Promoting bonding strength between internal Al-Si based gradient coating and aluminum alloy cylinder bore by forming homo-epitaxial growth interface

The reliability of interface bonding strength between internal thermal sprayed coating and cylinder bore substrate causes special concern in the field of internal thermal spraying. In this study, a new approach is proposed to create a large area of metallurgical bonding at the coating-substrate interface by the design of Al-Si based gradient coating on A383 alloy cylinder bore. The internal coating comprises hypereutectic Al-Si-Cu-Fe top layer, near-eutectic Al-Si bond layer, and transition layers, which were fabricated by an advanced internal rotating plasma spraying technology. The tensile adhesive and interfacial indentation tests were conducted to investigate the adhesion properties of the coating. The crystallographic relationship of the bond layer-substrate interface was further investigated using the focused ion beam and high-resolution transmission electron microscopy. Results show that the Al-Si based gradient coating presents both desirable surface hardness (385.1 ± 46.6 HV0.025) and bonding strength (46.4–58.1 MPa). The tensile failure mode is the fracture at the subsurface of the top layer, without fracture at the coating-substrate interface. The superior bonding strength results from the formation of chemical interdiffusion (estimated thickness of 700–1500 nm) and homo-epitaxial growth at the interface. This study provides a framework for forming a reliable metallurgical bonding interface of internal thermal-sprayed coating.