Thickness-dependent fracture characteristics of ceramic coatings bonded on the alloy substrates

The ceramic coatings with different thicknesses in the range of 150 μm to 490 μm, bonded on the same alloy substrates, were obtained by the air plasma spray method. The crack evolution and the fracture characteristics of the samples were observed synchronously under the three-point bending load using a scanning electron microscope. The results show different fracture modes for the thinner and the thicker coating samples; the multiple transverse cracks, vertical to the interface between the coatings and the substrates, in the coatings are the main fracture mode for the samples with coatings thinner than about 200 μm, while the interface crack is the main fracture mode for the samples with coatings thicker than about 300 μm. Different fracture modes were theoretically analyzed based on the nonlinear delamination model, and the calculated critical thickness is 255 μm, which agrees well with the experimental result. The finite element simulations for the corresponding coatings with a series of thicknesses were carried out by introducing the interface cohesive model, and the simulations show results similar to those of the experimental observations. The study indicates that the fracture modes depend on the thickness of the coatings, and the critical thickness depends on the intrinsic properties of coatings and the interfaces between the coatings and the substrates, such as the elastic modulus, the interface strength and the fracture energy.