Alloys based upon the Al-Y-Ni-Co system were air plasma sprayed to produce nanocrystalline and amorphous coatings. All coatings produced in this study were deposited on either 7075-T6 plate or on a water-cooled copper plate. Glass forming ability was observed to increase with yttrium, nickel, and cobalt content. The alloy Al91Y4Ni4Co1 produced a composite microstructure of metallic glass and nanocrystalline aluminum grains with a grain diameter of 10 – 15 nm. In contrast, the as deposited Al85Y8Ni5Co2 and Al81Y12 Ni5Co2 alloy coatings were almost entirely amorphous. Salt spray corrosion tests were performed on these coatings to investigate the influence of coating microstructure (nanocrystalline vs. amorphous) and alloy composition. Subcritical heat treatments were determined by differential calorimetry to investigate the effect of corrosion and abrasion resistance on the amount of nanocrystalline aluminum phase in the amorphous coating. Coatings were characterized using TEM and x-ray diffraction. Plasma spray experiments were also conducted on Al-Ge alloys and the undercooling achieved was determined from the formation of various Al-Ge metastable phases. The undercooling data obtained from Al-Ge system was then applied to the differential thermal and calorimetry results of Al-Y-Ni-Co coatings to investigate the formation of amorphous phase.