Synthesis of functionally graded metal-ceramic microstructures by chemical vapor deposition
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(Received 12 May 1995; accepted 24 August 1995)
A composite microstructure consisting of small a -A1203 particles dispersed in a P -NiAl coating matrix was synthesized by chemical vapor deposition (CVD). While the surface of a pure Ni substrate was being reacted with AlC13 and H2 to form P-NiAI at a temperature of 1050 “C, the partial pressure of C02 in the reactor was controlled via pulsing to nucleate and disperse 50 to 500 nm a-Al203 particles in the P-NiAl matrix. The relative amount of the a-AI203 phase increased with coating thickness as the rate of the P-NiAl formation decreased with time. These experimental observations suggest that the synthesis of a graded composite microstructure by the CVD method is feasible.
Improving the adhesion between two dissimilar materials such as a metal and a ceramic under thermochemically and thermomechanically severe environments is a major challenge for many reasons. One may find such an example in trying to protect Ni-based superalloys from high temperature environments encountered in gas turbines. The protection has been traditionally achieved by applying metallic coatings such as diffusion pack NiAl and plasmasprayed NiCrAlY coatings. These coatings typically form an adherent thin A1203 scale upon oxidation. Since A1203 is stable and relatively resistant to rapid oxygen diffusion, further oxidation is retarded once the scale is formed. However, with subsequent thermal cycles, the protective scale tends to microcrack and eventually spa11 off. This spallation re-exposes the underlying metal surface to oxidation. During service, this cycle of scale formation and spallation continues at the expense of the sacrificial coating layer. From a mechanical point of view, the principal driving force for scale spallation is the development of stresses at the interface between the oxide scale and the metal surface. The scale becomes severely strained with temperature cycles because of the large differences in material properties such as coefficient of thermal expansion (CTE) and Young’s modulus. In addition, if the scale continues to grow by the inward diffusion of oxygen through the scale, the oxide scale will become strained to accommodate the volume expansion at the metal-scale interface. In a qualitative sense, when the level of these stresses exceeds the bond strength of the bi-material interface, the scale and metal surface are debonded. From a standpoint of fracture mechanics, it is important to consider the resistance for crack initiation and propagation along the metal-scale interface. It has been known that scale adherence can be improved by several methods. First, the addition of small amounts of so-called “reactive” elements such as yttrium, hafnium, and zirconium to superalloys and coatings signifi3000
J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
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cantly increases scale adhesion and therefore their oxidative life.’-3 Second, the removal of sulfur impurities below several ppm levels also increases scale adhesion since
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