Thermal expansion coefficient of the coating

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Concrete surface temperature, ambient temperature, and relative humidity can affect the performance of the coating. If the thermal expansion coefficients of the coating and concrete are widely different, separation at the interface is likely. 

Fig. 7.14. Normalized radial residual stresses as a function of coating thickness, I/a, for varying coefficients of thermal expansion (CTE) of the coating, Oc = 10,70,130 x 10 /°C (a) Young s modulus ratio Ej/Em = 0.333 (b) Ei/En, = 1.0. After Kim and Mai (1996a, b).

There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to tailor the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint. 

The primary requirement is to adjust the thermal expansion coefficients of the frit and enamel to that of the metal. For this purpose and for the necessary corrections, use is mostly made of additive factors. In view of the imperfections involved in such calculations, the main criterion is always the laboratory test the enamel is applied on a metal specimen and fired to assess the resulting coating quality. 

From the cost point of view, precious metals (such as Au and Pt) are surely out of contention for practical coatings on SS substrates, although they might be used for short laboratory tests. In fact, electrochemical corrosion cells will be generated from the possible pinholes in the coatings due to the electrochemical dissimilarity of the precious metals and SSs in a PEMFC environment. Difficulties encountered with the carbon-based and conductive polymer-based coatings are application at intermediate temperatures, the cold-start issue, and the differences in thermal expansion coefficients between the coating itself and the substrate SS. The risk of... 

Stress can build up in a coated plastic part and can affect coating mechanical properties. Stress can accumulate during film formation and from variation in relative humidity and/or temperature (33). Even differences between thermal expansion coefficients of the substrate and the coating can induce stress. The dissipation of accumulated stresses is key to avoiding premature system failure. 

Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

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