Low pressure plasma spray coatings

A new technique - low-pressure plasma spray (LPPS) - has been used for deposition of high quality Al2O3 coatings on aluminum substrates for many different applications. The initial results on the properties and structure of the LPPS alumina coating are presented in this paper. Chemical compositional analysis using secondary neutral mass spectroscopy (SNMS) has shown that the coatings are close to stoichiometric with an oxygen to aluminum ratio of 1.65. The cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed that the structure of the coating is complex: layered splat structure with splats consisting of a polycrystalline core embedded in an amorphous matrix. Initial dielectric strength measurements show dielectric strengths of approximately 1000 kV/cm in some of the thinner (10-20 mum) LPPS coatings, a value significantly higher than in other thermal spray coatings. The dielectric strength of these LPPS coatings is related to the microstructure as determined by electron microscopy imaging. The studies of the adhesion of the coating to the aluminum substrate using scratch testing with a 5-mm ball indentor showed maximum adhesion (critical loads) for the 20-mum thick LPPS coatings. These adhesion data have been related to the microstructure of the coatings and the initial substrate preparation. The effect of different substrate preparation on improving adhesion is being investigated together with wear testing of the LPPS coatings. (C) 2000 Elsevier Science B.V. All rights reserved.