Microstructure And Wear Resistance Of Carbides Reinforced Coating By Plasma Melt Injection

To overcome problems of spallation, dissolution of carbides, carbides sinking or rising, and cracking in carbide coatings produced by thermal spraying, hardfacing and cladding, plasma melt injection(PMI) is presented in this thesis. Coatings metallurgically bonded to the substrate with evenly distributed carbides can be synthesized with PMI technology, and bonding strength of the coating is improved greatly. Dissolution of carbides, carbides sinking or rising,and cracking can be minimized with PMI. The wear resistance can be improved. Equipment of PMI is cost-effective and efficient, and the technology is easy to be industrialized.WC-Co, SiC particles were injected into Q235 steel,and carbide coating with good appearance were synthesized using PMI technology. The coatings are without macro defects. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy diffraction spectrum (EDS) were used to examine the microstructure and composition of the coatings. Wear resistance of the coatings was tested using dry sliding wear tester. The results show that the microstructure consists of WC, M6C(Fe3W3C or Co3W3C), and W2C. In the upper part of the specimen injected with 20~30μm WC-Co particles, the microstructure constists of some original WC particles, white bar-shaped and cross-shaped Fe3W3C, gray ion base and black lath Fe-based eutectic structure. White bar-shapedand crossshaped carbides in the middle part of the coating are bigger, amount of the gray ion base is more than that in the upper. White bar-shaped and cross-shaped carbides in the bottom are smaller, and amount of them is more than that in the middle. In the upper part of the specimen injected with 200~300μm WC-Co paticles, WC-Co particles keep their original block shape basically. Carbides crystallize in long pole shape on the edge of WC particles. The microstructure far from WC particles consists of ion based matrix and reticulate carbides. Microstructure in the middle and the bottom consists of ion based matrix and reticulate carbides. Graphite,Fe,SiC Microstructure of the coating injected with with 200~300μm SiC particles consists of iron, SiC and graphite. The SiC particles are embedded on the top surface of the specimen, and the graphite, ferrite, and pearlite are in the coating. The microstructure of the coating injected with 80~120μm SiC particles consists of Fe and Fe3C, forming pearlite and non-equilibrium phase. In the upper part of the coating injected with 20~30μm WC-Co particles, the microstructureconstists of gray, herringbone structure and black belt around the herringbone structure, when the substrate preheated temperature 200℃. In the middle part, the carbide dendrites are larger. In the bottom part, the carbides are fewer not evenly distributed, and in block shape. Some herringbone structure appeares.Microstructure of the coatings injected with 20~30μm WC-Co, 200~300μm SiC, and 80~120μm SiC particles is evenly distributed, and carbides sinking or rising due to density differences between the substrate and the carbides is avoided.Large amount of unmelted carbides can be found in the top part of the coating injected with 200~300μm WC–Co particles.The critical velocity of the 20~30μm WC-Co particles to overcome the melt surface barrier is 0.20m/s, and the injecting velocity is 2.6m/s. The critical velocity of the 200~300μm WC-Co particles is 0.063m/s, and the injecting velocity is 0.7m/s. When the nickel-based self-fluxing alloy is coated on the substrate before injection, wetability of the liquid metal to WC-Co particles is improved. The critical velocity of the 200~300μm WC-Co particles becomes zero. The initial velocity in the molten pool increased to 0.7m/s. The WC paticles are well distributed in the coating.Dissolution of the carbide particles decreases with increase of the particle's size. Dissolution of the 200~300μm WC-Co and the 200~300μm SiC is lower than that of the 20~30μm WC-Coand the 80~120μm SiC. With deepening of the molten pool and prolongation of the solidification time, the 20~30μm WC-Co particles tends to sink down, when the substrate is preheated 200℃. Abrasion is the main wearing mechanism for carbides coatings produced by PMI Wear resistance of the coatings increase greatly. Large amount of unmelted carbide particles exist in the coatings injected with large carbide particles, so their wear resistance is much higher than that of the coatings injected with fine carbide particles. Wear resistance of the coatings injected with 200~300μm WC particles, 20~30μm WC particles, 200~300μm SiC particles, and 80~120μm SiC particles are about 87, 14, 48 and 4 times higher than the reference substrate material, respectively.