Synthesis of In Situ SiC/Graphite/Al Hybrid Composite Coating by Laser Direct Energy Deposition

The present work is undertaken to highlight a novel in situ process to fabricate SiC/graphite/Al hybrid composite coating by laser direct energy deposition using Al–Si–C powder mixtures. Microstructures of the coatings, mainly the in situ formed reinforcing phases and their interface with the matrix, are comprehensively characterized by scanning electron microscopy, X-ray diffraction, analytical transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that almost all the graphite powders are dissolved in the molten pool during the deposition, except for a few large graphite powders that are only partially dissolved in the Al–Si melt. In the coating with a low content of Si and graphite, i.e., Al–19Si–4C with a C/Si mole ratio of 0.44, needle-like Al4C3 is the sole in situ formed carbide phase. Most of the Al4C3 are uniformly distributed in the Al–Si matrix. Al4C3 is also observed on the surface of partially dissolved graphite powder. In the coating with a high content of Si and graphite, i.e., Al–32Si–11C with a C/Si mole ratio of 0.75, SiC, plate-like Al4SiC4, and dissolved-and-precipitated feather-like graphite are the main in situ formed phases. The size and morphology of SiC are varied depending on the location in the coating. In the upper region of the coating, elongated SiC crystals with a typical length of 10 to 40 µm and a width of 2 to 3 µm are observed. A mixture of elongated SiC and blocky SiC crystals of 9 to 15 µm, together with precipitated feather-like graphite phases, are revealed in the middle region. In the bottom region, fine SiC particles with a size of 2 to 6 µm are formed. Besides, ultra-fine microcrystalline SiC phases are formed on the feather-like graphite, whereas a continuous SiC layer is formed on the surface of partially dissolved graphite powder. Several interfaces interfacial structures between the in situ formed phases and matrix have been identified, i.e., SiC/Al4SiC4/Al, SiC/graphite, SiC/Al4SiC4/graphite, and graphite/Al4SiC4/Al or Si. The thermodynamics and the mechanism for forming in situ carbide, i.e., solution-precipitation mechanism and solid–liquid diffusion-reaction mechanism, are discussed. Based on the experimental and theoretical results, a sequence of the reaction and solidification leading up to the in situ synthesis of SiC/graphite/Al hybrid composite coating is illustrated. The corresponding mechanism for the present process has been proposed.