Residual Coating Stresses

During plasma spraying, the particle-substrate interactions characterised by momentum and heat transfer from the solidifying particle after impacting the surface of the substrate lead eventually to the development of residual stresses in the coating. These stresses can be divided into microscopic, mesoscopic and macroscopic stresses. 

Since residual coating stresses influence the quality and the service life of coatings, in particular their adhesion and wear performance, it is the goal of any coating development to minimise such stresses. The origin of residual stresses is twofold. First, rapid quenching of the molten particles at the substrate interface results in frozen-in particle contraction according to 

The determination of the stress state is based on the measurement of the lattice deformation of a polycrystalline materials subjected to stresses. This is accomplished by measuring the change of the /J-values of the interplanar spacing of selected lattice planes hid relative to the stress-free state, D0  

Because the penetration of the radiation into the coating is rather limited (1 -10 pm) only the stress state of the coating surface can be measured with accuracy. To obtain a stress distribution profile the surfaces must be consecutively removed by polishing, sputtering or etching and the measurement be repeated. 

Differentiation of the well-known Bragg equation (Eq. (7.1a) nA = 2Ds m0 yields the (relative) lattice deformation 

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From the residual coating strain, the residual coating stress can be calculated assuming a two-dimensional stress state in the thin coating. 

A variant of the sin2lP method, likewise based on lattice deformation caused by residual coating stresses, is the direct calculation of stress based on the equation... 

In the past, although much effort has been expended to predict residual coating stresses by modeling the life expectancy of the TBCs, problems were encountered by the assumption of a continuum theory and the non-consideration of elastic finite elements, nonlinear processes, and the general fractal nature of plasma-sprayed coatings (Heimann, 2008). 

Residual Compressive Stress. Residual compressive stress ia commercial ground coat enamels varies with enamel thickness ... 

Thermal shock resistance is a direct function of enamel thickness. The greater the residual compressive stress in the porcelain enamel, the greater is the resistance to thermal shock failure. Thin coatings, such as one-coat enamels or the two-coat enamels having alow expansion titania covet coat, provide excellent thermal, shock resistance. 

Fig. 7.15, Normalized residual radial stress as a function of Young s modulus ratio, Ej/Em, for varying coating thickness, t/a = 0.05, 0.1, 0.2. Coefficients of thermal expansion (CTE) of the coating (a) Kc = 100 X 10-V°C (b) = 20 X 10-V°C. After Kim and Mai (1996a, b).

A candidate interlayer consisting of dual coatings of Cu and Nb has been identified successfully for the SiC-Ti3Al-I-Nb composite system. The predicted residual thermal stresses resulting from a stress free temperature to room temperature (with AT = —774°C) for the composites with and without the interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the composite constituents used for the calculation are given in Table 7.5. A number of observations can be made about the benefits gained due to the presence of the interlayer. Reductions in both the radial, and circumferential, o-p, stress components within the fiber and matrix are significant, whereas a moderate increase in the axial stress component, chemical compatibility of Cu with the fiber and matrix materials has been closely examined by Misra (1991). 

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