Plasma thermal coating stress

These factors lead to an inferior barrier and a more rapid reduction of the barrier effect. For a good barrier effect, co-extrusion or coating of the substrate, corona discharge, or plasma pretreatment, are, therefore, usually necessary. Vacuum metallization conditions, thermal stress of the substrate, and mechanical stress of the metal layer also play a fundamental role ... 

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 expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

The surface of upper disc is coated with a layer of TiC/Mo materials by a plasma spray method . The TiC surface has emissivity of approximately 0.9 and can be heated up to a temperature range of 1800 to 2000 [K] by solar concentration. Furthermore, such a FGM layer reduces the stress caused by difference of thermal expansion between TiC surface and Mo base. 

Thermal Expansion Matching. The coefficients of thermal expansion (CTE) of coating and substrate should match as closely as possible. The CTE of the coating is usually lower than that of a metallic substrate and, upon cooling from the deposition temperature, thermal stresses are produced which may cause cracks and delamination. Such considerations have led to the development of low-temperature deposition processes such plasma-CVD or metallo-organic CVD (MOCVD) which minimizes these stresses and reduces the chance of coating failure (see Sec. 3.1).P1... 

M. Ahrens, R. Vassen, D. Stover, Stress distributions in plasma sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness. Surface and Coatings Technology 161... 

Figure 14. Typical stress strain curve for plasma sprayed Zr02 8wt% Y2O3 thermal barrier coatings in compression".

Functional structured YSZ coatings have been produced by using thermal plasma physical vapor deposition (TP-PVD) [27]. In this process, a peculiar layered stmcture composed of both powder-sprayed splat and vapor-deposited nanoparticles or columns is achieved by PVD integrated with droplet deposition. The splats obtained by plasma spraying act as radiation shields, and the columns or nanoparticles with nanocracks and pores help to reduce both photon and phonon transportation and to release the stress that accompanies drastic temperature change. 

---