Microstructure and Thermal Diffusivity of Rare Earth Zirconate Powders for Thermal Barrier Coating Systems Obtained by APS Method

摘要

The paper presents results concerning thermal diffusivity and microstructural characteristics of the powders on the basis of rare earth zirconates with RE_2Zr_2O_7 type pyrochlore structure. Powders on the basis of Gd, La, Sm and Nd were used as the study material. The assessment of surface morphology and inner structure of powders (SEM) was made. The chemical composition (EDX, OES-ICP) of the powders was determined, with special attention being paid to uniformity in arrangement of alloy elements and content of carbon, sulphur, oxygen and nitrogen. Phase composition by using XRD and EBSD methods was also analysed. The study performed showed that the analysed powders are predominantly characterized by a spherical shape. Powder surface shows low smoothness and high porosity, both surface and inner porosity. The analysis of contamination content showed their similar content to the standard commercial powders of 8YSZ type. The analysis of phase composition depicted predominant share of RE_2Zr_2O_7 type cubic phases with low quantities of rare earth oxides and zirconium. Measurements of thermal diffusivity of powders by a laser-flash method (LFA 427 by Netzsch) were carried out on round pressed samples (Φ -12.7 mm, h - 2 mm). Neodymium powder (0.121 mm~2s~(-1)) had the lowest value for the thermal diffusivity at ambient temperature and samarium based powder had the highest value of diffusivity (0.149 mm~2s~(-1)). Decreasing thermal diffusivity of all powders up to a temperature of 750°C and increasing values were measured up to 1500°C. Within a range of potential working temperature of the TBC layers, neodymium powder had the best diffusivity and the other powders are ordered as following: lanthanum powder, samarium powder, gadolinium powder and 8YSZ commercial powder. Measured values of thermal diffusivity at a temperature of 1250°C were as follows: 0.097; 0.135; 0.146; 0.165 and 0.181 mm~2s~(-1). Porosity of tested samples was below 1%.