Fabrication of a novel molybdenum carbide composite coating with double-layer structure on cast iron via in situ solid-phase diffusion

The improvement of comprehensive properties including hardness, toughness, adhesion strength, and wear resistance of the hard coating is highly desirable for various applications. Herein, a new strategy involving in situ solid-phase diffusion (ISSD) is utilized to fabricate a novel molybdenum carbide composite coating (MCCC) with double-layer structure on cast iron (CI). MCCC is composed of two layers, the outermost layer (Layer I) is a completely dense Mo2C layer, and just beneath it is a Fe3Mo3C(Si) composite transitional layer (Layer II) which connects the outer layer and the CI substrate. The squares of the thickness of the Layer I, Layer II, and MCCC are found to be proportional to the ISSD time, thus following the classic parabolic law. The hardness, elastic modulus, and fracture toughness of the coating surface reach 24.6 ± 0.5 GPa, 483.5 ± 3.1 GPa, and 3.0 ± 0.1 MPa·m1/2. With the evolution of the microstructure in the coating along the thickness direction, the hardness and elastic modulus gradually decrease, while the fracture toughness gradually increases. Furthermore, the result of scratch test shows that the adhesion strength between Layer I and Layer II is approximately 93.4 N, while the adhesion strength between the coating and the substrate is greater than 93.4 N. Therefore, this work provides a new route for the fabrication of MCCC with double-layer structure capable of combining superhardness, excellent toughness, and superior adhesion strength.