Elastic and inelastic mechanical behaviour of free standing EB-PVD thermal barrier 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. 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, having about 1000°C, compressive residual stresses develop in the coating after cooling down. Since the deformation behavior of EB-PVD coatings is known to be 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. Specimens in as coated condition displayed non-linear stress-strain behavior, showing increasing stiffness with increasing compressive stress. Long term thermally aged specimens showed higher stiffness than as processed specimens and linear stress-strain behaviour. The data measured on the free standing tubular specimens are compared to the data obtained from the same coating system but using different testing methods.