Nonlinear behavior in compression and tension of thermally sprayed ceramic coatings

Mechanical properties of thermally sprayed coatings, especially of ceramics, are strongly influenced by a high density of mesoscopic defects, microcracks of dimensions between fractions of pm up to tens of tun. The anisotropic linear elastic stress-strain relations are valid only at very low deformations, e.g., lei < 0.05%, with small values of Young's moduli due to elastic openings and elastic partial closings of microcracks. At higher deformations, e.g., 0.05% < lel < 0.4%, the stress-strain relations are strongly nonlinear. Under compressive stresses, elastic closing of microcracks leads to a gradual decrease of the microcrack density and to an increase of Young's modulus in compression. Under tensile stresses, the microcracks slightly grow by inelastic processes; the microcrack density gradually increases and effective Young's modulus in tension decreases. A two-parametric equation containing linear and quadratic terms is used to describe the nonlinear stress-strain curves of plasma-sprayed ceramic coatings. The effect of nonlinearity on the bending of beams with coatings and the nonlinear combination of external and residual stresses are discussed. The fracture of coatings at higher tensile stresses due to coalescence of the microcracks is mentioned.