Review on self-healing thermal barrier coatings for space applications

The demand for reusable space transportation systems is increasing, which leads to the need for higher-performance, reliable rocket engines at the lowest possible cost. The steep thermal gradient in the combustion chamber of rocket engines is catered by certain cooling mechanisms. In the case of reusable rocket engines, a progressive deformation and thinning of the cooling passage wall during engine operation is encountered because of the plastic ratcheting after numerous thermal cycles. This phenomenon may lead to the catastrophic failure of the engine when in operation and it also limits the number of operation cycles, adding cost. Thermal barrier coating (TBC) was successfully employed initially, and the functionally graded coatings were subsequently employed. However, the spallation of TBCs exists upon repeated thermal cycles. It is felt as necessary to induct the self-healing concept in such applications to enhance the number of thermal cycles. Various researchers have investigated crack-healing behavior of ceramic materials in recent years. This article provides a comprehensive analysis on the findings in the self-healing TBC and discusses future perspectives in this research area. This article mainly focusses on self-healing systems based on molybdenum disilicide (MoSi2)/MoSi2(B) with yttria partially stabilized zirconia and MCrAlY. In addition, the isothermal kinetics of MoSi2/MoSi2B and the formation of zircon are also discussed. The methodology of encapsulating the self-healing particles in order to avoid premature oxidation of MoSi2 self-healing particles is also discussed.