Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning

Abstract

Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed.