Oxide layers for high and highest temperatures
Metal materials which, due to their application, are exposed to very high temperatures require a protective surface layer. Generally, chromium or aluminum is used for this. The layers of pure chromium oxide (Cr2O3) or pure aluminum oxide (Al2O3) formed at high temperatures must adhere well, be dense and free of cracks, grow sufficiently slowly and not evaporate significantly. At temperatures above 1000 °C, practically solely the thermodynamically stable high-temperature phase of aluminum oxide (α-Al2O3, corundum) fulfils these conditions.
The stable phase of the aluminum oxide can be produced thermally from suitable oxide formers such as MCrAlY. The growth of aluminum oxide takes place from the outside to the inside through the diffusion of oxygen ions to the interface with the metal. The addition of alloying elements, such as yttrium, to the oxide formers improves the adhesion and the service life of such protective coatings. The left Figure above shows a thermally grown aluminum oxide which has formed on an FeCrAl alloy at 1050 °C.
Sputtered aluminum oxide layers
Well-adhering protective layers of pure aluminum oxide can also be applied to heated metallic components by means of thin film techniques, e. g. reactive gas flow sputtering (see Figures above). The decisive factor hereby is that, if possible, the α-phase is already present in the layer prior to the first application at high temperatures. A heating of the low temperature phase (γ-Al2O3) above the threshold of 1000 °C would lead to a strong alteration in volume due to the phase transformation and therefore to the failure of the layer compound. Furthermore, pores can form within the layer which then impair the application. Tests at the Fraunhofer IST have shown that by means of ion assistance or suitable template layers, the formation of the α-phase is already promoted at deposition temperatures of approx. 830 °C. By means of X-ray diffractograms of reactive sputtered aluminum oxide layers, the right Figure above shows that a chromium template layer encourages the development of the rhombohedral corundum phase.
Outlook: oxide layers for high-temperature sensor technology
Crack-free and dense barrier layers of aluminum oxide are also an important component for the integration of additional electrical functions on components in a high-temperature environment. Sensors, actuators and electrical information transfer only become possible through high-quality, temperature- and cycle-resistant insulation layers. Good insulation properties are achieved with layer thicknesses of several micrometers. By means of conventional magnetron sputtering, good and adherent layers can be applied. The relatively low deposition rates and the complex process control are, however, disadvantageous. In this respect, reactive gas-flow sputtering offers the possibility of achieving high rates with simple process management. Parallel to the development of the insulation layers, the Fraunhofer IST is also investigating high-temperature sensor layers, e. g. for strain measurement.