SEGREGATION PHENOMENON DURING CO-DEPOSITION OF CERAMICS AND METALS

摘要

Thermal barrier coatings (TBC) are regularly used today to protect and extend the service life of several superalloys which are extensively used in high temperature applications. The existing TBCs are typically between 0.1 to 0.5 mm in thickness, are deposited on metal substrates using plasma spray or electron beam vapor deposition, and can reduce temperatures at the substrate surface by up to 300 °C. For greater temperature reductions, which would enable higher operating temperatures, there is a need for thicker TBCs. The building of thick TBCs has to date been stymied by poor adhesion, and cracking during deposition. It has been suggested that a functionally graded approach may reduce the residual stresses which result in these defects. However, to date these potential advantages remain undemonstrated. The additive manufacturing (AM) technologies used in laboratory settings to produce FG-ceramic layers lack the ability to produce components on the scale typical of TBC uses, and to date thicknesses have remained limited. Laser direct energy deposition (LDED) offers potential advantages over other AM technologies in having little restriction on build size and its abilities to build material at higher rates. However there have been few reported efforts on the use of LDED to build either monolithic or functionally graded TBCs, or on its use to deposit ceramics. We report on efforts to use LDED to co-deposit ceramics favored for TBCs with common superalloys representative of probable substrates for TBC protection. We will discuss the differential response of selected ceramic and metal materials to laser source and other processing parameters, as manifested in materials segregation phenomenon. Mechanistic explanations of ceramic particle movement and of dopant diffusion will be provided, as well as the potential ramifications on material selection and processing parameters.