CVD aluminide nickel
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I.
INTRODUCTION
NICKEL-basesuperalloys
need to be given aluminizing treatments to provide a barrier coating when they are used in environments which are known to cause severe surface degradation. The aluminizing process results in the formation of nickel aluminide phase layers on the alloy surface which can provide improved corrosion and oxidation resistance, thereby enhancing the service life and reliability of the coated alloy.1 The present method for applying aluminide coatings to blades is by a pack cementation process, but small or inaccessible passages do not lend themselves to deposition by this process, 2 as surfaces that require coating must be in intimate contact with the pack powder. Also, the internal coating thickness and morphology are a function of the ratio of the surface area to the pack powder available. Therefore with a complex internal geometry a uniform coating thickness and microstructure are difficult to obtain. 2 The chemical vapor deposition (CVD) process utilizes the same basic reactions as the pack cementation process except that the substrate to be coated is not in contact with a powder mixture. A gaseous transporting agent is generated in the coating chamber and reaches the substrate to be coated by diffusion. The gas phase process improved the controlling of coating thickness and microstructure by eliminating the dependence of coating formation on the pack powder to surface area ratio and by permeating small and inaccessible passages. 2,3 The purpose of this study is to determine the conditions of the growth of the aluminide coating by gas phase deposition on pure nickel.
II.
EXPERIMENTAL PROCEDURES
The nickel specimens, cut into 15 mm • 15 mm • 2 mm plates, were usually polished on carborundum paper to grit size 1000, degreased, and rinsed. The deposition of aluminum was performed by chemical vapor deposition (CVD) in a hot wall (INCONEL* tube) *INCONEL is a trademark of the INCO family of companies.
reactor (Figure 1) from an A1C13-H2 mixture at temperatures
WEN-PIN SUN, H. J. LIN, and MIN-HSIUNG HON are with Research Institute of Metallurgy and Materials Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. Manuscript submitted April 29, 1985. METALLURGICALTRANSACTIONS A
in the range of 973 K to 1373 K. In order to raise the aluminum activity in the bulk, the inlet gas mixture was passed over a powder mixture of aluminum (A1) and aluminum oxide (A1203), held at the same temperature as the substrate. The composition of powder mixture was 30 wt pct A1 and 70 wt pct A1203. A total of 20 g of powder mixture was placed at both sides of the sample holder. A1 was put here to form the gases of aluminum subchlorides such as A1C1 and A1C12. The AIC13 is carried by H2 (300 cc/min), and the A1C13 gas ratio is controlled by a heater at temperatures in the range of 303 K to 403 K. After aluminization, the samples were cleaned by ultrasonic treatment in acetone. Sample weight gains and surface composition were measured as a function of time to determine the growt
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