The Microstructure and Mechanical Properties of ZrO2-Al2O3 Thick Film Formed on the SUS304 Sheet Using Thermal Spray

A ZrO2-Al2O3-based thick film has been formed on the SUS 304 stainless-steel sheet using a thermal spray in air (atmospheric plasma spraying: APS). The chemical composition of starting raw material (cubic ZrO2 solid solution: c-ZrO2(ss)) is 75mol%ZrO2(1.5mol%Y2O3)-25mol%Al2O3, which has been prepared by the neutralization co-precipitation method followed by the calcination of 1373 K(1100°C) for several hours in air for controlling their particle size to 50~100 µm. Three kinds of films with their thickness around 30 to 120 µm have been fabricated by increasing the number of soaking from 1, 3 and 5. X-ray diffraction (XRD), laser microscopy, scanning electron microscopy (SEM) revealed that the middle portion of films consisted of about 55 mass% Zr0.33Fe0.67O1.67, 34 mas% tetragonal ZrO2 (t-ZrO2), 8 mass% monoclinic ZrO2 (m-ZrO2), and 4 mass% -Al2O3; these crystalline phases might be explained by the decomposition of melting c-ZrO2 (ss) passing through the high temperature plasma, followed by the rapid thermal quenching on the substrate. The volume t-ZrO2 ratios vs. m-ZrO2 in the films are 65~80%, suggesting that high mechanical properties could be expected. Nano-indentation analysis showed Vickers hardness Hv of 12.8 and 14.3 GPa were achieved for 3 and 5 soaking-pass films, respectively. And also high fracture toughness has been expected from the high areas within the load-displacement curves that were obtained during nano-indentation. Thermal conductivity t of 9.4 W·m-1·K-1, which is much reduced from t (14.0 W·m-1·K 1) of SUS304, was attained for the 5-pass film. From these good mechanical and thermal properties, it has been proved that the thermal sprayed ZrO2-Al2O3 thick films formed on the metal substrates could have higher application capability.