Stiffness of free-standing thermal barrier coating top coats measured by bending tests

Abstract

Free-standing miniature beam specimens of thermal barrier coating (TBC) top coats were prepared by metal dissolution from high pressure turbine blades coated with TBC by electron beam physical vapour deposition (EBPVD) and thermally cycled to 1150°C for various times. The beams comprised the yttria stabilized zirconia (YSZ) TBC and the thermally grown oxide (TGO) and their effective elastic modulus was measured using a miniaturized three-point bending test. The measured effective modulus was typically in the range of 10–22GPa, with large specimen-to-specimen variations. The modulus increased with thermal exposure of the coated blades up to 85 cycles, but decreased for a larger number of cycles. The Young’s modulus of the YSZ layer alone was derived from the effective modulus of the composite beams (YSZ and TGO) by taking into account the contribution of the TGO. The derived Young’s modulus of the YSZ was in the range 5–10GPa, and was verified independently by TGO residual stress measurement. Significant inelastic deformation was found to occur during the bending test when a relatively high load was applied and is speculated to be due to micro-fractures between columns in the YSZ. Specimens prepared from the concave part of the turbine blade were found to be approximately four times stiffer than those taken from a flat part of the blade, indicating that the modulus of the TBC is strongly dependent on the microstructure of the YSZ coating.