New plasma ceramic coatings

Abstract

It is expedient to use plasma methods for deposit ing protective coatings containing refractory compo nents. Plasma generators for coating deposition are selected on the basis of the composition, dispersion of initial powders, and properties that should be imparted to the coating. Wide potentialities of modern plasma generators enable the deposition of microcomposites of different types and compositions.In this paper, we summarize the results on fabrica tion of new materials as biceramic layered composites and boride composites on the stainless steel substrate sprayed by common (subsonic) and supersonic plasma generators.Initial materials were ceramic powders: oxide (based on Al 2 O 3 -13TiO 2 ) and boride (CrB 2 , TiB 2 ) compositions. Oxide microcomposites were fabricated from ultradisperse Al 2 O 3 -13TiO 2 powders with addi tions of SiO 2 and ZrO 2 . Boride based powders were preliminarily subjected to spheroidization. Coatings were deposited by spraying polydisperse compositions. The reference was a corundum coating. Powders were deposited on stainless steel and ceramic substrates by means of a subsonic (subsonic coatings) and super sonic (supersonic coatings) plasma guns. The Ni-Al alloy was used as a sublayer material.Metallographic studies showed that the interface of the deposited oxide layers with the basic metal is of high quality and has neither pores nor unfused areas (Fig. 1). The coatings consist of two layers: the sub layer and the basic ceramic layer. The basic layer has a layered composite structure (Fig. 1a): the light col ored matrix material contains evenly distributed darker, shiny, and white inclusions. Inclusions of dif ferent composition are irregular, partially fused, or spherical, or have a shape of layers. Such a structure is characteristic of the subsonic coatings. The coatings deposited by supersonic spraying contain spherical inclusions of only one type, ZrO 2 , in the structure of layers, and the phases of other compositions occur as separate irregular inclusions (dark phase) and fine grained regions (Fig. 1b). Such coatings contain a much larger number of zircon layers than the coatings produced by a subsonic plasma generator. This can be caused by the high flow rate and disintegration of par CHEMICAL TECHNOLOGY 10 μm (a) (b) Fig. 1. Microstructure of the oxide coating sprayed by (a) subsonic and (b) supersonic plasma generators.