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Research Papers: Materials and Fabrication

# Evaluation of Nanocrystalline Coatings for Coal-Fired Ultrasupercritical Boiler Tubes

[+] Author and Article Information
N. S. Cheruvu, R. Wei

Southwest Research Institute® , 6220 Culebra Road, San Antonio, TX 78238

M. R. Govindaraju

SAI Global Technologies, P.O. Box 690313, San Antonio, TX 78249

D. W. Gandy

Electric Power Research Institute, 1300 W. T. Harris Boulevard, Charlotte, NC 28262

J. Pressure Vessel Technol 132(6), 061403 (Oct 20, 2010) (9 pages) doi:10.1115/1.4001548 History: Received April 24, 2009; Revised March 30, 2010; Published October 20, 2010; Online October 20, 2010

## Abstract

Cyclic oxidation behavior and microstructural degradation of nanocrystalline Ni–20Cr–xAl (where $x=4 wt %$, $7 wt %$, and $10 wt %$) 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 $4 wt %$ 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 $4 wt %$ to $7 wt %$ or $10 wt %$, 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.

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## Figures

Figure 1

Microstructure of an as-coated sample showing typical lamellae structure

Figure 2

Typical XRD spectra obtained from the surface of the as-deposited coatings: (a) Ni–20Cr–4Al and (b) Ni–20Cr–10Al

Figure 3

Comparison of mass change results of uncoated and coated Ni–20Cr–xAl Haynes 230 samples at peak temperature of 750°C

Figure 4

Comparison of mass change results of uncoated and coated Ni–20Cr–xAl Haynes 230 samples at peak temperature of 1010°C

Figure 5

XRD spectra obtained from (a) Ni–20Cr–4Al and (b) Ni–20Cr–7Al coated samples in the as-coated and thermal exposed conditions at 1010°C showing stability of Ni-rich solid solution, formation of Al2O3 oxide scale during thermal exposure, and the variation in the width of the most intense γ peak

Figure 6

SEM micrographs of cross sections of samples after 2070 cycles exposure at the peak temperature of 750°C: (a) Ni–20Cr–4Al, (b) Ni–20Cr–7Al, and (c) Ni–20Cr–10Al coatings

Figure 7

SEM micrographs of cross sections of samples after 2070 cycles exposure to 750°C showing typical morphology of oxide scale on (a) Ni–20Cr–4Al and (b) Ni–20Cr–7Al coatings

Figure 8

Cross sections of Ni–20Cr–4Al samples (a) after 347 cycles and ((b) and (c)) after 1472 cycles at 1010°C

Figure 9

SEM micrograph of cross sections of Ni–20Cr–4Al samples after exposure to 1472 cycles at 1010°C. Arrows point to ultrafine particles.

Figure 10

Optical and SEM micrographs of the Ni–20Cr–7Al after (a) 347 cycles and (b) 1472 cycles exposure at 1010°C

Figure 11

Optical and SEM micrographs of the Ni–20Cr–10Al after (a) 347 cycles and (b) 1472 cycles exposure at 1010°C

Figure 12

SEM micrographs of cross sections of samples after 1472 cycles exposure to 1010°C showing morphology of oxide scale and condition of (a) Ni–20Cr–7Al and (b) Ni–20Cr–10Al coatings

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