ICMCTF2010 Session A1-2: Coatings to Resist High Temperature Oxidation

Spherical Al particles in the size range of 2 to 10 µm oxidise at high temperatures forming hollow spheres from alpha-Al₂O₃. Depositing such Al particles on a substrate alloy by brushing or spraying and subjecting them to a heat treatment, coating structures are obtained that consist of an aluminised diffusion zone and an adherent quasi-foam top coat from sintered hollow alumina spheres, which has the potential to effectuate as a thermal barrier by gas phase insulation. The present work aims at understanding the mechanisms and investigating the appropriate parameters for the design of multifunction coatings.

Spherical Al particles sized in the range of 2 to 10 µm were deposited with different binder/plastisizer systems by brushing on Alloy 347H and IN738 obtaining a coating with an average thickness of 60 µm. For understanding the influence of temperature on the oxidation of the particles, the samples were subjected to a step-wise heating from room temperature up to 1400°C with steps of 50°C. At each step an XRD pattern was recorded in situ. The results show a temperature range, in which transient alumina phases with an amorphous structure are formed, followed by the formation of alpha-alumina on further heating.

For investigating the heat treatment for the coating formation, the samples were subjected to 1 to 4 h at 400°C and 500°C for curing and then to 5 h at 700°C to 900°C for forming the coating structure. Coating structures are encountered that consist of a quasi-foam top coat from hollow alumina spheres and a diffusion layer below that forms a protective alumina scale. Depending on the parameters of the thermal treatment, the sintering of the hollow alumina spheres to each other and the adherence of the top coat to the substrate is more or less pronounced. The formation of the diffusion zone and the adherence is influenced by the binder.

The structure stability and oxidation behaviour of the system was studied up to 2000 h. The adherence of the top coat depends strongly on a good and homogeneous contact during the thermal treatment. This can be controlled by the selection of an appropriate binder system. Only small amounts of brittle aluminide phases were found in the diffusion zone and no cracking was observed. A protective alumina scale is formed on the diffusion zone surface.

Operating a large number of construction materials in very aggressive environments of diverse industrial processes demands to develop new functional coatings protecting effectively the surface of these materials against oxidation and corrosion at temperatures exceeding 1000°C. Such coatings have to be thermally stable without any degradation of their structure and properties. Recently, novel amorphous Si-B-C-N coatings, which may accomplish all of these criteria, have been successfully prepared by magnetron sputtering in our laboratories. Their excellent oxidation resistance, due to the formation of an amorphous surface SiO₂-based layer containing boron [1‑2], even above 1500°C during dynamical heating in flowing air, has stimulated the present study extending our knowledge about thermal stability and oxidation kinetics of these coatings. The Si-B-C-N coatings magnetron sputtered from a single B₄C-Si target in a nitrogen‑argon gas mixture at optimum process parameters were investigated by means of differential scanning calorimetry (Labsys DSC 1600) and symmetrical thermogravimetry (TAG 2400). The structure of the coatings was characterized primarily by X‑ray diffraction. Freestanding coating fragments without a substrate were used as specimens for the experiments in order to eliminate an influence of a substrate. It was found that the magnitude of the N/(Si+B+C) concentration ratio is of a key importance to achieve ultra-high thermal stability of the coatings. When this ratio is sufficiently high, the Si-B-C-N material retains its amorphous structure in inert gases up to 1600°C . Analysis of oxidation kinetics revealed concurrently that the oxidation rate constants at temperatures ranging from 1100 to 1300°C are very low, which is caused especially by a low number of diffusion channels in the protective oxide layer being formed on the surface of the Si-B-C-N coatings.

[1] J. Vlcek, S. Hreben, J. Kalas, J. Capek, P. Zeman, R. Cerstvy, V. Perina, Y. Setsuhara, J. Vac. Sci. Technol. A 26, 1101 (2008).

To prevent the diffusion out of cobalt from cemented carbides at high working temperature, TaN_x coatings were prepared as a diffusion barrier by direct current magnetron sputtering using a Ta target in an argon-nitrogen atmosphere. The nitrogen flow ratio, N₂/(Ar+N₂), in the sputtering process varied from 0.1 to 0.4. The deposition rate reduced as the nitrogen flow ratio increased. Silicon wafers and 6 wt.% cobalt cemented tungsten carbide were used as the substrates. Effects of nitrogen flow ratio on crystalline characteristics and mechanical properties of the TaN_x coatings were examined by X-ray diffraction and nano-indenter, respectively. The TaN_x coatings were annealed at 500, 600, 700, and 800^oC for 4 hours in air, respectively. The diffusion barrier performance was evaluated by Auger electron spectroscopy depth profiles and X-ray diffraction. Oxidation resistance of the TaN coatings was also investigated. Orthorhombic Ta₂O₅ was observed after annealing above 700^oC. The hardness declined with increasing annealing temperature due to the formation of the oxide compound with a lower hardness.