Supersonic plasma spraying is a process that uses an extremely high-energy-density supersonic plasma jet to heat and accelerate the spraying of materials to obtain high spray quality. The formation and presence of heterogeneous interfaces significantly affect the operational performance of refurbished parts. In this study, a Ni60A coating renowned for its robustness and wear resistance is employed as the substrate. The morphology of the highly disorganized and irregular coating / substrate bonding interface is quantitatively characterized by fractal theory. Additionally, the relationship between the interface state and the bonding strength of the supersonic plasma coating is explored. A comparative test is performed to generate coatings with distinct deposition morphologies. This is achieved by controlling the spraying distance and current to vary the melting and flight characteristics when the particles make contact with the matrix. The influences of parameters such as spraying distance, spraying current, and other process variables on the morphology and bonding strength of the coating / substrate bonding interface are studied. Fractal dimensions are calculated using the FracLac plug-in and box-counting methods. Furthermore, the corresponding relationships between the morphology of the binding interface, binding strength, and fractal dimensions are investigated. The results show differences between the pretreated substrate and the bonding interface of the coating / substrate after the spraying process, affecting the bonding strength of the coating. Correspondence related to the deposition process of the coating is found between the surface morphology and fractal dimension. The supersonic plasma-sprayed nickel-based alloy coating and substrate are predominantly mechanically bonded. The bonding occurs though different forms including mosaic, anchoring, spreading, occluding, and compound types. Among these, the mosaic and anchoring types feature barbs, enhancing the contact area between the matrix and the coating. This increased contact area serves to enhance stress distribution, making it more uniform and effectively dispersing concentrated stress. Within a certain range, as the fractal dimension increases, the morphology of the mosaic and anchoring types increases, and the contact area between the substrate and coating also expands. This expansion leads to improved distribution of concentrated stress and a more uniform stress distribution, thereby improving the bonding strength. The spraying distance has a more significant effect on the fractal dimension than the spraying current. At spraying distances of both 80 and 100 mm, an increase in the spraying current from 400 to 500 A initially leads to a reduction in the fractal dimension to a minimum of 1.115, followed by a subsequent increase. At a spraying distance of 120 mm, the flight time of the particles in the plasma flame flow increases, and the fractal dimension of the coating interface exhibits an initial increase followed by a subsequent decrease as the current increases. There is a positive correlation between the fractal dimension of the interface and the bonding strength of the coating when the tensile method is used to measure the coating bonding strength. As the fractal dimension increases within a certain range, the porosity at the coating / substrate interface decreases and the bonding strength increases. During the tensile process, cracks propagate readily at the interface where bonding strength is comparatively weaker. In addition, the presence of voids inside the coating leads to stress concentration, initiating cracks which eventually propagate, destabilize, and expands at the interface, culminating in fracture formation. Within a certain range, there is a positive correlation between the fractal dimension and bonding strength, indicating a degree of dependence between the bonding strength and the fractal dimension of the bonding-interface topography. However, the existence of a functional relationship between the fractal dimension and bonding strength, as well as the extent of the positive correlation, require further exploration. Therefore, studying the fractal characteristics of the coating / substrate bonding behavior is of great significance in the evaluation of coating quality.
