Thermal conductivity of yttria-stabilized zirconia thin films with a zigzag microstructure

Abstract

Yttria-stabilized zirconia (YSZ) is the most common material used as a thermal barrier in several engineering applications. In order to improve the insulator potential of these thin films, an oblique-angle deposition approach was used to grow YSZ with tilted columnar structures. Initially, the period (n) was defined as the repetition unit composed of two layers each with different columnar growth directions, and then, n was increased (n = 1, 2, 10, 30, and 50), keeping constant the total thickness (∼3.50 μm). The influence of (n) on the structure, roughness, grain size, microstructure, and thermal conductivity (κ) of thin films deposited was determined by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy, and hot-plate technique, respectively. For all the samples, XRD patterns indicate the presence of the characteristic 8YSZ tetragonal phase peaks. Through AFM analysis, it was established that the roughness of the films deposited decreases from (4.0 ± 0.6) to (2.0 ± 0.6) nm when n is increased. Cross-sectional images recorded by SEM corroborate the formation of marked interfaces when growth direction changes occur, allowing to identify a multilayer system with a “zigzag” microstructure and an evolution towards more refined and isolated columns. Moreover, the SEM images reveal that for n = 10, 30, 50, and 70, the growth direction of the column is perpendicular to the substrate plane, losing its tilted form and the “zigzag” behavior becomes intra-columnar, reaching nanometer scale. Finally, analyses by hot-plate technique for different n repetition units showed that the thermal conductivity of YSZ films decreases from 0.151 W/m K to 0.064 W/m K, establishing the direct influence of the “zigzag” microstructure on the κ value. This study shows the potential of growing YSZ thin films by oblique-angle deposition as an effective method to improving the thermal insulator potential of this material.