Study on enhancement of cyclic oxidation resistance of Pt-modified aluminide coating on Ni-based superalloy

Abstract

Pt-modified aluminide coatings are widely used to protect Ni-based superalloys at high temperatures. Various methods were used in this study to enhance the alloy’s resistance to cyclic oxidation, including of Zr coating by using EB-PVD, surface mechanical attrition treatment (SMAT), and air blasting. The SMAT-treated Pt-modified aluminide coating contained more diffused Al than did the untreated Pt-modified aluminide coating due to the more grain boundary, which is a short-circuit path that existed in the PtAl alloy before aluminizing. The SMAT-treated Pt-modified aluminide coating was thicker than was the untreated Pt-modified aluminide coating. Although the two-phase (PtAl2 + β-NiAl) SMAT-treated Pt-modified aluminide coating had a thickness similar to that of the untreated Pt-modified aluminide coating, the thickness of the one-phase layer (β-NiAl) was thicker than that of untreated Pt-modified aluminide coating. In Zr-coated Pt-modified aluminide coating, Zr was located at surface and at the grain boundary of the coating as an agglomerated form of the NiAlZr alloy. The SMAT-treated and air-blasted Pt-modified aluminide coatings had less surface roughness than did the untreated Pt-modified aluminide coating because the PtAl2 phase was uniformly distributed in the SMAT-treated Pt-modified aluminide coating. However, the air-blasted Pt-modified aluminide coating was mechanically leveled by using air blasting with glass beads. Because these coatings experience thermal shock due to thermal expansion between the thermally grown oxide (TGO) and the coatings during heating and cooling, cyclic oxidation is a considerable concern. The Zr-coated Pt-modified aluminide coating had a slightly increased resistance to cyclic oxidation than did the untreated Pt-modified aluminide coating. In the Zr-coated Pt-modified aluminide coating, many pegs composed of oxide related with Zr were observed near the interface between the coating and the TGO after cyclic oxidation tests for 1000 h. These pegs were a major factor in the enhancement of the adhesion between the coating and the TGO. Among the tested coatings, the SMAT-treated and air-blasted Pt-modified aluminide coatings provided oxidation resistance superior to that of Zr-coated and untreated Pt-modified aluminide coatings. This property was improved by using flat initial surface conditions which were commonly shown in SMAT-treated and air-blasted Pt-modified aluminide coating because as a low initial surface roughness reduces stress between the coating and the TGO during cyclic oxidation. During cyclic oxidation, the air-blasted Pt-modified aluminide coating had many Hf related oxide pegs caused by the precipitation of Hf at the grain boundaries. Such pegs were not seen in the SMAT-treated Pt-modified aluminide coating. However, when the Zr-coated Pt-modified aluminide coating was compared, the Hf-related oxide peg could not be the main factor of cyclic oxidation behavior because Zr-coated Pt-modified aluminide coating did not show cyclic oxidation resistance similar to that of the air-blasted Pt-modified aluminide coating. Therefore, the surface conditions prior to cyclic oxidation are the most influential factor in cyclic oxidation resistance.