Study of the mechanics of the delamination of ceramic functional coatings

Delamination experiments were performed on composite materials consisting of a thick ceramic coating on a metallic substrate as is common in thermal barrier coatings (TBC). TBCs consist of a ceramic coating (200–400 μm thick) deposited on a metallic substrate. Failure and delamination of the ceramic can be initiated by transverse cracks caused by sintering processes, thermal stresses and internal oxidation during prolonged exposure to high temperatures. Although these transverse cracks do not the function of the thermal barrier coatings, they may induce important stresses at the interface between substrate and coating, and delamination is initiated. The cracking and delamination after continued loading of ceramic plasma sprayed ZrO2 coatings with a thickness ranging from 250 to 400 μm was studied by four-point bending experiments. The estimation of the stress distribution and concentration at the interface during these experiments allows the failure stress of the ceramic and of the toughness of the interface coating–substrate to be evaluated. The interfacial toughness of a ZrO2 (Y) coating deposited on sand blasted Inconel 617 substrates was about two times higher than the same coating deposited on a polished CMSX4 substrate (KI=3.0 and 0.9 MPa m1/2, respectively).