Improvement of wear and corrosion resistance of gray cast irons by HVOF thermal spray coating method

In this thesis study, the microstructure, phase composition, mechanical hardness, wear, scratch, and corrosion behaviors of WC–Co and WC–Co(Cr) thermal spray coatings applied onto lamellar graphite grey cast iron (GG-20) substrates via the HVOF (High Velocity Oxygen Fuel) technique were comparatively investigated. The structural integrity of the coatings was evaluated using X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), while the mechanical performance was determined through micro-Vickers hardness, pin-on-disc wear, scratch testing, and electrochemical polarization methods. XRD results indicated that both coatings exhibited a single-phase WC carbide structure. It was observed that both coatings significantly improved the hardness, wear resistance, scratch performance, and corrosion properties of the GG20 grade flake graphite cast iron used as the substrate. While the WC-Co(Cr) coating demonstrated superior mechanical, wear, and scratch performance compared to the Cr-free WC-Co coating, the WC-Co coating showed better corrosion resistance. Microhardness measurements showed that the grey cast iron substrate had a value of 217 HB, while the WC–Co and WC–Co(Cr) coatings reached average values of 612 HV and 650 HV, respectively — corresponding to a threefold increase and an additional 6% improvement in hardness. In wear tests, Cr addition led to a reduction in the coefficient of friction and provided further improvement in volumetric wear resistance. Scratch test results also indicated that the Cr-containing coating exhibited higher critical load values, demonstrating enhanced adhesion strength. As a result, the pure WC–Co coating is more suitable for applications requiring high and stable hardness along with superior corrosion resistance, whereas the Cr-enhanced WC–Co(Cr) coating can be preferred in more demanding mechanical environments due to its improved microstructural integrity and wear resistance. The findings obtained in this study provide valuable insights for industrial coating selection and future advanced tribo-corrosion research.