Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the pr ocess parameters, coatings were deposited with 20 vol pct aluminum particle s. Coating morphology and composition were characterized using light optica l microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to s tudy reactive phase formation. The effect of heat treatment on coating phas e formation was studied in the temperature range 415 degreesC to 1000 degre esC. Long-time exposure at low temperature results in the formation of seve ral intermetallic phases at the Ni matrix/Al particle interfaces and concen trically around the original Al particles. Upon heating to the 500 degreesC to 600 degreesC range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperat ures (600 degreesC to 1000 degreesC), diffusional reaction between NiAl and nickel produces (gamma')Ni3Al. The final equilibrium microstructure consis ts of blocks of (gamma')Ni3Al in a gamma (Ni) solid solution matrix, with s mall pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural developm ent is discussed with reference to reaction synthesis of bulk nickel alumin ides.