The effects of platinum concentration on the oxidation resistance of superalloys coated with single-phase platinum aluminide
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.
References (9)
- et al.
- et al.
- K. Bungardt, G. Lehnert, H.W. Meinhardt, US Patent No. 3819338,...
- et al.
Cited by (72)
Theoretical examination of thermo-migration in novel platinum microheaters
2021, Microelectronics ReliabilityEffect of substrate alloy composition on the oxidation behaviour and degradation of aluminide coatings on two Ni base superalloys
2020, Corrosion ScienceCitation Excerpt :Aluminide coatings are commonly employed to extend the operating lifetime of Ni-base alloy components in high-temperature applications [16–19]. Depending on the actual requirements, the coatings may be modified by e.g. additions of Pt or Pd [20–22]. The chemical composition of both substrate alloy and the coating may influence the long term performance of the alumina-forming coating on the alloy.
Effect of platinum addition on oxidation behaviour of γ/γ′ nickel aluminide
2015, Acta MaterialiaCitation Excerpt :Therefore, an improved TGO/bond coat bonding has been considered as one of the key factors to control the durability of the TBCs. The Pt-modified nickel aluminide bond coats, β-(NiAl)Pt, demonstrate significant improvement in resistance to oxide spallation, which has been identified by extensive research [2–7]. However, this type of bond coat is prone to undesirable surface roughening/rumpling after cyclic oxidation due to its low creep resistance [8–11].
Microstructure and high temperature oxidation behavior of Pt-modified aluminide bond coats on Ni-base superalloys
2013, Progress in Materials Science