High Velocity Oxy-fuel Spray Technique Research Articles

Sliding friction and wear behavior of Al–Ni–Co–Si quasicrystalline coatings deposited by the high-velocity oxy-fuel spraying technique

The sliding friction and wear performance of Al–Ni–Co–Si quasicrystalline coatings deposited by the high-velocity oxy-fuel technique were investigated under dry sliding conditions. This study indicated that changes in the imposed sliding test conditions modified the friction and wear behavior of quasicrystalline coatings. Qualitative analysis of the contact interface and wear debris were performed with the aim of understanding the role of the third body on the friction and wear processes. The dependence of the coefficient of friction on the sliding velocity and counterpart material was explained by the stick-slip behavior. It was also shown that test conditions favorable for the formation of thick intermediate layers and the densification of the coating subsurface led to low wear rates. Large cylindrical particles, formed by agglomeration of small wear debris, were suggested as a beneficial factor for the reduction of the coefficient of friction.

Component repair using HVOF thermal spraying

Abstract The high velocity oxy-fuel (HVOF) thermal spraying technique has been widely adopted in many industries due to its flexibility and cost effectiveness. It is normally used to apply coatings to components to protect against wear, heat and/or corrosion, but also has potential to build up worn components. The main objective of this study was to optimise the HVOF spraying technique for the repair of damaged tool steel (D2) and stainless steel components. Experimental work presented involved the spraying of repair material on specimens in which `defects' had been introduced. These defects were grooves of various depths machined in the samples. Each sample was then sprayed with the repair material until the whole face of the sample was covered with the deposited material. Depth of groove and spraying parameters were varied. The machinability of each repaired component was examined. Techniques used include milling, grinding, turning and spark erosion. Metallography is used to evaluate the quality of adhesion of repair material both prior and subsequent to machining, and trends based on processing parameters are identified.

Tungsten carbide coatings deposited by high-velocity oxy-fuel spraying on a metallized polymeric substrate

A triplex system consisting of WC(Co) hard coating, metallic bond layers (Ni/Cu/Ni), and acrylonitrile-butadiene-styrene as substrate, was investigated. The WC (88 wt.%)-Co (12 wt.%) coating was deposited by the high-velocity oxy-fuel spraying technique modified by the addition of CO 2 gas flows. The microstructure, the composition, and the phases present in the coating were studied by means of transmission and scanning electron microscopies both combined with energy dispersive analysis, X-ray diffraction, and Auger electron spectroscopy. It was found that during the spraying a fraction of the WC phase partially decomposes into α- and β-W 2C and β-WC 1-x and reacts with cobalt to form the ternary carbide, Co 3W 3C. The decomposition occurred on the surface of the WC particle to a thickness of several tenths of a nanometer. The microstructure of the coating consisted of tungsten carbide particles bonded by a binder matrix composed mainly of Co. The microstructure of the binder phase was found to be dependent on the cooling rate. As a result of the variation in the particles' velocities and temperatures, different interface morphologies as well as different fractions of binder phase and pore densities were observed in the coating. A theoretical model describing the dynamic and the thermal performance of the powder particles in high-velocity oxy-fuel spraying was developed, correlating between the particles' properties (size, density, emissivity) and their velocity and temperature. The factors that influence the powder velocity and temperature and thereby the coating microstructure and composition are discussed.