HVOF is a process whereby particles present in a powder mixture are accelerated towards the surface of a component by gas, and subsequently deposited. The resultant coating increases the lifespan and durability of the component by improving the corrosion resistance, wear resistance and erosion resistance of the material. Typical substrate materials for HVOF coatings are gas turbines, power plant boilers and engines.
The properties and behavior of two HVOF thermally sprayed coatings were compared by Zhang et al. (2015). The two coating materials that were sprayed onto a H13 steel substrate were WC−10Co−4Cr and Cr3C2−25NiCr. The Cr3C2−25NiCr coating was able to withstand a greater degree of thermal shock than the WC−10Co−4Cr coating. In addition, the friction coefficient was found to be higher for the Cr3C2−25NiCr coating. Furthermore, the Cr3C2−25NiCr coating exhibited better wear resistance. In general, the choice of coating material to be used in the HVOF process plays a vital role in the quality and properties of the component that is being coated.
Myoung, Lua, Jung, Jang, & Paik. investigated the microstructural integrity of the bond coat as a function of the change in various process parameters. A bond coat is essentially a coating that has a dense microstructure and allows for a more effective adhesion of the top coat to the substrate material. The findings indicated that as the distance of the spray decreased, the density and hardness of the coating increased. Furthermore, when the concentration of hydrogen or oxygen in the gas mix increased, the hardness of the material decreased and the occurrence of defects such as oxides and global pores increased. A decrease in the step size also resulted in a likewise decrease in the extent of observed defects. Myoung et al. were able to identify optimal conditions for the coating process in order to attain improved material properties. HVOF is an emerging technology and the demand for this technology is expected to increase over the next few years.
References
Myoung, S.W., Lua, Z., Jung, Y.G., Jang, B.K., & Paik U. (2014). Control of bond coat microstructure in HVOF process for thermal barrier coatings. Surface & Coatings Technology, 260, 63-67.
Zhang, W.C., LIU, L.B., Zhang, M.T., Huang, G.X., Liang, J.S., LI, X., Zhang, L.G. (2015). Comparison between WC−10Co−4Cr and Cr3C2−25NiCr coatings sprayed on H13 steel by HVOF. Transactions of Nonferrous Metals Society of China, 25, 3700-3707.
