Heat engines are based on considering various factors such as durability, performance and efficiency with the objective of minimizing the life cycle cost. For example, the turbine inlet temperature of a gas turbine having advanced air cooling and improved component materials is about 1500oC. Metallic coatings were introduced to sustain these high temperatures. The trend for the most efficient gas turbines is to exploit more recent advances in material and cooling technology by going to engine operating cycles which employ a large fraction of the maximum turbine inlet temperature capability for the entire operating cycle. Thermal Barrier Coatings (TBC) performs the important function of insulating components such as gas turbine and aero engine parts operating at elevated temperatures. Thermal barrier coatings (TBC) are layer systems deposited on thermally highly loaded metallic components, as for instance in gas turbines. TBC’s are characterized by their low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow. The most commonly used TBC material is Yttrium Stabilized Zirconia (YSZ), which exhibits resistance to thermal shock and thermal fatigue up to 1150oC. YSZ is generally deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes. It can also be deposited by HVOF spraying for applications such as blade tip wear prevention, where the wear resistant properties of this material can also be used. The use of the TBC raises the process temperature and thus increases the efficiency.
APPLICATIONS OF TBC
Thermal barrier coatings can also be applied on furnace components, heat-treating equipments, chemical processing equipments, heat exchangers rocket motor nozzles, exhaust manifolds, jet engine parts and nuclear power plant components. It has also been applied to uncoated blades used to drive the high-pressure hydrogen turbo pump of the Space Shuttle Main Engine. Here the coating is applied to provide thermal lag.
DISADVANTAGES OF TBC
The main disadvantage is ceramic bond coat interface region which was determined to be the weakest link in the ZrO2-12Y2O3 / NiCrAlY system. Adhesive plus cohesive failure of the ceramic layer in this region occurred when the coating was subjected to a normal tensile load at room temperature as well as when applied on bars subjected to axial tension or compression at elevated temperature.
Detailed analysis of coating stresses and controlled process, plasma spray technology has significantly improved the reliability of TBC turbines, diesel engines and other heat engines. Processing improvement in the control and development of TBC are required. Further study on, the mechanisms controlling coating adherence and degradation in clean and dirty environments, the effects of coating composition and structure on coating properties and correlation of models of engine tests are necessary to obtain thermal barrier coating that have even better tolerance to high temperature and thermo mechanical stresses.