Materials for High-Temperature Technologies
The department investigates innovative high-performance coating techniques with a focus on thermal spraying for High-Temperature Technologies. Research and development of improved materials for these applications is closely linked. Typical applications are thermal and environmental barrier coating systems for stationary and aircraft gas turbines.
Thermal barrier coating systems
Thermal barrier coating (TBC) systems are an essential constituent of modern gas turbines since only these can achieve the high fuel gas temperatures necessary for efficient operation. During the volume expansion of a heated gas, these turbines transform the heat into kinetic energy. On the basis of very fundamental physical laws, a high temperature of the gas is decisive for a high efficiency of the energy transformation. Above approx. 600°C, the internal components of the turbine (usually metals) must be protected from the temperature because they do not withstand these conditions in the long run. This happens with thermal barrier coating systems.
The standard material that has been established worldwide for thermal barrier coatings is zirconia that has been partially stabilized with 7–8 wt% Y2O3 (YSZ). YSZ has a number of outstanding properties for this application, including low thermal conductivity, high expansion coefficients for reducing thermal stresses in the composite with the metallic substrate, and good fracture toughness.
Optimization of coating design and application of new processing technologies (see below) enables optimized materials microstructure which still expands the capabilities of this system for the application.
In addition to the development of extremely efficient YSZ layers, another area of work at IEK-1, Forschungszentrum Jülich, focuses on developing new materials for thermal barrier coatings. This is necessary because of the limited temperature stability of YSZ. Long-term applications are limited to temperatures of approx. 1200 °C because higher temperatures cause phase transitions and amplified compacting of the layers. Of particular interest are pyrochlores such as Gd2Zr2O7, complex perovskites, or (hexa-)aluminates. Due to the relatively low fracture toughness of these new TBC materials, the double-layer systems developed at Jülich are usually used. As figures show below, the bond coat is first covered with a layer of the tough YSZ, and then with a layer of the new ceramic.
For the coating of gas turbine components, mainly the atmospheric plasma spray process (APS) is used. This results in lamellar structures which are typically interspersed with pores and microcracks. Such microstructures have a low modulus of elasticity which leads to correspondingly low stresses. Moreover, a good strain tolerance is made possible by opening the cracks. This is important since the difference in the thermal expansion between the substrate and the TBC causes thermal stresses during the heating and the cooling. The porosity also reduces the thermal conductivity and thus improves the thermal insulation effect of the coatings.
In addition to the thermally insulating ceramic coating, a thermal barrier coating system also consists of an intermediate metallic layer, the so-called bond coat. It improves the fusion adherence between the metallic substrate and the ceramic top coating. Moreover, it protects the substrates from the oxidation and corrosion caused by the hot fuel gases. The coatings are manufactured often using various thermal spraying processes such as low-pressure plasma spraying (LPPS), high-velocity oxy-fuel spraying (HVOF), or cold gas spraying.
In addition to the refinement of the mentioned processes, new techniques are also being tried out. One particularly interesting process in this respect is suspension plasma spraying (SPS) in which suspensions are introduced into the plasma flame instead of input feedstock materials in powder form. One process which is also at the development stage is the Plasma Spray-Physical Vapor process (PS-PVD). At a low pressure and a high power, this leads to the formation of a very large, highly laminar plasma jet. In this case, the input materials in powder form are vaporized, thus resulting in the deposition of coatings which have a columnar structure and exhibit an outstanding efficiency in the thermal cycle test.
Ceramic Matrix Composites
Additionally to the TBCs and EBCs activities, we develop also Ceramic Matrix Composites (CMCs) for gas turbine components and/or other systems that operate under high temperature and harsh environmental conditions. The novelty of these composites is that the matrices are based on MAX phases, a novel family of layered materials that bridge the gap between ceramics and metals. Due to their ceramic character, MAX phases present low density, high elastic modulus, and good oxidation and corrosion resistance at high temperature, whereas as their metal character, they exhibit high electric and thermal conductivity, readily machinability, good thermal shock and damage tolerance. As a result, MAX phases are new and potential candidates for high-temperature applications. Among all the MAX phases, we are mainly focused on Cr2AlC, Ti3SiC2, Ti2AlC, and Ti2AlN. Regarding the fibers, short SiC, C, and Al2O3 fibers are used as reinforced phase.
The CMC team has patented a process to synthesize large amount (kilograms) of high pure MAX phases (> 98%) since highly pure powders are not commercially available. Processing of the composites by different techniques (such as infiltration, tape casting, injection molding, additive manufacturing, etc.) and sintering (i.e. by pressureless sintering, HP, FAST/SPS, HIP) are the main activities of the group. Furthermore, the dense monolithic MAX phases, as well as the CMCs, are tested under environmental conditions at high temperature. The group is financially supported by BMBF (“Bundesministerium für Bildung und Forschung”) under the MAXCOM project (03SF0534).
Assessment of high-temperature protective coatings
An increasingly important aspect of the application at even higher temperatures is the resistance of porous layers to calcium-magnesium-aluminium silicates (CMAS) penetrating into them. These silicates are present in air as solids and are also produced by abrasion in the hot gas path. They are deposited on the turbine components and can penetrate the thermal barrier coatings in their melted form. Once they solidify, they cause considerable damage by decreasing the strain tolerance. In addition to the Gd-containing pyrochlores, aluminates also show a better stability than YSZ.
In order to ensure fast development cycles, the functionality and lifetime of these novel thermal barrier coatings are tested in standard specimens as well as in parts with complex geometries at high temperatures in realistic cycling tests.