Evaluation of Nanocrystalline Coatings for Coal-Fired Ultrasupercritical Boiler Tubes

Cyclic oxidation behavior and microstructural degradation of nanocrystalline Ni–20Cr–xAl (where $x=4wt%$⁠, $7wt%$⁠, and $10wt%$⁠) coatings have been investigated. The coatings were deposited on Haynes 230 samples using a magnetron sputtering technique. Cyclic oxidation tests were conducted on the uncoated and coated samples at peak temperatures of $750°C$ and $1010°C$ for up to 2070 thermal cycles between the peak and room temperatures. The results showed that a dense $Al2O3$ scale was formed on the external surface of all coatings after exposure at both temperatures. All three coatings showed no evidence of internal oxidation after exposure at $750°C$⁠. Among the three coatings, only the coating containing $4wt%$ Al showed evidence of internal oxidation along the columnar grain boundaries after exposure at $1010°C$⁠. The $Al2O3$ scale exhibited good spallation resistance during cyclic oxidation tests at both temperatures. As the Al content in the coating increased from $4wt%$ to $7wt%$ or $10wt%$⁠, thermal exposure led to precipitation of coarse Al-rich particles at the coating/substrate interface. In addition, thermal exposure at both temperatures led to rapid depletion of Al in the coating and grain coarsening of the coatings. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are attributed to the ultrafine grain structure of the coating and oxide scale.