Thermal spray coatings ceramics, applications

In Thermal spray processes the nature of the process was described here, applications to adhesion are presented. Thermal spray coatings for applications involving adhesion are in three general areas, metallic bond or anchor coating, ceramic bond coat, and sprayed polymer as the adhesive. In many cases, the surface modification by the deposition of a thermally sprayed coating is an alternative to environmentally hazardous methods. The thermal spray process is a dry process in which the effluent is collected in a dry cartridge system and usually recycled. 

Coating and thin films can be applied by a number of methods. In thermal or plasma spraying, a ceramic feedstock, either a powder or a rod, is fed to a gun from which it is sprayed onto a substrate. For the process of physical vapor deposition (PVD), which is conducted inside an enclosed chamber, a condensed phase is introduced into the gas phase by either evaporation or by sputtering. It then deposits by condensation or reaction onto a substrate. A plasma environment is sometimes used in conjunction with PVD to accelerate the deposition process or to improve the properties of the film. For coatings or films made by chemical vapor deposition (CVD), gas phase chemicals in an appropriate ratio inside a chamber are exposed to a solid surface at high temperature when the gaseous species strike the hot surface, they react to form the desired ceramic material. CVD-type reactions are also used to infiltrate porous substrates [chemical vapor infiltration (CVI)]. For some applications, the CVD reactions take place in a plasma environment to improve the deposition rate or the film properties. 

A ceramic bond coat is almost exclusively used to modify a surface for adhesive bonding. The ceramics include, but are not limited to, alumina, zirconia, titania, spinels, carbides and combinations of these materials." The thermally sprayed bond coatings provide a rough surface on the order of 8 p,m R that has a three-dimensional morphology. Arc-plasma is traditionally used for deposition of the ceramics for these applications. 

A plasma spray can therefore be used to apply sintered coatings to substrates which would normally be adversely affected by the sintering temperature of the coating. However, the powders carried by the plasma are exposed to greater thermal stress than the substrate, and some experimenting with specific powders is usually necessary to define the best application conditions. Where molybdenum disulphide in a metal, resin or ceramic binder is applied by this technique, the optimum conditions will usually be different for the two materials. This difficulty has been overcome by the use of two separate entry ports into the nozzle for the two components. The use of this technique has even been applied to molybdenum disulphide in a polyethylene binder. 

The plasma spray technique offers a straightforward and cost-effective means to spray deposits of metals and ceramics that are tens to hundreds of micrometers in thickness onto a variety of substrates in applications involving thermal-barrier or insulator coatings. Typical plasma-spray deposits are porous, with only 85-90% of theoretical density. 

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