Critical coating thickness

Fig. 11. Dependence of the peel strength of tin plate/epoxy systems on coating thickness for coatings cast from DTPM and methyl cello-solve. These theoretical curves were constructed using experimental values for modulus, critical coating thickness, solvent evaporation rate, solution concentration and interfacial work of adhesion96 (Reprinted from Ref. 96, p. 123 by cautesy of Plenum Press)...

The most critical steps in the manufacturing process are casting and lamination, and tensions and pressures must be carefully controlled to produce a wrinkle-free laminate with uniform drug content and reproducible adhesive-coating thickness (Jenkins 1995). 

Coating thickness can interact with other factors in a complex manner to influence the critical load. Thicker coatings commonly have lower critical loads because of accumulated residual stresses. However, hard thicker films can also provide greater load-bearing capability if residual stresses are not too high. This is particularly important for hard, brittle coatings on soft, ductile substrates where considerable substrate deformation can occur before delamination failure. Under these conditions, the critical load for spallation may increase with coating thickness. 

Critical Cracking Thickness - The maximum thickness which can be coated in a single layer (pass) of polytetrafluoroethylene dispersion without crack formation. This thickness is measured after sintering has been completed. 

In the context of the interpretation of the critical loads, the wide variation in the mode of failure mentioned above and also the dependence of the critical load on the coating thickness [71] indicate that eqns. (9-11) need to be validated and updated experimentally. 

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