Due to the continuously increasing of material requirements, a variety of surface modification and coating techniques have been proposed to protect the engineering parts from the harsh environments such as high temperature, abrasion and corrosion. Thermal spraying is a type of surface coating process which uses combination of thermal and kinetic energy to deposit coatings of metals, cermets, ceramics and polymers on the substrate surface in layers of substantial thickness, typically 0.1 to 10 mm. Coating porosity, bond strength and oxide content are typical properties influenced by the coating procedure. High velocity oxygen fuel (HVOF) spraying, which is a relatively new method of thermal spray coating family, can produce coatings with outstanding characteristics including higher density, bond strength and toughness, and lower oxide content, as a result of a favorable combination of significantly high particle velocity (1 500 m/s) at impact and relatively low temperature (2 000 ℃). In addition, it benefits the advantage of high flexibility and low cost for mass production. Thus, HVOF coatings as a protective candidate have been widely used for many years in various industrial fields, such as aviation, aerospace, petroleum, and marine components as well as systems. Although considerable research studies were carried out previously to explore the HVOF coating, the challenges for achieving the coating quality are still in progress.

In this work, the effects of parameters on coating properties of HVOF coatings were summarized in the light of the previous studies. HVOF coating performance is strongly dependent on its microstructure, which is result of particle in-flight behavior influenced by temperature, velocity, melting and oxidation of particles. The coating properties are influenced not only by the nature of the used powders but also significantly by the spray parameters. In HVOF spraying, particle morphology and size distribution of the feed powder was found to be important parameters, strongly affecting the porosity of HVOF coating. Besides, the HVOF spraying parameters make a great contribution to the flame temperature, airflow velocity and decomposition during deposition, which could change the structure and properties of final coatings. Oxygen flow rate, fuel flow rate and fuel/oxygen ratio not only determine the heat generated by combustion, flame flow characteristics and the proportion of melted raw feed powder, but also affect the heat and momentum exchange between flame flow and particles, which directly works on the porosity and hardness of the coating. The spraying distance affects the speed and temperature of the spray droplets reaching the substrate, and it leads to the increase of oxide inclusions and high local thermal stress. If powder feeding rate was too large, it would lead to an increasing of unmelted powder particles, which resulted in increased inclusion and porosity. Moreover, unmelted particles would deposit on the inner wall of the Laval nozzle, causing damage to the spray gun.

Post-treatments for HVOF coating mainly include mechanical post- treatment, thermal post-treatments (in-furnace heat treatment, laser heat treatment, hot isostatic pressing, etc), high-energy beam surface modification technology (laser remelting, electron beam remelting), coating sealing treatment. The above treatments could greatly improve a certain aspect of the coating performance, enhancing the protective effect of the coating. The surface roughness of HVOF coating is usually between 5 and 20 μm, so mechanical finish is often conducted to meet the surface roughness requirements of engineering parts. The post-heat treatment can be used to improve the microstructural and chemical homogeneity of the HVOF coating and eliminate the residual stress. The mechanical bonding of the coating/substrate can be transformed into metallurgical bonding through the high-temperature thermal diffusion reaction, which can refine the coating microstructure, enhance the bonding force between the coating and the substrate, and finally improve the coating performance. Laser remelting is an effective method for the surface modification of HVOF coatings, enhancing the erosion and corrosion resistance by eliminating defects such as microcracks and pores. In addition, laser remelting could yield a much fine and homogeneous microstructure during the rapidly melting and solidifying. During laser melting, the phase transformations take place in the remelted layers. The transformation could be introduced to improve the hardness and wear resistance of the HOVF coatings.

By filling the pores and microcracks on the HVOF coatings surfaces, sealing treatment has been proved to be a kind of effective post-treatment process in terms of corrosion resistance. In this paper, the sealing process and effects of the sealing treatment with organic or inorganic sealants was also reviewed. The relationship among process parameters, particle characteristics, and the resulting coating properties is highly nonlinear and might not be thoroughly revealed by experimental studies. Further research efforts should focus on the development of mathematical modeling of HVOF process, which would provide a fundamental understanding of the underlying momentum and heat-transfer mechanisms.