Behavior and mechanism of the stress buffer effect of the inside ceramic layer to the top ceramic layer in a double-ceramic-layer thermal barrier coating

Abstract The Double-Ceramic-Layer Thermal Barrier Coating (DCL-TBC) consists of a top ceramic layer (TC1), an inside ceramic layer (TC2), bond coat (BC) and alloy substrate. The top ceramic layer is made by new ceramic materials which has the lower thermal conductivity, such as LZ, LZ7C3, LMA etc. Although these materials have good high temperature performance and thermal insulation properties, their thermal expansion coefficients are very low which cause higher degree of mismatch with material properties of alloy substrate. The behavior of the stress buffer effect of the layer of TC2 is investigated aiming at the DCL-TBC with the two ceramic layers as LZ7C3(TC1) and 8YSZ(TC2). A FE model of the DCL-TBC is established and the stress of the TC1 during the thermal cycles is calculated. Results show that the layer of TC2 has the behaviors of stress buffer effect on reducing the stress of TC1 and reducing the influence of the TGO growth upon the stress of TC1. The inner mechanism of the stress buffer effect is investigated based on the influence of the thermal expansion coefficient ρ of the TC2 layer. Results show that the TC1 layer varies with the TC2 layer, having different thermal expansion coefficients; and the stress buffer effect is dominant, affected by the thermal expansion coefficients of the TC2 layer. The thermal expansion coefficient should be neither too large nor too low; if its values are too low or too large, the stress levels of TC1 are all high. The reasonable range of the thermal expansion coefficient is investigated. Aiming at the DCL-TBC with the two ceramic layer thicknesses as 150 μm, the reasonable range of the thermal expansion coefficient of TC2 is from 10.35 °C−1 (×10−6) to 10.887 °C−1 (×10−6).