The Performance of Pt-Modified Alumina-Forming Coatings and Model Alloys

Abstract

The oxidation behavior of two-phase, γ+γ' Pt diffusion coatings and model alloys has been evaluated to better understand the potential of this class of coatings for power generation applications. Coatings were deposited on both directionallysolidified alloy 142 and single crystal N5 superalloys. Fabrication issues such as initial Pt thickness and annealing temperature were evaluated. Oxidation testing was performed in both dry O 2 and with the addition of H 2 O to study the performance of γ+γ' and conventional coating materials. After exposure for 1000, 1-h cycles at 1150°C in dry O 2 , a γ+γ' coating on N5 had evolved into a thicker, almost continuous γ' layer with the Pt content dropping to ~7at.% in this layer and diffusing ~250 µm into the substrate, disrupting the γ+γ' substrate microstructure. Characterization of the oxide formed on a γ+γ' composition showed that even 30 at.%Pt did not prevent the formation of a Ni-rich spinel layer at 1200°C. In order to assist the coating development process, model alloys also were evaluated. Other Pt group metals (Pd and Ir) were evaluated as well as the addition of Cr and various Hf levels. As expected, Cr and Hf were beneficial for oxidation resistance. However, Pd was not an adequate replacement for Pt because of the multi-phase microstructure that formed. A γ+γ' composition had a 1000°C yield stress in compression that was significantly higher than γ'-Ni 3 Al or β-NiAl. Also, the thermal expansion coefficient was very similar to other Ni-Al alumina-formers. The performance of γ+γ' compositions are very promising, however, sensitivity to superalloy substrate composition, Pt interdiffusion during long-term service, spinel formation and the potential need for a secondary processing step are areas of concern for this class of coatings.

Effect of water vapour content on thermal barrier coating lifetime

“…The mass change data for the coated (similar b and czc9) specimens was difficult to interpret and did not change monotonically with water content. The increased roughness (which was not observed at 1100uC, 18 consistent with other studies) 25 with the addition of water vapour for the aluminide bond coatings is consistent with the decrease in lifetime observed, especially for 10%H 2 O. A mechanistic explanation for the increased rate of roughening has not been identified.…”

“…19 A second prior study evaluated alumina scale formation at 1100uC in 0-90% water vapour containing environments on similar Pt containing bond coatings and model alloys but did not include YSZ top coatings. 8,18 Without the YSZ coating, these prior results were similar to other work suggesting increased scale spallation and more transient oxide formation with increasing water vapour content. 6 With a YSZ top coating in the present study, the lifetime with Pt diffusion bond coatings was unaffected by thermal cycling in air with 10-90% water vapour, but the Pt modified aluminide coating lifetimes were reduced, with the lowest water vapour content being the most aggressive.…”

The Effect of Water Vapor and Superalloy Composition on Thermal Barrier Coating Lifetime

“…The Pt diffusion coatings were the only bond coating that was virtually immune from water vapor in these experiments ( Figure 2). The Ni-rich oxide on the coatings ( Figure 10) was considerably less than observed on model γ+γ' NiPtAl cast alloys [23], perhaps due to a finer alloy grain size in the coatings.…”

Mitigation of Platinum Depletion in Platinum Diffused Single Phase Bond Coat on CMSX-4 Superalloy

“…However, concerns are always raised on a Pt-diffused γ/γ' bond coat due to the significant inwards diffusion of Pt towards the substrate during long-term heat treatment [11]. A depletion of Pt near the TGO/bond coat interface fails to maintain strong adhesion of TGO to bond coat, and it results in a compromised oxidation resistance and ultimate TBCs 2 of 13 failures [12][13][14][15]. In addition, the high material cost of Pt is another big concern.…”