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Electrodeposited metal matrbdmetal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 volume percent aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEMj, and electron probe microanalysis @?MA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 to 1000"C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al . particle interfaces and concentrically around the original Al particles. Upon heating to the 500-600"C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (6OO-1OOO"C), diffusional reaction between NiAl and nickel produces (y')Ni3Al. The final equilibrium microstructure consists of blocks of (y')Ni3Al in a y(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminizes.

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The interdiffision of Ni matrix/Al particle composite coatings and nickel substrates was studied using electron probe microanalysis (EPMA) and a one-dimensional diffusion model. The initial coating microstructure was a two-phase mixture of y(Ni) and y'(Ni3Al).The coating/substrate assemblies were aged at 800 to 11OO°C for times up to 2000 hours. It was found that aluminum losses to the substrate are significant at 1000"C and above. The "experimental results for the diffusion of Al into the substrate were compared to model predictions based on a diffusion equation for a finite layer on an infinite substrate. Using combined experimental and model results, the effects of temperature and coating thickness were determined and a rationale was developed for coating lifetime prediction.