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Thermal barrier coatings (TBCs) are used to protect mechanical parts and metal surfaces from mechanical, chemical and physical degradation. They are mainly employed in cases were very high temperatures exist such as diesel engines, turbines and other related applications. Their aim is to provide adequate resistance to the metal surface beneath which is exposed to high temperatures, a variety of chemical species for different time ranges. In order to do so, thermal barrier coatings require specific properties including very low thermal conductivity, absence of phase transformations within the temperature range of application, excellent adherence to the metal surface beneath, specific porosity, comparable thermal expansion coefficient to that of the substrate and very high melting. Chemical inertness is typically another requirement that has been reevaluated over the last years.

In a typical arrangement of a TBC, a cool fluid flow (air commonly) cools down a metal surface whereas on the opposite side the Thermal Barrier Coating is protecting the surface from a very high temperature air flow. The higher the thermal conductivity of thermal barrier coating is, the lower the temperature profile (as a function of time) of the thermal surface will be. At that point one has to examine what makes a material thermally insulating? Many factors affect the thermal conductivity of materials. Previous studies have indicated that the most important of them are the scattering that is induced by the amorphous structure of a material or a specific arrangement (like the t-tetragonal), filler (refractory) content and even porosity that cuts off thermal paths. Previous works of ours (http://www.sinodoschemistry.com/portfolio/282-enhancing-mechanical-and-molecular-properties-of-composites-via-orientation-at-atomistic-level) have shown the governing effect of order on thermal conductivity of polymeric chains.

Chemical inertness is another significant requirement for an excellent thermal barrier coating. Thermal barrier coatings are susceptible to phase transformations at high temperatures or even complete dissolution under attack of molten Calcium Alumina Magnesium Silicate deposits, also known as CAMS. The origin of these molten deposits is atmospheric debris in most cases, but other origins also exist. In short, when CAMS deposits melt on the thermal barrier coatings (or reach the TBCs having already melted) they instantly provoke a dissolution of thermal barrier coatings that leads to an irreversible deterioration of their insulating properties. Molten CAMS can result to thermal barrier coatings delamination, complete cracking and can even reach the metal surface inducing cracks and oxidation. A great amount of research has been devoted to solutions for the CAMS effect, and some of them seem to work even at very high temperatures and for long exposures. The most prominent solutions up to date include the use of Alumina- Titania combined layers and combinations of these layers with rare earth oxides and sillicates.

The thermal expansion factor of thermal barrier coatings is required to be close to the respective factor of the metal surface. This suggests that when heating and cooling, stresses will not transfer from sub layer to sub layer thus avoiding cracks and loss of adherence and coverage. Porosity of the thermal barrier coating and its pore size distribution affects the mechanism of stresses and strain generation due to the ability to absorb changes and generated forces. Porosity itself is also a governing factor of the overall thermal barrier coating thermal conductivity.

Thermal barrier coatings structure comprise four layers, beginning with the substrate, the binding layer, the grown oxide and the actual thermal barrier coating itself. All layers serve a purpose, and their inter connectivity is of the highest importance. TBCs are either used in high temperatures (around 700 °C) for long times (as in the case of diesel engines) or in very high temperatures (higher than 1200°C) for short times (in the range of 10-20 minutes or very short times in special applications). Depending on the application, different thermal barrier coatings exhibit optimum protective properties. In the low temperature scenario of 700 °C the Yttria stabilized zirconia (YSZ) demonstrates the most effective protection against the thermal gradient imposed on the metal surface. This is due to the very low thermal conductivity of Yttria stabilized Zirconia and its excellent stability at that temperature range. YSZ is found following a t-tetragonal arrangement almost up to 1200 °C, but at higher temperatures it transforms initially to tetragonal and lastly to cubic and monoclinic. All of these transformations are highly undesirable since they lead to a deterioration of key properties like thermal expansion factor, thermal conductivity and mechanical properties.

The real issue thus for thermal barrier coatings starts at very high temperatures (higher than 1200 °C). Modern gas turbines are required to operate at such temperatures and the future gas turbines will operate at even higher ones. The reason for that? Thermodynamic efficiency. The higher the temperature of operation for gas turbines is, the higher their efficiency and energy savings are. Several approaches have been published and patented regarding the optimization of thermal barrier coatings for such demanding applications. Some of them make use of Alumina thin layers, Alumina and Titania inclusion in the thermal barrier coating film, introduction of tailored porosity within the thermal barrier coating, rare earth oxides inclusion, Ceria, Mullite, rare earth zirconates and metal – glass composites.

Novel thermal barrier coatings are simulated and modeled via atomistic simulation and CFD approaches. Both these approaches aim at modification of properties at micro (atomistic level) and meso (bundle, aggregate level). Atomistic simulations of thermal barrier coatings structures lead to prediction of thermal conductivity, mechanical, chemical properties and even phase changes. CFD simulations of novel thermal barrier coatings demonstrate the time dependence of their physical and thermal attributes (film level simulation). Such atomistic simulations, atomistic modeling and CFD simulations of thermal barrier coatings produce novel solution for gas turbines and diesel engines. New applications of thermal barrier coatings are expected to emerge based on these properties predictions based on atomistic simulations and CFD simulations. Contact SinodosChemistry.com experts for more info on these approaches!