Tribo-corrosion behaviour of chromium carbide based coatings deposited by HVOF

• Tribocorrosion in NaCl 1M of NiCr-Cr3C2 Cermet at room temperature was studied.
• The Cermet showed better corrosion resistance than AISI 304 reference material.
• Coefficient of friction was 0.3 under cathodic and 0.1 under anodic polarisation.
• Wear losses under cathodic polarisation were twice respect to anodic conditions.

Carbide based coatings deposited by HVOF are one of the most common solutions for wear protection of mechanical components. Among them, Cermet constituted by Cr carbide in Ni–Cr matrix is widely used in applications where corrosion risk is high, because of their ability to form a passivation layer on the surface. On the other hand, the presence of the carbide particles can act as a preferential attack site of the corrosion.In the present work, the synergistic effect of corrosion and wear on Cr carbide Cermet coatings was investigated.Coatings were deposited by HVOF (High Velocity Oxy Fuel). Their microstructure, and chemical and phase composition were characterised by SEM-EDS. Coated samples were submitted to pin on disc test, to evaluate their tribological behaviour. Other samples were submitted to electrochemical tests in order to evaluate their corrosion resistance in a NaCl solution environment. Combined effects of wear and corrosion were investigated by carrying out tribocorrosion tests on coated samples, achieving potentiodynamic polarisation curves during pin on disc tests. After testing, wear tracks and corroded surfaces were characterised by SEM.The experiments confirmed the better corrosion resistance of Cermet coatings with respect to the AISI 304 substrate, also in the presence of wear. Moreover, a significant dependence of the wear and of the COF (coefficient of friction) from the corrosion potential was observed. In particular, the COF value was about 0.3 when the test was carried out under cathodic conditions, and about 0.1 under anodic polarisation.Such a behaviour could be attributed to the passivation layer grown on the wear surface. Further experiments are needed to confirm the proposed mechanism.