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7362 |
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HVOF NbC-NiCr coatings as an alternative to conventional hardmetals |
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Larissa Rossi Gehlen / University of Modena and Reggio Emilia and Federal University of Technology – Paraná, Italy Giovanni Bolelli / University of Modena and Reggio Emilia, Italy Giuseppe Pintaude/ Federal University of Technology – Paraná, Brazil Alberto Colella/ MBN Nanomaterialia S.p.A, Italy Enrico Forlin/ MBN Nanomaterialia S.p.A, Italy Luca Lusvarghi/ University of Modena and Reggio Emilia, Italy |
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Conventional hardmetal coatings by thermal spraying are based on WC-Co(Cr) and Cr3C2-NiCr compositions. WC-based hardmetals exhibit excellent wear resistance at room and moderate temperatures and, adding Cr to the matrix, good corrosion resistance. However, they are unsuitable for application at high temperatures due to thermal expansion mismatch with most substrate materials (including e.g. steels) and to catastrophic oxidation above 550 °C. Cr3C2-NiCr coatings exhibit excellent corrosion resistance and they are resistant to oxidation and wear up to 800 – 900 °C; however, due to their lower hardness, they offer less wear protection at low and moderate temperatures. To bridge the gap between these formulations, this work explores the use of NbC-based hardmetals, as NbC presents the possibility to form a protective layer able to increase the wear performance at moderate temperatures. Feedstock powders with NbC-25vol.%NiCr and NbC-40vol.%NiCr composition were obtained by high-energy ball milling and were HVOF-sprayed onto grit-blasted AISI 304 stainless steel plates using an H2-fuelled Diamond Jet 2600 system with different process parameter sets. Dense coatings with ~900 HV0.3 and a moderate amount of oxide inclusions were obtained with both feedstock powders and under all processing conditions explored in this work. The tribological responses were assessed in two systems: i) the sliding wear resistance under ball-on-disc configuration at room temperature and 200 °C using a sintered WC-Co as the counterpart, and at 600 °C utilizing a sintered Al2O3 ball, and ii) the abrasive wear resistance under steel-wheel apparatus fed with dry particles. Their corrosion resistance was evaluated by electrochemical polarization in 3.5 wt.% NaCl solution. The performance of the coatings was compared to that of WC-10Co4Cr, Cr3C2-25%(Ni-20Cr), and TiC-40vol.%NiCr references sprayed with the same HVOF system. |