Kee Sung Lee

In this study, the self‐healing capacity of Titanium Aluminum Carbide (Ti ₂ AlC, MAX phase) was investigated. Bulk coin samples were fabricated to evaluate the self‐healing capacity at different temperatures (1000, 1200, and 1400°C). The extensive self‐healing capacity of Ti ₂ AlC was confirmed on larger quasiplastic damage (diameter ≥1 mm) and radial cracks by covering and filling of oxides such as titanium oxide, aluminum oxide, and aluminum titanate oxide. Although the mechanical properties of Ti ₂ AlC after healing are similar or improved relative to the Ti ₂ AlC before healing at the microscale, some properties of Ti ₂ AlC after introducing larger damage and healing at 1400°C showed reduced values due to excessive oxide formation on the surface. For example, the strength of Ti ₂ AlC healed at 1400°C exhibited 151.4 MPa, which is relative to the original strength of 298.3 MPa. Alternatively, the mechanical properties such as strength, hardness, toughness, and relative modulus of elasticity of Ti ₂ AlC healed at 1000 or 1200°C and were restored to their original strength after healing. These findings suggest that Ti ₂ AlC can be used as a healing agent for high‐temperature applications, such as environmental barrier coating for gas turbine hot‐gas components.

Oxidation and indentation properties of silicon carbide-coated carbon composites were investigated to analyze its durability under atmospheric thermal shock conditions. The silicon carbide-coated samples were prepared either with chemical vapor deposition or chemical vapor reaction/chemical vapor deposition hybrid coating. The remnant weight of uncoated and coated samples was investigated after each thermal shock cycle. The surface and cross-section of coated samples were then analyzed to confirm morphological changes of the coating layers. The spherical indentation test for uncoated and coated samples were also performed. As a result, silicon carbide coating improved the oxidation resistance, elastic modulus, and hardness of carbon composites. Hybrid coating drastically enhanced the durability of samples at high temperature in atmospheric conditions.

Crack healing phenomena were observed in mullite and mullite + Yb₂SiO₅ environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiC_f/SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles; however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 °C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO₂ phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al₂O₃ in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y₂SiO₅. The indentation load–displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb₂SiO₅ EBC demonstrated the best durability during repetitive thermal shocks.

The effect of starting granules on the indentation properties of air plasma sprayed thermal barrier coatings (TBCs) is investigated in this paper. Various kinds of spray‐dried granules are prepared from different processing conditions, especially varying solvent and dispersant, showing a deformed hollow‐typed and a filled spherical‐typed granule. The similar coating thicknesses are prepared by adjusting process parameters during air plasma spray. All XRD peaks in phase analysis are tetragonal and cubic phases without any monoclinic phase after the starting granules were heat‐treated. A relatively porous microstructure of the coating layer could be obtained from the monodisperse granules, while a relatively dense microstructure resulted from the hollow‐typed granules. The morphology and distribution of the granules crucially affect the microstructure of thermal barrier coatings and thus have influences on indentation properties such as indentation stress‐strain curves, contact damage, and hardness. The implication concerning microstructure design of TBCs for gas turbine applications is considered.