Thermal barrier coating
In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications.
Due to increasing demand for more efficient engines running at higher temperatures with better durability/lifetime and thinner coatings to reduce parasitic mass for rotating/moving components, there is significant motivation to develop new and advanced TBCs.
In addition, such zirconates may have a high concentration of oxygen-ion vacancies, which may facilitate oxygen transport and exacerbate the formation of the TGO.
[5] The TBC can also be locally modified at the interface between the bond coat and the thermally grown oxide so that it acts as a thermographic phosphor, which allows for remote temperature measurement.
When it is cooled, a lattice mismatch strain arises between TGO and the top coat (TC) due to differing thermal expansion coefficients.
For this reason, in order to make a TBC that lasts a long time before failure, the thermal expansion coefficients between all layers should match well.
In fact, the repeated thermal shocks associated with turning the engine on and off many times is a main contributor to failure of TBC-coated turbine blades in airplanes.
These linked-up horizontal and vertical cracks due to thermal shock ultimately contribute to the failure of the TBC via delamination of the TC.
These columns start out with a feathery structure, but become smoother with heating due to atomic diffusion at high temperature in order to minimize surface energy.
As the YSZ sinters and becomes more dense in this fashion, it shrinks in size, leading to the formation of cracks via a mechanism analogous to the formation of mudcracks, where the top layer shrinks but the bottom layer (the BC in the case of TBCs, or the earth in the case of mud) remains the same size.
[10] [3] YSZ is the most widely studied and used TBC because it provides excellent performance in applications such as diesel engines and gas turbines.
By incorporating alumina in YSZ TBC, oxidation and corrosion resistance can be improved, as well as hardness and bond strength without significant change in the elastic modulus or toughness.
One challenge with alumina is applying the coating through plasma spraying, which tends to create a variety of unstable phases, such as γ-alumina.
When these phases eventually transform into the stable α-phase through thermal cycling, a significant volume change of ~15% (γ to α) follows, which can lead to microcrack formation in the coating.
Some negative effects of the addition of ceria include the decrease of hardness and accelerated rate of sintering of the coating (less porous).
Single and mixed phase materials consisting of rare earth oxides represent a promising low-cost approach towards TBCs.
The main challenge to overcome is the polymorphic nature of most rare earth oxides at elevated temperatures, as phase instability tends to negatively impact thermal shock resistance.
A powder mixture of metal and normal glass can be plasma-sprayed in vacuum, with a suitable composition resulting in a TBC comparable to YSZ.
As well as providing thermal protection, these coatings are also used to prevent physical degradation of the composite material due to friction.
This is possible because the ceramic material bonds with the composite (instead of merely sticking on the surface with paint), thereby forming a tough coating that doesn't chip or flake easily.
Combined with cool air flow, TBCs increase the allowable gas temperature above that of the superalloy melting point.
[12] To avoid the difficulties associated with the melting point of superalloys, many researchers are investigating ceramic-matrix composites (CMCs) as high-temperature alternatives.
At high temperatures, these CMCs are reactive with water and form gaseous silicon hydroxide compounds that corrode the CMC.
