Dipak K. Das

Role of Pt content in the microstructural development and oxidation performance of Pt-aluminide coatings produced using a high-activity aluminizing process

The present study highlights the effect of Pt content on the microstructure of Pt–aluminide coatings produced using a single-step high-activity aluminizing process. The amount of Pt in the coating was varied by changing the thickness of the initial electroplated Pt layer between 1 and 15 μm. The aluminium uptake from the pack was found to be almost the same for all the coatings produced using a Pt layer of thickness 2.5 μm and above, with a somewhat lower uptake for the coating corresponding to a 1 μm thick Pt layer. The coating microstructure, which consisted of an outer two-phase (PtAl2 in a matrix of NiAl) layer, an intermediate NiAl layer and an interdiffusion layer, was also found to be independent of the Pt layer thickness when it was in the range 2.5–10 μm. In the case of the 1 μm Pt layer, however, the whole of the Pt remained in solid solution in the NiAl phase. For a Pt layer thickness exceeding 10 μm, on the other hand, a continuous surface layer of PtAl2 phase was observed. The above mentioned influence of the thickness of the Pt plated layer on the microstructure of the Pt–aluminide coatings observed in the present investigation could be explained in terms of the Pt concentration in the diffusion layer resulting from the interdiffusion between the Pt layer and the superalloy substrate during the pre-aluminizing diffusion treatment. Cyclic oxidation tests on these Pt–aluminide coatings reveal that the presence of Pt in aluminide coatings, in general, enhances oxidation resistance. However, in order to fully realize the beneficial effects of Pt on oxidation behaviour, a certain minimum Pt content in the coating was found to be necessary.

Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation

AbstractIsothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2 O4 was also found in the case of the plain alumini...

Effect of Al content on microstructure and cyclic oxidation performance of Pt-aluminide coatings

The effect of Al content, i.e., the amount of Al picked up during aluminizing, on the microstructure and cyclic oxidation properties of Pt-aluminide coatings has been investigated. The cast Ni-base superalloy CM-247 was used as the substrate material and a single-step, high-activity pack aluminizing process was used to produce the Pt-aluminide coatings. The Al content of these coatings was varied by using packs with different compositions of the Al source. Pt-aluminide coatings having three different Al contents, namely 6.5, 16, and 21 mg cm-2, were evaluated for their cyclic oxidation resistance at 1200°C in air. It was found that the Pt-aluminide coatings, irrespective of their Al contents, evolve in the same manner during aluminizing and result in a three-layer structure with an outer PtAl2+NiAl two-phase layer, an intermediate NiAl layer, and the inner interdiffusion layer. The stability of this three-layer coating structure over long periods of aluminizing, however, is dependent on the availability of Al from the pack during this period. Below a certain threshold Al availability, the two-phase outer layer transforms to a single-phase NiAl structure causing the coating to change from its three-layer structure to a two-layer one. Cyclic oxidation results indicate that, while a minimum Al content in Pt-aluminide coatings is essential for deriving the best oxidation performance, increasing the Al content beyond a certain level does not significantly enhance oxidation behavior. The effect of Al content on aspects, such as coating degradation and nature of coating–surface damage during cyclic oxidation, is also discussed.

Surface roughness created by laser surface alloying of aluminium with nickel

Abstract Commercial pure aluminium was laser surface alloyed with nickel using an Nd-YAG pulse laser. It was observed that the roughness of the alloyed surface decreased with an increase in the extent of defocusing of the laser beam. The observation is explained in terms of the variation in laser power density due to defocusing, and the concentration of the alloying element in the alloyed layer.

Formation of secondary reaction zone in ruthenium bearing nickel based single crystal superalloys with diffusion aluminide coatings

Abstract Several Ru bearing experimental single crystal superalloys were coated with β-NiAl and Pt–Hf modified γ–γ′ coatings and subjected to elevated temperature exposure. The microstructure, oxidation resistance and the propensity for the formation of secondary reaction zones (SRZ) were investigated for these coatings. The presence of significant amounts of Ru in the superalloys did not prevent the cellular transformation leading to the formation of SRZ beneath the β-NiAl coating. Cyclic oxidation exposure of the β-NiAl coated alloys at 1100°C led to a significant further growth of the SRZ. In contrast, the γ–γ′ coating did not induce the undesirable cellular transformation, even after prolonged high temperature exposure. The γ–γ′ coating also provided good oxidation resistance to the superalloys. The driving forces for SRZ formation in case of the β-NiAl coating as compared to the γ–γ′ coating are discussed.

High temperature oxidation behaviour of directionally solidified nickel base superalloy CM–247LC

Abstract The present paper describes the isothermal and cyclic oxidation behaviour of the technologically important nickel base directionally solidified superalloy CM-247LC in air in the temperature range 1000-1200°C. This superalloy behaves as a transition nickel base alloy under isothermal oxidation conditions and exhibits a fairly long transient oxidation period (~20 h at 1100°C). Irrespective of the temperature of exposure and nature of oxidation (isothermal or cyclic), a composite oxide scale develops on CM-247LC. While the outer portion of the oxide scale consists of either spinel (NiAl2O4) or a mixture of spinel and NiO, depending on oxidation temperature, the inner portion is always constituted of alumina. Beyond the transient period, the alloy is found to follow parabolic oxidation kinetics. The oxide layer that forms is invariably very non-uniform in thickness, and is dispersed with two types of oxide particles. While tantalum rich oxide particles are found scattered in the outer zone of the oxide layer, hafnium rich oxide particles lie close to the oxide/metal interface. Results also reveal that the nature of oxidation associated with the CM-247LC superalloy causes entrapment of metal islands in the oxide layer.

Scratch adhesion testing of plasma-sprayed yttria-stabilized zirconia coatings

Abstract The suitability of scratch adhesion testing, usually used for determining the critical load for thin hard coatings like TiC and TiN, in characterizing plasma-sprayed yttria-stabilized zirconia coatings is demonstrated. The effects of loading rate and scratching speed on the critical load of these sprayed coatings were studied. Although some peculiarities in acoustic signal-load plot were observed at high values of loading rate and scratching speed, it was found that these intrinsic parameters, at low and medium values, do not have any prominent effect on the critical load.

Microstructure studies of air-plasma-spray-deposited CoNiCrAlY coatings before and after thermal cyclic loading for high-temperature application

In the present study, bond-coats for thermal barrier coatings were deposited via air plasma spraying (APS) techniques onto Inconel 800 and Hastelloy C-276 alloy substrates. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) were used to investigate the phases and microstructure of the as-sprayed, APS-deposited CoNiCrAlY bond-coatings. The aim of this work was to study the suitability of the bond-coat materials for high temperature applications. Confirmation of nanoscale grains of the γ/γ′-phase was obtained by TEM, high-resolution TEM, and AFM. We concluded that these changes result from the plastic deformation of the bond-coat during the deposition, resulting in CoNiCrAlY bond-coatings with excellent thermal cyclic resistance suitable for use in high-temperature applications. Cyclic oxidative stability was observed to also depend on the underlying metallic alloy substrate.

TIME DOMAIN TERAHERTZ NON‐DESTRUCTIVE EVALUATION OF AEROTURBINE BLADE THERMAL BARRIER COATINGS

Time domain terahertz (TD‐THz) non destructive evaluation (NDE) imaging is used to two‐dimensionally map the thickness of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on aircraft engine turbine blades. Indications of thermal degradation can be seen. The method is non‐contact, rapid, and requires no special preparation of the blade.

Microstructure and oxidation performance of a γ - γ ′ Pt-aluminide bond coat on directionally solidified superalloy CM-247LC

The microstructure of a Pt-modified γ-γ′ bond coat on CM-247LC Ni-base superalloy has been examined and its cyclic oxidation performance at 1100 °C in air is comparatively evaluated with that of a conventional β-(Ni, Pt)Al bond coat. The γ-γ′ bond coat was effective in imparting oxidation resistance to the CM-247LC alloy for about 100 h, whereas the β coating imparted oxidation resistance for significantly longer duration of about 1000 h. The nature of surface damage that occurred to the γ-γ′ coating during oxidation has been compared with that reported in the case of β coating.