| The process of chemical plating was used to form Co-P coating on steel-bonded carbide. The surface structure and morphology of the coating were investigated by using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). This paper mainly studied on the thermal fatigue behavior of the coating and substrate: initiation of thermal fatigue crack, propagation forms and mechanisms of thermal fatigue crack, effect of electroless plating process and the structure of substrate on thermal fatigue properties.The results show that oxidation happened on the surface of coating when thermal cycling times is less than 200, and no cracks is observed. While the plastic deformation of coating occurs when the thermal cycling times is beyond 200. With the increase in the number of cycle, the plastic strain energy of coating is gradually reduced. And cracks initiates at the root of notch finally. Thermal fatigue crack initiation and crack propagation are the combining results of cyclic thermal stress and high temperature oxidation.The initiation gestation period of thermal fatigue crack exists. The coating surface inclusions and holes form cracks under the cyclic stress. With the increase of micro-holes and aggregation, the adjacent micro-holes connect to form small cracks. With the increase in cycles, cracks propagate continuously by the connection of holes.Along the boundary of carbides and the matrix, oxidation corrosion on the boundary, separated matrix and carbides, through the blocky carbides, the fragmentation of carbides, and cracking-self of carbides, are mainly the propagation forms of thermal fatigue crack on the substrate.When the cracks passed through the coating and propagated on the substrate, the initiation of thermal fatigue relate with the aggregation of carbides, and the propagation relates with the connection of holes and micro-cracks. The propagation form can be explained by the plastic passivation model.
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