Multifunction High Temperature Coating System Based on Aluminium Particle Technology
Spherical Al particles sized in the range of 2 to 5 μm were deposited with an organic binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). The coated samples were annealed in air at 400°C for 1 h in order to expel the binder. For studying the oxidation behaviour in air, isothermal experiments were performed at 700°C and 900°C with oxidation times of 5 h, 100 h and 1000 h. The oxide formation was studied in situ by high temperature X-ray diffraction (HTXRD) up to 100 h. Field emission scanning electron microscopy (FE-SEM) was applied to investigate the surface and the cross-section of the particle coating. During oxidation, the stable α-Al2O3 was identified in situ by HT-XRD on all studied samples at both temperatures. No meta-stable alumina phases were found. In the initial state, 2 h at 900°C, the Al particles are completely oxidised to hollow alumina spheres, controlled predominantly by the reaction due to the small particle size and relatively high surface portion. Simultaneously, the Alrich diffusion layer is formed in the substrate. On further exposure, a thin protective alumina scale continues growing on the top of the diffusion layer. After exposure to both 700°C and 900°C, a coating structure was encountered, which consists of a quasi-foam top coat from conjoint hollow spherical alumina particles and an Al-rich diffusion layer below. The quasi-foam top coat has the potential to effectuate as thermal barrier by gas phase insulation, while the diffusion layer below serves as protective coating against oxidation. The approach by particle size processing opens a potential for obtaining a complete thermal barrier coating system in one manufacturing step. The coating properties can be adjusted by parameters like selection of source metal/alloy, particle size, substrate, binder and heat treatment.
Pierre Steinmetz, Ian G. Wright, Alain Galerie, Daniel Monceau and Stéphane Mathieu
V. Kolarik et al., "Multifunction High Temperature Coating System Based on Aluminium Particle Technology", Materials Science Forum, Vols. 595-598, pp. 769-777, 2008