Evaluation of thermal conductivity of zirconia coating layers deposited by EB-PVD

Abstract

Electron beam-physical vapor deposition (EB-PVD) is a widely used technique for depositing thermal barrier coatings (TBCs) on metal substrates for high temperature applications, such as gas turbines, in order to improve thermal efficiency [1]. Characterization of the thermal conductivity of the coating layers is therefore very important for developing superior thermal barrier coatings, but because of the irregular nature of the coated specimens it is difficult to derive the thermal conductivity of the coating layer from measurements of the thermal conductivity of the combined coating and substrate. Two steps are therefore involved in determining the thermal conductivity of a thin coating film: (i) separation of the coating film from the combined coating and substrate specimen, and (ii) measurement of the thermal conductivity of the film. With regards to the first step, it is known that coating layers deposited by EB-PVD have a porous structure so that they are easily damaged because of their poor strength [2, 3]. In other words, in practice it is not easy to physically separate the coating film from the coated substrate by machining or some other method without damaging it. Regarding the second step, even if the coating can be successfully separated from the substrate, it is not a simple matter to measure directly the thermal conductivity of the coating. The laser flash method is generally used to accurately measure the thermal diffusivity and specific heat capacity of materials, from which the thermal conductivity can be calculated. The technique was developed by Parker et al. [4], and is usually carried out assuming the specimen to be uniformly dense and opaque. However, coated layers deposited by EB-PVD have a columnar non-uniform structure, making it difficult to measure the thermal conductivity directly. The aim of the present work is therefore to derive a practical method for determining the thermal conductivity of coating layers based on theoretical calculations, and comparing the values obtained with direct experimental measurements. We have therefore adopted the response function method as a means of determining the thermal conductivity of the coating layers. It has been reported that the response function method is a powerful method to analyze one-dimensional heat diffusion across multi-layer materials [5]. We also present the experimental results from thermal conductivity measurements of coated substrates as well as coating layers detached from their substrates as a function of substrate thickness. In this work, ZrO2-4 mol% Y2O3 coatings have been applied by EB-PVD to zirconia substrates with the same composition as the coating material to minimize interface effects on thermal conductivity. Disc-type zirconia substrates were prepared by pressureless sintering at 1600◦C. The sintered substrates were machined to 10.0 mm diameter and 0.1–3 mm thickness. The substrates were first preheated at 900–1000 ◦C in a heating chamber using a graphite heating element. An electron beam evaporation process was used to deposit the film in a coating chamber under a vacuum level of 10−4 Pa using a 45 kW electron gun at a rate of 4 μm/min and substrate rotation speed of 5 rpm. The average coating thickness was about 300 μm. The density of each specimen was determined by measuring its mass on an electronic balance and its volume with a micrometer. All thermal diffusivity and specific heat capacity measurements were carried out three times for each specimen at room temperature by the laser flash method. The microstructure of the coated specimens was observed by SEM. A typical microstructure of a specimen coated on a zirconia substrate is shown in Fig. 1. The crosssectional surface of the coated specimen clearly reveals the columnar microstructure, with all columnar grains oriented in the same direction, i.e., perpendicular to the substrate. This columnar structure is very similar to those reported for metal substrates coated by EB-PVD [3]. In other words, the distinctive columnar microstructure can be obtained regardless of whether the substrate being used is metal or ceramic. The correlation between temperature rise at the rear surface of a specimen and time is shown in Fig. 2 when the front surface of the specimen is uniformly heated using a laser pulse. For bulk materials, the thermal diffusivity (α) is described by the following equation: