deposited yttria-stabilized zirconia (YSZ) top coats were applied. Three coatings of each type were furnace cycled to failure to compare the average lifetimes obtained in dry O2, using the same superalloy batch and coating types. Average lifetimes with Pt diffusion coatings were unaffected by the addition of water vapor. In contrast, the average lifetime of Pt-modified aluminide coatings was reduced by more than 50% with 10% water vapor but only slightly reduced by 90% water vapor. Based on roughness measurements from similar specimens without a YSZ coating, the addition of 10% water vapor increased the rate of coating roughening more than 90% water vapor. Qualitatively, the amount of β-phase depletion in the coatings exposed in 10% water vapor did not appear to be accelerated.

The Influence of Deposition Process on Structure of Platinum-Modifed Aluminide Coatings O Ni-Base Superalloy

The modern jet engines used in commercial and military aircrafts are characterized by operating temperature in turbine section above 1000oC. The Ni-base superalloy turbine blades and vanes working in high temperature in very aggressive environment require using of protective coatings. The aluminide coatings are widely used to protect this engine parts. The pack cementation, out of pack and chemical vapour deposition (CVD) technologies are usually used to produce this type of coating. The aluminide coatings can be modified by platinum or other elements. The Pt-modified aluminide coatings are characterized by better oxidation and corrosion resistance in comparison with conventional aluminide coatings and can be used as a bond coat for Thermal Barrier Coatings deposited by EB-PVD technology. In present study the influence of deposition technology and their’s parameters on structure and chemical composition of Pt-aluminide coatings are presented. The base material for coatings was a Inconel 738 Ni-base superalloy. The first step of coatings production were Pt electroplating with different thickness of platinum layer. The second step of coating production was aluminising process. The aluminide coatings were produced by pack cementation and out of pack technologies. Additional the influence of heat treatment of base alloy with coatings was investigated. The structure of all deposited coatings was observed by scanning electron microscopy and the chemical and phase composition of coatings were investigated by EDS and XRD methods. The observed coatings were characterized by two types of structure: first based on NiAlPt phase obtained on thin Pt layer and the second with additional presence of PtAl2 phase on the thick Pt layer.

Cyclic oxidation behavior of Pt-modified aluminide coating treated with ultrasonic nanocrystal surface modification (UNSM) on Ni-based superalloy

The effect of ultrasonic nanocrystal surface modification (UNSM) on Pt-modified aluminide coatings on Ni-based superalloy was investigated. UNSM was applied to make the grain size finer and to release compressive stress on the Al oxide film by reducing surface roughness. A Pt layer, with a thickness of 10–11 μm, was coated on a superalloy and transformed to a Ni-Pt alloy layer by annealing, at 1050 °C for 3 h. Before pack aluminizing, the surface of the Ni-Pt alloy layer was shocked by UNSM. The grain size of this UNSM-shocked Ni-Pt alloy layer was finer than the grain size on the untreated specimen. During pack aluminizing, the treated Pt-modified aluminide coating had more Al uptake and greater thickness than the untreated Pt-modified aluminide coating, because many grain boundaries and volume increase were incurred by UNSM. Furthermore, the treated coating displayed a smoother surface before, and after, pack aluminizing. The treated coating showed superior cyclic oxidation resistance. The decrease of surface roughness in the treated coatings diminished compressive stress, which caused spallation of the thermally grown oxide (TGO) and faster depletion of Al. Faster Al depletion in the untreated coating led to a phase transformation, from β-NiAl to γ′-Ni 3Al, and then to changes in volume and solubility of the alloying element, Cr. The increase of the surface and interface roughness was a result of the change in volume as well as the increased stress and strain between the TGO and the coating. Additionally, the increasing solubility of the alloying element, Cr, in the coating, resulted in the formation of a large amount of Cr- and Ni-related oxides, which are unstable in the TGO during cyclic oxidation. Spallation of the TGO caused an accelerated rate of Al depletion in the untreated coating, and a faster degradation rate than in the treated coatings.