Elastic and inelastic deformation properties of free standing ceramic EB-PVD coatings

Abstract

Thermal barrier coatings (TBC) processed by electron beam physical vapor deposition (EB-PVD) show complex deformation behavior due to their columnar microstructure and multi-scale porosity. Limited information is available regarding the inelastic deformation behavior. Furthermore, the existing data for the elastic properties of EB-PVD coatings varies in a wide range, depending on the measurement method. In most cases, the fragile EB-PVD coatings are tested on the substrate, onto which they were deposited. Since the ceramic coatings are deposited onto metallic substrates, which have about 1000°C, compressive residual stresses develop in the coating after cooling down. Since the deformation behavior of EB-PVD coatings is stress dependent, the residual stresses cannot be calculated without knowledge of the stress-strain response, and without knowledge of the residual stress the elastic properties cannot be measured correctly. To overcome this dilemma, tubular free standing EB-PVD coating samples with a thickness of 250µm have been processed and tested in compression at room temperature. For introducing the mechanical load safely into the gauge length a soft clamping has been developed, and the strain was measured using a contact-free laser extensometer. Several subsequent loading and unloading cycles were performed on specimens with different thermal pre heat treatments. Non-linear stress-strain behavior was observed, showing increasing stiffness with increasing compressive stress. Cycle by cycle the stiffness increased, indicating inelastic deformation, which has also been observed in creep experiments in the same test configuration. Long term thermally aged specimens showed higher stiffness than as processed specimens.