Microstructure, formation mechanism and mechanical properties of ZrC–Fe coating on cast iron via in situ solid-phase diffusion

To improve the mechanical properties of the surface of iron-based alloys, a ZrC–Fe coating is produced on cast ion via in situ solid-phase diffusion (ISSD) at 1130 °C. The coating is completely dense, reaching a thickness of approximately 18 μm after 10 h of ISSD. The volume fraction of the ZrC phase in the coating reaches approximately 92.3%. The size of the long axis (short axis) of columnar ZrC particles gradually decreases from approximately 4 μm (1.5 μm) to 1 μm (0.3 μm) as the thickness of the coating increases, displaying a gradient microstructure. The formation mechanism of the coating is mainly attributed to the carbon atoms diffuse into the octahedral vacancies of the zirconium lattice to undergo a diffusion-type solid state phase transformation to form ZrC, and α-Fe phase are distributed among the ZrC particles through the Fe atoms diffusion. Due to the carbon concentration gradient, the nucleation rate is reduced, thereby forming a gradient microstructure on the grain scale. The growth of the coating obeys the growth kinetic model d2=30.5t. In addition, the nanohardness and elastic modulus of the surface of the ZrC–Fe coating reach 29.7 GPa and 425.1 GPa, respectively, showing remarkable improvement compared to those of the cast iron substrate (4.5 GPa and 229.5 GPa, respectively). The fracture toughness gradually increases from 2.9 ± 0.1 MPa m1/2 to 3.6 ± 0.1 MPa m1/2 as the thickness of the coating increases. Moreover, the coating exhibits the superior coating/substrate adhesion. The excellent comprehensive mechanical properties are tightly related to a small amount of high toughness α-Fe phase, high volume fraction of in situ ZrC particles and the gradient change of the ZrC grain scale.