Tribocorrosion behaviour of HVOF cermet coatings

The main purpose of this work is to analyze the degradation mechanisms induced on industrial HVOF cermet coatings by tribocorrosion. Tribocorrosion of cermet coatings is a subject that has not been widely analyzed in research studies: in fact, while many works dealing with wear or corrosion of HVOF cermet coatings are published, studies relevant to the combined processes (wear and corrosion) are relatively few. The tribocorrosion mechanisms of the cermet coatings were studied in a sodium chloride solution under sliding wear, trying to combine and integrate differently produced mechanical and electrochemical damage phenomena. Electrochemical techniques such as potentiodynamic polarization curves as well as potentiostatic (I vs t) or galvanostatic (E vs t) methods were used in order to stimulate and to interprete tribocorrosion degradation mechanisms. It was shown that coating post grinding, which is a mechanical operation usually performed after the deposition of conventional cermet coatings in order to obtain a desired roughness, could produce structural damages, which can greatly affect the mechano-chemical behaviour of the cermet coatings. Mainly abrasive–adhesive wear mechanisms were observed on the coating surface and sometimes, depending on coatings mechanical properties (fracture toughness), cracks developed during wear causing the coating continuity breaking. In the latter case, the degradation mechanism is no longer governed only by surface tribocorrosion, but undermining corrosion can occur, greatly affecting sample performances and promoting coating detachment. Cr3C2–NiCr coatings, under all the selected experimental conditions, showed good barrier properties and substrate corrosion was never observed. Moreover, when chromium was added to the metal matrix of WC–Co based systems, tribocorrosion behaviour was enhanced and the lower tribocorrosion rates were measured. Finally, it was shown that electrochemical techniques can be used to govern the coating corrosion processes and to interpret the main degradation mechanisms, even though they seem not to provide a precise quantitative analysis of tribocorrosion.