Large degradation in thermal insulation and strain tolerance is a main headache and a primary cause of the failure for plasma-sprayed thermal barrier coatings (TBCs) during service. One mechanism behind such degradation is the healing of interlamellar pores formed by multiple connections between edges of a pore, which significantly speeds up healing during thermal exposure. The objective of this study is to obtain sintering-resistant TBCs by tailoring the width of interlamellar pores to avoid multiple connections. Firstly, the mechanism responsible for the multiple connections was revealed. The splat surfaces before and after thermal treatments were characterized via an atomic force microscope (AFM). The roughening of the pore surface occurs during thermal exposure, along with the grain growth inside the splats. Consequently, the local surface height increases, which causes multiple connections and healing of the interlamellar pores. Secondly, critical widths of the interlamellar pores for avoiding the multiple connections during thermal exposure are established by correlating the extent of surface roughening with the growth of individual grains. The height increase of the splat surface and the growth of the grain size (D) were found to increase with the exposure temperature and duration. A relationship linking the height increase and the growth of the grain size induced by thermal exposure in plasma-sprayed ceramic splats was obtained. Finally, composite TBCs were prepared to form wide interlamellar pores in the coatings. Using this design, the increases in the thermal conductivity (λ) and the elastic modulus (E) can be prevented to a large extent. Thus, sintering-resistant TBCs that maintain high thermal insulation and strain tolerance, even after long thermal exposure, can be created.

Although magnesium (Mg) alloys are the lightest among structural metals, their inadequate corrosion resistance makes them difficult to be used in energy-saving lightweight structures. Moreover, the improvement in corrosion resistance by the conventional surface treatments is always achieved at the expense of sacrificing the fatigue lifetime. In this study, high purity aluminum (Al) and AlMgSi alloy coatings were deposited on Mg alloys via an in-situ micro-forging (MF) assisted cold spray (MFCS) process for simultaneous higher corrosion resistance and longer fatigue lifetime. Besides contributing to a highly dense microstructure, the in-situ MF also greatly refines the grain of the deposited Al alloy coating to the sub-micrometer range due to the enhanced dynamic recrystallization and also generates notable compressive residual stress up to 210 MPa within the AlMgSi coating. The absence of secondary phases in the AlMgSi alloy coatings enable the coated Mg alloy with corrosion resistance, which is even better than its bulk AlMgSi counterparts. The unique combination of refined microstructure and the prominent compressive residual stress within the AlMgSi coatings, effectively delayed the crack initiation upon repeated dynamic loading, thereby leading to ~10 times increase in the fatigue lifetime of the Mg Alloy. However, although residual stress is also generated in the submmicro-sized grained pure Al coating, the low intrinsic strength of the coating layer leads to a lower fatigue lifetime than the uncoated Mg alloy substrate. The present work is aimed to provide a facile approach to break the trade-off between corrosion resistance improvement and fatigue lifetime of the coated Mg alloys.

Thermal barrier coatings (TBCs) can effectively protect the alloy substrate of hot components in aeroengines or land-based gas turbines by the thermal insulation and corrosion/erosion resistance of the ceramic top coat. However, the continuous pursuit of a higher operating temperature leads to degradation, delamination, and premature failure of the top coat. Both new ceramic materials and new coating structures must be developed to meet the demand for future advanced TBC systems. In this paper, the latest progress of some new ceramic materials is first reviewed. Then, a comprehensive spalling mechanism of the ceramic top coat is summarized to understand the dependence of lifetime on various factors such as oxidation scale growth, ceramic sintering, erosion, and calcium-magnesium-aluminium-silicate (CMAS) molten salt corrosion. Finally, new structural design methods for high-performance TBCs are discussed from the perspectives of lamellar, columnar, and nanostructure inclusions. The latest developments of ceramic top coat will be presented in terms of material selection, structural design, and failure mechanism, and the comprehensive guidance will be provided for the development of next-generation advanced TBCs with higher temperature resistance, better thermal insulation, and longer lifetime.