Surface Cracking Mechanism of SP2215/Stellite-6 Components Fabricated by Laser-Directed Energy Deposition During High-Temperature Service

Laser-directed energy deposition (L-DED) is a key technology for fabricating wear-resistant Stellite-6 coatings on power-plant components. This study aims to clarify the surface cracking mechanism of L-DED Stellite-6 coatings deposited on 22Cr15Ni3.5CuNbN (SP2215) boiler tubes during long-term high-temperature service. The coatings were exposed at 650 °C for up to 5000 h, and their microstructural evolution and mechanical degradation were systematically characterized using SEM/EDS, TEM, EBSD, microhardness testing, and impact testing. High-temperature service induces the decomposition of M23C6 precipitates at dendrite boundaries, releasing Cr, C, and W atoms that subsequently migrate toward the coating surface. Owing to the rapid interstitial diffusion of C, a carbon-enriched surface region preferentially develops, accompanied by progressively increasing coverage of surface Cr2O3 and subsurface M23C6. Consequently, the surface hardness increases from 441.6 HV0.1 at 0 h to 613.1 HV0.1 after 5000 h, whereas the impact-absorbed energy decreases from 82.8 ± 4.2 J to 5.9 ± 2.5 J. An increase of 1 HV0.1 in hardness corresponds to an approximately 0.448 J reduction in impact-absorbed energy. Mechanistically, local stress concentration associated with Cr2O3 formation, interfacial sliding promoted by lattice mismatch, and crack nuclei originating from pores between chain-like M23C6 precipitates collectively promote crack initiation and propagation. These results demonstrate that precipitate decomposition, elemental redistribution, and subsequent oxide/carbide evolution govern the progressive surface embrittlement and cracking of L-DED Stellite-6 coatings during long-term high-temperature service. This study provides mechanistic insight into the coupling between microstructural evolution and surface failure and offers a theoretical basis for microstructural regulation and long-term reliability assessment of wear-resistant Co-based coatings used in power-plant components.