Elsevier

Surface and Coatings Technology

Volumes 146–147, September–October 2001, Pages 1-6
Surface and Coatings Technology

The effects of platinum concentration on the oxidation resistance of superalloys coated with single-phase platinum aluminide

https://doi.org/10.1016/S0257-8972(01)01362-7Get rights and content

Abstract

Superalloy samples were coated with a low activity platinum aluminide applied by chemical vapor deposition (CVD). Different platinum thickness values were obtained on each group of samples by electroplating prior to CVD aluminizing and resulted in an array of platinum concentrations in the product coatings. The platinum aluminide coatings were single phase, (Ni,Pt)Al, with platinum concentrations varying from 8 to 38 wt.%. The coated samples were subjected to cyclic oxidation testing at 1175°C until failure. As expected, it was discovered that higher concentrations of platinum resulted in a coating more resistant to degradation during the cyclic oxidation testing. Electron microprobe composition profiles across the coatings for aluminum, platinum and substrate alloying elements were performed before and after oxidation testing. The results were compared to classical literature on two-phase platinum aluminide coatings. The ramifications of diffusion of many substrate alloying elements are discussed with regard to their relative effects on oxidation resistance of the coating.

Introduction

Aluminide coatings for the protection of superalloy turbine hardware have been utilized since the 1950s. Groundbreaking work by Bungardt et al. [1] on the introduction of noble metals (primarily platinum) revolutionized diffusion coating technology by providing superior resistance to oxidation at high temperatures and type I hot corrosion. Debates continue over whether the presence of Pt slows the growth of the Al2O3 scale, slows the diffusion of aluminum into the substrate, slows diffusion of substrate elements into the coating or prevents internal oxidation of other alloying elements in the substrate [2], [3]. Bungardt's original work stated that as the Pt concentration increased, the resistance to oxidation also increased. This work was carried out on dual phase (PtAl2+NiAl) coatings grown by inward diffusion of aluminum and consequently, the coatings contained high concentrations of substrate alloying elements. Dual phase coatings typically have Pt and Al concentrations on the order of 30 wt.% or higher for each element, and significant differences in both concentrations between phases. Single phase (Ni,Pt)Al coatings generally have lower concentrations of Al, in the order of 15–20% and approximately 20–25% Pt. The benefits of using single-phase vs. duplex coatings are based on the brittle nature of the two-phase structure, which undergoes a volume change during dissolution of the PtAl2. The single phase platinum aluminide coatings also provide better phase stability, better mechanical properties for handling and fatigue crack resistance, plus better overall resistance to oxidation due to outward coating growth and purification of the additive aluminide layer during deposition [4]. Based on work by Chan et al. [5] the minimum threshold level of Al in the coating to provide adequate high temperature oxidation protection is 19.5 at.%. Since transport of Al is occurring both into the substrate and out to the oxidizing surface, it is desirable to have as high an Al concentration as possible prior to any high temperature exposure. This must be balanced, however, with the concentrations of both Pt and other alloying elements from the substrate, including nickel.

This research program involved work similar to that of Bungardt, except using single-phase platinum aluminide coatings on numerous nickel base superalloy substrates. A relationship between the platinum deposit thickness, the subsequent concentration of Pt in the coating and the relative resistance to oxidation of the coated substrate is established.

Section snippets

Experimental procedure

Rectangular tab specimens of dimensions (mm) 25.4×12.2×3.2 with rounded corners and edges were obtained for three alloys: IN 100; MarM 247; and PWA 1480, whose nominal compositions are given in Table 1. Within each alloy group, the samples were obtained from the same alloy heats. The samples were blasted with 220 grit brown alumina at a blast pressure of approximately 276 kPa (40 psi). Layers of platinum, with four different nominal thickness values outlined in Table 2, were electroplated onto

Thickness and initial composition

The thickness and elemental composition of the additive layer was determined following CVD aluminizing for slave samples prior to cyclic oxidation testing. The results obtained from the various alloys are summarized in Table 3. As expected, as plated platinum thickness increased, the resultant coating thickness and the coating Pt concentration increased as well. Representative microstructures in the as-coated condition are displayed in Fig. 1, Fig. 2. Every sample examined in this program

Conclusions

Several conclusions can be drawn from the experimental results. First, for single phase platinum aluminide coatings, increasing concentration of platinum generally result in higher resistance to cyclic oxidation conditions. Single-phase platinum aluminide coating structures were possible with up to approximately 40 wt.% platinum. Second, the presence of titanium, both in the substrate and the coating following high temperature exposure, appears to be a major detriment to cyclic oxidation life.

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There are more references available in the full text version of this article.

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