Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

High-temperature environments pose significant challenges to the durability of metallic alloys. Oxidation resistance is a critical factor in maintaining structural integrity under these conditions. Prior research has shown that enriching surfaces with elements like Al, Si, and Cr can enhance oxidation resistance. However, the application of Cr-based coatings via slurry techniques has been limited due to Cr's high melting point. Pack cementation remains the dominant method for Cr deposition. This gap motivated the development of a novel slurry-based process for Cr-Si coatings. The need for simpler, cost-effective coating methods has driven recent investigations into alternative approaches. Existing methods often require complex equipment or elevated temperatures. The current study addresses this limitation by introducing a new slurry technique. This approach aims to lower the melting point of Cr through the addition of Si.

The aim of this study was to develop and demonstrate a novel Cr-Si slurry coating process for Ni-based superalloys. The specific problem addressed is the difficulty in applying Cr-based coatings via slurry techniques due to Cr's high melting point. The motivation stems from the need for simpler and more cost-effective coating methods. The study focuses on whether a Cr-Si eutectic can enable liquid-phase formation during heat treatment. This would allow for the deposition of Cr-Si diffusion layers through a slurry process. The goal is to evaluate the oxidation resistance of the resulting coatings. The study also seeks to compare the performance of coated and uncoated alloys. The ultimate objective is to provide an alternative to pack cementation for Cr-based coatings.

The study employed a novel Cr-Si slurry coating process on two Ni-based superalloys, Rene 80 and Inconel 740H. The process involved the addition of Si to the slurry to lower the melting point of Cr. The slurry was applied to the alloy surfaces and then heat-treated to form diffusion layers. The heat treatment enabled the formation of a liquid phase due to the Cr-Si eutectic. The resulting diffusion layers were enriched with Cr and Si, reaching a thickness of about 150 µm. Oxidation behavior was evaluated through isothermal exposures at 900 °C for 1000 hours in lab air. The study compared the oxidation resistance of coated and uncoated samples. The analysis focused on the formation of oxide scales and internal oxidation zones.

The Cr-Si slurry coating process successfully formed diffusion layers enriched with Cr and Si, reaching a thickness of approximately 150 µm. Uncoated samples of Rene 80 and IN740H developed a Ti-containing Cr₂O₃ scale beneath a thin TiO₂ top layer. These samples also exhibited internal oxidation of Al over time, reducing the load-bearing cross-section. In contrast, the Cr/Si-coated samples showed a Si-rich oxide film beneath the external Cr₂O₃ scale. This subscale acted as an additional oxygen diffusion barrier. The coated samples demonstrated significantly lower weight gain during the 1000-hour exposure. The Si-rich subscale slowed the oxidation process compared to uncoated materials. The absence of internal Al oxidation in coated samples suggests improved protection. The results indicate that the Cr-Si slurry process enhances oxidation resistance.

The study demonstrated that the novel Cr-Si slurry coating process can form diffusion layers on Ni-based superalloys. The addition of Si to the slurry lowers the melting point of Cr, enabling the formation of a liquid phase during heat treatment. The resulting Cr-Si diffusion layers showed improved oxidation resistance compared to uncoated materials. The coated samples exhibited a Si-rich oxide film beneath the external Cr₂O₃ scale, which acted as an additional oxygen diffusion barrier. This subscale reduced weight gain during isothermal exposure at 900 °C. The absence of internal Al oxidation in coated samples suggests enhanced protection. The study supports the viability of the Cr-Si slurry process as an alternative to pack cementation. The findings indicate that this method can be applied to improve the high-temperature performance of Ni-based superalloys.