Development of Near-Nanostructured Coatings by High Velocity Oxy-Fuel Spraying
Creators: name: Ben Mahmud, Tarek
Publisher: name: Graduate Studies
Year: 2018
Subjects: subject: Near-Nanostructure
subject: Coatings
subject: HVOF spraying
subject: Heat treatment
subject: Abrasive wear
subject: Microhardness
subject: Dry sliding wear
subject: Cermet
subject: Materials Science
subject: Metallurgy

description: Oil sands development makes Canada the world’s second largest holder of recoverable oil reserves and an increasingly important part of global energy security. The oil sands environment consists of a mixture of abrasive/erosive wear which affects all components engaged in the extraction and processing of the heavy oil from the up-stream reservoir to downstream slurry processing. The potential to increase the wear resistance of machinery will not only increase the wear life of components, but reduce plant downtime and costs associated with constant maintenance and replacement of parts. Various hard facing technologies have been employed to extend the wear life of surfaces using traditional fusion welding processes, however, thermal spray processes offer unique versatility and the ability to protect surfaces of different morphology and contours. Furthermore, the thickness of the coatings (200-500 m) does not add weight to the components being protected and thereby reduce fuel costs. Therefore, the High Velocity Oxy-Fuel (HVOF) spraying process has been used to deposit microstructured and nanostructured coatings based on the X-Co-Cr and X-Co compositions in which X has been a nanosized dispersion such as Cr3C2 or WC. In previous research, the dispersion has been in the100 nm range or less and during thermal spraying, severe decarburization of the tungsten carbide (WC) was observed. This resulted in the nanostructured coating having wear resistance that was inferior to that of the microstructured coatings. In this research a novel coating composition is investigated based on WC-17%NiCr. In this study a near-nanosized WC dispersion makes up the NiCr metal matrix core of the feedstock powder. The particle also contains a Co binder on the outer surface of each powder particle. As a comparison, a microstructured coating based on the WC-15%NiCr composition was also studied. The experimental results showed that both coatings could be sprayed by the HVOF technique, but the near-nanostructured coatings showed less decarburization of the WC particles with lower % porosity and better coating homogeneity than the microstructured coatings. The differences in quality of the coatings produced had a direct effect on the hardness and wear resistant properties of the two types of coatings. Hardness measurements showed that the Vickers hardness of the near-nanostructured coating was 20% greater than that of the microstructured coating. Various sliding wear tests were used to examine the wear behaviour of the coatings (ASTM-G133 and ASTM G-65) under different loads and sliding distances. The results showed an increase in wear resistance of the near-nanostructured coating by about 30% compared to the conventional coating. The mechanism of wear at the near-nanostructured coating surface was dominated by plastic deformation and removal of WC particles by a debridement process. The conventional coatings showed a wear process dominated by brittle fracture of the coating initiated around WC particles that had fractured and suffered decarburization resulting in W2C formation around particles and intersplat boundaries.