Platinum-modified Aluminide Coatings Research Articles

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.

Thermal cycling behavior of EB-PVD TBCs on CVD platinum modified aluminide coatings

Thermal barrier coating systems (TBCs) including of chemical vapor deposited (Ni, Pt)Al bond coat with grit blasting process and electron beam physical vapor deposited Y2O3-stabilized-ZrO2 (YSZ) ceramic coating were investigated. The phase structures, surface and cross-sectional morphologies, cyclic oxidation behaviors and residual stresses of the TBCs were studied in detail. It was found that the fracture path traverses through the ceramic coating to TGO interface, as well as at the TGO to bond coat interface is obviously detected. The change in fracture plane occurs at grain boundaries. The ridge top spallation leads to separate of sufficient size to result in unstable fracture driven by the strain energy stored in the TGO. The bond coat can undergo a volume increase upon oxidation, so that a cavity, enlarged strictly by oxidation would be full to overflowing with TGO layer. The spalled location of the TBCs probably occurs either at the interface of TGO layer and bond coat or inside of TGO layer. The lower strain energy release rate within TGO layer during thermal cycling is beneficial to prolong of TBCs life. The lower is the compressive stress within TGO layer, the longer is the lifetime of TBCs.

Precious-metal-modified nickel-based superalloys: Motivation and potential industry applications

Nickel-based superalloys are extensively used in the hot sections of gas turbine engines and other propulsive power machines because they possess an excellent combination of high-temperature strength and resistance to oxidation and hot corrosion degradation. The γ-γ′ microstructure inherent in nickel-based superalloys is designed with respect to composition and morphology so as to achieve a balance of strength versus environmental resistance. Often, aluminide and platinum-modified aluminide coatings are applied to the component surface to further improve the resistance to environmental degradation by supporting the formation of a protective aluminum oxide scale. The potential exists to utilize alloying concepts from novel platinum and hafnium-modified γ-γ′ diffusion coatings so as to create in-situ a new class of superalloy that combines enhanced environmental resistance while maintaining sufficient strength at high temperatures. This paper describes how precious-metal-modified superalloys can offer advantages for structural applications in gas turbine engines. Several examples that illustrate component performance benefits are also presented.

On oxidation behaviour of platinum aluminide coated nickel based superalloy CMSX-4

The effects of platinum modification on the isothermal oxidation performance of a chemical vapour deposition aluminide coating are examined. Platinum was electrodeposited to varying thicknesses (to a maximum of 16 μm) onto the CMSX-4 single crystal superalloy, before applying a high temperature/low activity aluminisation process. The oxidation performance at 1100°C improved monotonically with increasing Pt thickness; this was confirmed by thermal gravimetric analysis. The degree of rumpling of the alumina scale was also decreased with increasing platinum. The superior oxidation resistance of platinum modified aluminide coatings is a consequence of the β-NiAl formed during aluminisation containing an enhanced Al/Ni ratio, reduced levels of Co and Ti and decreased concentrations of the refractory elements Re, W and Ta. This seems to be due at least in part to the microstructural effects caused by the prior diffusion of platinum into the surface, which causes the generation of a γ-(Ni,Pt)/γ′-(Ni,Pt)3Al microstructure; the upper portion of the Pt diffused coating comprises predominantly the γ′-(Ni,Pt)3Al phase which is rich in Pt and Al and depleted in Co and the refractory elements W and Re. The ramifications of our findings for the design of bond coats required for thermal barrier coating systems are discussed.

Factors affecting the microstructure of platinum-modified aluminide coatings during a vapor phase aluminizing process

Platinum (Pt)-modified aluminide coatings were developed by electroplating a thin layer of Pt followed by an industrial vapor phase aluminizing process. The goal of this work was to systematically investigate the effect of critical coating process parameters (such as the electroplated Pt thicknesses, Al contents in Cr–Al nuggets, diffusion heat treatments) and substrates on the final Pt-modified aluminide coatings. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Optical Microscope, Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that the Al and/or Pt increase shall favor the formation of ξ-PtAl 2 phase; transformation of ξ-PtAl 2 into β-(Ni,Pt)Al phase can be obtained via a heat treatment process; Cr, Co elements in the studied Ni-base superalloy substrates did not show significant influence on coating outer layer microstructure; while substrate elements affect the microstructure of the coating interdiffusion layer.

Synthesis of simple and platinum-modified aluminide coatings on cobalt (Co)-base superalloys via a vapor phase aluminizing process

An investigation was conducted to synthesize simple β-CoAl or β-(Co,Ni)Al coatings and platinum (Pt)-modified aluminide coatings on cobalt-base superalloys via an industrial vapor phase aluminizing process. Surface morphology and cross-section microstructure of the developed coatings were studied and compared by using optical microscope, scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that Al increase in Al–Cr nuggets increases the coating thickness; substrate elements influence the formation of coatings; the increase of the electrodeposited Pt thickness increases the amount of ξ-PtAl 2 phase after aluminizing process; and the transformation of ξ-PtAl 2 phase into β-(Co,Ni,Pt)Al phase can be obtained via a heat treatment process.

Rumpling of Platinum Modified Aluminide Coatings during Thermomechanical Testing

The evolution in surface morphology of platinum modified nickel aluminide (Ni,Pt)Al oxidation coatings during thermo-mechanical testing has been evaluated. One type of test consisted of cyclic oxidation between an upper temperature of 1150°C and a lower temperature varying from room temperature to 1050°C. The other type of test was cycling between 1000°C/1150°C under an applied compressive stress. Profilometry using optical interferometry was used to quantify the surface “rumpling”. First and second-order statistical parameters including RMS roughness and the auto-correlation function were calculated from the profilometry measurements. The results indicate that the grain structure of the aluminide coating plays a major role in the early stages of rumpling and set its wavelength. Also, the superimposed compressive stress during thermal cycling leads to an asymmetry in the rumpling pattern with respect with the loading axis as well as cracking along the applied stress direction.

Deformation and phase transformations during the cyclic oxidation of Ni−Al and Ni−Pt−Al

The reversible high-temperature γ′ to β phase transformation may be critical to explaining the unusual high-temperature oxidation behavior of (Ni,Pt)Al alloys and coatings. During high-temperature, high-frequency (1 h) cyclic oxidation in dry, flowing O2, unprecedented macroscopic deformation was observed in two-phase (γ′+β) cast specimens of Hf-doped Ni−Al at 1,150°C and Hf-doped Ni−Pt−Al at 1,100° and 1,150°C, Outside of this two-phase field or when the cycle frequency was decreased to 100h, no deformation was observed. Using high-temperaturex-ray diffraction in an inert environment, the β-to-γ′ phase ratio was observed to increase above 1,000°C, causing a 2.5% volume change. The addition of platinum appeared to lower the transformation temperature consistent with the deformation observed in castalloys and rumpling of simple and platinum-modified aluminide coatings.

The effect of TCP morphology on the development of aluminide coated superalloys

Aluminisation gives a cost-effective coating for turbine components subjected to hostile environments. However, inter-diffusion between coating and substrate causes changes in microstructure, which destroy the creep resistance of the alloy. We compare low-temperature, high-activity aluminide and platinum-modified aluminide coatings on the single crystal, nickel-based alloys, RR3000, CMSX-4 and TMS75, and describe the evolution of the microstructure following exposure at 1100 °C. In particular, the precipitation of topologically close packed (TCP) phases varies with alloy composition. We suggest that the morphology of the precipitates may influence the rate of ingress of the coating into the substrate. For example, the alloy RR3000 forms the σ-phase, which has an excellent fit with the alloy {1 1 1} planes and can nucleate as very fine small precipitates. In contrast, the μ-phase formed in the alloy CMSX-4 forms much larger, more widely spaced, TCP phases which necessitate a longer diffusion path for the slow diffusing refractory elements of which they are principally composed.

Investigation of the microstructure of platinum-modified aluminide coatings

The high temperature performance of Pt-modified aluminide coatings is directly related to their composition and microstructure. In this work, several of the factors that control such features are investigated on a single crystal Ni base superalloy. It will be shown that the bi-phase PtAl 2+(Ni,Pt)Al composition is more prone to occur at higher Pt surface concentrations, lower temperatures and shorter aluminising treatments. The alloying elements also seem to play a role in the achievement of either a single or a bi-phase structure. Finally, the incorporation of different elements to the Pt/Al coatings brings about shifting of the X-ray peaks, which in turn may indicate the presence of residual stresses.

Synthesis and cyclic oxidation behavior of a (Ni, Pt) Al coating on a desulfurized Ni-base superalloy

The influences of sulfur impurities and Pt incorporation on the scale adhesion behavior of aluminide coatings were studied and compared. Low-sulfur NiAl coatings were prepared on a desulfurized, yttrium-free, single-crystal Ni-based superalloy by a modified version of a conventional aluminizing procedure based on chemical vapor deposition. The sulfur level in the resulting NiAl coatings was measured to be less than ∼0.5 ppmw by glow-discharge mass spectroscopy. Platinum-modified aluminide coatings were synthesized by first electroplating a thin layer of Pt (∼7 µm) on the superalloy, followed by the same low-sulfur aluminizing procedure. The measured sulfur content in the (Ni, Pt)Al coating was substantially higher than that of the low-sulfur NiAl coating due to contamination during the Pt electroplating process. A very adherent α-Al2O3 scale formed on the grain surfaces of the low-sulfur NiAl coating during cyclic oxidation testing at 1150 °C, but scale spallation eventually occurred over many of the NiAl grain boundaries. In contrast, despite the higher level of sulfur in the (Ni Pt)Al coating, a very adherent scale was formed over both the coating grain surfaces and grain boundaries during thermal cycling. These results suggest that Pt additions can mitigate the detrimental influence of sulfur on scale adhesion.

Use of experimental designs to evaluate formation of aluminide and platinum aluminide coatings

Abstract Aluminide and platinum-modified aluminide coatings are widely used for protecting nickel and cobalt-based superalloy gas turbine engine components from oxidation and corrosion. Conventional methods for producing these coatings include pack cementation, `above-the-pack', and CVD aluminization processes. An alternative aluminization process was investigated utilizing deposition of a metallic slurry on the part surface and formation of an aluminide coating during a subsequent high temperature diffusion operation. As part of the initial investigation of two candidate slurry formulations, a series of statistically designed experiments were conducted to determine how key process factors influence aluminide coating formation. One experiment utilized a three-level response surface design capable of modeling process factor effects on aluminide coating thickness. A second experiment utilized a two-level screening design to sort process factor effects on platinum aluminide coating structure and thickness. Process time and temperature were the most significant process factors affecting aluminide coating formation using the slurry process. One slurry formulation produced similar aluminide coating thicknesses over a wide range of applied slurry amounts. Both formulations produced two-phase platinum aluminide coating structures.

Growth Mechanisms and Hot Corrosion Resistance of Palladium Modified Aluminide Coatings on Superalloys

Abstract Recently palladium has generated interest as a precoating for aluminizing superalloys used in gas turbine blades. Little work has however been done on the mechanisms of formation of palladium modified aluminide coatings, and also on the constitution of the ternary Ni-Pd-Al system. The present work was initiated to fill this gap. It was first devoted to a study of the ternary phase diagram, with synthesis of alloys in order to evaluate the stability domains of the different known phases: Ni3 Al, NiAl, Ni2Al3, Pd2Al, PdAl, Pd2Al2. Annealing of diffusion couples made with different alloys of the two binary Pd-Al and Ni-Al systems enabled to complete this study. Formation mechanisms of the coatings were also determined, with aluminization of binary PdNi alloys in different conditions (low and high aluminum activity). Resistance to hot corrosion of palladium modified aluminide coatings was evaluated with burner rig tests, and was compared to that of simple and platinum modified aluminide coatings.

Structure and high temperature performance of various palladium-modified aluminide coatings: A low cost alternative to platinum aluminides

Of the protective coatings applied to turbine hot stage components (generally superalloys), platinum-modified aluminide coatings have proved to be satisfactorily hot corrosion resistant (type I hot corrosion). More recently, cost-saving measures have motivated studies on the replacement of platinum by a cheaper metal: palladium. The structures of three different types of palladium-modified aluminide coating on Inconel 100 (IN 100) substrates are described; it is shown that the integrity of the coatings critically relies on an adequate processing route (preventing hydrogen embrittlement). Cyclic oxidation tests at 1100 °C and 850 °C hot corrosion tests (in air and air plus 0.01% SO 2) have been performed on simple aluminides, commercial RT22 platinum-modified coatings and palladium-modified coatings applied to IN 100. Palladium not only stabilizes the protective alumina scale formed in cyclic oxidation, but also dramatically increases the hot corrosion resistance of the coatings. Indeed, the performance of palladium-modified coatings compares very well with that of commercial platinum-modified coatings in terms of hot corrosion resistance.

High temperature-resistant palladium-modified aluminide coatings for nickel-base superalloys

Platinum-modified aluminide coatings have shown increased resistance to high-temperature corrosion. The high price of platinum metal has motivated investigations of the potential for palladium-modified aluminide coatings. In this paper, we have shown that the poor hot corrosion resistance of PdAl coatings reported in the literature was mainly due to inadequate processing, leading to significant incorporation of hydrogen into the coating and the subsequent formation of pores and blisters. This can be avoided by replacing a pure palladium predeposit by a Pd-20wt.% Ni alloy predeposit in which hydrogen is far less soluble. By this new technique, PdNi-modified aluminide coatings of the inward and outward type were obtained on IN 100. Their structure consists of a single β(Pd, Ni)-Al phase, with a strong palladium concentration gradient across the coating thickness. The composition in the outer part of the coating corresponds to (Ni 0.4, Pd 0.6)-Al. Corrosion tests were performed on these coatings at 850°C in air and in air + 0.01% SO 2, with periodic Na 2SO 4 contamination (0.5 mg cm −2 every 50 1 h cycles). All palladium-containing coatings showed an excellent resistance to this environment (lifetime greater than 1000 cycles). Cyclic oxidation tests performed at 1100 °C also showed a beneficial effect of palladium on the protective Al 2O 3 scale integrity and adhesion. The results obtained so far indicate that palladium aluminide coatings behave at least as well as typical platinum-modified aluminide coatings under the same testing conditions.

Oxide adhesion and growth characteristics on platinum-modified aluminide coatings

The improved lifetime of platinum-modified aluminide coatings under conditions of high temperature, hot corrosion and cyclic oxidation has been attributed in large part to the excellent oxide adherence exhibited by these systems. Although these benefits have been well documented, the reasons for the increased adherence requires additional investigation. In this study the effects of pre-aluminizing surface smoothness and coating structure for both platinum-modified and unmodified aluminide coatings on IN-738 were examined in a cyclic oxidation environment at 1100 °C. Weight change measurements were used to determine the coating scale adherence characteristics and to quantify the cyclic oxidation behavior effect of the various coating structures.

Surface instability of platinum modified aluminide coatings during 1100 °C cyclic testing

Platinum additions to aluminide coatings have been reported to be beneficial in both hot corrosion and oxidation environments. The data, however, are somewhat limited and strongly dependent upon structure and test conditions. Therefore, a study of the effects of surface structure under 1100 °C cyclic oxidation conditions of the platinum modified and the unmodified aluminide coatings on the IN-738 substrate was initiated. During the early stages of this evaluation surface upheavals on the order of 50 μm, often called ‘‘rumpling,’’ were observed for both systems. The degree of rumpling was greater in the platinum modified coatings but was observed to a significant extent with the unmodified coatings as well. The amount of rumpling observed during cyclic testing can be attributed to a number of possible effects including (i) coefficient of thermal expansion mismatch, (ii) thermal gradient across the coated specimen, and (iii) coating mechanical properties. An effect of coating thickness is also observed although it must be noted that for diffusion-type coatings such as the Pt–Al system, coatings with different thicknesses have different compositions. Therefore, the resulting performance of the coating is a complex interplay of the above-mentioned variables. Excellent Al2O3 adherence is seen for all the platinum modified aluminides tested, even on the highly convoluted surfaces. The role of rumpling in the high temperature protectively is not known but should be given more consideration as a potential high-temperature degradation mechanism of these coatings.

Structure and hot corrosion behavior of platinum-modified aluminide coatings

The well-documented differences in performance of platinum-modified aluminide coatings are studied in the light of the unique structural and diffusional features of these systems. Changes in the platinum and substrate element concentration and in the NiAl phase matrix lattice parameter suggest that one important function of the platinum is the rapid self-healing of the formed oxide scale owing to aluminum diffusion to the surface. Results of these analyses are reviewed in the light of exposition test data and theories.

Platinum aluminide coating structural effects on hot corrosion resistance at 900 °C

Studies have shown that platinum modified aluminide coatings can exhibit a range of structural variations depending on initial platinum thickness, prealuminizing heat treatment, aluminizing cycle, and postcoating heat treatment. It had been speculated earlier that such structural variation could account at least in part for some of the reported variation of performance of these coatings. A series of archetype structures was prepared on IN738 by two aluminizing processes: (i) low aluminum activity and (ii) high aluminum activity; and tested at 900 °C under conditions producing so called Type I high temperature hot corrosion. The results of the present investigation showed that the addition of platinum greatly improved the hot corrosion resistance. It is observed that the low aluminum activity process has better corrosion resistance than that of the high aluminum activity process. It is also found that the surface degradation of both aluminizing processes is quite severe when the specimens were given prolonged prealuminizing diffusion heat treatment. Microstructural observation revealed a significant difference in coating thickness in both the processes and it depends on prealuminizing diffusion and the subsequent aluminizing treatment employed.

Surface morphology of platinum modified aluminide coatings

The processing variables which lead to the rough, convoluted surface topologies of platinum modified aluminide coatings were studied. Platinum deposition techniques such as electroplating, ion plating, and sputtering were examined as were postplating diffusion treatments and aluminizing processes. It was found that while the initial Pt surface could be varied by the deposition process and procedures selected, the principal factor influencing surface topology was Pt substrate (IN-738) interdiffusion. Subsequent aluminizing processing had a much smaller additional effect. The effects of the surface roughness and phases and porosity found in the Pt layer on subsequently aluminized coatings performance are not known.