The stresses and cracks in thermal barrier coating system: a review

Gas turbine is a class of clear and efficient power and energy machines, which has wide applications to air engine, power generation, chemical and oil engineering, etc. The manufacture of gas turbine is a mark of the manufacturing capability of a nation and is regarded as a pearl on the crown of manufacturing technology. Advanced gas turbines are featured by the high inlet gas temperature, as well as its continuous increase due to its key role in the turbine performance. The current inlet temperature of the advanced gas turbine has reached 1600℃ and is expected to be 1700℃ and much higher in the future. It is challenging to design and manufacture the high-temperature turbine blades working in such an extreme temperature environment. Thermal barrier coating system (TBCs) is one of the core technologies for the design and manufacturing of advanced gas turbines and has been playing important roles in the development history of gas turbines. TBCs can increase the inlet temperature and protect the turbine blades from oxidation, corrosion, foreign object impact, particle erosion and so on. Therefore, thorough investigations on the failure mechanisms and the influence factors of TBCs are crucial to the design, manufacturing and strength assessment of TBCs and furthermore ensure the safe service of gas turbines. In this review article, we present the more recent advances in the studies on stresses and cracks in TBCs of gas turbine blades, including theoretical, experimental and numerical analyses. This paper covers the failure modes in TBC, the thermal stresses generated in the fabrication of TBC, the thermally grown oxide (TGO) and its resulting growth stress, the surface cracks, interface cracks and their competition in TBCs, the evaluation method of biaxial strength of TBCs, the delamination induced by the penetration of environmental CaO-MgO-Al₃O₂-SiO₂ (CMAS) into coatings, and the sintering effect on the cracking of ceramic layer. Finally, potential directions for further research are concluded.