CVD chromized nickel
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CVD Chromized Nickel WEN-PIN SUN, TING-HWANG LIN, and MIN-HSIUNG HON Chromized coatings form an important class of protective coatings for high temperature and hard wear-resistant alloys. Chromaluminide coated superalloys are used in gas turbine engines, which can provide improved hot corrosion and oxidation resistance, c2 thereby enhancing the service life and reliability of the coated alloy. For the hard wearresistant application, the hardness of Cr coated layers varies in the range HV 1000 to 2100 and is thus comparable with that of cemented carbides.3 The conventional method for applying chromized coating is by the pack cementation process, but small or inaccessible passages do not lend themselves to deposition by this process, as surfaces that require coating must be in intimate contact with the packed powder. Therefore, with a complex internal geometry, a uniform coating thickness and clean surface are difficult to obtain, 4 and that limits the property of the chromized coatings. 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 the powder mixture. The gas phase process improved the control of the coating thickness and microstructure by eliminating the dependence of coating formation on the pack powder to surface area ratio and by permeating the small and inaccessible passages. 5'6 The purpose of this study is to determine the conditions of the growth of chromized coating by gas phase deposition (CVD) on pure nickel. The nickel specimens, cut into 15 mm x 15 mm x 2 mm plate, were usually polished on carborundum paper to grit size 1000, and rinsed. The deposition of chromium was performed by chemical vapor deposition (CVD) in a hot wall reactor (Figure 1) in the temperature range of 950 to 1050 ~ In order to raise the chromium activity in the bulk, the inlet gas mixture was passed over a powder mixture of Cr and A1203, held at the same temperature as the substrate. The composition of powder mixture was 80 wt pct Cr and 20 wt pct A1203. A total of 30 g of powder mixture was placed at both sides of the sample holder. The transport agent (NH4C1) was in contact with chromium powder to form CrClz gas during coating. Above 800 ~ the NH4C1 activator was allowed to sublime and decompose: 7 NH4CI(g) ~
NH3(g ) + HCI(g)
[ 1]
Hydrogen chloride reacted with the chromium powders in the sample holder. 5 2HCI(g) + Cr(.,i .
" CrC12(g) + Hz(g)
[2]
WEN-PIN SUN is Associate Metallurgist, Chun-Shan Institute of Science and Technology, R.O.C. TING-HWANG LIN, Student, and MIN-HSIUNG HON, Chairman and Advisor, are with Graduate School of Mineral, Metallurgy and Materials Science, National Cheng Kung University, Tainan, Taiwan, R.O.C. Manuscript submitted August 6, 1986. METALLURGICALTRANSACTIONSB
Fig. 1 - - Schematic diagram of experimental apparatus for CVD chromization: (1) flow-meters, (2) NH4CI saturator, (3) shut-off valves, (4) sample, (5) Cr + AlzO3 mixture pow
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