Velocity Oxygen Fuel Research Articles

Fretting wear behavior of aluminum coatings

The aluminum bronze coating is a type of soft solid lubricant film with excellent performance. In this work, to improve the wear resistance and tribological behavior of the aluminum alloy, the aluminum bronze coatings were prepared on aluminum alloy substrates using the High- Velocity Oxygen Fuel (HVOF) system. Fretting wear tests were performed on the coatings at 200 °C with different frequencies to analyze the wear behavior. The worn surface was characterized by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that as the frequency increased from 5 Hz to 15 Hz, the coefficient of friction increased from 0.653 to 0.929. It then decreased to 0.890 at 20 Hz. There was an overall trend of the coefficient of friction increasing and then decreasing. The worn surface’s morphology changed from pits to bulges. At a low frequency, the wear mechanism was abrasive wear with delamination. As the frequency increased, adhesive wear was observed and gradually became the dominant wear form. Oxidation occurred on the surface of the worn tracks caused by the high temperature and thermal effect of high-frequency micromotion. Due to the intensification of the oxidation phenomenon, oxidation films formed on the worn surface. The wear rate of the coating was significantly reduced. The oxide was identified as mainly CuO and Cu2O. The presented results confirmed that the aluminum bronze coating fabricated via HVOF exhibited a great resistance performance even at a high frequency.

High-temperature corrosion behavior of selected thermally sprayed coatings in corrosive aggressive environment

The aim of this paper is to evaluate and compare the corrosion resistance of Hastelloy C-276 coating with Stellite 6 and Cr3C2%-25% NiCr coatings. The coatings were deposited using HP/HVOF (High Pressure/High Velocity Oxygen Fuel) thermal spraying technology. The corrosion test was conducted in the corrosive-aggressive environment of molten salts mixture in the composition of 40% Na2SO4–60% V2O5, simulating the corrosion environment of boilers for residual fuel oils. The corrosion test under cyclic conditions included 50 cycles at the temperature of 750 °C and subsequent cooling at the room temperature for 20 min. The corroded specimens were analyzed using scanning electron microscope (SEM) and elemental analysis (EDS). In comparison with Cr3C2-25% NiCr coating, both Stellite 6 and Hastelloy C-276 coatings provided the substrate material with sufficient protection against high-temperature corrosion. The corrosion mechanism of Hastelloy C-276 coating was based on the formation of protective oxide layer which prevented further penetration of corrosive media. Stellite 6 coating exhibited the corrosion attack along splat boundaries resulting in spat delaminating into the build-up. The corrosion mechanism of Cr3C2-25% NiCr coating included the attack on NiCr matrix resulting in gradual separation of carbide phases into the corrosive environment.

Polarization Resistance of Stainless Steel-Coated Rebars

Stainless steel (316L) coated coupons and rebars were prepared using the twin-wire electric arc (TWEA) and high pressure/high velocity oxygen-fuel (HP/HVOF) processes. Metallographic examination was performed to characterize the coating density and the oxide content. The corrosion performance of the coatings was evaluated using linear polarization, AC impedance, and salt spray techniques. It appears that the coating prepared using the HP/HVOF process is far superior in terms of polarization resistance performance than that prepared using the TWEA process. The former process produces a dense, low-oxide-content coating while the latter produces relatively porous coatings.