Coating surface cracks

In the above analyses, we have only considered interfacial fracture. Several other cracking patterns have been observed in coated systems subject to residual stresses. The more common examples include coating surface cracks (Fig. 13), coating channelling cracks (Fig. 14), substrate cracks (Fig. 15), and spalling... 

The thermal properties of C3 materials at high temperatures are most remarkable if protected from oxidation. This issue is discussed below in more detail. If they are not oxidized, the C3 materials exhibit similar stability data as ceramics [22], in particular at temperatures above 1500 K where protective coatings applied behave like a plastic and close developing surface cracks against air attack. C3 materials expose the advantages of their hierarchical structure being present in both filler and binder phase and develop wood-like properties under ambient conditions. A descriptive pa-... 

Survey coatings for surface cracking, delamination, rust staining, or bulging. 

Major causes for coating failure are surface cracking and undetected pinholes or voids. These can be repaired and serious problems avoided. Coatings generally fail in different modes, these are chemical failure, abrasion failure, adhesive failure, cohesive failure and undercoat corrosion. For performance evaluation of coatings on experimental basis on these parameters various ASTM and BS specifications are presently being used. 

Optical microscopy can be used to characterize the surface of the charred residue after heat treatment. As an example, Figure 10.18 compares the optical microscopy of virgin PU coatings with that of fire-retarded coatings. Whereas cracks are observed on the surface in the case of PU, large... 

Figure 1. Mode I stress intensity factor for a surface crack in FGM coating, hi = hi.

The experimental thermal load consisted of a line heat flux focused along the center of the specimen. The specimen geometry was a multi-layered beam consisting of a mullite coating and intermediate layers which were composed of mixtures of mullite and CoCrAlY (the bond coat material). The resulting fracture mode was multiple surface cracking formed in the region near the thermal load. 

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