| Materials for high-temperature turbine environments face rigorous operating conditions. Not only are components exposed to high-temperature oxidation, but corrosive gasses that evolve during combustion and service operation can also lead to degradation. Ni-based superalloys are used in turbines for their strength and creep resistance at high temperatures. The compositions of these superalloys are such that they do not provide adequate environmental protection alone; therefore, coatings are used to protect the underlying materials from degradation. Currently, there is no coating that provides adequate protection to both oxidation and hot corrosion. In high-temperature areas where the main degradation mode is oxidation, beta-NiAl-type diffusion coatings are often used. These metal coatings have a significant Al content to insure Al 2O3-scale formation, which provides excellent oxidation protection but variable corrosion resistance that is highly dependent on composition. Conversely, MCrAlY overlay coatings (M=Ni, Co, or NiCo) provide superior corrosion resistance due to their relatively high Cr content (up to about 30at%), but cannot be used in applications exceeding 1000°C due to limited oxidation protection. A new generation of Al2O3-scale-forming coating, based on Hf+Pt-modified gamma'-Ni + gamma'-Ni3Al, can show excellent oxidation protection and increased corrosion protection compared to beta-type coatings. This research modifies the Hf+Pt-modified gamma-Ni + gamma'-Ni3Al coating composition with Cr and Si to increase the hot-corrosion resistance, while maintaining superior oxidation resistance. The best-performing gamma-Ni + gamma'-Ni3Al composition was found to be Ni-18Al-10Cr-3Pt-2Si-0.1Hf alloy (at%). Tests were conducted on both bulk alloys and coatings. Thermal spraying was used to generate the coatings for testing. A Pt-modification procedure was devised for the thermal-spray coatings. Oxidation testing and accelerated hot corrosion testing were used to assess performance. Metallography, SEM and EDS techniques were used to characterize samples and rank performance. As will be shown, the Ni-18Al-10Cr-3Pt-2Si-0.1Hf composition provided the best combination of oxidation and hot corrosion protection when compared to Ni-50Al-15Pt (Pt-modified beta), Ni-20Al-20Pt-1wt%Hf (gamma-Ni + gamma'-Ni3Al), Ni-21.7Cr-19.5Al-0.5Y (beta+gamma MCrAlY), and Ni-17.6Cr-12Al-0.3Y (gamma+gamma' MCrAlY), with all compositions listed in at%. |
