This proposal introduces a new concept of high temperature protective coating.
In hot harsh environments some components require surface protection: typical applications are thermal protection systems (TPS), hot structures for hypersonic vehicles and hot sections of gas turbines.
In oxidizing atmospheres, above 1300°C, ceramics or ceramic matrix composites are selected for their oxidation resistance and mechanical strength at high temperature. However their durability is adversely affected by water vapour from combustion or atmosphere.
In the proposed thermal and environmental barrier coating (TEBC), deposited by plasma spray, the multilayer modular architecture assigns to each layer a distinctive protective function. The external layer is an innovative Ultra High Temperature Ceramics (UHTCs) coating with an outstanding oxidation resistance at extreme temperatures; this layer acts as an environmental barrier coating (EBC), protecting the inner layers. Two or more ceramic-metallic layers complete the system architecture: the ceramic layers provide the thermal insulation and the metallic ones promote the compliance inside the TEBC and the adhesion to the substrate.
Current EBC limits are the maximum service temperature and the low thermal shock and thermal cycling resistance: the proposed system is designed to improve toughness, adhesion and cohesion using multiple metallic bond coats, providing an overall thermal conductivity tailorable by a proper stacking design of ceramic layers.
The proposal involves the preliminary study, characterization, design of the new system. A Finite Element Model will be created to optimize coating architecture, simulating the service conditions. A significant experimental effort will be devoted to evaluate physical and mechanical properties of studied materials as a function of temperature.
Finally an optimized coating will be the produced with a properly designed microstructure and its behavior will be evaluated in relevant environment
The protection requirements for hot structures in aggressive environments are more and more challenging due to the increase in operating temperature and severe limitations in terms of service life and severity of chemical attacks. The protective coatings used for these applications (see state of art) are approaching their limits and numerous researches (also by proponents) are currently ongoing in order to modify existing or introduce new compositions for these materials.
A less-traveled-road is the approach here proposed, i.e. that of radically modifying the architecture of coatings trying to overcome the limitations of well-established materials. The idea basically is to separate the protection functions (e.g. insulation, oxidation, hot corrosion) facing each one with the more appropriate material. At the moment of proposal preparation no comparable systems are available in the literature. The major drawback is represented by the increased microstructural complexity, which needs a properly addressed study in order to avoid undesired failures; e.g. the increased number of interfaces and the use of materials with different CTE could generate excessive interlayer stresses at high temperatures or during heating and cooling cycles. This requires an accurate study on system architecture taking into account and reducing these effects.
The major expected result of this research is indeed the characterization of the behavior of this system in temperature, and the demonstration of the the feasibility of the concept. To obtain a reliable result, useful for the design of optimized TEBC, a fully coupled thermo-mechanical, non-linear, time-dependent FEM analysis has to be introduced, with a material database built by experiments on real samples.
A huge amount of theoretical or simplified studies on high temperature behavior of thermal spray coatings is available in the literature. On the other hand a lack of experimental activities can be evidenced on this aspect, mainly due to the complicated preparation and testing route needed to obtain reliable results, also in the absence of specific standard (e.g. ASTM) test methods. This research is intended to fill up this empty space and to gather and make available real experimental evidence of the high temperature mechanical, chemical and thermal properties of specific metal-ceramic systems.