The physical bonding of spray particles to the substrate is a main factor which contributes to the high adhesive strength of HVOF WC-Co coatings besides mechanical interlocking effect. The adhesive strength between substrate and coatings deposited with partially melted particles under the HVOF spray conditions will be contributed significantly by the physical bonding compared with convention thermal spray process by molten particles with relatively low velocity. The physical bonding is resulted from the high impact velocity of partially-molten droplets during the HVOF spraying process.

In this paper, the bonding mechanisms of sprayed coatings were studied on basis of the high velocity partially-molten droplets impact on smooth substrate. The substrates in the contact zone were deformed due to the droplets stuck into/embed in the substrate. It is shown that the substrate deformation and embedded droplet play a major role in the bonding of a particle with substrate. The Van der Waals force as a one of the physical bonds between the atoms in the spray particle and these in the surface of substrate will occur.