The preparation of hard coatings on the steel/iron surface can significantly overcome the bottleneck of low service life under friction conditions due to the low surface hardness and poor wear resistance. However, for traditional homogeneous coatings, the increase in surface hardness and wear resistance is usually accompanied by a significant drop in toughness, that is, there is a trade-off relationship between hardness (wear resistance) and toughness. Therefore, the design and development of coatings with high toughness and high wear resistance are of great significance for expanding the application range of steel/iron materials. The work aims to prepare a new type of molybdenum carbide composite coating with a double-layer structure on the surface of HT300 at 1 050 ℃ by in-situ solid-phase diffusion technology, in order to achieve the synergistic improvement of surface hardness and toughness of the coating.
The cross-sectional microstructure, chemical composition, and thickness of the coating were characterized by combined scanning electron microscopy (SEM) and X-ray energy dispersive spectrometery (EDS). The phase constitution of the surface of coating was analyzed by X-ray diffraction (XRD) over 2θ angles ranging from 20° to 90°. The grain morphology and the corresponding growth orientation of the cross section of coating were investigated by electron backscatter diffraction (EBSD) technique. The nanohardness and indentation modulus of the cross section of the coating and the substrate were measured with a nanoindentation tester. Furthermore, the fracture toughness of the coating was evaluated by the Vickers indentation method.
Results showed that when the coating was held at 1 000 ℃ for 10 h, it presented a dense structure and there was an obvious interface interstice between the coating and the substrate. When the preparation process parameters were optimized to 1 050 ℃for 10 h, the cross-sectional microstructure of the coating presented a typical double-layer structure. From the composite coating surface to the substrate, the first layer (Layer Ⅰ) was a completely dense Mo2C layer, and the second layer (Layer Ⅱ) was a Fe3Mo3C(Si) transition layer that connected the Mo2C layer and the cast iron substrate. The Mo2C/Fe3Mo3C(Si) layer interface and Fe3Mo3C(Si) layer/matrix interface showed good metallurgical bonding. Moreover, as the holding time (t) increased from 4 h to 10 h, the thickness (d) of the composite coating increased from (15.9±0.46) μm to (25.3±0.85) μm, and the square of the thickness of the composite coating was proportional to the holding time t (d2=Kt, K=64.35 μm2·h). The indentation test results showed that the hardness and fracture toughness of the composite coating surface reached (24.6±0.5) GPa and (3.0±0.1) MPa·m1/2. Compared with the uncoated HT300 substrate ((3.5±0.5) GPa), the hardness of the coated surface increased by about 6 times. Moreover, along the thickness direction, the hardness of the composite coating decreased gradually, while the fracture toughness increased gradually.
The study demonstrates that the formation of the double-layer structure in the composite coating significantly reduces the stress concentration at the composite coating/substrate interface, thereby greatly improving the interfacial bonding between the composite coating and the substrate. Therefore, this work provides a novel preparation strategy for a double-layer structure coating on the surface of steel/iron, effectively improving the problems of low surface hardness and insufficient wear resistance of the steel/iron.
Key words
solid-phase diffusion /
double-layer structure /
molybdenum carbide composite coating /
microstructure /
mechanical properties
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Funding
National Natural Science Foundation of China (52461030); The Key Research and Development Program Project of Shaanxi Province (2025CY-YBXM-020); The Project of the Science and Technology Bureau of Yulin (2024-CISY-065); The Project of Education Department of Shaanxi Provincial Government (25JK0757); The Project of Yulin Association for Science and Technology Youth Talent Support Program (20240622)
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