The morphology (surface and cross-section) of the high-velocity oxygen–fuel coupled physical vapor deposition (HVOF-PVD) duplex coating and its mechanical (stress and load-bearing capacity), tribological (wear and lubrication), and corrosion (chemical and electrochemical) behavior are described and discussed for surface protective concern. The duplex coating involves a combination of a thick, hard HVOF bottom layer and a thin, hard yet tough PVD top layer. The hardness and modulus of the HVOF coating are between those of the soft substrate and the hard PVD coating, which thereafter provides a smooth transition and effective support between them. As a result, the HVOF interlayer can eliminate the plastic deformation of the substrate due to stress concentration under heavy load, effectively avoiding the “eggshell effect”. As expected, the duplex coating system would provide excellent load-bearing capacity. In terms of tribology, the wear resistance of the HVOF-PVD duplex coating is significantly improved compared to that of PVD monolayer coating.
Thermal spraying is a general term for a series of coating processes. The process involves the powder material being given thermal and kinetic energy in a hot air flow medium and sprayed onto the substrate at high speed to form a coating . Over the past few decades, a series of thermal spray technologies have been developed, such as electric arc spraying (EAS), plasma spraying (PS), detonation spraying (DS), and high-speed oxy-fuel (HVOF) spraying . In particular, HVOF spraying achieves a favorable combination of thermal and kinetic energy due to its high speed (~1500 m/s) and relatively low temperature (~2000 °C) . In addition, it benefits the advantage of high flexibility (such as the deposition of cermets, oxide and oxide-free ceramics coatings) and low cost for mass production . Thus, HVOF coating as a protective candidate has been widely used in various industrial fields, such as aviation, aerospace, petroleum, and marine components as well as systems . It can be observed that the raw material powder and the spraying process are two key factors for the coatings prepared by HVOF spraying. Therefore, designing and optimizing the raw material powder and spraying process parameters is quite crucial before coating deposition. Typically, the powder material consists of a metal/ceramic primary phase and metal binder phase (Co, Cr, Ni, etc.) with a particle size distribution of 10–40 μm . At present, a variety of commercial feedstock powders have been developed, mainly including but not limited to WC-based, Cr3C2-based, and Fe-based powders . Moreover, many researchers have conducted structural and compositional modifications based on commercial feedstock powders to obtain high-quality HVOF spray coatings . In addition, the HVOF spraying parameters particularly involving the feature and flow rate of working gas and the ratio of oxygen/fuel gas make a great contribution to the temperature, airflow velocity and decomposed composition during deposition, which could change the structure and properties of coatings, such as strength, toughness and residual stresses, to a great extent . As a result, it is empirically known that the performance of HVOF spray coating shows strong dependence upon the combined effect of feedstock powder materials and the spraying processes.