Effect of HVOF processing parameters on Cr₃C₂-NiCr hard coatings deposited on AISI 4140 steel

Abstract

High Velocity Oxygen Fuel (HVOF) thermal spray coating is a deposition process that is widely employed at present to improve surfaces and the remanufacturing of components subject to severe wear, offering an alternative to reduce the cost of spare parts and used as a substitute for electro-chromium, also generating greater thickness than chemical vapor deposition and physical vapor deposition. This study evaluates the influence of the projected application parameters for Cr₃C₂ - 20 (Ni20Cr) coatings on AISI 4140 steel using the HVOF technique. To do so, a 24 factorial experimental design was used to evaluate the effect on the thickness, porosity, surface hardness, and microhardness in the cross-section, produced by four of the main operational factors in the HVOF process: the fuel/oxygen (F/O) ratio entering the combustion chamber, powder flow, the roughness of the substrate before deposition, and spray gun speed. The morphology of the Cr₃C₂ - 20 (Ni20Cr) powders and the microstructure of the coatings were studied through a scanning electron microscope and energy dispersive spectroscopy, while the composition were analyzed by X-ray diffraction. It was determined that: (i) the F/O ratio is the operation parameter with the most significant influence on the properties studied; (ii) the gun speed has a significant effect on the thickness generated and porosity; and (iii) the powder flow and roughness of the substrate have a greater effect on the surface hardness and microhardness. Finally, it was determined that coatings with great thickness, low porosity, and great hardness, ideal for the recovery of parts, can be achieved from a F/O ratio of 0.45 and a powder flow with the system feeder rotating at 12 rpm, applied to a substrate with a roughness of Rₐ = 18 μm, combined with a spray gun speed of 5 mm/s.