The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

Abstract

An 8-9 mu m thick Pt layer was coated on a superalloy and transformed to a Ni-Pt alloy layer by the interdiffusion of Ni and Pt at 1050 degrees C for 3 h. The surface of the Ni-Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 degrees C were studied. The treated Ni-Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 degrees C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 degrees C were composed of only a two-phase (NiAl + PtAl2) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 degrees C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 degrees C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 degrees C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 degrees C. (C) 2013 Elsevier Ltd. All rights reserved.